Numerical analysis and optimization of a two-phase water ejector

Kurzfassung

Conventional heat pump systems that employ water as the working fluid require multiple stages of compression to achieve high pressure ratios. This incorporation of multiple compression stages adds complexity, on top of the added requirement of re-cooling at the compressor outlet, result ing in pressure loss. In this context, an ejector can partially replace a multi-stage compression system to serve the practical purpose of cooling in addition to compression, without adding complexity to the high-temperature heat pump system. However, the limited availability of validated models that predict the flow physics, phase interactions, and shock dynamics in a two-phase water ejector remains a notable limitation in the existing literature. The current work converges towards developing a well-validated Computational Fluid Dynamics model to reliably predict two-phase water ejector performance using a convergent-divergent nozzle ejec tor and subsequently perform parametric studies on the CD nozzle ejector with an intent to explore performance enhancement, positioning it as a potential alternative to the convergent nozzle ejector and thereby contributing towards bridging the gap in research. The commercially available CFD solver Ansys Fluent is employed with a compressible multiphase mixture model, a transient time formulation, and a pressure-based solver to investigate and accurately predict the transient behavior of flow physics and shock mechanics while concurrently capturing physically realistic results. The simulations are performed on a 2-Dimensional axisymmetric domain, and the k −ω shear stress transport model is chosen for its ability to accurately predict turbulence. Furthermore, the Lee model, in conjunction with the water saturation curve, is chosen to capture non-equilibrium mass and heat transfer phenomena, including condensation and evaporation in duced by phase interactions between high-pressure water and superheated steam (modeled as an ideal gas) entering the ejector through the motive and suction inlets, respectively. Additionally, geometric and operational parametric studies are conducted to evaluate their influence on ejector behavior and to deduce its optimal operating configuration within the scope of the parameters considered. Initial simulations indicated the presence of anticipated shock waves originating in the diffuser, given the low speed of sound in a two-phase mixture formed through momentum exchange in the mixing chamber. The shock waves are followed by a favorable pressure rise, resulting in a pressure ratio of approximately 1.04. Performance metrics such as entrainment ra tio, compression ratio, and steam mass flow rate at the outlet provided a clear understanding of how each parameter influenced ejector performance. Furthermore, flow field distributions of ve locity, pressure, Mach number, and void fraction are analyzed to gain insights into the physically relevant phenomena that prevail in the two-phase water ejector. Eventually, the CFD model is validated by verifying its ability to accurately capture the key physical mechanisms governing the operation of the two-phase water ejector against experimental outcomes documented under similar operating conditions.