Calculations of Flow of Air and Diatomic Gases

Abstract

Measurement of airflow is frequently required in connection with aeronautical projects. Conventional practice, as exemplified by the A.S.M.E. Test Code for Flow Measurement, provides two different formulas (or sets of formulas) for the calculation of flow from pressure and temperature measurements made with standard nozzles, orifices, and the like, the proper formula for use in any particular case depending upon whether the pressure ratio across the is greater or less than the critical value. In most cases of aeronautical interest the formula for subcritical (or restricted) flow is applicable. However, the exact formula for this case is quite cumbersome, and often requires much laborious calculation. Without sacrifice of exactness, the much simpler formula for critical flow may be generalized and used in all cases merely by including a restriction factor that is a function of pressure ratio only, and so may be tabulated once and for all. This paper proposes such a formula and presents a table of factors applicable to air and other diatomic gases.