Paweł Jakończuk

This paper describes the procedure of design and manufacture of a micro-ejector proposed for miniature ejection refrigeration systems. It describes the procedure of design, fabrication, and experimentation on supersonic micro-ejectors and makes the case for isobutane as a working fluid for such systems. It was demonstrated that it is possible to design and fabricate a micro-ejector with a cooling capacity of approximately 3 W. The discussed micro-ejector was driven by a heat source with temperature below 60 °C. The evaporation temperature was approximately 15 °C. For these operating parameters, the reported entrainment ratio was approximately 0.20. The difficulties in fabricating the micro-ejector due to its small dimensions are discussed in the paper. Additionally, the potential difficulties and solutions related to ensuring and maintaining stable operation of the testing stand are presented. The performance of the proposed system is demonstrated and discussed, including relations between mass entrainment ratio, compression ratio, cooling capacity, and temperature.

Fin-and-tube heat exchangers have been extensively used in many fields, especially in heat, ventilation, air-conditioning, and refrigeration systems. In the case of the operation of a fin-and-tube heat exchanger as an air cooler, frost formation is an important effect that should be taken into account. The frost accumulation process is undesirable since it deteriorates heat transfer due to the insulation of the frost layer as well as causing excessive pressure loss. The analysis of the effect of the frosting process on a fin-and-tube air cooler performance is presented in this paper. Based on long-term experimental investigations applied to the air cooler in a cold storage chamber, the general degradation of the heat exchanger performance is discussed. The influence of frost on the cooling capacity, by-pass factor, and thermal resistance is analysed. The temperature distribution of the air passing through the air cooler before and after the defrosting process is presented and discussed. A method for the assessment of the amount of frost formed at the air cooler surface, based on visualisation of the air cooler during operation and synchronised with the thermal measurements, is developed. The results show that the frosting process causes deterioration of the cooling capacity by up to 40% in the analysed case. Correlation is demonstrated between frost formation and heat transfer degradation in the air cooler.

Two-phase ejectors as well as single phase ejectors can be applied in many branches of industry: refrigeration and heat pump systems, chemical engineering, food processing, and others. Due to the complicated nature of the process of momentum transfer in two-phase ejectors, their design procedure based on the accurate theoretical prediction of the ejector performance is still an open issue. The paper provides its own experimental results of the velocity coefficients of the components of the two-phase ejector, i.e., the motive nozzle, suction chamber, mixing chamber, and diffuser. The results were obtained in the case of isobutane as the working fluid. It was demonstrated that the velocity coefficients may not be treated as constant quantities. Therefore, our own proposed dimensionless relationships describe the velocity coefficients of the components of the ejector that may be applied in the design procedure of the ejector. The two physical parameters, the wet vapour quality and the volumetric entrainment ratio, were selected as the key parameters. In addition, the aspects of the prediction of the critical mass flow rate of the motive nozzles was considered on the basis of the Henry–Fauske model. It was demonstrated that the model accurately predicts the two-phase critical flow under the conditions of a higher range of wet vapour quality.

The conventional refrigeration ejector cycle utilizes a constant temperature process during motive heat consumption needed for vapor generation. This may be thought of as the main restriction concerning the application of low-grade heat as a motive source by these systems since it results in required significant mass flow rate with a small temperature drop of heating fluid. The first proposed novel approach is an ejector flash cycle (EFC) in which the working fluid is heated up to the saturation temperature only at the inlet to the primary nozzle of the ejector. The second possible approach is the application of a zeotropic mixture as the working fluid for the conventional ejector cycle. The performance of an EFC is compared to the ejector refrigeration cycle (ERC) working with zeotropic refrigerant. Three motive heat source temperatures (60°C, 75°C, 90°C) and two evaporation temperatures (0°C, 6°C) were tested. Eight pure low-GWP refrigerants and four low-GWP refrigerant blends were analyzed. It was demonstrated that the flash cycle can efficiently operate with low-temperature motive heat source (60°C) with refrigerants R1234yf and R1234ze(E). The highest coefficient of performance (COP) is attainable for propane with a motive source temperature of 60°C and R1234ze(Z) for the motive temperature of 75°C and 90°C. The differences of liquid pump power consumption of both discussed cycles compared to the conventional compression cycle are significant and reach up to 90 percent in the case of EFC and up to 96 percent in the case of the zeotropic cycle. The electric power savings in the case of the zeotropic cycle are slightly higher, but COP and drop of the heating water temperature are notably lower. Therefore, the EFC system is suggested to be a better solution for low-grade heat utilization in the cases when a significant decrease of the motive source fluid temperature is required.