A heat transfer model for two-phase flow in an ejector refrigeration system

Abstract

Sustainable refrigeration technologies with low electrical energy consumption such as the Ejector Refrigeration System (ERS) can contribute to meet carbon reduction goals. In this study, the thermodynamic performance is assessed by means of a validated mathematical dynamic model. The proposed model integrates simultaneously the operation of each part of the ERS (composed basically by a generator, condenser, evaporator, expansion valve and recirculation pump); being capable of predicting the global behavior of the cycle by describing mass, momentum and energy transport in each subsystem and by coupling their inlet and outlet conditions. In particular, the heat exchangers model used in this work is a simplified two-phase model where the phase change is considered as an homogeneous flow of a mixture liquid/vapor under the assumption of one-dimensional flow, that permits to estimate the phase change and the pressure drop, which directly influence the operation of the ejector and the overall performance of the system. After validating the numerical approach by comparison to experimental data from the literature, the behavior of the ERS is simulated for different operational conditions, obtained from changes in generator heat input, that affects the temperature and pressure at the ejector inlet. From these simulation results, thermal performance can be written in terms of ejector entrainment ratio (ER) and system coefficient of performance (COP). For high generator heat flow, the pressure and temperature of the primary flow at the ejector inlet are higher due to the overheating, and the system’s operating range in the critical mode is reduced, causing a considerable decrease in the ER and the COP. For the same condensing pressure, reductions of 58% and 45% are observed in the ER and the COP, respectively, when the generator heat input increases by 40% from the design conditions, showing a great system’s sensitivity to the available heat. Besides proposing an accurate computational tool to permits achieve more efficient ERS designs, obtained results show the need for more adaptable systems in terms of heat input conditions, such as solar or industrial waste sources.