Performance evaluation of a variable geometry ejector applied in a multi-effect thermal vapor compression desalination system

Abstract

Nowadays, desalination is the primary alternative to increase water supply for domestic, agriculture and industrial use. Energy demanding desalination could be a sustainable solution in places where renewable energy resources are abundant. Multi-effect thermal vapour compression desalination is widely applied; however, its integration with solar energy requires more research effort. One of the key components of these systems is the steam ejector. The present study focuses on the geometry modification of an ejector, designed for a small-scale multi-effect thermal vapour compression unit, by means of computational fluid dynamics simulations. Its performance is evaluated considering a range of operating temperatures of the primary flow (i.e. 120 °C–180 °C) that would be suitable for running the system using low-grade solar thermal energy. Results show that for relatively high primary inlet temperatures, the primary jet undergoes a considerable expansion after leaving the nozzle. Consequently, the effective area for the secondary fluid becomes reduced, leading to a poor ejector performance and the entrainment ratio decreases from 1.2 to 0.3. In order to improve the ejector performance, variable geometry features are tested by allowing two geometry components to vary: the nozzle exit position and the area ratio (rA). The results show that nozzle exit position influences the ejector performance only about 16% within the considered range. The influence of the area ratio on the entrainment ratio is however very significant. The results indicate that the improvement of the entrainment ratio for high primary inlet temperatures could be as high as 400% when compared to a fixed geometry design.