Numerical and Experimental Investigations on Supersonic Ejectors

Numerical assessment of ejector operation for refrigeration applications based on CFD

Condensing ejectors utilize the beneficial thermodynamics of condensation to produce an exiting static pressure that can be in excess of either entering static pressure. The phase change process is driven by both turbulent mixing and interphase heat transfer. Semi-empirical models can be used in conjunction with computational fluid dynamics (CFD) to gain some understanding of how condensing ejectors should be designed and operated. The current work describes the construction of a multidimensional simulation capability built around an Eulerian pseudo-fluid approach. The fluid is treated as being in a non-thermodynamic equilibrium state, and a modified form of the homogenous relaxation model (HRM) is employed. The CFD code is constructed using the open-source OpenFOAM library. Using carbon dioxide as the working fluid, the results of the simulations show a pressure rise that is comparable to experimental data. It is also observed that the flow is near thermodynamic equilibrium in the diffuser, suggesting that turbulence effects present the greatest challenge in modeling these ejectors.

/pdf/multidimensional-modeling-of-condensing-two-phase-ejector-2bt3kbxco4.pdf

Multidimensional Modeling of Condensing Two-Phase Ejector Flow

Ejectors used in refrigeration systems as entrainment and compression components or expanders, alone or in combination with other equipment devices, have gained renewed interest from the scientific community as a means of low temperature heat recovery and more efficient energy use. This paper summarizes the main findings and trends, in the area of heat-driven ejectors and ejector-based machines, using low boiling point working fluids, which were reported in the literature for a number of promising applications. An overall view of such systems is provided by discussing the ejector physics principles, as well as a review of the main developments in ejectors over the last few years. Recent achievements on thermally activated ejectors for single-phase compressible fluids are the main focus in this part of the review. Aspects related to their design, operation, theoretical and experimental approaches employed, analysis of the complex interacting phenomena taking place within the device, and performance are highlighted. Conventional and improved ejector refrigeration cycles are discussed. Some cycles of interest employing ejectors alone or boosted combinations are presented and their potential applications are indicated.

https://www.mdpi.com/2411-5134/4/1/16/pdf

Current Advances in Ejector Modeling, Experimentation and Applications for Refrigeration and Heat Pumps. Part 1: Single-Phase Ejectors

A study of the conjugate effects of ejector performance characteristics, the activation pressure–temperature conditions at the generator and the interaction with the compressor on refrigeration systems was performed. Besides the conventional compression cycle which was used as a reference, three more configurations were selected for this study: a hybrid ejector–compressor booster and two cascade compressor–ejector cycles. The modelling methodology developed to handle such systems was based on the principles of thermodynamics and fluid dynamics and included two separate sub-cycles in constant interaction: a loop driven mechanically by a compressor, coupled with a thermally activated ejector loop. This cycle model was combined with a validated ejector model and manufacturer's compressor performance maps. A performance analysis and a parametric optimization were performed to select promising cycles. For given working conditions, substantial improvement in COP of the selected cycles over the conventional mechanical compression cycle was found.

Numerical evaluation of ejector-assisted mechanical compression systems for refrigeration applications

The start condition of a second-throat ejector-diffuser (STED) system has been studied by solving the axisymmetric Navier-Stokes equations with a high order conservative supra-characteristics method (CSCM). According to the conventional concept, when a STED is started, the flow passing through the second-throat contraction should be supersonic so that the normal shock swallowing condition applying for a supersonic windtunnel holds. The present numerical results, however, suggest that a STED can be started with a normal shock forming ahead of the second-throat contraction because the high speed flow in the region close to the wall of the diffuser serves to release the mass blocked by the normal shock, leading to a decrease in chamber pressure as well as the start of the system. This not only contradicts the conventional concept of the start of a STED, but also provides an explanation for the experimental discrepancy between the start conditions of a STED and a supersonic windtunnel.

J. J. Wang

Papers

On the start condition of a second-throat ejector-diffuser