Flow coefficients of critical flow venturi nozzles calibrated with hydrogen and other gases

Critical flow Venturi nozzles (CFVNs) are very stable and widely used secondary standards for gas flow rate measurements. The current study presents the first step in introducing CFVNs in the traceability scheme for gaseous hydrogen. The study was arranged in the framework of a Joint Research Project (EMPIR – MetHyInfra) and deals with the characterisation of the hydrogen discharge coefficient and the identification of a potential alternative gas for calibration of nozzles. The presented experimental study was made for two CFVNs with throat diameters of 0.175 mm and 0.436 mm. Tests were carried out using six different gases including hydrogen for the inlet pressures between 200 kPa and 700 kPa thereby covering the Re number range from 2  10 3 to 6  10 4 . The results for both tested nozzles demonstrate the dependence of the discharge coefficient on the isentropic coefficient of the gas. With the exception of nitrous oxide, this behaviour can be explained by the theoretical model accounting for the isentropic coefficient, which presents good prospects for calibrating the nozzles intended for hydrogen processes with alternative inert gases.