Numerical Study On Flow And Heat Transfer Of Supercritical R1233zd(E) And R444A In Tubes

Supercritical fluid is a kind of fluid in a special state that is neither gas state nor liquid state.It is very sensitive to changes in pressure and temperature and has special thermophysical properties.Its density is relatively high,close to liquid,and its viscosity is relatively low,close to gas,which make it have good flow and heat transfer characteristics,so it is widely used in industry and life.The ozone depletion potential(ODP)of the fourth-generation HFOs organic refrigerant R1233zd(E)is almost zero,and the ODP of the mixed organic refrigerant R444A is zero,and both of them have low global warming potential(GWP)values,making them new environmentally friendly organic refrigerants.In this paper,R1233zd(E)and R444A are taken as the main research objects,and their flow and heat transfer characteristics under supercritical pressure are studied by numerical simulation.The compactness of heat exchangers in supercritical cycle systems is very important,and mini tubes usually have better heat transfer capacity than traditional ordinary tubes.Therefore,in this paper,the flow and heat transfer characteristics of supercritical fluid in mini tubes are emphatically discussed.The main work and research results of this paper are divided into the following three parts:(1)The flow and heat transfer of supercritical R1233zd(E)in four horizontal cooling tubes with different inner diameters are numerically simulated,including a mini tube with an inner diameter of 0.9 mm and three ordinary tubes with inner diameters of 1.55 mm,2.26 mm and 3 mm,respectively.The flow and heat transfer characteristics and mechanism of supercritical R1233zd(E)in the mini tube and ordinary tubes are analyzed and compared in detail,and the advantages of the mini tube are found;The applicability of several typical buoyancy criteria is discussed,and the thresholds applicable to this study are obtained;Finally,some existing heat transfer prediction correlations are discussed,and a new heat transfer prediction correlation of supercritical R1233zd(E)with high prediction accuracy is developed by using the piecewise fitting method.(2)The flow and heat transfer of supercritical R1233zd(E)in two horizontal tubes with different inner diameters under heating conditions are numerically simulated,and the results obtained are compared with those obtained under cooling conditions.The two tubes are a mini tube with an inner diameter of 0.9 mm and an ordinary tube with an inner diameter of 3 mm,respectively.The supercritical R1233zd(E)under heating and cooling conditions is compared from four aspects of heat transfer performance,fluid distributions,buoyancy criteria and heat transfer correlations.The comparison results show that the flow and heat transfer characteristics of supercritical R1233zd(E)fluid under heating and cooling conditions are significantly different,and the mini tube has obvious advantages under both heating and cooling conditions.(3)The flow and heat transfer characteristics of supercritical R444A and R134a in a horizontal mini tube under heating conditions are compared to explore the feasibility of replacing R 134a with R444A.The flow and heat transfer characteristics of R444A and R134a under different mass fluxes,supercritical pressures and heat fluxes are analyzed and compared,respectively.The comparison results show that the overall heat transfer performance of supercritical R444A is better than that of supercritical R 134a under the same working conditions.The reasons for the differences of heat transfer performance between the two are analyzed,and the applicability of the existing heat transfer prediction correlations proposed based on supercritical R134a to supercritical R444A and R134a in this study is also analyzed and compared,and it is found that the heat transfer prediction correlations proposed based on supercritical R134a are also applicable to supercritical R444A.