Numerical investigation of the expansion devices applied in modern vapour compression refrigeration unit considering specific entropy and entropy generation analyses

Ejector-based refrigeration cycles working with natural refrigerants have already gained industry attention. Replacing throttling valve with an ejector in vapour-compression cycles brings high improvement of the cycle efficiency due to the ejectors potential of recovering part of throttling losses. With the rapidly growing market for heat pumps, they are also being implemented in those cycles, but this requires redesigning the ejector geometry for new natural working fluids for different operating conditions and applications. Typical approach to perform the ejector shape optimization is to use the ejector mass entrainment ratio or overall efficiency as an objective function. However, an entropy generation analysis seems to be more efficient. For this reason, the aim of this work was to perform the numerical analysis of the entropy generation of two two-phase ejectors for R744 and R290 refrigerants in order to assess its potential as a tool for efficiency improvement in the shape optimization algorithms. The ejectorPL approach utilizing the HEM approach was complemented with the entropy generation model and implemented using an additional transport equation to the CFD software. The CFD models were validated based on experimental results obtained from ejectors working in R744 transcritical refrigeration system and R290 heat pump. Each ejector was simulated for set of operating conditions representing the system working in both supercritical and near critical point conditions. The CFD results of the mass flow rates for both devices were used for validation purposes. The results of the pressure and temperature were compared with experimental data. The entropy generation module allowed for the entropy generation analysis in terms of maximum values and their location showing critical areas of irreversibility characterizing different working fluids usage and ejector applications.