The Calculation Of Thermodynamic Properties Of Air With The Dissociation Of Oxygen

When an aircraft flies at a hypersonic speed, the temperature of gas inner boundary layer near the wall is so high because of the existence of shock wave and the effect of viscosity that part kinetic energy of the air transform into the energy of molecular random motion. The freedom degree of molecular vibration is excitation and then the dissociation or ionization of the molecular and excitation of electronic. The air is no longer a perfect gas any more but a real gas constituted by the molecules, atoms, ions and electrons. The equation of gas state and the thermodynamic properties such as compressibility factor, energy, enthalpy, the specific heat and so on are deviate from values of perfect gas.The thermodynamic properties of air with dissociation should be studied in order to study the stability of hypersonic boundary layer and predict the transition location well. In this paper, the thermodynamic properties are studied with the methods of the degree of dissociation and the equilibrium constant. The following conclusions can be drawn:1、The degree of dissociation of oxygen and the compressibility factor of the air are functions of pressure and temperature. The degree of dissociation and the compressibility factor of the air increase as the temperature increases at the same pressure. And the degree of dissociation and the compressibility factor of the air decrease as the pressure increases at the same temperature.2、The internal energy and the enthalpy of the air increase as the temperature increases, and get a maximum value in the end of dissociation reaction. The specific heat per mol at constant density and the specific heat per mol at pressure of mixed gas are decrease after the first increase as the raise of temperature. And the specific heat gat the maximum value at the temperature where the reaction rate of dissociation is largest. The gas considering the dissociation absorbs more heat than that of perfect gas with the temperature raises the same amount, namely, the gas considering the dissociation raises less degrees of temperature than that of perfect gas with the gas absorbs the same amount of heat.3、The ratio of specific heat and isentropic exponent of mixed gas change as the dissociation of oxygen. They decrease after the first increase as the raise of temperature. And the isentropic exponent gets the minimum value at the temperature where the reaction rate of dissociation is largest. The ratio of specific heat gets the minimum value before that temperature.4、The closer to one the pressure is, the smaller the compressibility factor of the air is, and the curves of thermodynamic properties of air are more gradual. So the deviation of air from the perfect gas is less, and the changes of thermodynamic properties are less. Therefore the dissociation of oxygen should be considered in the case of high temperature or low pressure, especially in the case of hypersonic aircraft.