The fluid mechanics of non-nucleating wet steam flows

A semi-analytical, theoretical treatment of high speed, non-nucleating wet steam flows is presented and demonstrates both qualitatively and quantitatively the salient features of such flows. Departures from both thermal and inertial equilibrium occur, each characterized by an individual relaxation time, but in general thermal nonequilibrium effects dominate over inertial ones. The analysis shows that for flows characterized by a constant rate of expansion an initial transient phase (governed by the thermal relaxation time) is followed by a region of dynamic equilibrium where the degrees of nonequilibrium (the supercooling and slip velocity) remain approximately constant. The nonequilibrium parameters in the regime are shown to be functions of the pressure and expansion rate, as well as the relaxation times. If the expansion rate is continually changing the supercooling and slip velocity adjust to these changes (with an exponential lag) in an attempt to maintain a state of dynamic equilibrium. The departures from equilibrium are also characterized by a production of entropy which is responsible for the thermodynamic and inertial wetness losses observed in steam turbines. Loss coefficients specifying these quantities are defined and the analysis shows how they may be calculated.