DOI: 10.14457/tu.the.2022.811
title: Comparisons between constant pressure mixing (CPM) and constant rate of momentum change (CRMC) ejectors on performances of steam ejector refrigeration systems

description: An ejector refrigeration system can efficiently convert low grade heat to useful refrigeration. The system is relatively simple compared to other heat-powered refrigeration systems. Its system performance depends mainly on the performance of the ejector used. The most widely used design model of the ejector is the one-dimensional compressible flow theory. In this theory, mixing in the mixing chamber occurs at constant pressure. The major compression effect is created by a normal shock. Ejectors designed based on this model are known as CPM ejectors (constant pressure mixing). However, the shock presents very high thermodynamic losses. To increase efficiency, the constant rate of momentum change (CRMC) design model was purposed by I. W. Eames in 2002, aiming to eliminate the shock wave from the flow process. It was claimed that CRMC ejector provided superior performance over the conventional ejector (CPM ejector). However, there are a limited number of publications associated with performance comparison between these two ejectors and only a few are experimental studies.According to the design criteria for the CRMC ejector, based on the same input data (designed working conditions), the CRMC ejector provides a smaller ejector throat diameter. This results in producing significantly different ejector area ratios between the two ejectors. It is well known that using ejectors with different area ratios yields a significant difference in ejector performance (trade-off between the mass entrainment ratio (Rm) and the critical condenser pressure (Pcri) as supported by many researchers. With the difference between the two design models, there is a lack of the experimental work to prove the improvement potential of the CRMC ejector compared to the conventional ejector (CPM). The study of Eames in 2002 showed that the CRMC ejector outperforms the CPM ejector. However, such studies were implemented under different experimental units which might not reflect the real improvement potential via the CRMC ejector. In this dissertation, a CRMC ejector was experimentally investigated and compared with the widely used CPM ejector under the same ejector area ratio. The impact of the boiler temperature, evaporator temperature, and primary nozzle size on the performance of both the CRMC and CPM ejectors were studied. The experimental results revealed that at the same ejector area ratio, the CRMC ejector always produced a higher mass entrainment ratio compared to that of the CPM ejector. However, the critical condenser pressure was still identical. The entrainment ratio of the CRMC ejector was, on average, 18.9% higher than that of the CPM ejector. A compression shock wave was still found in the flow process of the CRMC ejector and observed from the transparent sight of the ejector. This shows that the improvement potential of the CRMC ejector is not the result of the elimination of the compression shock wave as proposed in the design theory. The key improvement is suggested to be its ability to produce a lower momentum loss during the mixing process. A simulation work based on computational fluid dynamics (CFD) revealed that the compression shock wave was indeed still present in the CRMC ejector and not eliminated from the mixing process of the ejector as suggested by the theory. The mixing processes within the mixing chamber of the two ejectors (CRMC and CPM) were found to be very similar, both experimentally and in the simulation, since the major compression effect was created by a shock wave. The difference between the two ejectors was that the curved profile mixing chamber of the CRMC ejector provided a lower momentum loss and consequently higher mixing chamber efficiency. This is suggested to be the main reason for the superior performances of the CRMC ejectors over those of the conventional CPM ejectors.