The Choking Pressure Ratio of a Critical Flow Venturi

Abstract

The critical flow venturi has many important applications in the measurement and control of gas flow. In many of these applications, it is desirable to minimize the pressure loss required to maintain critical flow conditions. The performance of the venturi may be characterized by the ratio of outlet static pressure to inlet total pressure just sufficiently small to produce critical flow. This ratio is called choking pressure ratio (CPR). The optimization of diffusers for critical flow Venturis is discussed and suggestions for designs practice are presented. Test results are given for six different diffuser configurations, and a comparison is made with data on 11 configurations from other investigators. This work was done under contract to the National Aeronautics and Space Administration—Marshall Space Flight Center. It is shown that, for the small divergence angles considered, a simply defined diffuser effectiveness parameter is approximately independent of flow conditions and may be used to predict choking pressure ratio. Even very short diffusers greatly improve performance, and, for longer diffusers, critical flow can be maintained at total pressure losses of 5 percent.