141 research outputs found
LES of Turbulent Premixed Flame Kernel Formation and Development
No full textSpark ignition of flammable mixtures is highly sensitive to early and local conditions. Kernel formation and subsequent flame development are largely governed by turbulent conditions and interactions with igniter geometry. In order to investigate this phenomenon, the use of Large Eddy Simulation (LES) is examined for (1) modelling spherical turbulent flame development, and (2) simulating spark ignition in a channel with either laminar or turbulent inflow.
A comparison between LES spherical flame simulation is made to FSD-LES results as well as experimental measurements from previous studies.
For spark ignition experiments, we characterize the temporal evolution of the ignition process, and demonstrate the dependence on early velocity fluctuations and local conditions.M.S
Prediction of Soot Formation in Laminar Opposed Diffusion Flame with Detailed and Reduced Reaction Mechanisms
No full textThe present work focuses on a computational study of a simplified soot model to predict soot production and destruction in methane/oxidizer (O2 and N2) and ethylene/air flames using a one-dimensional laminar opposed diffusion flame setup. Two different detailed reaction mechanisms (361 reactions and 61 species for methane/oxidizer flame and 527 reactions and 99 species for ethylene/air flame) are used to validate the simplified soot model in each flame. The effects of strain rate and oxygen content on the soot production and destruction are studied, and the soot related properties such as soot volume fraction, particle number density and particle diameter are compared with published results. The results show reasonable agreement with data and that the soot volume fraction decreases with higher strain rate and lower oxygen content. The simplified soot model has also been used with two reduced reaction mechanisms (12-step, 16-species for methane flame
and 20-species for ethylene flame) since such reduced mechanisms are computationally more efficient for practical application. The profiles of the physical properties and the major species are in excellent agreement with the results using the detailed reaction mechanisms. However, minor hydrocarbon-species such as acetylene (C2H2) that is the primary pyrolysis species in the simplified soot model is significantly over predicted and this, in turn, results in an over-prediction of soot production. Finally, the reduced reaction mechanism is modified to get more accurate prediction of the minor hydrocarbon-species. The modified reduced reaction mechanism shows that the soot prediction can be improved by improving the predictions of the key minor species.M.S
Parametric study of liquid fuel jet in crossflow at conditions typical of aerospace applications
No full textDue to the fact that cross flow fuel injection is widely used in gas turbine engines combustors, it is important to understand the mechanisms that control the spray breakup within the cross flow. In spite of a lot of work done in this field, very few studies have been carried out under conditions typical of aerospace applications. This thesis describes a series of experiments carried out to simulate these conditions in order to characterize the formation of spray within a high speed, high pressure and high temperature cross flow close to conditions typical of aerospace applications.
Fuel spray characteristics were studied for Jet-A fuel injected into a crossflow (M=0.2 and M=0.35) of preheated (T=555K) air at a chamber pressure of 4 atm. It was seen that larger droplets could be found in the periphery of the spray while smaller droplets could be found closer to the injection plate. In most cases, the droplet velocities were seen to lag the incoming air flow velocity by 20-40% and a spray hat structure was created by the jet in crossflow near the injection wall most likely caused by vortex flow created around the liquid column (jet).
The influence of Weber number was then studied. It was seen that shear breakup mechanism dominates at We greater than about 100. Droplets diameters were found to be in the range of 15-30 microns for higher values of We, while larger droplets (100-200 microns) were observed at Weber number of 33. The initial sharp-edged injector was then replaced by a smooth-edged injector having. Spray characteristics from the two injectors were compared. The spray produced by the smooth countersunk injector penetrated further into the test section away from the injector orifice by approximately 2mm. This injector also produced droplets with a significantly smaller mean diameter (D10). The average droplet velocities in the vertical direction deviated from the incoming air flow velocity to a lesser degree using the countersunk injector. Meanwhile, droplets from this injector had a higher average velocity in the direction of fuel injection between the core of the spray and the orifice wall.M.S
Large eddy simulation of heated pulsed jets in high speed turbulent crossflow
No full textThe jet-in-crossflow problem has been extensively studied, mainly because of its applications in film cooling and injector designs. It has been established that in low-speed flows, pulsing the jet significantly enhances mixing and jet penetration.
This work investigates the effects of pulsing on mixing and jet trajectory in high speed (compressible) flow, using Large Eddy Simulation. Jets with different density ratios, velocity ratios and momentum ratios are pulsed from an injector into a crossflow.
Density ratios used are 0.55 (CH4/air), 1.0 (air/air) and 1.5 (CO2/air). Results are compared with the low speed cases studied in the past and then analyzed for high speed scaling. The simulations show that the lower density jet develops faster than a
higher density jet. This results in more jet spread for the lower density jet. Scaling for jet spread and the decay of centerline jet concentration for these cases are established, and variable density scaling law is developed and used to predict jet penetration in the far field.
In most non-premixed combustor systems, the fuel and air being mixed are at different initial temperatures and densities. To account for these effects, heated jets at temperatures equal to 540K and 3000K have been run. It has been observed that, in addition to the lower density of heated jets, the higher kinematic viscosity effects the jet penetration. This effect has been included and validated in the scaling law for the heated jet trajectory.M.S
Investigating Lean Blowout of an Alternative Jet Fuel in a Gas Turbine Combustor
No full textIn the global effort to reduce the climate impact of combustion emissions, sustainable
aviation fuels offer the ease and reliability of conventional petroleum-derived jet fuels without
the significant pollutant effects. Ongoing research efforts include experimental testing
of alternative jet fuels to identify fuel candidates that produce less pollutant combustion
products and are cheaper and environmentally cleaner to source than conventional jet fuels.
Fuel lean combustion already reduces the emissions of jet engines and increases fuel
efficiency, but lean blowout (LBO) can occur at reduced throttle and minimum power scenarios
such as descent. Lean blowout (LBO) has been identified as a critical figure of merit
to ensure the stability of alternative jet fuels in the place of conventional fuels.
This work aimed to further understand the LBO phenomenon, leveraging computational
studies of the alternative fuel designated C-5 by the National Jet Fuel Combustion
Program (NJFCP). The fuel sensitivity of LBO has been established by the NJFCP’s participants
recently. In this thesis, the chemical kinetics for C-5 is first verified using zerodimensional
(0-D) and one-dimensional (1-D) studies and then this is followed by three
dimensional (3D) large-eddy simulations (LES). In LES to observe LBO, a direct-step and
gradual equivalence ratio reduction were separately employed to assess fuel sensitivity of
LBO against available experimental data. The time histories of pressure, temperature, and
composition were analyzed for precursor signatures of LBO both inside and outside the
flame. Localized extinction, a reduction in the vortex breakdown bubble size and magnitude,
and a reduction in the exhaust velocity were all observed to occur during the LBO
event.M.S
Chemiluminescence Measurements and Modeling in Syngas, Methane and Jet-A Fueled Combustors
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