Abstract
Ejector-equipped vapor-compression systems for refrigeration and cooling, relying solely on CO2 (R744) as a natural working fluid, are perceived to be an eco-friendly and highly efficient solution for many applications. However, the complexity of two-phase ejector flows makes it very challenging to find realiable and efficient ejector designs. Improved design methods are necessary in order to achieve higher performance in R744 units compared to the traditional compressor-based systems with refrigerants that put a high strain on the environment. Consequently, the development of advanced models and tools for an accurate design of the R744 ejectors has been a highly prioritized research topic. To the best of the authors’ knowledge, the current status of R744 ejector models and their limitations has not been thoroughly evaluated yet. To summarise the current state of the art and knowledge gaps, this work presents an exhaustive overview of the available numerical models applied to R744 two-phase ejectors, i.e. multiphase flow modeling, turbulence aspects, numerical solution methods, applications of models, to further encourage the adoption of R744 vapor-compression solutions. Finally, a thorough discussion of different focus points for future research as well as the main challenges in the field is presented.
Highlights
Growing concern for human impact on the environment has brought about a major shift in the field of Heating, Ventilation, Air Conditioning, and Refrigeration (HVAC&R)
To summarise the current state of the art and knowledge gaps, this work presents an exhaustive overview of the available numerical models applied to R744 two-phase ejectors, i.e. multiphase flow modeling, turbulence aspects, numerical solution methods, applications of models, to further encourage the adoption of R744 vapor-compression solutions
The characteristics of R744 favorable for expansion recovery, combined with the high energy efficiency offered by the adoption of a two-phase ejector, are further promoting the application of R744 refrigeration system at an industrial scale [21,22,23,24,25,26]
Summary
Growing concern for human impact on the environment has brought about a major shift in the field of Heating, Ventilation, Air Conditioning, and Refrigeration (HVAC&R). Work-recovery devices, such as expanders or two-phase ejectors are used This is especially important for refrigerants that operate at high pressures (such as CO2). The refrigerant flow and a qualitative pressure and velocity profile are presented in Fig. 1 (a) In this solution, CO2 exiting the gas cooler as vapor (thermodynamic state 3, identifying the high pressure) is referred to as the primary or motive flow. The characteristics of R744 favorable for expansion recovery, combined with the high energy efficiency offered by the adoption of a two-phase ejector, are further promoting the application of R744 refrigeration system at an industrial scale [21,22,23,24,25,26]. It is costly to produce results with large variations in operating conditions, ejector design, and system layout Many of these limitations can be bridged using experimentally validated numerical modeling. The available modeling strategies and suggestions for further work are summarised
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