Study on irreversibility in an ejector with zeotropic mixtures using direct entropy analysis method

Abstract

Ejector refrigeration attracts more attention due to the intense increase of space cooling requirements and energy crises. Significant irreversibility inside the ejector hinders its applications by lowering ejector efficiency and system performance. Thus, it is important to understand the distribution of irreversibility inside the ejector and the effects of operating conditions and ejector geometries on irreversibility. A two-dimensional numerical model is developed considering direct entropy analysis method and real gas refrigerant property. Zeotropic mixture R134a/R32 is selected. Effects of primary and secondary flow pressure, ejector back pressure, nozzle throat and mixing chamber diameter on three types of entropy generation within the ejector are investigated. Results show that the turbulent dissipation entropy generation dominates the total entropy generation in all cases. As the primary flow pressure varies from 2588.80 kPa to 3188.80 kPa, the entrainment ratio first increases from 0.127 to 0.461 and then decreases to 0.400. The direct dissipation entropy generation mainly occurs in the divergent part of the nozzle and the mixing chamber, while the heat transfer entropy generation mainly occurs in the mixing shear layer and where the shock train occurs. As the secondary flow pressure changes from 334.61 kPa to 494.61 kPa, the entrainment ratio increases from 0.155 to 0.589, and the turbulent dissipation entropy generation decreases from 3.557 W/K to 2.667 W/K and then remains almost constant. As the ejector back pressure changes from 726.57 kPa to 900.57 kPa, the minimum total entropy generation is achieved at critical point.