Performance comparison of fixed- and controllable-geometry ejectors in a CO2 refrigeration system

• The fixed ejector offers high efficiency for all the considered operating conditions.
• The fixed ejector efficiency is practically independent on the ejector size.
• It is difficult to determine optimal needle position of the controllable ejector.
• The controllable ejector offers high efficiency for a low-reduced motive flow.
• The controllable ejector offers low efficiency for a highly reduced motive flow.

This paper presents a performance comparison of fixed- and controllable-geometry ejectors equipped with convergent and convergent-divergent nozzles installed in a CO2 refrigeration system. The coefficient of performance (COP) of an ejector-based refrigeration system depends on the control of the mass flow rate through the device. Thus, two different solutions were analysed, i.e., ejectors with fixed and controllable geometries. A set of fixed-geometry ejectors working in parallel provides incremental regulation of the refrigerant flow, whereas a controllable-geometry ejector with a needle provides more flexible regulation of the mass flow rate. To compare the potential of both approaches, the device performance was numerically simulated using a validated homogeneous equilibrium model (HEM) for the same typical transcritical parameters. For the fixed-geometry ejectors, the performances of devices of various sizes were simulated and compared with the corresponding controllable-geometry ejector performances at several needle positions. For both ejector types, the global efficiency as a function of the mass flow rate is presented and discussed. The results show that each fixed-geometry ejector configuration exhibits high efficiency over the entire range of operating conditions. In the case of the controllable-geometry ejectors, the efficiency is even higher for a reduction in the motive nozzle throat area of up to approximately 35%, after which the efficiency gradually decreases.