It is shown that critical flow Venturi nozzles need time intervals, i.e., more than five hours, to achieve steady state conditions. During these intervals, the discharge coefficient varies gradually to reach a value inherent to the pressure ratio applied. When a nozzle is suddenly put in the critical condition, its discharge coefficient is trapped at a certain value then afterwards approaches gradually to the inherent value. Primary calibrations are considered to have measured the trapped discharge coefficient, whereas nozzles in applications, where a constant pressure ratio is applied for a long time, have a discharge coefficient inherent to the pressure ratio; inherent and trapped coefficients can differ by 0.03–0.04%.
Super-Fine Structure in the Critical Flow-Rate of Critical Flow Venturi Nozzles
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Ishibashi, M. "Super-Fine Structure in the Critical Flow-Rate of Critical Flow Venturi Nozzles." Proceedings of the ASME 2002 Joint U.S.-European Fluids Engineering Division Conference. Volume 1: Fora, Parts A and B. Montreal, Quebec, Canada. July 14–18, 2002. pp. 105-114. ASME. https://doi.org/10.1115/FEDSM2002-31079
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