Nozzles are used in steam turbines for expanding and accelerating the steam to transfer power into the rotor. Low-grade steam, namely, steam of low superheat and high wetness, may readily give rise to a non-equilibrium condensation expansion during its passage through the turbine nozzle in waste-heat recovery applications, which may severely impair flow-path efficiency in these machines. To investigate this type of operating condition, an Eulerian–Eulerian model is adopted to establish a numerical simulation with consideration of the actual operating conditions in the present paper. The work aims to interpret the effect of the population of the high wetness-created droplets on the behavior of the condensation inside the nozzle. With higher inlet wetness, more droplets are introduced, which also suppresses the overall non-equilibrium condensation process and induces heterogeneous condensation. Under the same wetness conditions, higher superheat produces a stronger suppressive effect. Within the range of parameters examined in this study, the combination of 20 K superheat and 5.0% wetness showed the most significant inhibition effect. It is found that the main reason for the loss in static pressure and dynamic energy in the flow field is due to the formation of non-equilibrium condensation, which can be effectively suppressed by reasonably matching the degree of superheat and mass fraction at the entrance: the pressure damping coefficient and kinetic energy damping coefficient are reduced by 16.44% and 13.30%. However, it is worth noting that increasing inlet wetness leads to decreasing steam enthalpy and, therefore, lower potential for doing work by the turbine.
Li et al. (Sun,) studied this question.