Real-air effects in propelling nozzles

Abstract

Properties of 'Real Air' in isentropic nozzle flow are compared with those of either 'Classic Air' for which y = 1.4 exactly or 'Ideal Air' for which y may vary, but which obeys the Ideal Gas law Pv = RT. Having shown that there is little difference between Classic Air and Ideal Air in the total temperature range from 270 to 400 deg K, the present results are presented as ratios of Real Air/Classic Air mass-flow intensity, and again as ratios of Real Air/Classic Air isentropic velocity. Operating conditions cover pressure ratios from 2 to 100, levels of inlet total pressure of 1, 4, 7 and 10 atm, and levels of inlet total temperature of 270, 290, 300, 350 and 400 deg K. At inlet conditions of 10 atm and 290 deg K, Real Air isentropic velocity falls about 0.4 per cent below Classic Air, while Real Air mass-flow intensity rises about 0.6 per cent above Classic Air. Strictly, the calculated results are for expansion of the respective fluids over exactly the same pressure ratio, which requires a flexible nozzle, but fixed nozzle operation is discussed.