Research On Response Characteristic And Nonlinear Analysis Of Stability Of Liquid Rocket Engine

Dynamic characteristic includes response characteristic and stability. The shortcoming and problems of these two parts are resolved in the thesis by means of theoretical analysis, numerical simulation, and experiment research.The cryogenic throttling venturi is designed for liquid oxygen in the thesis. Uniform dynamic models of throttling Venturi are established when Venturi is cavitated or not. Large numbers of cold tests and firing tests are taken based on the mass flow control system, which show the system can control the mass flow of liquid oxygen accurately and change the operating conditions successfully. By the tests, the rules of discharge coefficient and pressure recovery factor changing with inlet pressure of Venturi and the displacement of the tapered rod are found.The dynamic models of the electro-pneumatic valve and the lever pneumatic liquid valve are established. The effects of the pressure of control gas and voltage etc. on the electro motion are analyzed in the thesis. And the effects of the pressure of control gas, radius of the control cavity, the length between the solenoid valve and the pneumatic valve, the inlet pressure of the propellant etc. on the startup of the pneumatic valve are also analyzed in the thesis. Many experiments are taken to study the response characteristic of the electro-pneumatic valve and the lever pneumatic liquid valve.Uniform dynamic models of the combustion chamber are established for tripropellant mode and twipropellant mode. The dynamic response processes of the ground test system of the tripropellant model engine are studied detailedly, which include the priming process of normal temperature and cryogenic propellants, the timing sequence of startup and mode-transition. The liquid volume fraction is proposed to describe the priming process of the cavity. Large numbers of ground firing tests are taken to validate the simulation results.Nonlinear time series analysis based on phase space reconstruction is introduced to analyze the stability of liquid rocket engine for the first time, which is a new visual angle to study the stability of complicated engine system. The new method can give the boundary of stability and reveal the dynamics essence of the engine system. It can provide a brand-new method to resolve system dynamic instability and combustion instability, which are difficult problems in theory and engineering.The foundation of nonlinear time series analysis is clean and real data. A new wavelet-denoising method is proposed in the thesis, which is based on wavelet transform modulus maximum and nonlinear threshold denoising method on wavelet. This method absorbs the advantages and overcomes the disadvantages of traditional methods. Lorenz system is used to validate the improved method, which shows the new method has the advantage of the high precision and the simple arithmetic and the high computing efficiency. And the improved method is used to denoise the test data. The results show it can denoise very well and also keep the important information of the test data.The in-depth theory study of nonlinear time series analysis based on phase space reconstruction is taken in the thesis. The program is developed to compute the reconstruction parameter including the time delay and the embedding dimension, correlation dimension and maximum Lyapunov exponent. For the ground test system of the tripropellant model engine, it's the first time to compute correlation dimension between 2.9 and 3.2 and maximum Lyapunov exponent between 0.0006 and 0.001 when the state of engine is normal. The abrupt change of correlation dimension and maximum Lyapunov exponent can detect the fault of the engine and decide the boundary of stability.Nonlinear time series analysis based on phase space reconstruction is used to analyze the data from experiments of the ground test system of the tripropellant model engine. The results show the stability boundary of hydrogen percentage is about 12% and hydrogen temperature is about 112K and hydrogen injection pressure drop is about 16 percent of the combustion chamber pressure. Once these parameters are under the boundary, instability may be excitated. The range of stability of the oxygen-fuel ratio is broad. Engine can work normally when the oxygen-fuel ratio is between 0.6 and 1, which is advantageous for liquid rocket engine.