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Title: Back Pressure Ratio And The Transonic Resonance Mechanism Of Low Unchoking In Critical Flow Venturis
Author: J. D. Wright B. W. Sims R. J. Mckee K. A. Gillis A. N. Johnson m. S. Carter Ida I. Shinder
Source: Flomeko 2016
Year Published: 2016
Abstract: 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 so nic velocity is referred to as the maximum back pressure ratio (MBPR). Being able to accurately predict the MBPR for a sp ecific 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 50 000, 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

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