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Back Pressure Ratio and the Transonic Resonance Mechanism of Low Unchoking in Critical Flow Venturis



John D. Wright, Keith A. Gillis, Aaron Johnson, Ida I. Shinder


Reliable Critical Flow Venturi (CFV) operation requires sonic velocity at the throat of the device. The maximum ratio of exit pressure to inlet pressure that ensures this sonic velocity is referred to as the maximum back pressure ratio (MBPR). Being able to accurately predict the MBPR for a specific CFV as well as design a CFV to have a high MBPR allows diverse application and confidence in CFV flow measurements. At Reynolds numbers based on throat diameter below 50000, CFVs can display low unchoking and the standard equation over-reports the flow by 1 % or more. We show that MBPR for a particular, 0.8 mm throat diameter CFV using dry air, argon, helium, and sulfur hexafluoride is well correlated by the "fully expanded jet Mach number". Sound detected by microphones placed up and downstream from a CFV show high correlation between low unchoking and the presence of powerful transonic resonances (oscillations at audio frequencies in pressure and the position of a lambda shock in the CFV diffuser) described by Zaman et al. We propose a mechanism in which pressure fluctuations from the transonic tones lead to intermittent unchoking of the CFV throat. This paper also presents unchoking test results from 270 unchoking tests on 79 CFVs with a wide range of throat diameters, Reynolds numbers, diffuser lengths, half angles, and gases. Correlations for avoiding low unchoking and predicting MBPR for broader application are presented that incorporate the necessary effects due to Re, diffuser length, diffuser area ratio, and specific heat ratio. We advocate inclusion of these correlations in documentary standards.
Proceedings Title
Conference Dates
September 26-29, 2016
Conference Location
Sydney, AU


critical flow venturi, low unchoking, premature unchoking, transonic resonance, conical diffuser


Wright, J. , Gillis, K. , Johnson, A. and Shinder, I. (2016), Back Pressure Ratio and the Transonic Resonance Mechanism of Low Unchoking in Critical Flow Venturis, FLOMEKO 2016, Sydney, AU, [online], (Accessed May 17, 2024)


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Created September 28, 2016, Updated April 7, 2022