Ejector Profile Modelling for Heat-Driven Ejector Refrigeration System Without Involving Shock

Abstract

Heat-driven ejector refrigeration system is one of the fastest emerging technologies in cooling applications for years. This is due to the fact that it can harness cooling capacity from waste heat sources at above 80 °C. Low coefficient of performance (compared to commercial vapor compression systems) is the major disadvantage of the said system, and thus it became a topic of research studies in the field of cooling. The work required by the compressor in a vapor compression cycle (VCC) can be eliminated by using waste heat from any available heat source. Although a relatively lower COP was obtained, the savings using the ejector refrigeration system can cover all the disadvantages and proved that this system can be actually helpful if implemented in the real working systems with waste heat. In this study, a mathematical model for determining ejector parameters and performance was developed and applied to a system where shock was tried to be avoided. The model was coded into a computer program to allow easier computation of the ejector geometric and thermo-fluid dynamic parameters with varying input data such as the refrigerant to be used, evaporator and condensing temperatures, entrainment ratio, and velocity of the fluid flows. An ejector refrigeration system using ammonia, propane, R22, R134a, R1234yf, and R245fa as refrigerants was simulated using the said model. A boiler or generator temperature of 90 °C, a condenser temperature of 40 °C, and a refrigerating capacity of 35kW were maintained for all the refrigerants; however, the evaporator temperature was varied within the range of −10 °C to 10 °C, depending on the behavior of the system. A combination of a short straight section and then a converging-diverging profile was used for the combined mixing section and diffuser to smoothly decelerate the fully mixed supersonic flow exiting the short mixing section and thereby avoid shock waves in the section. The resulting parameters including the ejector dimensions, pressure and Mach number were determined along the length of the ejector. For all the simulation runs, the fluids respond as expected and the expansion energy was utilized from the high pressure side of the ejector as shown in the trend of pressure along the length of the ejector. Ejector size varies a little for different refrigerants; the calculated range of length is from 0.14 m to 0.36 m — this range shows the compactness of the resulting ejectors. The results show that a VCC refrigeration system can be replaced by a heat-driven ejector refrigeration system with the ejector that was designed from the simulations. Since the two systems are designed to have the same refrigerating capacity and working temperatures, it can be projected that savings can be made by using the ejector system. The compactness of the ejector produced in the simulations show a good potential for this kind of refrigerating system to be manufactured and mass produced.