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Energies 2021, 14, 8238 9 of 25 The coefficient of performance of the system operating according to the chiller config- uration was then given by Equation (11). .. Qeva(1) +Qeva(2) COPchiller = P (11) el,comp Table 3 summarises the maximum uncertainties evaluated during the experiments presented in Section 4; on the high pressure side, whenever the heat flow was calculated on CO2 side, the uncertainty related to the CO2 mass flow calculation with compressor curves was always applied, in favour of safety. In the case of the calculation performed on the water side, which occurred whenever the evaporator exit was not superheated, the uncertainty was heavily affected by the very limited water temperature difference. Table 3. Maximum uncertainties of the main heat flow rate evaluation. Evaluation Method Max Uncertainty [%] 5.0 5.5 0.7 16.0 . .Qgc QIHX Q. eva 3. Numerical Model Development CO2 side CO2 side CO2 side Water side After the experimental characterization of the refrigerating system operating according to the different configurations, the mathematical model of the system was developed with the commercial software Simcenter AMESim v.17. The libraries of the software are composed of basic elements designed to model the transient behaviour of the internal and external flows, i.e., refrigerant, air and water. On the refrigerant side, components were modelled based on a homogeneous fluid model, i.e., the two-phase flow was considered as a single phase fluid, displaying the mean fluid properties. Refrigerant and water pressure losses and heat transfer coefficients were evaluated with empirical correlations from the literature. In addition, the model neglected the effect of gravity; the natural convection heat exchanger was thus neglected in this work and preliminary results of the chiller configuration will be presented including only the ejector driven evaporator. The two-phase flow library provided the thermophysical properties of carbon dioxide, water, and air; the thermodynamic properties of CO2 were evaluated with the Modified Benedict Webb Rubin (MBWR) equation [23], considering the pressure as a function of the fluid’s density and temperature. Water was assumed to be pure and its thermodynamic properties were calculated with Bode’s formulation, and air was considered as a mixture, composed of a combination of pure fluids weighted by their concentration. Basic elements were coupled to develop the submodel of the main components of the system, which were connected to form the complete numerical model of the refrigerating unit. The system included mass flow devices (compressor, expansion devices, and two- phase ejector) and energy flow devices (evaporators, gas-coolers/condensers, and internal heat exchangers); as the dynamics of the mass flow devices are generally an order of mag- nitude faster than those of the heat exchangers, according to Rasmussen et al., 2005 [17], the compressor, expansion devices, and two-phase ejector were considered static com- ponents and were modelled using steady-state empirical equations. On the other hand, the modelling of the heat exchangers, which are characterized by a more complex nature, was conceived with a discretization of the devices into smaller volumes, each treated with a lumped parameters approach. Finally, the pressure drops of the refrigerant flowing through the pipes connecting different components was neglected, as the assumption of short pipes was made.PDF Image | Dynamic Modelling and Validation of an Air-to-Water Reversible R744
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