Improvement of ejector performance by using two stage ejector principle

TitleImprovement of ejector performance by using two stage ejector principle
Publication Typeวิทยานิพนธ์/Thesis
Year of Publication2019
DegreeDoctor of Philosophy -- Major in Mechanical Engineering
InstitutionFaculty of Engineering, Ubon Ratchathani University
CityUbon Rachathani
Call NumberTP N271 2019
Keywordscomputational fluid dynamics, ejector refrigeration system, Refrigeration and refrigerating machinery, two-stage ejector

This study aims to investigate the use of the two-stage ejector to improve the ejector performance of the refrigeration system and gas/gas ejector application. The computational fluid dynamics (CFD) results were validated with the experimental values. The ejector refrigeration system and gas/gas ejector application, the two most significant parameters used to describe the performance of an ejector, were specified in terms of entrainment ratio (Rm) and critical back pressure (CBP)
In the ejector refrigeration system, the CFD was used to investigate the performance to increase operational flexibility and COP. A 2D-axisymetric model of a two-stage ejector (TSE) was developed and its performance was compared to that of the commonly used single-stage ejector (SSE). The shear-stress-transportation k-omega (k-omega-sst) model was applied as a turbulence model. The simulation of the TSE in the refrigeration system was analyzed for performance using generator temperatures between 100 and 130 ๐C and evaporator temperatures between 0 and 15 ๐C. The CFD simulation results showed that the TSE provided high entrainment ratios up to 77.2%, while showing a marginal decrease in the critical back pressure up to a maximum value of 21.9%. Therefore, it can be concluded that the TSE can significantly benefit refrigeration systems requiring high refrigerating capacity while maintaining a slightly low condensing pressure.
Furthermore, the study of the TSE performance in the gas/gas ejector system compared with the SSE simulations using CFD shows that when the primary inlet pressure is 4 bar, the secondary inlet pressure is 1 bar. The TSE provides a marginal decrease entrainment ratio of 3.73% but increases critical back pressure of 35.92%. It was found that the predicted performances of the CFD simulated models agreed well with the experimental values. Average errors of the predicted entrainment ratio and the critical back pressure were 18.54% and 2.00%, respectively.
It can be concluded that the TSE increases in critical back pressure and can improve the ejector performance in terms of the entrainment ratio (Rm) during choked flow. The findings of this study can contribute toward advances in the field of ejector refrigeration.

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