A Study Of Flow Stability In Hypersonic Flat Boundary Layer Flow With Air Dissociation

When the aircraft flies at high hypersonic speed,due to the existence of shock waves and the reduction of the velocity of the boundary layer near the solid walls,the gas will reach a very high temperature.When the temperature is higher,such as an atmospheric pressure,the temperature is above 2500 K,the oxygen and nitrogen molecules in the air will dissociate successively.The component of air and the value of thermal physical parameters will vary with pressure and temperature.The influence of the change of thermal physical parameters on flow stability,aerodynamic force and aerodynamic heat needs to be studied.Firstly,the calculation method of thermophysical parameters of air dissociation is studied.For the four component model which only considers the dissociation of oxygen and nitrogen molecules,the equation satisfying the degree of dissociation is established,and the relationship between thermal physical parameters and dissociation degree is obtained.For more components,that is,in addition to dissociation,there are also reactions such as ionization,etc.The traditional equilibrium constant method is used to analyze the calculation of thermal physical parameters,and the results are compared with the one of existing thermal physical parameters table and the fitting method.The influence of different component models on thermophysical parameters is studied.On this basis,the basic flow of the air dissociated hypersonic plate boundary layer is calculated,the flow stability is analyzed,and the wall heat flow and the wall friction drag coefficient are calculated and analyzed.The main conclusions obtained are as follows:1.The dissociation degree method is reliable for calculating the value of thermal physical parameters only with dissociation,and the method is simple and intuitionistic.The dissociation degree of oxygen and nitrogen molecules is a function of temperature and pressure.When the pressure is the same,the degree of dissociation and compressibility will increase with the increase of temperature.When the temperature is the same,the higher the pressure is,the lower the degree of dissociation and compressibility factor are.2.In view of the above five components,the relationship between thermal physical parameters and temperature and pressure is established by using the traditional equilibrium constant method,and an analytical method for calculating the parameters of thermal physical properties at any pressure and temperature is obtained.3.The calculation of different component models shows that,for the pressure range of 0.01-1atm,different component models can be used for different temperature ranges,and the results of thermal physical parameters are consistent.For example,when the temperature is lower than 4000 K,the five component model is adopted.The temperature is within the range of 4000-6500 K,the seven component model is adopted.The temperature is over 6500 K,the thirteen component model is used.4.Considering the dissociation of air,there is still a similarity solution for hypersonic flat plate boundary layer flow.Comparing with the results of calorimetric complete gas discoveries that the basic flow profile,flow stability characteristics,wall heat flux and friction coefficient of the air dissociation boundary layer are quite different.The air dissociation reduces the thickness of the boundary layer and decreases the temperature.The second mode wave instability,considering air dissociation,moves in the low frequency direction,the frequency band narrows and the growth rate curve moves upstream,and the peak of growth rate increases.Air dissociation reduces the heat flux on the wall and increases the friction coefficient.