A Numerical Method For Fuel Spray Combustion In The Scramjet Combustor

Liquid hydrocarbon fuel is widely used in the scramjet combustor because it is safe,portability and easy to store.To better understanding working processes of the scramjet combustor,research on liquid hydrocarbon fuel evaporation,ignition and combustion process in the supersonic flow is helpful.Thus,study of the numerical method for fuel spray combustion in the scramjet combustor is significant.Based on Eulerian-Lagrangian method,a mathematical-physical model were proposed to simulate two phase combustion flow under supersonic conditions: The carrier fluid surrounding the liquid droplets was described by multi-component Navier-Stokes equations with chemical source term,and the discrete droplet method was employed to represent the behavior of atomized droplets in present work,and several sub-models were introduced to control the behavior of the spray droplets,including evaporation model,breakup model and collision model.The performance of individual spray submodels involved in above mathematical models was validated through multiple perspective quantitatively,and the results are in accordance with the experimental data.Additionally,effects of breakup model,injection model and evaporation model on the prediction of atomization processes in supersonic cold flow field were compared and analyzed.The main results were as follows: more accurate prediction values were achieved under supersonic flow conditions with K-H/R-T hybrid model comparing to other breakup models;the droplet size of the injected droplets had a significant impact on predicted values of the droplet size,location and penetration height;the main source of fuel vapor in supersonic flow field were droplets with small size.For better describing the combustion characteristics of liquid kerosene in the scramjet combustor after atomization and evaporation,further study on chemical kinetic mechanism is significant.To reduce the computational cost of numerical simulation of the combustion process in scramjet combustor,considerable simplification of chemical kinetic mechanism for practical calculations is required.A detailed chemical reaction model for the combustion of RP-3 surrogate fuel(73.0% n-dodecane,14.7%1,3,5-trimethylcyclohexane,12.3% n-propylbenzene,mass fraction)was simplified to obtain a new reduced model(26species and 89 reactions)based on the sensitivity analysis and reaction path analysis,depicted combustion characteristic of supersonic reacting flow field accurately.The ignition and combustion characteristics of this surrogate fuel at various conditions were simulated using the reduced reaction model,and the simulation results were compared with the experimental data and the calculated results by the detailed reaction model.Furthermore,numerical simulations were implemented using a coupled nonequilibrium reaction solver with the proposed reduced model,and the combustion characteristic of a scramjet combustor was analyzed in detail.The predicted values of steady combustion are validated by the experimental data with good agreement,and the results compared with quasi-global chemical reaction mechanism show that the reduced mechanism has better accuracy in combustion simulation.Furthermore,the unsteady combustion characteristics of a scramjet combustor with two different chemical reaction mechanisms are analyzed in detail.Predictions of the mass fractions of major species,such as alkanes,alkenes,and cycloalkanes species are presented and discussed.The calculations show that the reduced mechanism predicts the myriad details of the reacting flow field accurately,including the effect of free radicals and their related chemical reactions on the process of heat release,which improved the combustion simulation accuracy.On the basis of the above-mentioned research,numerical investigations on the combustion process of a liquid hydrocarbon fueled scramjet,ignited by hydrogen pilot flame were conducted using the Eulerian-Lagrangian method with the proposed reduced chemical reaction model.The combustion characteristic of a scramjet combustor was analyzed and the effect of several aspects,including the equivalence ratio,injection position of liquid kerosene,on the process of breakup,evaporation,combustion was discussed in detail.The main results were as follows: increasing fuel equivalence ratio and introducing hydrogen flame could improve the mixing efficiency and the combustion efficiency;the combustion of kerosene was enhanced by the high concentration of free radicals near the pilot hydrogen flame,and the enhanced mixing and evaporation process by the shock wave induced by hydrogen injection was another mechanism for piloted ignition.