Experimental study on improving the critical back-pressure ratio using a step in a critical-flow Venturi nozzle. (March 2020)

• Record Type: Journal Article
• Authors: Takegawa, Naoki; Ishibashi, Masahiro; Morioka, Toshihiro
• Is Part Of: Flow measurement and instrumentation. Volume 71(2020)
• Publication Date: 2020-03
• Subjects: Critical-flow Venturi nozzle -- Critical back-pressure ratio -- Step -- Diffuser -- Premature unchoking phenomenon -- Normal shock; Fluid dynamic measurements -- Periodicals; Flow meters -- Periodicals; Fluides, Dynamique des -- Mesure -- Périodiques; Débitmètres -- Périodiques; 681.2805
• DOI: 10.1016/j.flowmeasinst.2019.101682
• Languages: English
• ISSNs: 0955-5986

Abstract

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. Highlights: A step in a diffuser can maintain a high critical back-pressure ratio at low Re (=5500). The shock generation is fixed at the step, and the subsonic region, which is important for pressure recovery, is expanded. The back-pressure ratio considering energy dissipation due to a normal shock decreases as the shock moves downstream.