Effects of step in CFVN on Premature Unchoking Phenomena

N. Takegawa, M. Ishibashi, T. Morioka

Abstract

CFVNs (Critical Flow Venturi Nozzles) are widely used as transfer standards and can realize the accurate measurement of the flow rate. However, previous research has reported that the pressure recovery effect of the diffuser deceases significantly in some CFVNs, and it has been called PUP (Premature Unchoking Phenomenon). In this study, we conducted experiments and investigated the CBPR (Critical Back Pressure Ratio) of the CFVNs. Additionally, in order to solve the PUP, we focused on the step in the diffuser and verified the its effect. As a result, although a backward facing step (causing increase of diameter) at the throat is effective for improving the CBPR at the Reynolds number of about 44000, it does not provide the improvement when the Reynolds number decreases to 11000. A forward facing step (causing decrease of diameter) in the diffuser delivers the desired pressure recovery even in the low Reynolds number of about 5000, and the CBPR can be improved. Therefore, the forward facing step in the diffuser is suggested to be a new countermeasure against the PUP in the low Reynolds number.

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DOI

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IMEKO TC

TC9 - Flow Measurement

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