Aim of this thesis is to present an experimental investigation of standard R744 supermarket refrigeration system, with the high-pressure electronic valve (HPV), and refrigeration system with multi-ejector expansion pack on the same vapour compression rack. Comparison of the R744 multi-ejector refrigeration system, was carried out based on energy performance characteristics: refrigeration capacity, power consumption, COP, and exergy efficiency. Apart from the system performance comparison, influence of the pressure level in the flash tank on the system performance for both alternatives was analysed.The experimental results indicated COP and exergy efficiency improvement of the multiejector refrigeration system up to 7% and 13.7%, respectively. The multi-ejector system was able to operate in smaller range of the tanks pressure lift than the standard system dependent on the refrigeration load and the exit gas cooler section parameters. The highest values of COP and exergy efficiency were obtained by the multi-ejector refrigeration system for the tanks pressure lift value close to the limit value. The values of the overall compressor efficiencies were significantly differentiated, dependent on the operation module (cooling load and heat rejection conditions), which strongly influenced the values of COP and exergy efficiency. Therefore, it was not possible to clearly define the optimum pressure in the flash tank. It was concluded that improvement of compressors efficiencies utilized in the multiejector system will indicate high energy performance of the refrigeration system.

Highlights The modified homogeneous relaxation model of the R744 two-phase ejector was proposed. The application range of the proposed model is for a motive nozzle pressure above 59 bar. The proposed model improved the motive nozzle mass flow rate accuracy when compared to the homogeneous equilibrium model.

In this study, the accuracies of the homogeneous equilibrium (HEM) and homogeneous relaxation models (HRM) were compared. Both models were implemented in the ejectorPL computational tool. The HEM and HRM were used to simulate the carbon dioxide flow in ejectors that were designed for supermarket refrigeration systems. The model accuracy was evaluated by comparing the computational results with the experimental data. The discrepancy between the measured and computed motive nozzle mass flow rates was analysed. In addition, the difference between the experimental and computational mass entrainment ratios was calculated. The operating regimes in this study ranged from 47 bar to 94 bar and from 6 • C to 36 • C for the pressure and temperature, respectively. The model accuracy strongly depends on the distance between the operating regime and the critical point of the refrigerant. The discrepancy for the selected operating regimes ranged from 0.3% to 43.3% and 0.7% to 42.0% for HEM and HRM, respectively. For lower pressures and temperatures, the HRM has higher accuracy than the HEM. The errors of the HRM results were approx. 5% lower than those of the HEM results. The accuracy improvement of the HRM was considered unsatisfactory. The low accuracy improvements were possibly caused by the relaxation time formulation in the homogeneous relaxation model.

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.

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