A new method of computing the instantaneous polytropic exponent of the discharge process of a tank with compressed air is proposed. The state equation of ideal gas and the time-variation polytropic process equation are used to derive the differential equation of the pressure. The Nusselt criterion based on the natural convection heat transfer is used for the determination of the coefficient of heat transfer. The instant polytropic exponent can be obtained by substituting the coefficient into pressure differential equations. Calculation examples are given and the results are graphicallyillustrated. Experimental results show a good agreement with the simulation data.

Keywords: polytropic exponent, discharge process, time-variation, natural convection heat transfer, gas