Multi-scale modelling of ejector refrigeration systems

It is known that the efficiency of ejector-systems (the so-called "system-scale") relies on the behaviour of the ejector itself (the so-called "component-scale"), which is in turn, imposed by to the local flow phenomena (the so-called "local-scale"). As a consequence of these multiscale relationships, the precise prediction of the "local-scale" is needed to sustain the design and the large-scale deployment of commercially-viable ejector-systems. To achieve such an ambitious goal, computational fluid dynamics (CFD) and lumped-parameter models are needed. In this perspective, a comprehensive validation of a CFD approach for single-phase ejectors has been achieved, encompassing a broad range of refrigerant, boundary conditions as well as ejector designs. Besides, a comprehensive set of sensitivity studies have been completed, encompassing geometrical modelling (3-Dimensional vs 2- Dimensional approaches), solver settings (pressure-based vs density-based), mesh criteria and turbulence models (k-ε RNG vs k-ω SST). The validated CFD approach has been coupled with a lumped-parameter model of the refrigeration system to clarify the multi-scale influences of the refrigerants and ejector design, encompassing the local-, component- and system-scales. To this end, natural refrigerants and fourth-generation have been compared with third-generation refrigerants on different ejector geometries (obtained changing the mixing chamber dimension and nozzle exit position ). One of the tested geometries has been then selected along with R290 as a refrigerant and has been equipped with a spindle. Subsequently, it has been tested for different spindle positions and under a broad range of boundary conditions, obtaining a complete performance map. This outcome is of practical interest, to be implemented in predictive and multi-scale ejector control systems.