Dissertations / Theses on the topic 'Computational Combustion'

Implicit Large Eddy Simulation (ILES) with high-resolution and high-order computational modelling has been applied to flows with turbulent mixing and combustion. Due to the turbulent nature, mixing of fuel and air and the subsequent combustion still remain challenging for computational fluid dynamics. However, recently ILES, an advanced numerical approach in Large Eddy Simulation methods, has shown encouraging results in prediction of turbulent flows. In this thesis the governing equations for single phase compressible flow were solved with an ILES approach using a finite volume Godunov-type method without explicit modelling of the subgrid scales. Up to ninth-order limiters were used to achieve high order spatial accuracy. When simulating non chemical reactive flows, the mean flow of a fuel burner was compared with the experimental results and showed good agreement in regions of strong turbulence and recirculation. The one dimensional kinetic energy spectrum was also examined and an ideal k−5/ 3 decay of energy could be seen in a certain range, which increased with grid resolution and order of the limiter. The cut-off wavenumbers are larger than the estimated maximum wavenumbers on the grid, therefore, the numerical dissipation sufficiently accounted for the energy transportation between large and small eddies. The effect of density differences between fuel and air was investigated for a wide range of Atwood number. The mean flow showed that when fuel momentum fluxes are identical the flow structure and the velocity fields were unchanged by Atwood number except for near fuel jet regions. The results also show that the effects of Atwood number on the flow structure can be described with a mixing parameter. In combustion flows simulation, a non filtered Arrhenius model was applied for the chemical source term, which corresponds to the case of the large chemical time scale compared to the turbulent time scale. A methane and air shear flow simulation was performed and the methane reaction rate showed non zero values against all temperature ranges. Small reaction rates were observed in the low temperature range due to the lack of subgrid scale modelling of the chemical source term. Simulation was also performed with fast chemistry approach representing the case of the large turbulent time scale compared to the chemical time scale. The mean flow of burner flames were compared with experimental data and a fair agreement was observed.

The thesis comprises of a thorough assessment of turbulent non-premixed combustion modelling techniques, emphasising the fundamental issue of turbulence-chemistry interaction. The combustion models studied are the flame-sheet, equilibrium, eddy breakup and laminar flamelet models. An in-house CFD code is developed and all the combustion models are implemented. Fundamental numerical issues involving the discretisation schemes are addressed by employing three discretisation schemes namely, hybrid, power law and TVD. The combustion models are evaluated for a number of fuels ranging from simple H2/CO and CO/H2/N2 to more complex Cl4/H2 burning in bluff body stabilised burners at different inlet fuel velocities. The bluff body burner with its complex recirculation zone provides a suitable model problem for industrial flows. The initial and boundary conditions are simple and well-defined. The bluff body burner also provides a controlled environment for the study of turbulence-chemistry interaction at the neck zone. The high quality experimental database available from the University of Sydney and other reported measurements are used for the validation and evaluation of combustion models. The present calculations show that all the combustion models provide good predictions for near equilibrium flames for temperature and major species. Although the equilibrium chemistry model is capable of predicting minor species, the predictive accuracy is found to be inadequate when compared to the experimental data. The laminae flamelet model is the only model which has yielded good predictions for the minor species. For flames at higher velocities. the laminar flamelet model again has provided better predictions compared to predictions of other models considered. With different fuels, the laminar flamelet model predictions for CO/H2/N2 fuel are better than those for CH4/H2 fuel. The reasons for this discrepancy are discussed in detail. The effects of differential diffusion are studied in the laminar flamelet modelling strategy. The flamelet with unity Lewis number is found to give a better representation of the transport of species. The laminar flamelet model has yielded reasonably good predictions for NO mass fraction. The predictions of NO mass fraction are found to be very sensitive to differential diffusion effects. This study has also considered the issue of inclusion of radiative heat transfer in the laminar flamelet model. The radiation effects are found to be important only where the temperature is very high. The study undertaken and reported in this thesis shows that the presently available laminar flamelet modelling concepts are capable of predicting species concentrations and temperature fields with an adequate degree of accuracy. The flamelet model is also well suited for the prediction of NO emissions. The inclusion of radiation heat transfer has enhanced the predictive capability of the laminar flamelet model.

The combustion process of liquid conventional and biofuels depend on factors ranging from the thermophysicochemical properties associated with such fuels to the combustion infrastructure used to burn them. A third class of fuels commonly referred to as surrogate fuels can be obtained by mixing conventional and biofuels. It is thought that the existence of oxygen atoms in biofuels play a crucial role in the way they burn in a stream of air, in uencing not only the e ciency of the combustion process of such class of fuels but also the emissions. The mechanisms through which the existing oxygen atoms in uence the combustion process of biofuels (and its surrogates) are still debatable and unestablished. This thesis sheds light on the points mentioned in the paragraph above. Extensive computational and experimental work was done to elucidate the combustion process of conventional, surrogate and biofuels. Some of the reaction mechanisms used in modelling the current reactive ow simulation are already tested while others were developed during the course of this work. The computational results have shown good agreement with the available experimental data. One of the most important observations and ndings reported in this work was that when comprehensive reaction models were used, the injected fuels burned at a slower rate compared to the situation when reduced models were employed. While such comprehensive models predicted better ame structure and far better by-products compared to the existing experimental results, it has also led to di erences in some parameters, especially the temperature eld. The computational prediction has also shown that biodiesel produces a marginally higher rate of COx compared to diesel which was also observed experimentally using a Compression Ignition Engine (CIE). Having said so, the experimental work also showed that surrogate fuels perform far better than pure diesel and biodiesel in CIE) in terms of emissions. The experimental work further addressed some phyisical and spectral analysis of diesel, biodiesel and nine blends as well as assessing the performance of a combination of these fuels in a compression ignition engine. The results are in line with what has reported in the literature but also sheds light on important features related to surrogate fuels and explain better the expected structure of such blends which may in uence the way they burn under di erent environments. With regards to the harmfull emissions of the combustion of liquid fuels, biodiesel was found to produce harmful emissions in a lower quantity compared to conventional diesel which is in line with the ndings of many experimental data. The computational ndings have also predicted less energy content and temperature range for biofuels of order 10-15% which is also in agreement with many experimental ndings cited in the literature.

Las crecientes exigencias de la industria están cambiando la forma en que entendemos la sociedad y el entorno en el que vivimos. Frente a la necesidad de un comercio rápido y globalizado, están emergiendo varios problemas de sostenibilidad. Por una parte, ciertos sectores resultan favorecidos, como es el caso del transporte y su radical incremento de actividades. Por otra parte, esto causa un impacto negativo considerable en los ecosistemas terrestres. En este marco, los efectos negativos de la contaminación ambiental y sonora están llegando a límites realmente preocupantes, siendo estos especialmente visibles en los principales núcleos urbanos, donde las autoridades están incluso restringiendo la circulación de los vehículos térmicos. Particularmente, el ruido producido por la quema del combustible en vehículos propulsados por motores de combustión interna alternativos, siendo una de las principales fuente acústicas por delante de otras como la aviación o el ferrocarril, está siendo objeto de recientes estudios para reducir sus efectos perjudiciales en la población. El objetivo principal de esta tesis se centra en el estudio y caracterización de la combustión como fuente de emisiones acústicas. Concretamente, esta investigación tiene como propósito dar respuesta a cuáles son los fenómenos físicos asociados a la generación del ruido en motores de encendido por compresión, así como proponer algunas directrices que ayuden a entender y mejorar -desde el punto de vista de emisiones acústicas y consumo- el diseño de los motores actuales. En una primera aproximación, se recurre a técnicas experimentales de medida para, con el registro de la presión instantánea dentro de la cámara de combustión, caracterizar el origen de las perturbaciones acústicas. A pesar de que la información aportada por estos métodos es relevante, existen limitaciones para recrear la espacialidad del campo acústico y, por tanto, dificultan la comprensión de los fenómenos no estacionarios asociados a este. Por esta razón, en posteriores estudios se recurre al uso de la dinámica de fluidos computacional o CFD, superando así las limitaciones de las técnicas experimentales y permitiendo una visualización completa del problema. Como paso previo e indispensable, se procede a implementar y validar del modelo CFD para asegurar una buena precisión en los resultados y un tiempo de cálculo razonable. La aplicación de métodos de análisis en frecuencia y descomposición modal han permitido estudiar el campo de presiones en el interior de la cámara y así entender mejor su comportamiento. De este modo, ha sido posible encontrar relaciones entre la combustión y la respuesta espectral del campo acústico interno. Los patrones de oscilación de la presión muestran que las estructuras más energéticas, y que por tanto contribuyen a la emisión acústica en mayor medida, están centradas en estructuras macroscópicas de tamaño similar a la geometría de la cámara. Además, se ha demostrado que la posición de la ignición del combustible tiene un efecto directo sobre la amplitud de los modos resonantes y su distribución espacial. Por último, en cuanto a la evaluación de estrategias para mitigar el ruido, se proponen distintos estudios en los que se analizan las tendencias en la emisión acústica al modificar la fuente sonora, mediante la configuración de la inyección y la geometría del sistema de combustión.
Les creixents exigències de la indústria estan canviant la forma en què entenem la societat i l'entorn en què vivim. Davant la necessitat d'un comerç ràpid i globalitzat estan sorgint diversos problemes de sostenibilitat que, per una part afavoreixen que sectors com el del transport incrementen les seues activitats de forma radical, però que per l'altra, causen un impacte negatiu en els ecosistemes terrestres. En aquest context, els efectes negatius de la contaminació ambiental i sonora estan arribant a límits realment preocupants, sent aquests especialment visibles als principals nuclis urbans on les autoritats estan inclús restringit la circulació dels vehicles tèrmics. Particularment, el soroll causat per la crema de combustible en vehicles propulsats per motors de combustió interna alternatius, sent una de les principals fonts acústiques per davant d'altres com l'aviació o el ferrocarril, està sent objecte de recents estudis per tal de reduir els efectes perjudicials en la població. L'objectiu principal d'aquesta tesi es centra en l'estudi i caracterització de la combustió com a font d'emissions acústiques. Concretament, aquesta investigació té com a propòsit donar resposta a quins són els fenòmens físics associats a la generació de soroll en motors d'encès per compressió, així com proposar algunes directrius que ajuden a entendre i millorar -des del punt de vista de les emissions acústiques i consum- el disseny dels motors actuals. En una primera aproximació, es recorre a tècniques experimentals de mesura per a, amb el registre de la pressió instantània en la cambra de combustió, caracteritzar l'origen de les pertorbacions acústiques. Tot i que la informació aportada per aquests mètodes és rellevant, existeixen limitacions per a reconstruir l'espacialitat del camp acústic i, per tant, dificulten la comprensió dels fenòmens no estacionaris associats a aquest. Per aquesta raó, en posteriors estudis es recorre a l'ús de la dinàmica de fluids computacional o CFD, superant així les limitacions de les tècniques experimentals i permetent una visualització completa del problema. Com a pas previ i indispensable, es procedeix a implementar i validar el model CFD per assegurar una bona precisió en els resultats i un temps de càlcul raonable. L'aplicació de mètodes d'anàlisi en freqüència i descomposició modal ha permès estudiar el camp de pressions en l'interior de la càmera i així entendre millor el seu comportament. D'aquesta forma, ha sigut possible trobar relacions entre la combustió i la resposta espectral del camp acústic intern. Els patrons d'oscil·lació de la pressió mostren que les estructures més energètiques, i que per tant contribueixen a l'emissió acústica en major mesura, estan centrades en estructures macroscòpiques de grandària similar a la geometria de la càmera. A més, s'ha demostrat que la posició de la ignició del combustible té un efecte directe sobre l'amplitud dels modes ressonants i la seua distribució espacial. Per últim, pel que fa a l'avaluació de diverses estratègies per a mitigar el soroll, es proposen distints estudis en què s'analitzen les tendències en l'emissió acústica en modificar la font sonora mitjançant la configuració de l'injector i la geometria del sistema de combustió.
The ever-increasing demands of industry are changing the way we understand society and the environment in which we live. In the face of the need for rapid and globalised trade, a number of sustainability issues are emerging which, on the one hand, encourage sectors such as transport to radically increase their activities, but, on the other hand, cause a negative impact on terrestrial ecosystems. In this context, the negative effects of environmental and noise pollution are reaching really worrying limits, these being especially visible in the main urban areas where the authorities are even restricting the circulation of vehicles powered with thermal engines. In particular, the noise produced by the fuel burning in vehicles powered by reciprocating internal combustion engines, being one of the main acoustic sources ahead of others such as aviation or railways, is being the focus of recent studies to reduce its harmful effects on the population. The main objective of this thesis focuses on the study and characterization of combustion as a source of noise emissions. Specifically, this research focuses on addressing the physical phenomena associated with noise generation in compression-ignited engines, as well as proposing some guidelines in order to better understand and improve -from the point of view of noise emissions and fuel consumption- the design of current engines. In a first approach, experimental techniques are used to characterise the source of the acoustic disturbances by recording the instantaneous pressure inside the combustion chamber. Although the information provided by these methods is relevant, there are some limitations to recreate the spatiality of the acoustic field and, therefore, make it difficult to understand the non-stationary phenomena associated with it. For this reason, in subsequent studies the Computational Fluid Dynamics or CFD approach is utilized, thereby overcoming the limitations of experimental techniques and allowing a complete visualization of the problem. As a preliminary and indispensable step, we proceed to implement and validate the CFD model to ensure a good accuracy in the results and a reasonable calculation time. The application of frequency analysis and modal decomposition methods has made it possible to study the pressure field inside the chamber and thus better understand its behaviour. In this way, it has been possible to find relationships between the combustion and the spectral response of the internal acoustic field. The pressure oscillation patterns show that the most energetic structures, and thus contributing the most to the acoustic emission, are centred on macroscopic structures of similar size to the chamber geometry. In addition, the ignition position of the fuel has been shown to have a direct effect on the amplitude of the resonant modes and their spatial distribution. Finally, regarding the evaluation of different strategies for mitigating noise, different studies are proposed in which the trends in noise emission are analysed by modifying the sound source through the injection configuration and the geometry of the combustion system.
Gómez Soriano, J. (2018). Computational assessment of combustion noise of automotive compression-ignited engines [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/112726
TESIS

A numerical study of turbulent combustion systems was pursued to examine different computational modeling techniques, namely computational fluid dynamics (CFD) and chemical reactor network (CRN) methods. Both methods have been studied and analyzed as individual techniques as well as a coupled approach to pursue better understandings of the mechanisms and interactions between turbulent flow and mixing, ignition behavior and pollutant formation. A thorough analysis and comparison of both turbulence models and chemistry representation methods was executed and simulations were compared and validated with experimental works. An extensive study of turbulence modeling methods, and the optimization of modeling techniques including turbulence intensity and computational domain size have been conducted. The final CFD model has demonstrated good predictive performance for different turbulent bluff-body flames. The NOx formation and the effects of fuel mixtures indicated that the addition of hydrogen to the fuel and non-flammable diluents like CO2 and H2O contribute to the reduction of NOx. The second part of the study focused on developing chemical models and methods that include the detailed gaseous reaction mechanism of GRI-Mech 3.0 but cost less computational time. A new chemical reactor network has been created based on the CFD results of combustion characteristics and flow fields. The proposed CRN has been validated with the temperature and species emission for different bluff-body flames and has shown the capability of being applied to general bluff-body systems. Specifically, the rate of production of NOx and the sensitivity analysis based on the CRN results helped to summarize the reduced reaction mechanism, which not only provided a promising method to generate representative reactions from hundreds of species and reactions in gaseous mechanism but also presented valuable information of the combustion mechanisms and NOx formation. Finally, the proposed reduced reaction mechanism from the sensitivity analysis was applied to the CFD simulations, which created a fully coupled process between CFD and CRN, and the results from the reduced reaction mechanism have shown good predictions compared with the probability density function method.
Ph. D.

This work focuses on aspects of combustion in a spark ignition engine. A pent-roof research engine was used to generate an experimental data set which was combined with a preexisting data set from a disc-chamber research engine. The combined dataset was used to refine a thermodynamic spark ignition engine combustion code which could operate in either a three-zone entrainment and bum up, or a two-zone direct combustion, configuration. The pent-roof engine was skip-fired to ensure residual gases were purged and care was taken to ensure that the thermodynamic state and chemical composition of the intake mixture were well defined. The combustion chamber, which featured near complete optical access, was illuminated using a sheet of laser light. Mie scattered laser light from fine seed particles was recorded allowing the position of the flame front to be tracked. These images were then ensemble averaged and combined to give a three-dimensional reconstruction of the mean combustion progress variable field for three different engine speeds. As the flame approached the combustion chamber walls it was found to decelerate. A relationship between the burning velocity of an unconstrained flame and a flame approaching a wall was derived which agreed well with the experimental results. This relationship was incorporated into the engine simulation code and found to improve greatly the predictions of the two-zone combustion model. New flame acceleration and laminar burning velocity submodels reported in the literature were added to the engine simulation code. The suitability of these models for simulating spark ignition engine combustion was evaluated using the disc and pent roof experimental data sets and model constants adjusted to optimise the performance of each submodel. Although there were substantial differences between the individual submodels, over the range of operating conditions for which experimental data was available changes to the submodel used had a negligible effect on the combustion model predictions. The work concludes with an evaluation of the performance of the three-zone combustion model for simulating a pent-roof engine. The model was modified for the pent-roof engine to include suitable assumptions for turbulence and the position of the centre of the flame. Using constants which were chosen to fit data recorded in the disc chamber engine, the model predictions for the pentroof engine were comparable in accuracy to predictions for the disc roof engine. The model was incorporated in a commercially available manifold gas dynamics simulation software to allow the predictive simulation of complete engine cycles.

The combustion of fossil fuels, in particular coal, meets the majority of energy demand worldwide, but produces carbon dioxide, which is believed to be the main cause of climate change. Since the majority of energy comes from coal-fired power stations, the deployment of carbon capture and storage (CCS) technologies, which remove the CO2 by either utilisation or storage, are necessary to mitigate climate change. Oxy-fuel combustion is one of the leading options for CCS. The fuel combusts in a mixture of oxygen and recycled ue gas, rather than in air and the change in the oxidiser environment poses questions relating to combustion characteristics such as heat transfer, emissions and burnout. To gain a further understanding of the process, the use of modelling and simulation techniques can be employed and in this thesis, Computational Fluid Dynamics (CFD) is used to model air and oxy-fuel environments using advanced combustion sub-models. An in-house Large Eddy Simulation (LES) CFD code has been updated to include models suitable for the prediction of NO. The model is verified and compared against available experimental data for three cases involving methane, coal and oxycoal combustion. Advanced simulations of a 250 kWth combustion test facility (CTF) are validated against experimental measurements of air-coal combustion. The geometry set-up and simplifications are discussed followed by a sensitivity study of grid refinement, turbulence models and approaches in modelling gaseous radiative properties. The validated CFD simulation of the facility were then numerically examined under a number of oxy-fuel environments. Finally, CFD simulations were performed on a full-scale utility boiler at 500MWe to examine the effects of firing coal and biomass under air and oxy-fuel environments. This included an assessment of the heat transfer as a method of addressing the performance of the boiler under these conditions.

This thesis presents the results and analyses of computational studies on certain problems of combustion instability in solid propellants. Specifically, effects of relaxing certain assumptions made in previous models of unsteady burning of solid propellants are investigated. Knowledge of unsteady burning of solid propellants is essential in studying the phenomenon of combustion instability in solid propellant rocket motors. In Chapter 1, an introduction to different types of unsteady combustion investigated in this thesis, such as 1) intrinsic instability, 2) pressure-driven dynamic burning, 3) extinction by depressurization, and 4) L* -instability, is given. Also, a review of previous experimental and theoretical studies of these phenomena is presented. From this review it is concluded that all the previous studies, which investigated the unsteady combustion of solid propellants, made one or more of the following assumptions: 1) quasi-steady gas-phase (QSG), 2) quasi-steady condensed phase reaction zone (QSC), 3) small perturbations, and 4) unity Lewis number. These assumptions limit the validity of the results obtained with such models to: 1) relatively low frequencies (< 1 kHz) of pressure oscillations and 2) small deviations in pressure from its steady state or mean values. The objectives of the present thesis are formulated based on the above conclusions. These are: 1) to develop a nonlinear numerical model of unsteady solid propellant combustion, 2) to relax the assumptions of QSG and QSC, 3) to study the consequent effects on the intrinsic instability and pressure-driven dynamic burning, and 4) to investigate the L* -instability in solid propellant rocket motors. In Chapter 2, a nonlinear numerical model, which relaxes the QSG and QSC assumptions, is set up. The transformation and nondimensionalization of the governing equations are presented. The numerical technique based on the method of operator-splitting, used to solve the governing equations is described. In Chapter 3, the effect of relaxing the QSG assumption on the intrinsic instability is investigated. The stable and unstable solutions are obtained for parameters corresponding to a typical composite propellant. The stability boundary, in terms of the nondimensional parameters identified by Denison and Baum (1961), is predicted using the present model. This is compared with the stability boundary obtained by previous linear stability theories, based on activation energy asymptotics in the gas-phase, which employed QSC and/or QSG assumptions. It is found that in the limit of large activation energy and low frequencies, present result approaches the previous theoretical results. This serves as a validation of the present method of solution. It is confirmed that relaxing the QSG assumption widens the stable region. However, it is found that a distributed reaction in the gas-phase destabilizes the burning. The effect of non-unity Lewis number on the stability boundary is also investigated. It is found that at parametric values corresponding to low pressures and large flame stand-off distances, small amplitude, high frequency (at frequencies near the characteristic frequency of the gas-phase) oscillations in burning rate appear when the Lewis number is greater than one. In Chapter 4, the effect of relaxing the QSG assumption is further investigated with respect to the pressure-driven dynamic burning. Comparison of the pressure-driven frequency response function, Rp, obtained with the present model, both in the QSG and non-QSG framework, with those obtained with previous linear stability theories invoking QSG and QSC assumptions are made. As the frequency of pressure oscillations approaches zero, |RP| predicted using present models approached the value obtained by previous theoretical studies. Also, it is confirmed that the effect of relaxing QSG is to decrease the |Rp| at frequencies near the first resonant frequency. Moreover, relaxing QSG assumption produces a second resonant peak in |Rp| at a frequency near the characteristic frequency of the gas-phase. Further, Rp calculated using the present model is compared with that obtained by a previous linear theory which relaxed the QSG assumption. The two models predicted the same resonant frequencies in the limit of small amplitudes of pressure oscillations. Finally, it is found that the effect of large amplitude of pressure oscillations is to introduce higher harmonics in the burning rate and to reduce the mean burning rate. In Chapter 5, first the present non-QSC model is validated by comparing its results with that of a previous non-QSC model for radiation-driven burning. The model is further validated for steady burning results by comparing with experimental data for a double base propellant (DBP). Then, the effect of relaxing the QSC assumption on steady state solution is investigated. It is found that, even in the presence of a strong gas-phase heat feedback, QSC assumption is valid for moderately large values of condensed phase Zel'dovich number, as far as steady state solution is concerned. However, for pressure-driven dynamic burning, relaxing the QSC assumption is found to increase |RP| at all frequencies. The error due to QSC assumption is found to become significant, either when |Rp| is large or as the driving frequency approaches the characteristic frequency of the condensed phase reaction zone. The predicted real part of the response function is quantitatively compared with experimental data for DBP. The comparison seems to be better with a value of condensed phase activation energy higher than that suggested by Zenin (1992). In Chapter 6, burning rate transients for a DBP during exponential depressurization are computed using non-QSG and non-QSC models. Salient features of extinction and combustion recovery are predicted. The predicted critical initial depressurization rate, (dp/dt)i, is found to decrease markedly when the QSC assumption is relaxed. The effect of initial pressure level on critical (dp/dt)i is studied. It is found that the critical (dp/dt)i decreases with the initial pressure. Also, the overshoot of burning rate during combustion recovery is found to be relatively large with low initial pressures. However as the initial pressure approached the final pressure, the reduction in initial pressure causes a large increase in the critical (dp/dt)i. No extinction is found to occur when the initial pressure is very close to the final pressure. In Chapter 7, a numerical model is developed to simulate the L* -instability in solid propellant motors. This model includes a) the propellant burning model that takes into account nonlinear pressure oscillations and that takes into account an unsteady gas- and condensed phase, and b) a combustor model that allows pressure and temperature oscillations of finite amplitude. Various regimes of L* -burning of a motor, with a typical composite propellant, namely 1) steady burning, 2) oscillatory burning leading to steady state, 3) oscillatory burning leading to extinction, 4) reignition and 5) chuffing are predicted. The predicted dependence of frequency of L* -oscillations on mean pressure is compared with one set of available experimental data. It is found that proper modeling of the radiation heat flux from the chamber walls to the burning surface may be important to predict the re-ignition. In Chapter 8, the main conclusions of the present study are summarized. Certain suggestions for possible future studies to enhance the understanding of dynamic combustion of solid propellants are also given.

Thesis (MScEng)--University of Stellenbosch, 2003.
ENGLISH ABSTRACT: The automotive industry in South Africa is expanding as a result of pressure on the world economy that forces vehicle manufacturers to outsouree work to developing countries. In order to add value to automotive engine development, the capability to perform state-of-the-art engineering must be developed in this country. Threedimensional fluid flow simulation is one such area and is being developed in this study in order to enhance the ability to develop combustion systems. Another capability being developed at the University of Stellenbosch is the simulation of valve train dynamics. It was realised that there is a lack of research results of in-cylinder flow characteristics and how they influence combustion chamber processes. This project focuses on the investigation of two different combustion chamber geometries and how they influence the flow and combustion processes in two different combustion chambers. The aim is to gain a better understanding of combustion chamber flow as an indirect result from comparing the flow in two fundamentally different engines under similar operating conditions. The difference in the engines is that one was developed for reduced exhaust gas emissions while the other was developed to achieve high performance. The numerical simulation capability is developed in the process of achieving this goal. To achieve the above-mentioned aim, a literature study was performed on the different combustion chamber flow characteristics and how they are influenced by different configurations. An experimental method of measuring combustion characteristics is studied in order to establish the ability to perform the latter. Theory of numerical flow simulation is also studied with this same goal in mind. Experimental testing is performed and combustion analysis is done on the results. In conjunction to the experimental work, numerical flow simulations are performed on the two different combustion chambers. The results from experimental testing and numerical simulations have shown that obstructions in the flow into the combustion chamber, together with a port configuration that cause flow around the longitudinal axis of the cylinder, increases the rate at which fuel burns in the combustion chamber and thereby reduce the production of toxic emissions from the engine. The study also proved that reducing resistance to flow increases the amount of air that is breathed by the engine and thereby results in increased torque generation. Through this study, opportunities for further research are identified. The results of the study can be used when new combustion systems are developed, especially in the light of ongoing tightening of emission regulations. The contribution to numerical flow simulation capabilities developed in this study add value to the ability to develop new combustion systems in the future, especially when complimented by some of the further research topics identified.
AFRIKAANSE OPSOMMING: Die motorbedryf in Suid-Afrika is besig om vinnig te ontwikkel as direkte gevolg van druk op die wêreldekonomie wat internasionale motorvervaardigers forseer om werk na ontwikkelende lande uit te kontrakteer. Hoogs gesofistikeerde ingenieurstegnieke moet ontwikkel word in Suid-Afrika met die doelom waarde toe te voeg aan enjin ontwikkeling. Drie-dimensionele vloei simulasie is een van hierdie vermoëns en word tydens hierdie studie ontwikkelom die verbrandingstelsel ontwikkelings-vaardighede te bevorder. Nog 'n vaardigheid wat tans ontwikkel word aan die Universiteit van Stellenbosch is die vermoë om nok-en-klepstelsel dinamika te simuleer. Daar bestaan egter 'n leemte in navorsingsresultate van vloei eienskappe binne in die verbrandingsruim en hoe dit verbrandingsruim prosesse beïnvloed. Die projek fokus dus op 'n ondersoek van twee verskillende geometriese konfigurasies van die verbrandingsruim en hoe dit die vloei- en verbrandingsprosesse in die twee konfigurasies beïnvloed. Die doel is om 'n beter begrip te ontwikkel van verbrandingsruim prosesse as 'n indirekte gevolg van die vergelyking tussen twee fundamenteel verskillende enjins onder eenderse bedryfstoestande. Die verkil tussen die twee enjins is dat een ontwikkel is met die doelop verlaagde uitlaatgas emmissies en die ander ontwikkel is om verbeterde werkverrigting. Die numeriese simulasie vermoë is ontwikkel in die proses om die doel te bereik. Om bogenoemde doel te bereik is 'n literatuurstudie gedoen wat verskillende vloeieienskappe in die verbrandingsruim ondersoek, asook hoe dit deur verskillende konfigurasies beïnvloed word. 'n Eksperimentele metode III die bepaling van verbrandingseienskappe is ook bestudeer met die doelom laasgenoemde uit te voer. Teorie aangaande numeriese vloei simulasie is ook bestudeer met bogenoemde doel. Eksperimentele toetse is gedoen en verbrandingsanalise uitgevoer op die resultate. In kombinasie met die eksperimentale werk is numeriese simulasies van die prosesse in die twee verbrandingsruim konfigurasies uitgevoer. Die resultate van die eksperimentele toetse en numeriese simulasies toon dat obstruksies in die vloei na die verbrandingsruim, gesamentlik met die poort konfigurasie wat veroorsaak dat lug om die longitudinale as van die silinder vloei, die tempo waarteen die lug-brandstof mengsel verbrand verhoog en sodoende die vrystelling van skadelike uitlaatgasse na die atmosfeer verminder. Die studie het ook getoon dat die vermindering van weerstand teen vloei, die hoeveelheid lug wat in die verbrandingsruim invloei vermeerder en sodoende die wringkrag wat deur die enjin gelewer word verhoog. Deur die studie is verdere navorsingsgeleenthede uitgewys. Die resultate van die studie kan gebruik word in die ontwikkeling van nuwe verbrandingstelsels, veral in die lig van verstrengende regulasies rakende uitlaatgas emmissies. Die bydrae tot numeriese vloei simulasie vermoëns ontwikkel in hierdie studie voeg waarde toe tot die vermoë om nuwe verbrandingstelsels te ontwikkel, veral wanneer dit gekomplimenteer word met van die verdere navorsingsonderwerpe wat geïdentifiseer is.

The process of shock-induced ignition has been investigated both computa- tionally and experimentally, with particular emphasis on the concept of radical farming. The first component of the investigation contained Computational Fluid Dynamic (CFD) calculations of an ignition delay study, a 2D pre-mixed flow over flat plate at a constant angle to the freestream, and through a generic 2D scramjet model. The focal point of the investigation however examined the complex 3D flow through a generic scramjet model. Five experimental test conditions were ex- amined over flow enthalpies from 3.4 MJ/kg to 6.4 MJ/kg. All test conditions simulated flight at 21000 metres ([symbol=almost equal to] 70000 ft), while the equivalent flight Mach number varied from approximately 8.5 at the lowest enthalpy, to approximately Mach 12 at the highest enthalpy condition. The presence of H2 fuel injected in the intake caused a separated region to form on the lower surface of the model at the entrance to the combustor. A fraction of the total mass of fuel was entrained in this separated region, providing long residence times, hence increased time for the chemical reactions that lead to ignition to occur. In addition, extremely high temperatures were found to exist between each fuel jet. Both fuel and air are present in these regions, therefore the chance of ignition in these regions is high. Streamlines passing through the recirculation zone ignited within this zone, while streamlines passing between the fuel jets ignited soon after entry into the combustor. The first instance of a pressure rise from combustion was observed on the centreline of the model where the reflected bow shock around the fuel jets crossed the centreline of the combus- tor. Upstream of this location the static pressure of the flow was too low for the chemical reactions that release heat to occur. The comparison between the experimental and computational results was lim- ited due to inaccuracies in modelling the thermal state of the gas in the CFD calculations. The gas was modelled as being in a state of thermal equilibrium at all times, which incorrectly models the freestream flow from the nozzle of the shock tunnel, and also the flow downstream of oblique shock wave within the scramjet model. As a result combustion occurs sooner in the CFD calculations than in the experimental result.

A novel concept which couples ID and 2D CFD models in a simulation of unsteady lC engine flows was investigated, and such a coupled model was developed. Two unified solution procedures which are capable of predicting mixed compressible and incompressible flow fields found in an engine were developed and comparatively studied. One is the pressure correction algorithm, the other is the block implicit algorithm. They provided platforms for the implementation of coupled models. Second order spatial and Euler backward time differencing schemes were adopted. The comprehensive comparative studies were performed on a variety of benchmark flows ranging from steady to unsteady, incompressible to compressible. The data documented have shown that the prediction qualities of the two algorithms were comparable in all calculations. The block implicit procedure required more storage memory generally but it converged faster in all cases except the incompressible flow calculations. General strategies to couple the ID CFD model with the 2D CFD model in one calculation were proposed. They were successfully incorporated in both of the unified solution procedures. The predictions from these coupled models for a series of unsteady benchmark flows were competitive in quality with those from single 2D CFD models, however, the computing costs involved were comparatively much lower. In these calculations, the coupled models integrated in the block implicit procedure produced faster convergence than those in the pressure correction procedure, but required more computing resource. In addition, the implicit coupling stragety was more efficient compared to the explicit counterpart. A ID and 2D coupled model integrated in the pressure correction procedure was applied to simulate a realistic cylinder-valve-pipe flow. The overal prediction quality is satisfactory compared with experimental measurements. Some discrepancies which occurred were largely attributed to numerical representations of valve mechanism and the lack of turbulence models. For this engine application, the coupled model has shown advantages in computing cost or straightforwardness over a conventional uniform 2D model or boundary condition model.

The design of aeronautical engines is subject to many constraints that cover performance gain as well as increasingly sensitive environmental issues. These often contradicting objectives are currently being answered through an increase in the local and global temperature in the hot stages of the engine. As a result, the solid parts encounter very high temperature levels and gradients that are critical for the engine lifespan. Combustion chamber walls in particular are subject to large thermal constraints. It is thus essential for designers to characterize accurately the local thermal state of such devices. Today, wall temperature evaluation is obtained experimentally by complex thermocolor tests. To limit such expensive experiments, efforts are currently performed to provide high fidelity numerical tools able to predict the combustion chamber wall temperature. This specific thermal field however requires the consideration of all the modes of heat transfer (convection, conduction and radiation) and the heat production (through the chemical reaction) within the burner. The resolution of such a multi-physic problem can be done numerically through the use of several dedicated numerical and algorithmic approaches. In this manuscript, the methodology relies on a partitioned coupling approach, based on a Large Eddy Simulation (LES) solver to resolve the flow motion and the chemical reactions, a Discrete Ordinate Method (DOM) radiation solver and an unsteady solid conduction code. The various issues related to computer resources distribution as well as the coupling methodology employed to deal with disparity of time and space scales present in each mode of heat transfer are addressed in this manuscript. Coupled application high performance studies, carried out both on a toy model and an industrial burner configuration evidence parameters of importance as well as potential path of improvements. The thermal coupling approach is then considered from a physical point of view on two distinct configurations. First, one addresses the impact of the methodology and the thermal equilibrium state between a reacting fluid and a solid for a simple flame holder academic case. The effect of the flame holder wall temperature on the flame stabilization pattern is addressed through fluid-only predictions. These simulations highlight interestingly three different theoretical equilibrium states. The physical relevance of these three states is then assessed through the computation of several CHT simulations for different initial solutions and solid conductivities. It is shown that only two equilibrium states are physical and that bifurcation between the two possible physical states depends both on solid conductivity and initial condition.Furthermore, the coupling methodology is shown to have no impact on the solutions within the range of parameters tested. A similar methodology is then applied to a helicopter combustor for which radiative heat transfer is additionally considered. Different computations are presented to assess the role of each heat transfer process on the temperature field: a reference adiabatic fluid-only simulation, Conjugate Heat Transfer, RadiationFluid Thermal Interaction and fully coupled simulations are performed. It is shown that coupling LES with conduction in walls is feasible in an industrial context with acceptable CPU costs and gives good trends of temperature repartition. Then, for the combustor geometry and operating point studied, computations illustrate that radiation plays an important role in the wall temperature distribution. Comparisons with thermocolor tests are globally in a better agreement when the three solvers are coupled.

Doctor of Philosophy (PhD)
In recent times significant public attention has been drawn to the topic of combustion. This has been due to the fact that combustion is the underlying mechanism of several key challenges to modern society: climate change, energy security (finite reserves of fossil fuels) and air pollution. The further development of combustion science is undoubtedly necessary to find improved solutions to manage these combustion science related challenges in the near and long term future. Combustion is essentially an exothermic process, this exothermicity or heat release essentially occurs at small scales, by small scales it meant these scales are small relative to the fluid length scales, for example heat release layer thicknesses in flames are typically much less than the fluid integral length scales. As heat release occurs at small scales this means that in turbulent combustion the small scales of the turbulence (which can be of the order of the heat release layer thickness) can possibly interact and influence the heat release and thus chemistry of the flame reaction zone. Premixed combustion is a combustion mode where the fuel and oxidiser are completely premixed prior to the flame reaction zone, this mode of combustion has been shown to be a promising method to maximise combustion efficiency and minimise pollutant formation. The continued and further application of premixed combustion to practical applications is limited by the current understanding of turbulent premixed combustion, these limitations in understanding are linked to the specific flame phenomena that can significantly influence premixed combustion in a combustion device, examples of such phenomena are: flame flashback, flame extinction and fuel consumption rate – all phenomena that are influenced by the interaction of the small scales of turbulence and chemistry. It is the study and investigation of the interaction of turbulence and chemistry at the small scales (termed finite-rate chemistry) in turbulent premixed flames that is the aim of this thesis which is titled “Finite-rate chemistry effects in turbulent premixed combustion”. Two very closely related experimental burner geometries have been developed in this thesis: the Piloted Premixed Jet Burner (PPJB) and the Premixed Jet Burner (PJB). Both feature an axisymmetric geometry and exhibit a parabolic like flow field. The PPJB and PJB feature a small 4mm diameter central jet from which a high velocity lean-premixed methane-air mixture issues. Surrounding the central jet in the PPJB is a 23.5mm diameter pilot of stoichiometric methane-air products, the major difference between the PPJB and the PJB is that the PJB does not feature a stoichiometric pilot. The pilot in the PPJB provides a rich source of combustion intermediates and enthalpy which promotes initial ignition of the central jet mixture. Surrounding both the central jet and pilot is a large diameter hot coflow of combustion products. It is possible to set the temperature of the hot coflow to the adiabatic flame temperature of the central jet mixture to simulate straining and mixing against and with combustion products without introducing complexities such as quenching and dilution from cold air. By parametrically increasing the central jet velocity in the PPJB it is possible to show that there is a transition from a thin conical flame brush to a flame that exhibits extinction and re-ignition effects. The flames that exhibit extinction and re-ignition effects have a luminous region near the jet exit termed the initial ignition region. This is followed by a region of reduced luminosity further downstream termed the extinction region. Further downstream the flame luminosity increases this region is termed the re-ignition region. For the flames that exhibit extinction and re-ignition it is proposed that intense turbulent mixing and high scalar dissipation rates drives the initial extinction process after the influence of the pilot has ceased (x/D>10). Re-ignition is proposed to occur downstream where turbulent mixing and scalar dissipation rates have decreased allowing robust combustion to continue. As the PJB does not feature a pilot, the flame stabilisation structure is quite different to the PPJB. The flame structure in the PJB is essentially a lifted purely premixed flame, which is an experimental configuration that is also quite unique. A suite of laser diagnostic measurements has been parametrically applied to flames in the PPJB and PJB. Laser Doppler Velocimetry (LDV) has been utilised to measure the mean and fluctuating radial and axial components of velocity at a point, with relevant time and length scale information being extracted from these measurements. One of the most interesting results from the LDV measurements is that in the PPJB the pilot delays the generation of high turbulence intensities, for flames that exhibit extinction the rapid increase of turbulence intensity after the pilot corresponds to the start of the extinction region. Using the LDV derived turbulence characteristics and laminar flame properties and plotting these flames on a traditional turbulent regime diagram indicates that all of the flames examined should fall in the so call distributed reaction regime. Planar imaging experiments have been conducted for flames using the PPJB and PJB to investigate the spatial structure of the temperature and selected minor species fields. Results from two different simultaneous 2D Rayleigh and OH PLIF experiments and a simultaneous 2D Rayleigh, OH PLIF and CH2O PLIF experiment are reported. For all of the flames examined in the PPJB and PJB a general trend of decreasing conditional mean temperature gradient with increasing turbulence intensity is observed. This indicates that a trend of so called flame front thickening with increased turbulence levels occurs for the flames examined. It is proposed that the mechanism for this flame front thickening is due to eddies penetrating and embedding in the instantaneous flame front. In the extinction region it is found that the OH concentration is significantly reduced compared to the initial ignition region. In the re-ignition region it is found that the OH level increases again indicating that an increase in the local reaction rate is occurring. In laminar premixed flames CH2O occurs in a thin layer in the reaction zone, it is found for all of the flames examined that the CH2O layer is significantly thicker than the laminar flame. For the high velocity flames beyond x/D=15, CH2O no longer exist in a distinct layer but rather in a near uniform field for the intermediate temperature regions. Examination of the product of CH2O and OH reveals that the heat release in the initial ignition region is high and rapidly decreases in the extinction region, an increase in the heat release further downstream is observed corresponding to the re-ignition region. This finding corresponds well with the initial hypothesis of an extinction region followed by a re-ignition region that was based on the mean chemiluminescence images. Detailed simultaneous measurement of major and minor species has been conducted using the line Raman-Rayleigh-LIF technique with CO LIF and crossed plane-OH PLIF at Sandia National Laboratories. By measuring all major species it is also possible to define a mixture fraction for all three streams of the PPJB. Using these three mixture fractions it was found that the influence of the pilot in the PPJB decays very rapidly for all but the lowest velocity flames. It was also found that for the high velocity flames exhibiting extinction, a significant proportion of the coflow fluid is entrained into the central jet combustion process at both the extinction region and re-ignition regions. The product of CO and OH conditional on temperature is shown to be proportion to the net production rate of CO2 for certain temperature ranges. By examining the product of CO and OH the hypothesis of an initial ignition region followed by an extinction region then a re-ignition region for certain PPJB flames has been further validated complementing the [CH2O][OH] imaging results. Numerical modelling results using the transported composition probability density function (TPDF) method coupled to a conventional Reynolds averaged Naiver Stokes (RANS) solver are shown in this thesis to successfully predict the occurrence of finite-rate chemistry effects for the PM1 PPJB flame series. To calculate the scalar variance and the degree of finite-rate chemistry effects correctly, it is found that a value of the mixing constant ( ) of approximately 8.0 is required. This value of is much larger than the standard excepted range of 1.5-2.3 for that has been established for non-premixed combustion. By examining the results of the RANS turbulence model in a non-reacting variable density jet, it is shown that the primary limitation of the predictive capability of the TPDF-RANS method is the RANS turbulence model when applied to variable density flows.

The present work is based on the need for understanding the in-cylinder flow and its subsequent effects on combustion in a valved-two-stroke spark ignition engine with fuel injection using Computational Fluid Dynamics (CFD) and experimental techniques. In this context, the CFD code KIVA-II has been modified to model the two-stroke engine gas exchange and combustion processes. A 3-D Cartesian grid generation program for complex engine geometry has been added to the KIVA code which has been modified to include intake and exhaust flow processes with valves. New and improved sub models for wall jet interaction, mixing controlled combustion and one dimensional wave action have also been incorporated. The modified version of the program has been used to simulate a fuel injected two-stroke spark ignition engine and parametric studies have been undertaken. The simulated flow, combustion and exhaust emission characteristics over a wide range of operating conditions show the expected trends in behaviour observed in actual engines. In the second phase of this study, the air-assisted-fuel-injection (AAFI) process into a cylinder has been simulated with a high resolution computational grid. The simulation results are presented and compared with experimental data obtained using the Schlieren optical technique. An approximate method based on the conservation of mass, momentum and energy of the spray jet and using a comparatively coarse grid has been suggested for simulating the AAFI process. The simulation study predicts a high degree of atomisation of fuel spray with Sauter mean diameter around 10 μm even with moderate air and fuel pressures. The penetration and width of spray are simulated within 15% of the experimental values. In the last phase of this study, the flow and combustion processes have been studied for a four-stroke spark ignition engine with the AAFI process. The simulation results obtained using this approximate method have been validated with experimental data generated for the same engine configuration.

A computational study has been carried out to investigate smoldering ignition and propagation in polyurethane foam. The onedimensional, transient, governing equations for smoldering combustion in a porous fuel are solved accounting for improved solid-phase chemical kinetics. A systematic methodology for the determination of solid-phase kinetics suitable for numerical models has been developed and applied to the simulation of smoldering combustion. This methodology consists in the correlation of a mathematical representation of a reaction mechanism with data from previous thermogravimetric experiments. Geneticalgorithm and trail-and-error techniques are used as the optimization procedures. The corresponding kinetic parameters for two different mechanisms of polyurethane foam smoldering kinetics are quantified: a previously proposed 3-step mechanism and a new 5-step mechanism. These kinetic mechanisms are used to model one-dimensionalsmoldering combustion, numerically solving for the solid-phase and gasphase conservation equations in microgravity with a forced flow of oxidizer gas. The results from previously conducted microgravity experiments with flexible polyurethane foam are used for calibration and testing of the model predictive capabilities. Both forward and opposed smoldering configurations are examined. The model describes well both opposed and forward propagation. Specifically, the model predicts the reaction-front thermal and species structure, the onset of smoldering ignition, and the propagation rate. The model results reproduce the most important features of the smolder process and represent a significant step forward in smoldering combustion modeling.

Coal consumption is predicted to account for about 21% of the total global primary energy in 2040 and this continues to be a challenge for global warming and air pollution. Oxyfuel combustion is one of the leading options for carbon capture and storage (CCS) technologies to reduce the impact on the environment. Initially this technology has been studied successfully on small-scale facilities but it needs to be developed for large-scale applications. CFD has been demonstrated to be a key tool for the development and optimisation of pulverised coal combustion processes and it is still an important tool for new designs and retro-fitting of conventional power plants for oxyfuel combustion. Radiation heat transfer plays an important role, influencing the overall combustion efficiency, pollutant formation and flame ignition and propagation. This thesis focuses on the radiation properties of the particles as well as gas property models on the overall influence of the prediction of the formation of NOx pollutants in pulverised coal combustion. The radiative properties of the particles are investigated with a focus on the effect of the optical properties and approximate solutions to determine the radiative properties, with different experimental data for the optical properties and approximate solutions being employed. The effects of the radiative properties on the radiative heat transfer are investigated in three dimensional enclosures for small and large-scale furnaces and implemented on a 250 kW pilot scale combustion for both air and oxyfuel conditions. The results from the study highlights the best selection for the particle properties for simulations in small and large-scale pulverised coal furnaces and employing radiation models for the gases and particles to improve the NOx predictions in pulverised coal combustion under air and oxy-fired environments.

Computational modeling was completed on a simplified downdraft gasifier to be implemented at the University of Iowa Oakdale Power Plant. The model was created in Gambit and exported to FLUENT, a computational fluid dynamics software program, in order to process non-premixed combustion on biomass fuels and better understand the combustion and gasification zones. The fuels were modeled as coal particles with the empirical formula of the biomass found from off-site proximate and ultimate analyses. The coal model inherent to FLUENT contains the same chemicals (C, H, N, O, and S) as the biomass tested, so this model was determined to be accurate. The model was tested for varying packing densities, oxidizer inlet velocities and fuel type to describe the effects on the combustion zone. It was concluded that packing densities around 0.5 with oxidizer inlet velocities less than 5 m/s would be ideal for modeling wood. The temperature distribution was the most even in this environment and produced a large rich fuel combustion (RFC) zone where gasification and pyrolysis could occur. The different fuels were found to have similar temperature and mean mixture fraction patterns, although the maximum temperatures attained were very different (1080K for seed corn and 678K for wood), the wood showed a greater area of RFC for gasification and pyrolysis.

Computational Fluid Dynamics (CFD) is a field that is constantly advancing. Its advances in terms of capabilities are a result of new theories, faster computers, and new numerical methods. In this thesis, advances in the computational fluid dynamic modeling of moving bodies and combustive flows are investigated. Thus, the basic theory behind CFD is being extended to solve a new class of problems that are generally more complex. The first chapter that investigates some of the results, chapter IV, discusses a technique developed to model unsteady aerodynamics with moving boundaries such as flapping winged flight. This will include mesh deformation and fluid dynamics theory needed to solve such a complex system. Chapter V will examine the numerical modeling of a combustive flow. A three dimensional single vane burner combustion chamber is numerically modeled. Species balance equations along with rates of reactions are introduced when modeling combustive flows and these expressions are discussed. A reaction mechanism is validated for use with in situ reheat simulations. Chapter VI compares numerical results with a laminar methane flame experiment to further investigate the capabilities of CFD to simulate a combustive flow. A new method of examining a combustive flow is introduced by looking at the solutions ability to satisfy the second law of thermodynamics. All laminar flame simulations are found to be in violation of the entropy inequality.

The effect of temperature on fuel-air ignition and combustion (thermal effects) have been widely studied and well understood. However, a comprehensive understanding of nonequilibrium plasma effects (in situ generation of reactive species and radicals combined with gas heating) on the combustion process is still lacking. Over the past decade, research efforts have advanced our knowledge of electron impact kinetics and low temperature chain branching in fuel-air mixtures considerably. In contrast to numerous experimental investigations, research on modeling and simulation of plasma assisted combustion has received less attention. There is a dire need for development of self-consistent numerical models for construction and validation of plasma chemistry mechanisms. High-fidelity numerical models can be invaluable in exploring the plasma effects on ignition and combustion in turbulent and high-speed flow environments, owing to the difficulty in performing spatially resolved quantitative measurements. In this work, we establish a multi-scale modeling framework to simulate the physical and chemical effects of nonequilibrium, nanosecond plasma discharges on reacting flows. The model is capable of resolving electric field transients and electron impact dynamics in sub-ns timescales, as well as calculating the cumulative effects of multiple discharge pulses over ms timescales. Detailed chemistry mechanisms are incorporated to provide deep insight into the plasma kinetic pathways. The modeling framework is utilized to study ignition of H₂-air mixtures subjected to pulsed, nanosecond dielectric barrier discharges in a plane-to-plane geometry. The key kinetic pathways responsible for radicals such as O, H and OH generation from nanosecond discharges over multiple voltage pulses (ns-ms timescales) are quantified. The relative contributions of plasma thermal and kinetic effects in the ignition process are presented. The plasma generated radicals trigger partial fuel oxidation and heat release when the temperature rises above 700 K, after which the process becomes self-sustaining leading to igntion. The ignition kernel growth is primarily due to local plasma chemistry effects rather than flame propagation, and heat transport does not play a significant role. The nanosecond pulse discharge plasma excitation resulted in nearly simultaneous ignition over a large volume, in sharp contrast to hot-spot igniters. Next, the effect of nanosecond pulsed plasma discharges on the ignition characteristics of nC₇H₁₆ and air in a plane-to-plane geometry is studied at a reduced pressure of 20.3 kPa. The plasma generated radicals initiate and significantly accelerate the H abstraction reaction from fuel molecules and trigger a “self-accelerating” feedback loop via low-temperature kinetic pathways. Application of only a few discharge pulses at the beginning reduces the initiation time of the first-stage temperature rise by a factor of 10. The plasma effect after the first stage is shown to be predominantly thermal. A novel plasma-flame modeling framework is developed to study the direct coupling of steady, laminar, low-pressure, premixed flames to highly non-equilibrium, nanosecond-pulsed plasma discharges. The simulations are performed with and without a burst of 200 nanosecond discharge pulses to quantify the effect of non-equilibrium plasma on a pre-existing lean premixed H₂/O₂/N₂ (ϕ = 0.5) flame at 25 torr. Simulation results showed a significant increase in O and H densities due to plasma chemistry, with peak values increasing by a factor of 6 and a factor of 4, respectively. It is demonstrated that Joule heating alone cannot move the temperature and species profiles as far upstream (i.e. closer to the burner surface) as the pulsed plasma source of the same total power. LES (large eddy simulation) of ignition and combustion of H₂ jets injected into a supersonic O₂ crossflow is performed. Nanosecond plasma discharges are studied for their potential to produce radicals and impact on the flame-holding process. The plasma has a significant effect on the O atom distribution near the discharge domain as well as in the leeward side of the second jet. The other species distributions, however, remained unchanged with or without plasma. We believe the reason for this behavior was the high jet momentum ratios considered in the present study. The plasma generated radicals were unable to have an effect on the flame development downstream because of the strong penetration of the cold fuel jet.

Large Eddy Simulation (LES) has potential to address unsteady phenomena in turbulent premixed flames and to capture turbulence scales and their influence on combustion. Thus, this approach is gaining interest in industry to analyse turbulent reacting flows. In LES, the dynamics of large-scale turbulent eddies down to a cut-off scale are solved, with models to mimic the influences of sub-grid scales. Since the flame front is thinner than the smallest scale resolved in a typical LES, the premixed combustion is a sub-grid scale (SGS) phenomenon and involves strong interplay among small-scale turbulence, chemical reactions and molecular diffusion. Sub-grid scale combustion models must accurately represent these processes. When the flame front is thinner than the smallest turbulent scale, the flame is corrugated by the turbulence and can be seen as an ensemble of thin, one-dimensional laminar flames (flamelets). This allows one to decouple turbulence from chemistry, with a significant reduction in computational effort. However, potentials and limitations of flamelets are not fully explored and understood. This work contributes to this understanding. Two models are identified, one based on an algebraic expression for the reaction rate of a progress variable and the assumption of fast chemistry, the other based on a database of unstrained flamelets in which reaction rates are stored and parametrised using a progress variable and its SGS variance, and their potentials are shown for a wide range of premixed combustion conditions of practical interest. The sensitivity to a number of model parameters and boundary conditions is explored to assess the robustness of these models. This work shows that the SGS variance of progress variable plays a crucial role in the SGS reaction rate modelling and cannot be obtained using a simple algebraic closure like that commonly used for a passive scalar. The use of strained flamelets to include the flame stretching effects is not required when the variance is obtained from its transport equation and the resolved turbulence contains predominant part of the turbulent kinetic energy. Thus, it seems that SGS closure using unstrained flamelets model is robust and adequate for wide range of turbulent premixed combustion conditions.

Thermoacoustic instabilities arise and sustain due to the coupling of unsteady heat release from the flame and the acoustic field. One potential driving mechanism for these instabilities arise when velocity fluctuations (u') at the fuel injection location causes perturbations in the local equivalence ratio and is convected to the flame location generating an unsteady heat release (q') at a particular convection time delay, τ. Physically, τ is the time for the fuel to convect from injection to the flame. The n-τ Flame Transfer Function (FTF) is commonly used to model this relationship assuming an infinitesimally thin flame with a fixed τ. In practical systems, complex swirling flows, multiple fuel injections points, and recirculation zones create a distribution of τ, which can vary widely making a statistical description more representative. Furthermore, increased flame lengths and higher frequency instabilities with short acoustic wavelengths challenge the 'thin-flame' approximation. The present study outlines a methodology of using distributed convective fuel time delays and heat release rates in a one-dimensional (1-D) linear stability model based on the transfer matrix approach. CFD analyses, with the Flamelet Generated Manifold (FGM) combustion model are performed and probability density functions (PDFs) of the convective time delay and local heat release rates are extracted. These are then used as inputs to the 1-D Thermoacoustic model. Results are compared with the experimental results, and the proposed methodology improves the accuracy of stability predictions of 1-D Thermoacoustic modeling.
Master of Science

Homogeneous charge combustion in a four stroke cycle spark-ignition engine was studied using through-piston-photography with a gated-intensified CCD camera. Analysis of computer stored multiple exposed flame front images was carried out for various engine conditions, in conjunction with the test data and cylinder pressure signals. Representative turbulence scales were inferred from the flame propagation and cylinder pressure data. Fractal analysis of flame edge contours resulted in a fractal dimension D3 in the range 2.12 to 2.23 corroborating data presented elsewhere. A correlation is presented here between the standard deviation of peak cylinder pressure and the fractal dimension D3.

A computational study on two-dimensional laminar premixed combustion has been conducted. A working model was developed that fully coupled a comprehensive chemical kinetic mechanism with computational fluid dynamics in the commercial software program FLUENT. The physical model for the simulations consisted of an adiabatic tube with a constant velocity inlet and an atmospheric pressure outlet. For all cases, the flame waves were shown to be stabilized by the developing boundary layer near the inlet. The combustion of methane with air was studied in depth and compared with the combustion of three different biofuels: landfill gas and two varieties of syngas. Additionally, combustion with a mixture of O2 and CO2 as an oxidizer was proposed as a way to facilitate carbon dioxide capture and sequestration. Flames produced by this combustion technique were then compared with traditional combustion oxidized with air. Results for methane combustion compared closely with experimental work and one-dimensional numerical work in predicting flame shape, laminar flame speed, and flame thickness. It was shown that the presence of the tube wall affected the flame thickness, but not the laminar flame speed, at sufficiently high inlet velocities. The results from the combustion of landfill gas showed that its laminar flame speed is lower than methane but that its flame shape is similar in nature to that of methane. Simulations of syngas combustion proved to be troublesome for the computational model, which struggled to converge to reasonable solutions, indicating that more work is needed with the numerical modeling method. Results from combustion simulations with the O2/CO2 oxidizer revealed that the flame characteristics were affected by the lower thermal diffusivity of the oxidizer, resulting in lower laminar flame propagation speeds and thicker combustion waves. The flame shape remained similar to combustion oxidized with air.

Thesis (M.S.)--Case Western Reserve University, 2009
Abstract Department of Mechanical & Aerospace Engineering Title from PDF (viewed on 20 April 2009) Available online via the OhioLINK ETD Center

Thesis (Ph. D.)--Ohio State University, 2005.
Title from first page of PDF file. Document formatted into pages; contains xvii, 259 p.; also includes graphics (some col.). Includes bibliographical references (p. 233-259). Available online via OhioLINK's ETD Center

Thesis (M.S. in Engineering Science (Mechanical))--Naval Postgraduate School, December 2003.
Thesis advisor(s): Chris M. Brophy, Garth V. Hobson. Includes bibliographical references (p. 103). Also available online.

Schopnost predikovat tepelné toky do stěn v oblasti spalování, konstrukce pecí a procesního průmyslu je velmi důležitá pro návrh těchto zařízení. Je to často klíčový požadavek pro pevnostní výpočty. Cílem této práce je proto získat kvalitní naměřená data na experimentálním zařízení a využít je pro validaci standardně využívaných modelů počítačového modelování turbulentního vířivého difúzního spalování zemního plynu. Experimentální měření bylo provedeno na vodou chlazené spalovací komoře průmyslových parametrů. Byly provedeny měření se pro dva výkony hořáku – 745 kW a 1120 kW. Z měření byla vyhodnocena data a odvozeno nastavení okrajových podmínek pro počítačovou simulaci. Některé okrajové podmínky bylo nutné získat prostřednictvím dalšího měření, nebo separátní počítačové simulace tak jako například pro emisivitu, a nebo teplotu stěny. Práce zahrnuje několik vlastnoručně vytvořených počítačových programů pro zpracování dat. Velmi dobrých výsledků bylo dosaženo při predikci tepelných toků pro nižší výkon hořáku, kde odchylky od naměřených hodnot nepřesáhly 0.2 % pro celkové odvedené teplo a 16 % pro lokální tepelný tok stěnou komory. Vyšší tepelný výkon však přinesl snížení přesnosti těchto predikcí z důvodů chybně určené turbulence. Proto se v závěru práce zaměřuje na predikce vířivého proudění za vířičem a identifikuje několik problematických míst v použitých modelech využívaných i v komerčních aplikacích.

Thesis (M.S.)--Southern Illinois University Carbondale, 2006.
"Department of Mechanical Engineering and Energy Processes." Includes bibliographical references (leaves 171-174). Also available online.

Natural gas/diesel dual fuel engines offer a path towards meeting current and future emissions standards with lower fuel cost. However, numerous technical challenges remain that require a greater understanding of the in-cylinder combustion physics. For example, due to the high compression ratio of diesel engines, substitution of natural gas for diesel fuel at high load is often limited by engine knock and pre-ignition. Additionally, increasing the natural gas percentage in a dual fuel engine often results in decreasing maximum load. These problems limit the substitution percentage of natural gas in high compression ratio diesel engines and therefore reduce the fuel cost savings. Furthermore, when operating at part load dual fuel engines can suffer from excessive emissions of unburned natural gas. Computational fluid dynamics (CFD) is a multi-dimensional modeling tool that can provide new information about the in-cylinder combustion processes causing these issues.

In this work a multi-dimensional CFD model has been developed for dual fuel natural gas/diesel combustion and validated across a wide range of engine loads, natural gas substitution percentages, and natural gas compositions. The model utilizes reduced chemical kinetics and a RANS based turbulence model. A new reduced chemical kinetic mechanism consisting of 141 species and 709 reactions was generated from multiple detailed mechanisms, and has been validated against ignition delay, laminar flame speed, diesel spray experiments, and dual fuel engine experiments using two different natural gas compositions. Engine experiments were conducted using a GM 1.9 liter turbocharged 4-cylinder common rail diesel engine, which was modified to accommodate port injection of natural gas and propane. A combination of experiments and simulations were used to explore the performance limitations of the light duty dual fuel engine including natural gas substitution percentage limits due to fast combustion or engine knock, pre-ignition, emissions, and maximum load. In particular, comparisons between detailed computations and experimental engine data resulted in an explanation of combustion phenomena leading to engine knock in dual fuel engines.

In addition to conventional dual fuel operation, a low temperature combustion strategy known as reactivity controlled compression ignition (RCCI) was explored using experiments and computations. RCCI uses early diesel injection to create a reactivity gradient leading to staged auto-ignition from the highest reactivity region to the lowest. Natural gas/diesel RCCI has proven to yield high efficiency and low emissions at moderate load, but has not been realized at the high loads possible in conventional diesel engines. Previous attempts to model natural gas/diesel RCCI using a RANS based turbulence model and a single component diesel fuel surrogate have shown much larger combustion rates than seen in experimental heat release rate profiles, because the reactivity gradient of real diesel fuel is not well captured. To obtain better agreement with experiments, a reduced dual fuel mechanism was constructed using a two component diesel surrogate. A sensitivity study was then performed on various model parameters resulting in improved agreement with experimental pressure and heat release rate.

Lean premixed combustors have been designed to lower NOx and other pollutant levels in land based gas turbines. These combustors are often susceptible to thermo-acoustic instabilities, which manifest as pressure and heat release oscillations in the combustor. To be able to predict and control these instabilities, it is required that both the acoustics of the system, and a frequency-resolved response of the combustion process to incoming perturbations be understood. Currently, a system-level approach is being used widely to predict the thermoacoustic instabilities. This approach requires simple, yet accurate models which would describe the behavior of each dynamic block within the loop. The present study is directed toward using computational fluid dynamics (CFD) as a tool in developing reduced order models for the dynamics of laminar flat flames and swirl stabilized turbulent flames. A finite-volume based approach is being used to simulate reacting flows in both laminar and turbulent combustors. The study has been divided into three parts -- the first part involves the modeling of a self-excited combustor (the acoustics of the combustor are coupled with the unsteady heat release); the second part of the research aims to study the effect of velocity perturbations on the unsteady heat release rate from a burner stabilized laminar flat flame; the third and final part of work involves an extension of the laminar flat flame study to turbulent reacting flows in a swirl stabilized combustor, and study the effects on the turbulent heat release due to the velocity perturbations. A Rijke tube combustor was selected to study self-excited combustion phenomenon. A laminar premixed methane-air flat flame was stabilized on a honeycomb flame-stabilizer. The flame stabilizer was placed at the center of the 5 feet vertical tube. The position of the flame at the center of the tube leads to a thermoacoustic instability of the 2nd acoustic mode. The fundamental thermoacoustic frequency was predicted accurately by the CFD model and the amplitude was reasonably matched (for a flow rate of Q = 120 cc/s and equivalence ratio phi = 1.0). Other characteristics of the pressure power spectrum were captured to a good degree of accuracy. This included the amplitude modulation of the fundamental and the harmonics due to a subsonic pulsating instability. The flat flame study has been being conducted for Q = 200 cc/s and equivalence ratio phi = 0.75. The objective has been to obtain a frequency response function (FRF) of the unsteady heat release rate (output) due to incoming velocity perturbations (input). A range of frequencies (15 Hz - 500 Hz) have been selected for generating the FRF. The aim of this part of the study has been to validate the computational model against the experimental results and propose a physics based interpretation of the flame response. Detailed heat transfer modeling (including radiation heat transfer) and two-step chemistry models have been implemented in the model. The FRF generated has been able to reproduce the experimentally observed phenomena, like the low frequency pulsating instability occurring at 30 Hz. A heat transfer study has been conducted to explain the pulsating instability and a fuel variability study has been performed. Both the heat transfer study and the fuel variability study proved the role of heat transfer in creating the pulsating instability. The final part of the study involves simulation of reacting flow in a turbulent swirl stabilized combustor. The effect of velocity perturbations on the unsteady heat release has been studied by creating an FRF between the unsteady velocity and the unsteady heat release rate. A Large Eddy Simulation (LES) approach has been selected. A swirl number of S = 1.19 corresponding to a flow rate of Q = 20 SCFM with an equivalence ratio of phi = 0.75 have been implemented. Reduced reaction chemistry modeling, turbulence-chemistry interaction and heat transfer modeling have been incorporated in the model. The LES of reacting flow has shown vortex-flame interaction occurring inside the combustor. This interaction has been shown to occur at 255 Hz. The FRF obtained between unsteady velocity and unsteady heat release rate shows good comparison with the experimentally obtained FRF.
Ph. D.

The investigation reported in this thesis is concerned with the topic of soot formation and soot particle motion in the cylinder of a light duty automotive diesel engine. CFD has been employed to simulate in-cylinder conditions and to investigate the source of particles which are transferred to the oil. The accumulation of soot in the lubricating oil of diesel engines is one of the factors limiting the interval between oil changes and hence service interval. Soot particles can be transferred to oil film on the cylinder wall layers through the complex motion of the fluid flow in the cylinder. The paths of soot particles from specific in-cylinder locations and crank angle instants have been explored using the results for cylinder charge motion predicted by the Kiva-3v CFD code. Using the velocity fields from the simulation data, massless tracking of the in-cylinder soot particles in space and time is carried out employing a particle tracking with trilinear interpolation technique. From this investigation, new computational codes for the prediction of soot particle paths and soot particle size change along a specific path in a diesel engine have been developed. This investigation is the first numerical study into soot particle trajectories within an engine and thus opens up a novel branch of research of soot formation within internal combustion engines. Computed soot paths from the investigation show that soot particles formed just below the fuel spray axis inside the middle bowl area during early injection period are more likely sources of soot particles on the cylinder wall layers than those formed later. Soot particles that are formed above the fuel axis have less tendency to be transported to the cylinder wall layers thus are not likely to be the main source of soot at the cylinder walls. Soot particles that are from the bowl rim area are found to be another source of soot transfer to the boundary layer, as they are directly exposed to reverse squish motion during the expansion stroke. Soot particles that are formed near the cylinder jet axis during fuel injection tend to move into the bowl. These soot particles are found to be from the relatively less concentrated area. In contrast, particles from the most concentrated areas tend to be moving into the bowl and pose least risk of contaminating oil films on the liner. Sensitivity studies of soot particle paths to swirl show that engine operating with low swirl ratios are more vulnerable to soot in oil problem as low swirls cause the bulk fluid flow to be moving closer to the cylinder walls due to fuel jet velocity and reverse squish motions. Decreasing the spray angle lessens the possibilities of soot particles from being transported close the cylinder wall layers while increasing the spray angle increases the possibilities of soot from the bowl region to be transported close to the cylinder wall layers. The temporal and spatial evolution of soot particle size can be predicted by using the history of temperature, pressure and gas species along the paths. An explorative investigation has been carried out to determine the most suitable method to tackle this soot particle evolution. With proper multipliers, all approaches perform quite satisfactorily in terms of predicting the trend of size change. Soot particles that are likely to be transferred to the cylinder wall layers are predicted to change in size parallel to the average mass profile in the whole cylinder where they quickly peak to maximum at around 18° CA ATDC, and gradually decrease in size through EVO.

The present research describes an investigation of the flow through the inlet port and the cylinder of an internal combustion engine. The principal aim of the work is to interpret the effects of the port shape and valve lift on the engine's "breathing" characteristics, and to develop a better understanding of flow and turbulence behaviour through the use of Computational Fluid Dynamics (CFD), using a commercial available package STAR-CD. A complex computational mesh model was constructed, which presents the actual inlet port/cylinder assembly, including a curved port, a cylinder, moving valve and piston. Predictions have been carried out for both steady and transient flows. For steady flow, the influence of valve lift and port shape on discharge coefficient and the in-cylinder flow pattern has been examined. Surface static pressures predicted using the CFD code, providing a useful indicator of flow separation within the port/cylinder assembly, are presented and compared with experimental data. Details of velocity fields obtained by laser Doppler anemometry in a companion study at King's College London, using a steady flow bench test with a liquid working fluid for refractive index matching, compared favourably with the predicted data. For transient flow, the flow pattern changes and the turbulence field evolutions due to valve and piston movement are presented, and indicate the possible source of cyclic variability in an internal combustion engine.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.
Cataloged from PDF version of thesis.
Includes bibliographical references (p. 229-239).
Oxy-fuel combustion of solid fuels, often performed in a mixture of oxygen and wet or dry recycled carbon dioxide, has gained significant interest in the last two decades as one of the leading carbon capture technologies in power generation. The new combustion characteristics in a high-O₂ environment raise challenges for furnace design and operation, and should be modeled appropriately in CFD simulation. Based on a comprehensive literature review of the state-of-the-art research on the fundamentals of oxy-coal combustion, sub-models for the critical physical processes, such as radiation and char combustion, have been properly modified for the O₂-rich environment, and the overall performance of CFD simulation on oxy-coal combustion has been validated using Large-Eddy Simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) approaches. The predicted distributions on velocity, species, and temperature were compared with experimental results from the literature in order to validate the CFD simulation. Results show that although agreeing reasonably with the measured mean axial and tangential velocity, all the RANS turbulence models used in this study underestimate the internal recirculation zone size and the turbulence mixing intensity in the char combustion zone, while LES improves the predictions of internal recirculation zone size, the entrainment of oxygen from the staging stream, and the overall flame length than the RANS approaches. Special attention was given to the CO₂'s chemical effects on CO formation in oxy-fuel combustion, and its modeling approaches in CFD simulations. Detailed reaction mechanism (GRI-Mech 3.0) identifies that the reaction H+CO₂ -->/<-- OH+CO enhances the CO formation in the fuel-rich side of the diffusion flame due to the high CO₂ concentration, leading to a significantly higher CO concentration. Reasonable CO predictions can only be obtained using finite-rate mechanisms combining with reaction mechanisms considering the above-mentioned reaction in CFD simulations. The validated CFD approach was used to investigate the pressure's effects in a pressurized oxy-coal combustion system. The results show that, given a fixed reactor geometry and burner velocity, the particle residence time does not change with operating pressure due to its small Stokes number; on the other hand, the coal conversion time decreases significantly because of the enhanced reaction rates at elevated pressures. Therefore, the burner can be operated at a higher burner velocity at elevated operating pressure, which results in a much higher coal throughput using the same reactor size. For instance, the thermal load can be increased from 3 MWth to 60 MWth using a pressurized oxy-coal reactor, when the operating pressure increases from 4 bar to 40 bar. In order to investigate the slag behaviors in the pressurized oxy-coal combustor, a first-of-its-kind three-dimensional slag model has been developed, which can be applied in slagging coal combustion/gasification with any geometry. The method couples Volume of Fluid (VOF) model and Discrete Phase Model (DPM), and fully resolves the slag's behaviors such as the slag layer buildup, multiphase flow, as well as heat transfer. The results are in good agreement with experimental observations, and can be taken as a design tool for coal furnace/gasifier development.
by Lei Chen.
Ph.D.