The Mechanism Of Refrigerant Flash Expansion And The Critical Flow In The Nozzle

Replacing the throttle valve with an ejector is a resultful way for recovering some of the lost energy of the expansion process, and then improve the cycle performance.In the ejector,the nozzle is the key component, and the energy efficiency of pressure in it has an important effect on the performance of the ejector and the refrigerating cycle.The thermodynamic analysis of the compression/ejector refrigeration cycle is carried out.Compared with the basic cycle, the performance coefficient(COP) of the compression/ejector refrigeration cycle is improved by 4%~33%, the total exergy destruction is reduced by 12.1%~17.2%under the normal air-conditioning working conditions. The results show that there exits an optimum suction pressure drop which makes a maximum COP improvement under a specified condition.A theoretical study of the refrigerant flashing expansion process in the nozzle is carried out. It is considerded that the non-equilibrium thermodynamics and the critical flow phenomenon exists in the course. At present, for two-phase critical flow, especially in the nozzle exists the imbalances of the thermodynamics and the dynamics and the process is more complex, so researches of its is still not perfect. The speed of the refrigerant in the acceleration process is calculated in the nozzle, and the results of the six kinds of sound velocity formula are compared. It is found that the two fluid model of the sound value is too large, which is not suitable. Two kinds of critical flow models, the Homogeneous Equilibrium model and the Moody model, were simulated and the critical mass flow velocity was calculated respectively.A nozzle is designed and processed, the construction of the test bench is completed. The experimental data shows that: with the increase of the inlet pressure, the pressure of each section is increased in the nozzle; the pressure decreases along the axial direction of the nozzle, and the pressure drop occurs mainly in the throat, which is about 80% of the total pressure drop of the nozzle; with the increase of the inlet pressure, the mass flow is increasing, and presents a linear growth trend. The critical mass flow velocity calculated by the two models are studied, and found that: for HEM model,the calculation values are smaller than experimental values, and the maximum error reaches above-40%; for Moody model, there is also a difference between the calculation value and the experimental valuet, and the error range is between-40% to 10%, the overall prediction effect is better than HEM Model.