Abstract：A discussion of some concepts such as critical back pressure ratio in detail for venturi sonic nozzles was presented. A flaw of the definition of back pressure ratio in ISO 9300 was pointed out. Then it has been mathematically proved, according to the one-dimensional isentropic flow theory, that when the ratio of the throat pressure to the upstream stagnation pressure of venturi nozzles reaches the critical pressure ratio, the fluid flow reaches a sonic speed at the throat with a maximum mass flow rate through the nozzle . A formula for flow through the venturi nozzle under real conditions was mathematically derived based on the previous discussion.Compared with ISO 9300, the derived formula includes a correction of compressibility factor at throat condition 1/Znt. At last, a discussion of the mechanism of shock generation in the diffuser was also presented from the basic equations of gas dynamics with the aim to develop the one-dimensional flow computational models for the shockwave generation position, pressure, and Mach number before and after the shockwave. The computational results were verified by numerical simulation and compared with the experimental data of Craig A. The results show that the maximum error of the minimum exit pressure ratio between the computational results and the experimental data is less than 17%.
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2023, Vol. 44 
