Analysis on the optimal mixing pressure and efficiency limit of an ideal ejector

Abstract

Ejector is a component using high-pressure fluid to aspirate low-pressure fluid, whose potential in energy saving is increasingly emphasized. The upper limit of ejector performance is still an interesting and instructive issue. In this paper, an ideal ejector is constructed, in which the working process is thermodynamically optimized to achieve minimal entropy generation. While working with ideal gas, the optimal mixing pressure can be expressed analytically as a function of inlet fluid parameters if the inlet temperatures of primary and secondary fluids are different. Furthermore, the analytical solution for the maximum of outlet pressure or entrainment ratio is deduced. The model of ideal ejector is compared with existing ejector models to illustrate its reasonability as a thermodynamic benchmark for ejector performance. For ejector working with real gas, a semi-perfect gas approach is introduced to estimate the upper limit of ejector performance. The calculation results are compared with other theoretical ejector models as well as the experimental data at typical refrigeration conditions. The optimal mixing pressure and maximal entrainment ratio are also obtained for ejector having polytropic processes in nozzles and diffuser. The thermodynamic upper limits provided in this paper offer a more reasonable reference for evaluating ejector performance than those from previous models.

Keywords

• Ejector
• Efficiency limit
• Thermodynamic analysis
• Optimization