Numerical Investigation of Two-Phase Water Ejectors for High-Temperature Heat Pumps: Insights Into Flow Behavior and Shock Wave Dynamics

Abstract Two-phase water ejectors can serve as a secondary steam compression mechanism in high-temperature heat pump (HTHP) systems. By integrating an ejector, high-pressure water can be combined with hot steam from the compressor, achieving simultaneous cooling and pressure increase. This integration offers the potential to reduce both the power and the number of stages required to obtain the specified compression. However, the complex flow behavior within two-phase water ejectors, especially under high-pressure, high-temperature conditions, remains insufficiently explored in the literature. This study addresses this gap by conducting a detailed numerical investigation of two-phase flow and shock wave behavior using the compressible two-phase mixture approach in computational fluid dynamics (CFD) commercial solver Ansys Fluent. The ejector geometry and critical operating points were derived from a prior study using one-dimensional (1D) analysis, which provided the design and operational conditions used in the current simulations. The ejector was modeled in two-dimensional (2D) axisymmetric configurations. The Lee model, in conjunction with the water saturation curve, was applied to capture non-equilibrium mass transfer processes, including evaporation and condensation. During the simulations, the presence of a liquid-vapor mixture may cause the speed of sound to drop, making the flow locally supersonic. Initial simulations revealed a series of shock waves in the mixing section, which elevated the mixture flow pressure to the designated set back-pressure value, thereby achieving an ejector pressure ratio of approximately 1.3. These findings provide crucial insights into how thermodynamic conditions influence two-phase flow behavior, particularly in the generation and intensity of shock waves. This work advances the modeling procedures for two-phase ejectors and enhances the understanding of the physical phenomena occurring within two-phase water ejectors designed for HTHP applications.