Critical flow of liquid‐vapor mixtures

Abstract

AbstractA unified theory of one‐dimensional, adiabatic, separated, two‐phase flow is presented. To describe the flow adequately, four mixture specific volumes are defined. They are based on area, momentum, kinetic energy, and velocity averages. Increasing relative velocity between the phases initially lowers all mixture specific volumes except the velocity average. The momentum average specific volume minimizes when the slip ratio equals (Vg/Vf)1/2, while the kinetic energy average specific volume reaches its minimum value at a slip ratio of (Vg/Vf)1/3. Area average specific value does not minimize with slip ratio.Because a higher slip ratio would decrease the entropy of a closed system, (Vg/Vf)1/3 is the maximum slip ratio attainable in two‐phase critical flow. Based on the maximum slip ratio and isentropic flow, a new critical flow model was developed and compared with the steam‐water critical flow data of four recent investigations. While the predicted flow rates followed well the pressure behavior of the experimental data, they were too low at high qualities and too high at low qualities. The average percentage difference between experimental and predicted critical flow rates was −8.5% (three hundred and seventy‐six data points).Differences in the approach to critical flow between a gas and a vapor‐liquid stream appear to be caused by the latter's increased frictional and gravitational pressure drops and relative velocity effects.