Critical conditions and the choking mass flowrate in non-equilibrium wet steam flows

Choking in steady, one dimensional, nonequilibrium, wet steam flows is analyzed. It is shown that the most useful, and physically informative, definition of the critical condition is that condition when the vapor phase velocity is equal to the frozen speed of sound. One dimensional wet steam flows become choked when the critical velocity is attained somewhere in the system. In this context the term choking implies that the upstream flow pattern cannot be altered by small adjustments of the back pressure and the mass flowrate is close to, although not necessarily identical with, its maximum value. The equilibrium speed of sound has no physical relevance in such flows. The critical conditions in a choked converging nozzle exhausting to a region of constant static pressure always occur in the exit plane of the nozzle. The analysis demonstrates that the critical pressure and velocity and the choking mass flowrate are all functions of the critical supercooling and inlet stagnation conditions only and shows how they may be calculated.