Numerical estimation of non-equilibrium condensation of steam in supersonic nozzles

摘要

Condensation phase change during the expansion of steam is an inevitable phenomenon encountered in several engineering applications. Under supersonic flow conditions, the expansion rate of flow will be very steep so that, steam continues to expand even after local pressure crosses the saturation curve. Steam gets supersaturated rapidly and the condensation departs far away from equilibrium state. By complicated sequences of spontaneous nucleation, several nucleation sites will be formed in the flow field. Water droplets may form at this sites by mass addition due to condensation. The latent heat liberated during phase change results in the formation of a condensation shock wave across which flow velocity gets reduced. Precise knowledge of this process is important since the presence of water droplets in steam can lead to erosion and subsequent reduction of performance. Non-equilibrium condensation can be numerically modeled using additional transport equations which solve for the mass fraction of condensate generated from the phase change process. In the present work, a five-equation non-equilibrium condensation model available in ANSYS FLUENT is studied and used. The methodology is validated against case studies reported in the literature. Later, parametric studies were conducted to investigate the effect of inlet saturation ratio of steam on the flow characteristic in two well-known steam nozzles.