Research On Large Eddy Simulation Of Ignition Sequence Of Combustor

The ignitability of combustion chamber is an important performance of aero-engine,which has a large impact on the reliability of ground start and the safety of re-ignition at high altitude.With the development of aviation technology and the expansion of operating range of aero-engine,it is very necessary to study the ignition process to enlarge the lean ignition limit of combustor as far as possible.Duo to the complexity of ignition process which include the fuel atomization,turbulent motion,combustion chemical reactions.It is costly to research the ignition process by experiment and can only get the ignition information macroscopically.The research by numerical simulation is more detail,and can get more ignition characteristic.Therefore,it i s very important to accurately predict the ignition performance by the numerical simulation for the design and operation of aero-engine combustion chamber.In this paper,a large-eddy numerical simulation of the CH4-Air non-premixed combustion process with a cone-shaped blunt body in a central reflow chamber is carried out by a Dynamic Thickening Flame Model.The DTFM can improve the simulation accuracy of the flame front.The turbulence sub-grid model adopts the WALL model considering the viscosity of the near-wall surface and simulates the heat release process of the spark plug by adding the Gauss heat source to the energy equation.The dynamic diffusion process of the flame in 40 ms is calculated.The large eddy simulation can capture the small-scale turbulence in the flow field.The dynamic thickening flame model can predict the ignition time and the flame size more accurately.It is found that the flame spreading into the recirculation zone and igniting the mixed gas in the recirculation zone is the key o f success of ignition.The existence of flame in the recirculation zone can be used as the criteria for the ignition success.Then,the dynamic thickening flame model is used to simulate the liquid mist ignition process of a refluxing combustor with a swirler.The flame can be stabilized in the back-flow zone at 100 ms,which can be regarded as ignition success.It is found that the best ignition parameters of the igniter are as follows: ignition energy 0.6J,fire core radius 1mm,ignition duration 1ms by researching the coupling influence of ignition parameters on the performance of ignition of a refluxing combustor.Increasing the inlet air temperature or the fuel temperature can effectively improve the ignition limit of the return burner.When the inlet air temperature increased from 580 K to 640 K,the minimum ignition gas-oil ratio decreased by 17.8%.When the fuel temperature increased from 340 K to 470 K,the minimum ignition gas-oil ratio decreased by 35.9%.Finally,a refluxing combustor chemical reactor network is established.Based on the complex reaction mechanism,the ignition process of the backflow combustor is calculated and analyzed.The results show that the variation of average temperature with time consisting with the results of large eddy simulation.On the basis of this,the influence of intermediate products on ignition delay time and flame propagation velocity is analyzed by chemical reactor model.The results show that the increase of OH,H,O concentration can shorten the ignition delay time and accelerate the flame propagation speed in the low temperature region,which is beneficial to improve the ignition performance.