Experimental study on improving the critical back-pressure ratio using a step in a critical-flow Venturi nozzle

Abstract

Critical-flow Venturi nozzles (CFVNs) are used widely as transfer standards and can measure flow rates accurately. However, previous research has reported that in some CFVNs, the pressure-recovery effect of the diffuser is poor, and it is called the premature unchoking phenomenon. In the present study, experiments were conducted to investigate the critical back-pressure ratio (CBPR) of a CFVN. Additionally, to solve the problem of premature unchoking phenomenon, we focused on the step in the diffuser and verified its effect. The diffuser step can maintain a high CBPR at values of the Reynolds number (Re) of around 22,000 and 5500. For Re = 5500, although the CBPR of a typical CFVN is 0.46, providing the step increases it to 0.75. Notably, we are able to increase the CBPR at low Re, at which previous studies could not suppress premature unchoking phenomenon. A diffuser step is believed to improve the CBPR because the shock location is fixed at the step, and the region of subsonic flow, which is important for pressure recovery, is expanded.