مدل شبیه سازی عددی برای نوع صفحه، اواپراتور و رول باند

This study presents a first-principles mathematical model developed to investigate the thermal behavior of a plate-type, roll-bond evaporator. The refrigerated cabinet was also taken into account in order to supply the proper boundary conditions to the evaporator model. The mathematical model was based on the mass, momentum and energy conservation principles applied to each of the following domains: (i) refrigerant flow through the evaporator channels; (ii) heat diffusion in the evaporator plate; and (iii) heat transmission to the refrigerated cabinet. Empirical correlations were also required to estimate the shear stresses, and the internal and external heat transfer rates. The governing partial differential equations were discretized through the finite-volume approach and the resulting set of algebraic equations was solved by successive iterations. Validation of the model against experimental steady-state data showed a reasonable level of agreement: the cabinet air temperature and the evaporator cooling capacity were predicted within error bands of ±1.5 °C and ±6%, respectively.

Refrigerators and freezers are responsible for approximately 8.5% of energy consumption in Brazil (PROCEL, 1998). As the major part of this energy is wasted by the system components (compressor, condenser, evaporator, and capillary tube) due to irreversible processes, studies to understand such thermodynamic losses may lead to the development of higher efficiency products. Jakobsen (1995) quantified the thermodynamic losses in a 325-l refrigerator and investigated various means of energy optimization. He found that such losses occurred mainly in the hermetic compressor and in the evaporator, and also that the latter showed the best system/component performance ratio, i.e., the highest system performance improvement with the lowest component-level investment. In Brazil, the most widely used evaporator for household refrigerators is known as the plate-type, roll-bond evaporator. Basically, it consists of a plate formed by two powder-coated aluminum sheets, with channels in which the refrigerant evaporation takes place, while a buoyancy-driven air circulation occurs at the outer side. The combination of low cost and reasonable performance – compared to plate-and-tube heat exchangers – has led to a steady increase of its application.

A computer model to predict the refrigerant flow characteristics and temperature distributions along plate-type, roll-bond evaporators was developed and validated against steady-state experimental data. The evaporator and the refrigerator compartment models were coupled in order to provide more realistic results for a 230-l all-refrigerator appliance. The steady-state predictions of cooling capacity and cabinet air temperature were 6% and 1.5 °C, respectively, lower than their measured counterparts. The model can also be used to estimate the transient behavior of the pair evaporator/cabinet, although a proper transient validation exercise has not being carried out. All the transient simulations reported in this work were carried out with pre-defined fictitious boundary conditions at the entrance of the evaporator. The model was also used to assess the performance capabilities of the evaporator/cabinet design. The main conclusions are as follow: • The simulations of plate temperature fields indicate a significant temperature difference (∼6 °C) between the central region (−20 °C), where the channels are located, and the top and bottom edges, where the temperature is around −14 °C. The evaporator plate area could be better used by extending the channels in top–bottom direction by 20 mm, in order to obtain a uniform temperature plate. • The radiative portion responds for almost half of the overall thermal load, i.e., only 50% of the evaporator cooling capacity is being used to cool the internal air, whereas the other half is being used to reduce the inner surface temperatures. The net effect of it is an increase in the heat gained through the cabinet walls. • The empirical model used to compute the heat transmission through the gasket sealing is a limiting factor for more accurate predictions of indoor temperature and cooling capacity.