Investigation On Simplified Modeling Of Fuel-rich Gas Thermal Property And Acceleration Process Simulation For Air Turbo Rocket Engines

The Air Turbo Rocket engine(ATR)is expected to be the suitable power device for near space vehicles due to its wide operation envelope,relatively high specific impulse and thrust,and excellent self-starting ability.In order to simulate the ATR engine acceleration process quickly and accurately,and hence improve its acceleration performance,achieve automatic control and optimize the control parameters,the modeling and simulation for the acceleration process of the ATR engine is studied in this paper.The main work is as follows:First,the component models that compose the ATR engine,such as the intake,the compressor,the gas generator,the turbine,the combustor and the nozzle,are built.In which the variable specific heat of the working fluids is considered,and the combustion process in the gas generator and the combustor is evaluated via a chemical equilibrium method based on minimum Gibbs free energy theory.Then the simulation model of the ATR engine acceleration process is established which employs plena to describe the volume dynamic characteristics of the gas generator and combustor in the engines.The control parameters of the close-loop PI controller adapted in the model are also discussed and analyzed.Results show that this model is capable of simulation the acceleration process of the ATR engine without iteration,hence the dynamic performance can be computed quickly and preciously.If the characteristics of the components are given,then the engine acceleration performance is directly influenced by the proportional factor K_p and the integral factor K_i of the PI controller.If K_p and K_i are too large,then the rotation speed of the engine would increase rapidly in the initial stage,while the compressor would have less surge margin,and the rotor would have bigger overshoot.The oscillation in rotation speed causes engine instability,which could harm the operation safety.And there would be bigger over-temperature in the combustor,thus the thermal protection should be taken into account.If K_p and K_i are too small,them the rotor acceleration would be too small,causes longer acceleration time.Therefore,if the compressor has enough surge margin,then the values of K_p and K_i should be optimized to maintain acceleration safety while minimize the stabilization time.Secondly,a component map exponent extrapolation method based on flow similarity is conducted and the low-speed component performance maps of the compressor and turbine are obtained,based on which the ATR engine start-up performance model is built.A hybrid control scheme which combines the open-loop controller with a close-loop PI controller is adopted to control the propellant mass flow at the gas generator inlet.With fixed open-loop control parameter,the engine start-up time,rotor overshoot,the combustor over-temperature is directly affected by the close-loop control parameters,hence the proportional factor K_p and the integral factor K_i are also discussed and analyzed.Results show that the ATR engine has excellent self-starting ability.And with the adopted hybrid control scheme,the ATR engine could start up from zero rotation speed to design rotation speed safely and quickly.With the optimal control parameters,the engine could start up within 8s,while there is almost no overshoot on the rotor and no over-temperature in the combustor.Finally,the air flow path,the fuel-rich gas flow path and the combustion gas flow gas in the ATR engine are analyzed respectively.Then the modeling method of each component is simplified based on the characteristic of the working fluids in the corresponding flow path.Hence the ATR engine real-time performance model is established.By comparing the simulation results with that of the non-real-time model,it shows that the real-time model has fine accuracy in predicting the rotor rotation speed,while has relative lower fidelity in turbine performance.This is because in the ATR engine,the turbine expansion ratio is much higher than the compressor pressor ratio,and the temperature varies greatly from the turbine inlet to the turbine outlet,which makes the thermodynamic parameters of the fuel-rich gas change intensely,hence the average specific heat method adopted here would cause higher relative error.Compared to the non-real-time model,the execution speed is improved substantially,and the real-time is achieved.