Journal articles on the topic 'Combustion front quenching'

The mechanism of self-propagating high-temperature synthesis (SHS) of AlB2‒Al2O3 composite powders was studied by means of a combustion front quenching method (CFQM). The results showed that combustion reaction started with the melting of B2O3 and Al particles. As the combustion reaction proceeded, the interpenetration of Al and B2O3 in melts happened. The XRD results of the product revealed the reflections of Al2O3, suggesting there had been an exchange of oxygen atoms between Al and B, and evidencing the reaction, B2O3 (l) + 2Al (l) → 2B (s) + Al2O3 (l). Under higher temperature, some of B2O3 volatilized and reacted with B forming gaseous B2O2, which deposited on the surface of Al to precipitate Al2O3 and B. Then B made available dissolved into Al melt, and reacted with the Al in melt to precipitate AlB12 particles. Finally, AlB12 transforms to AlB2 at the peritectic temperature under high cooling rate. Thus, this combustion reaction can be described by the dissolution-precipitation mechanism. In the final products, besides AlB2 and Al2O3 particles, some of Al was also detected. A model corresponding to the dissolutionprecipitation mechanism was proposed, and the ignition temperature of the combustion reaction was determined to be around 800 °C. Ill. 13. Ref. 47.

The behavior of small oil sand samples was monitored experimentally when subjected repeatedly to low-velocity heated air streams, with either prompt quenching or slow cooling of the samples to their initial temperatures between these exposures. The stream temperature was either in the range of 300°C to 500°C in air or at higher temperatures of up to 760°C, while being exposed to the products of combustion of very lean hydrogen-air mixtures. This was done in relation to understanding better the associated processes in some in-situ recovery trials where stoppage of the combustion front and/or the flow of the injected fluids can occur. A variety of multi-exposure-cooling schemes was considered and their effects on the extent of volatilization and ignition established. Comparison to the corresponding behavior of similar samples under uninterrupted exposure to the heated streams was made throughout.

Flame flashback from the combustion chamber into the mixing zone limits the reliability of swirl stabilized lean premixed combustion in gas turbines. In a former study, the combustion induced vortex breakdown (CIVB) has been identified as a prevailing flashback mechanism of swirl burners. The present study has been performed to determine the flashback limits of a swirl burner with cylindrical premixing tube without centerbody at atmospheric conditions. The flashback limits, herein defined as the upstream flame propagation through the entire mixing tube, have been detected by a special optical flame sensor with a high temporal resolution. In order to study the effect of the relevant parameters on the flashback limits, the burning velocity of the fuel has been varied using four different natural gas-hydrogen-mixtures with a volume fraction of up to 60% hydrogen. A simple approach for the calculation of the laminar flame speeds of these mixtures is proposed which is used in the next step to correlate the experimental results. In the study, the preheat temperature of the fuel mixture was varied from 100°C to 450°C in order to investigate influence of the burning velocity as well as the density ratio over the flame front. Moreover, the mass flow rate has been modified in a wide range as an additional parameter of technical importance. It was found that the quenching of the chemical reaction is the governing factor for the flashback limit. A Peclet number model was successfully applied to correlate the flashback limits as a function of the mixing tube diameter, the flow rate and the laminar burning velocity. Using this model, a quench factor can be determined for the burner, which is a criterion for the flashback resistance of the swirler and which allows to calculate the flashback limit for all operating conditions on the basis of a limited number of flashback tests.

The flow physics controlling the stabilisation of a methane/air laminar premixed flame in a narrow channel (internal width $\ell _{i}=5~\text{mm}$) is revisited from numerical simulations. Combustion is described with complex chemistry and transport properties, along with a coupled simulation of heat transfer at and within the wall. To conduct a thorough analysis of the flame–wall interaction, the steady flame is obtained after applying a procedure to find the inlet mass flow rate that exactly matches the flame mass burning rate. The response of the premixed flame shape to various operating conditions is then analysed in terms of flame propagation velocity and flow topology in the vicinity of the reactive front. We focus on the interrelations between the flame speed, the configuration taken by the flame surface, the flow deviation induced by the heat released and the fluxes at the wall. Compared to an adiabatic flame, the flame speed increases with edge-flame quenching at an isothermal cold wall in the absence of a boundary layer, decreases with a boundary layer, to increase again with heat-transfer coupling within the wall. A regime diagram is proposed to delineate between flame shapes in order to build a classification versus heat-transfer properties. Under a small level of convective heat transfer with the ambient air surrounding the channel, the larger the thermal conductivity in the solid, the faster the reaction zone propagates in the vicinity of the wall, leaving the centreline reaction zone behind. The premixed flame front is then concave towards the fresh gases on the axis of symmetry (so-called tulip flame) with a flame speed higher than in the adiabatic case. Increasing the heat loss at the wall through convection with ambient air, the flame shape becomes convex (mushroom flame) and the flame speed decreases below its adiabatic level. Scaling laws are provided for the flame speed under these various regimes. Mesh resolution was calibrated, with and without heat loss, from simulations of one-dimensional detailed chemistry flames, leading to mesh resolution of $12.5~\unicode[STIX]{x03BC}\text{m}$ for detailed chemistry and $25.0~\unicode[STIX]{x03BC}\text{m}$ with a skeleton mechanism. The quality of the resolution was also assessed from multi-physics budgets derived from first principles, involving upstream-flame heat retrocession by the wall leading to flow acceleration, budgets bringing physical insights into flame/wall interaction. Additional overall mesh convergence tests of the multi-physics solution would have been desirable, but were not conducted due to the high computing cost of these fully compressible simulations, hence also solving for the acoustic field with low convective velocities.

Atmospheric, laminar, stoichiometric, premixed hydrogen-air flames in a diverging channel are investigated by means of Computational Fluid Dynamics. This configuration has been recently used in a series of experimental investigations to determine the burning velocities and quenching distances for premixed flames of different fuels. The purpose of the present investigation is the validation of the prediction procedures for the burning speeds and quenching distances for hydrogen flames by comparing them with these measurements. Global and detailed reaction mechanisms are applied to describe the combustion process. For assuring an adequately fine resolution of the flame fronts, adaptive grid refinement techniques are applied. A reasonable agreement is observed with the experiments, where the detailed and global mechanisms are slightly overpredicting and underpredicting the quenching distance, respectively.

A flame front is quenched when approaching a cold wall due to excessive heat loss. Accurate computation of combustion rate in such situations requires accounting for near wall flame quenching. Combustion models, developed without considering wall effects, cannot be used for wall bounded combustion modelling, as it leads to wall flame acceleration problem. In this work, a new model was developed to estimate the near wall combustion rate, accommodating quenching effects. The developed correlation was then applied to predict the combustion in two spark ignition engines in combination with the famous Bray–Moss–Libby (BML) combustion model. BML model normally fails when applied to wall bounded combustion due to flame wall acceleration. Results show that the proposed quenching correlation has significantly improved the performance of BML model in wall bounded combustion. As a second step, in order to further enhance the performance, the BML model was modified with the use of Kolmogorov–Petrovski–Piskunov analysis and fractal theory. In which, a new dynamic formulation is proposed to evaluate the mean flame wrinkling scale, there by accounting for spatial inhomogeneity of turbulence. Results indicate that the combination of the quenching correlation and the modified BML model has been successful in eliminating wall flame acceleration problem, while accurately predicting in-cylinder pressure rise, mass burn rates and heat release rates.

ABSTRACTTwo complementary experimental techniques are presented that describe the mechanisms during the combustion synthesis of NiAl and Ti5Si3. The first involves quenching a reacting wedge-shaped sample imbedded in a copper block where the propagating combustion front extinguishes while traveling to the apex. The second technique, time-resolved X-ray diffraction (TRXRD), provides a direct in-situ observation of the sequence of high temperature phase transformations. The information obtained from this investigation will be useful in developing improved process models of combustion synthesis, which can lead to the production of advanced materials with tailored microstructure and properties.

Turbulent jet ignition (TJI) is a promising strategy to ignite diluted air-fuel mixtures; this is usually generated by igniting a fraction of the mixture inside a small pre-chamber. Nevertheless, the processes that take place inside the pre-chamber, as well as the injection of the turbulent jet into the main chamber and its subsequent re-ignition are not fully understood. The current work presents an experimental investigation that studies the effects of the nozzle size, turbulence level, and air-fuel mixture on the pre-chamber ignition and main chamber re-ignition and combustion. To accomplish this, a series of experiments have been carried out under different boundary conditions. To understand the phenomena taking place in the pre- and main chamber, two different approaches were taken: On one hand, (1) pressure-based diagnostics were applied by fitting a pressure sensor in each of the chambers. This was done to trace the pressure evolution during the whole combustion event and to calculate the heat-release. On the other hand, (2) optical diagnostics were setup on both combustion chambers, using dual schlieren setups synchronized at the same frame rate. The optically accessible test rig and the combination of schlieren in the pre-chamber (PC) & main-chamber (MC) allows to visualize the ignition, flame propagation, quenching mechanisms and re-ignition under a wide range of boundary conditions. This combined with the pressure traces and heat-release give a full understanding of the ignition and combustion processes. Higher turbulence levels and equivalence ratios increase the propagation of the flame front and the peak pressure in the pre-chamber. The resulting higher nozzle-exit velocities lead, on one hand, to faster mixing and therefore to a larger portion of main chamber fuel within the jet, which decrease the main chamber combustion duration. On the other hand, to high quenching and longer re-ignition times, which show the adverse effect.