A computational study of real gas flows through a critical nozzle

Abstract

A critical nozzle is used to measure the mass flowrate of gas. It is well known that the coefficient of discharge of the flow in a critical nozzle is a single function of the Reynolds number, in which the discharge coefficient approaches unity as the Reynolds number increases. However, it has recently been reported that at very high Reynolds numbers, which correspond to high-pressure supply conditions, the discharge coefficient exceeds unity. This impractical value in the discharge coefficient is vaguely inferred to be due to real gas effects. The purpose of the present study is to investigate high-pressure hydrogen gas flow through a critical nozzle. A computational analysis has been carried out to simulate a critical nozzle flow with real gas effects. Redlich—Kwong's equation of state is incorporated into the axisymmetric, compressible Navier—Stokes equations to account for the inter-molecular forces and molecular volume of hydrogen. The computational results show that the critical pressure ratio and the discharge coefficient for ideal gas assumptions are significantly different from those of the real gas, as the Reynolds number exceeds a certain value. It is also known that the real gas effects appear largely in terms of the compressibility factor and the specific heat ratio, and these become more remarkable as the pressure of hydrogen increases.