Effect of gas type on the thermal properties of small sonic nozzles

摘要

A previous study (Bignell and Choi, Flow Meas. & Instrum. 13 (2002) 17-22) of the effect of temperature on the coefficient used to characterise small sonic nozzles is reviewed. Adiabatic cooling of the gas stream in the throat causes the body of nozzles to be cooled but a heater and temperature control system allow the temperature of the nozzle to be held constant. Using a gas flow standard that can operate in continuous mode, measurements were made of nozzle coefficients at different temperatures using air, argon, nitrogen and carbon dioxide. The nozzle coefficient changes with the gas type and linearly with the temperature of the body of the nozzle. The first of these changes is explained by changes in the real gas correction factor for argon but not for carbon dioxide. The temperature changes are much greater than those due to the area, the discharge coefficient and the real gas correction factor. It is necessary to invoke the properties of the thermal boundary layer to explain these changes, which are found to be greater for gases having a higher specific heat ratio.