This report describes the activity carried out in the second year of a three-year project which aims to develop an innovative heat pump architecture equipped with an ejector, so as to contribute to the penetration of electrical technologies for air conditioning in the civil sector, according to growth estimates contained in the PNIEC PNIEC [Italian Integrated Energy and Climate Plan] and necessary to achieve the national decarbonization targets by 2030. During the first year, an extensive literature analysis was carried out on ejector refrigeration systems which made it possible to highlight their peculiar characteristics, such as for example the dependence of system performance on the fluid dynamic phenomena within the ejector (‘local-scale’) and on the characteristics of the refrigerant fluids. Several modeling techniques suitable for describing these complex systems were also investigated and compared. The second year’s activity focused on the definition of a modeling approach for the simulation of supersonic ejectors. Both lumped parameter (LPM) and thermo-fluid-computational (CFD) approaches, examined in the first year, were defined and validated through numerous simulations, which showed the superiority of the CFD approach for its ability to predict with satisfactory precision the main performance parameter of the ejector (the drag ratio) with an error of less than 15%. The CFD model also allows you to define a multi-scale approach to calculate the machine’s performance and link it to local fluid dynamic phenomena. This model allowed us to analyze in detail the impact of the working fluid, geometry and operating conditions on the internal fluid dynamics of the ejector and therefore on the COP of the cycle. In detail, we screened refrigerants by comparing third generation fluids with natural and fourth generation

fluids, in view of the imminent transition provided for by European regulations to combat the emission of greenhouse gases. The results show how different expansion conditions of the primary fluid significantly influence the global results, in terms of COP and critical temperature. At the same time, a sensitivity analysis was carried out on the main geometric factors of the ejector. These studies have allowed us to perfect a prototype of a variable geometry ejector (by means of a movable pin), for which performance maps have been obtained and which will be built and experimentally tested in a circuit that will use propane as the working fluid in the third year of research.