HEM and HRM accuracy comparison for the simulation of CO2 expansion in two-phase ejectors for supermarket refrigeration systems

Abstract

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.

Experimental and computational analysis of a R744 flashing ejector

“…This means that the model correctly reproduces the tendency of a near-critical flow to become closer to an equilibrium phase change. Similar conclusions were found by Palacz et al [33] by comparing the results obtained with a HEM and an HRM.…”

Recent Advances in Transcritical CO2 (R744) Heat Pump System: A Review

“…They developed a widely excepted correlation for relaxation time considering the critical flow parameters, vapor quality, and temperature profile. Through a comparison study, Palacz et al [92] found that HRM could predict the flow characteristics better than HEM (overall a 5% increase in accuracy). In addition, Zheng et al [93] and He et al [94] proposed dynamic numerical models, which are useful in predicting the dynamic system response in different operating conditions to optimize the ejector expansion performance.…”

Numerical investigation of the effect of the shape of the nozzle on the flash boiling phenomenon

“…The turbulent dynamic viscosity and turbulent thermal conductivity, which appeared in eq. (6) and 7, were calculated by applying formulae relevant for used turbulence model.…”

“…Successful validation procedure was also carried out by comparing obtained results with the experimental data for the primary and secondary mass flow rates in the ejector device as well as the pressure at the selected points in the computational domain. Next, Palacz et al [6] proposed a more complex two-phase flow model of the CO2 ejector, i.e., the HRM which included metastable effects. Then they evaluated the accuracy of the new model against the previously proposed HEM [5] by comparing calculated results with the experimental data for the typical operating regimes of CO2-based refrigeration systems.…”

“…5% lower than those of the HEM. Subsequently, the HRM from [6] was modified by applying the genetic algorithm optimization of the relaxation time correlation by Haida et al [7]. Then the studies on the impact of the relaxation time definition on the CO2 two-phase flow behaviour as well as on the model accuracy were carried out.…”