Development and performance mapping of a multi-ejector expansion work recovery pack for R744 vapour compression units,
AbstractA multi-ejector expansion pack, intended as a substitute for a standard high-pressure electronic expansion valve (HPV), was designed, manufactured and experimentally investigated. Four different ejector cartridges were sized to enable a discrete opening characteristic with a binary profile for a parallel-compression R744 system. The system is rated for 70 kW at a 35 °C gas cooler outlet temperature and a -3 °C evaporation temperature. High values of ejector efficiency, exceeding 0.3 over a broad operation range, were recorded for all four of the cartridges tested under vapour compression conditions. The applicability of the multi-ejector pack as a main flashing device was verified experimentally. Similar profiles of the discharge pressure control error were recorded for both alternative options: expansion purely in the HPV vs. HPV-assisted expansion in the multiejector pack.
The continuous derivation of the ambient temperature and cooling demand in CO 2 refrigeration and air-conditioning systems equipped with multi-ejector modules for supermarkets requires the analysis of the fixed ejector utilisation in a very wide range of the operational envelope. Therefore, performance mapping of the four R744 ejectors installed in the multi-ejector pack was performed. The investigations of a single ejector's work were performed based on the proposed hybrid reduced-order model to predict the performance of each ejector under arbitrary operating conditions. The proposed model was validated and generated by use of the experimental data together with the computational fluid dynamic model results. The ejector efficiency mapping indicated the area of the best ejector performance in the range from approximately 50 bar to 100 bar. The mass entrainment ratio of all four ejectors was presented for different ambient temperatures and the pressure lift. An area of the mass entrainment ratio greater than 0.3 was obtained by each ejector at ambient temperature above approximately 15 • C for pressure lift below 10 bar. The approximation functions of the ejector pressure lift in terms of the ambient temperature for air-conditioning operating conditions to reach the best efficiency of each ejector are proposed.
In this paper, the influence of heat transfer in the walls of an R744 two-phase ejector on ejector performance was investigated. A numerical investigation was performed using a computational fluid dynamic (CFD) model of the R744 two-phase flow coupled with the heat transfer inside the ejector. An ejector equipped with thermocouple channels was designed and manufactured to investigate temperature distribution in the inner walls under boundary conditions typical for a refrigeration and air-conditioning application in a supermarket. The ejector was installed on the test rig to perform a test series that evaluated the outer walls of the ejector with and without insulation. The experimental results were used to validate the proposed CFD model, and a numerical investigation was performed to analyse the influence of heat transfer on ejector performance. The motive nozzle and suction nozzle mass flow rates accuracies were within ±7% and ±15%, respectively. In addition, the proposed CFD model predicted the wall temperatures with ±5 K accuracy for most of the validated points. The heat transfer coefficient of the R744 two-phase flow inside the ejector is presented. The non-adiabatic inner walls degraded ejector performance. The maximum reduction of the mass entrainment ratio reached approximately 13%.
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