Academic literature on the topic 'Mean swirl number'

Flame response to imposed velocity fluctuations is experimentally measured in a single-nozzle, turbulent, swirling, fully-premixed combustor. The flame transfer function is used to quantify the flame’s response to imposed velocity fluctuations. Both the gain and phase of the flame transfer function are qualitatively similar for all operating conditions tested. Flame transfer function gain exhibits alternating regions of decreasing gain with increasing forcing frequency followed by regions of increasing gain with increasing forcing frequency. This alternating behavior gives rise to gain extrema. Flame transfer function phase magnitude increases quasi-linearly with increasing forcing frequency. Deviations from the linear behavior occur in the form of inflection points. Within the field, the current understanding is that the flame transfer function gain extrema are caused by the constructive/destructive interference of swirl number fluctuations and vortex shedding. Phase-synchronized images of forced flames are acquired to investigate the presence/importance of swirl number fluctuations, which manifest as fluctuations in mean flame position, and vortex shedding in this combustor. Analysis of phase-synchronized flame images reveals that mean flame position fluctuations are present at forcing frequencies corresponding to flame transfer function gain minima but not at forcing frequencies corresponding to flame transfer function gain maxima. This observation contradicts the understanding that flame transfer function gain maxima are caused by the constructive interference of mean flame position fluctuations and vortex shedding since mean flame position fluctuations are shown not to exist at flame transfer function gain maxima. Further analysis of phase-synchronized flame images shows that the variation of mean flame position fluctuation magnitude with forcing frequency follows an inverse trend to the variation of flame transfer function gain with forcing frequency, i.e. when mean flame position fluctuation magnitude increases flame transfer function gain decreases and vice versa. Based on these observations it is concluded that mean flame position fluctuations are a subtractive effect. The physical mechanism through which mean flame position fluctuations decrease flame response is through the interaction of the flame with the Kelvin-Helmholtz instability of the mixing layer in the combustor. When mean flame position fluctuations are large the flame moves closer to the mixing layer and damps the Kelvin-Helmholtz instability due to the increased kinematic viscosity, fluid dilatation, and baroclinic production of vorticity with opposite sign associated with the high temperature reaction zone.

In this study, experiments have been performed to investigate effects of pressure-swirl atomizer geometry on SMD. Different pressure-swirl atomizers were applied to study the effect of geometry on the SMD. Based on the experimental results, an empirical correlation was obtained to relate SMD with the Weber number characterized by film thickness. Meanwhile, a semi-empirical model which was improved from the surface wave breakup theory was established to predict the SMD of pressure-swirl atomizers. The model provides the droplet diameter as a function of atomizer geometry, operation condition and liquid properties. It is proved that the model is qualified for predicting SMD of pressure-swirl atomizers among wide range.

An aerodynamic design method is described and used to implement a parametric study of radial turbomachinery blade design in three-dimensional subsonic flow. Given the impeller hub and shroud, the number of blades and their stacking position, the design method gives the detailed blade shape, flow and pressure fields that would produce a prescribed tangential averaged swirl schedule. The results from that study show that decreasing the number of blades increases the blade wrap, and that the blade loading is strongly affected by the rate of change of mean swirl along the mean streamlines. The results also show that the blade shape and the pressure field are rather sensitive to the prescribed mean swirl schedule which suggests that, by carefuly tailoring the swirl schedule, one might be able to control the blade shape and the pressure field and hence secondary flow.

Abstract Precessing vortex cores (PVC), arising from a global instability in swirling flows, can dramatically alter the dynamics of swirl-stabilized flames. Previous study of these instabilities has identified their frequencies and potential for interaction with the shear layer instabilities also present in swirling flows. In this work, we investigate the dynamics of precessing vortex cores at a range of swirl numbers and the impact that turbulence, which tends to increase with swirl number due to the increase in mean shear, has on the dynamics of this instability. This is particularly interesting as stability predictions have previously incorporated turbulence effects using an eddy viscosity model, which only captures the impact of turbulence on the base flow, not on the instantaneous dynamics of the PVC itself. Time-resolved experimental measurements of the three-component velocity field at ten swirl numbers show that at lower swirl numbers, the PVC is affected by turbulence through the presence of vortex jitter. With increasing swirl number, the PVC jitter decreases as the PVC strength increases. There is a critical swirl number below which jitter of the PVC vortex monotonically increases with increasing swirl number, and beyond which the jitter decreases, indicating that the strength of the PVC dominates over turbulent fluctuations at higher swirl numbers, despite the fact that the turbulence intensities continue to rise with increasing swirl number. Further, we use a nonlinear van der Pol oscillator model to explain the competition between the random turbulent fluctuations and coherent oscillations of the PVC. The results of this work indicate that while both the strength of the PVC and magnitude of turbulence intensity increase with increasing swirl number, there are defined regimes where each of them hold a stronger influence on the large-scale, coherent dynamics of the flow field.

Swirlers are very important elements of gas turbine combustion systems, necessary for the proper operation of a modern gas turbine engine. In this paper the performance of air swirlers with a range of swirl angles and diameters are examined and discussed in the context of recently published papers. Parameters which are included in this discussion are swirl number, swirler thrust, swirler torque, swirler solidity, etc. Results suggest that swirler “see through” is not necessarily bad, but may be beneficial with respect to swirl number and other parameters. The use of a mean swirler radius to calculate the swirl number and the resulting benefits are discussed and demonstrated.

A hollow-cone spray with a nominal cone angle of 30 degrees from a pressure-swirl fuel atomizer was used in a swirl-stabilized combustor. The combustor is circular in cross section, with a swirl plate and fuel nozzle axis coinciding with the axes of the chamber. kerosene is injected upward inside the chamber from the fuel nozzle. Separate swirl and dilution air flows are distributed into the chamber that pass through honeycomb flow straighteners and screens. A calculated swirl number of 1.5 is generated with the design swirl plate exit air velocity of 30 degrees with respect to the chamber axis. Effects of swirl and dilution air flow rates on the shape and stability of the flame are investigated. A Phase Doppler Particle Analyzer (PDPA) is used to measure drop size, mean and rms values of axial drop velocity, fuel volume flux, drop velocity and size distributions, and size-classified drop velocity profiles for two cases of with and without combustion and at six different axial locations from the nozzle. For the no-combustion case all air and fuel flow rates were kept at the same values as the combusting spray condition. Results for mean axial drop velocity profiles indicate widening of the spray, with slight increase in the magnitudes of the peak drop velocities due to combustion. Root mean square (RMS) values of drop velocity fluctuations decrease due to a combination of increase in gas kinematic viscosity and elimination of small drops at high temperatures. Sauter mean diameter (SMD) radial profiles at all axial locations increase with combustion due to preferential burning of small drops. Fuel volume flux profiles indicate negligible drop vaporization and/or burning up to a distance of 25mm from the nozzle. Velocity number distributions at different radial points for without combustion at an axial distance of 55mm from the atomizer are symmetric in shape only close to the peak of the mean drop velocity and show a bimodal shape around the maximum mean drop axial velocity gradient. Corresponding number distributions for the combustion case are fairly symmetric and quite different in behavior at all radial positions. Size-classified drop velocity profiles are also plotted and discussed.

An experimental work was carried out on confined swirling flows under non-combusting conditions in a reverse flow annular gas turbine combustor. Flow measurements with a five hole pitot probe are carried out in a flow apparatus of a geometrical configuration similar to the model of a swirl combustor. Mean flow results are obtained for different flow conditions to determine the effect of swirl on the recirculation zone and the variation of the swirl strength along the axis of the gas turbine combustion chamber. The boundaries of the recirculation region are plotted to compare the size and length of the zone with various swirlers. Minimum flow Reynolds number is required for flow recirculation; the effect of Reynolds number on determining which flow class is present for flow of interest in combustion chamber was investigated. The inlet swirl number is optimized for higher swirl strength and the inlet swirl number for which recirculation completely vanishes is also estimated.

This work presents measurements of the dynamic and mean pressure distributions on the stator wall of a 50% eccentric whirling annular seal for three different pre-swirl conditions: −45, 0 and +45 degrees and two flow cases, Taylor number of 6,600/Reynolds number of 12,000 and Taylor number of 3,300/Reynolds number of 24,000. A data reduction calculation of the forces and moments generated by the pressure field has been performed, and a description of the effect of swirl on the pressure field has been presented.

A single-component PDPA is used to evaluate the spray characteristics of a simplex pressure-swirl atomizer when operating at high liquid flow rates and elevated ambient air pressures. Attention is focused on the effects of air pressure on mean drop size, drop-size distribution, mean velocity, volume flux, and number density. Using a constant flow rate of 75 g/s, measurements are carried out along the spray radii at a fixed distance downstream from the atomizer face of 50 mm. The air pressures of 1, 8, and 12 bars chosen for these tests correspond to air densities of 1.2, 9.6, and 14.4 kg/m3. The purpose of the investigation is to supplement the existing body of information on pressure-swirl spray characteristics, most of which were obtained at normal atmospheric ambient pressures, with new data that correspond more closely to the conditions prevailing in the primary combustion zones of modern gas turbines. The results obtained are explained mainly in terms of the influence of air pressure on spray structure, in particular spray cone angle and Weber number.