Two-phase, critical flow data are reported for initially saturated and subcooled liquid Freon-11 flowing through sharp-edged entrance tubes for length-to-diameter (L/D) ratios from 2.82 to 100. Comparisons among various analytical models and these data show that nonequilibrium models describe the critical flow phenomenon more accurately than equilibrium models. It is shown that to obtain reliable exit plane pressure measurements, it is necessary to have a gradual divergence at the exit of the constant-area tube. This minimizes the expansion of the exiting fluid, which would otherwise result in an exit pressure measurement much lower than the one-dimensional value governing the flow. Utilizing data from this study, quantitative estimates of the individual phase velocities at the condition of critical flow indicate that “vapor choking” is not the mechanism by which two-phase critical flow occurs in this investigation. The flow pattern in a transparent test section, with an L/D ratio of 2.82, has been observed and photographed, and this shows that the separated flow condition (liquid jet surrounded by vapor) begins to break up into a dispersed two-phase mixture approximately one equivalent diameter downstream of the entrance.
