Research On Heat And Flow Characteristic Of Falling Film Evaporation Of Refrigerant On Vertical Tube

With the advent of the refrigerant substitution,although there are some better refrigerants which are implemented in the refrigeration field.It is reported that there is no single refrigerant or mixture available to satisfy both the ozone depletion potential(ODP)and global warming potential(GWP)demands.As a result,changing the form of heat and mass transfer in the heat exchanger and reducing the refrigerant charge requirements may be a better way to solve the problem.Because of its high heat and mass transfer coefficients,falling film evaporation process is widely used in many industrial processes.Moreover,the process of falling film evaporation needs less hydrostatic liquid volume.To improve the heat transfer performance of falling film heat exchangers used in the air conditioning system,it is important to first analyse the flow and heat transfer behaviour of the refrigerant on the vertical surface of the evaporator.The wettability characteristic of liquid medium plays an important role in generating the dry spot in the falling film flow process.The contact angle is the most important measurable characteristic to describe wettability.Therefore,in this paper the contact angle of refrigerants(R134a,R22,R290,R600 a,R32)with the metal copper/ stainless steel 304/ aluminium 6063 has been measured.The contact angle measurements show the super wettability of the refrigerants in regards to common material surfaces.Moreover,comparing the curve fittings of Laplace and automatic polynomial,the defect of the automatic polynomial curve fitting in prediction area is exposed.The results calculated by Young-Laplace curve fitting with the physical significance are more precise.The contact angle measurement's results show that the dry spot is difficult to appear in falling film evaporation process of refrigerant.However,the experiment investigation indicates that the rivulet of R134 a will shrinkage obviously as a rivulet when heated and the heat exchange area deteriorates seriously.In this paper,a new model based on the stable rivulet and micro-layer evaporation theories was built to explain this phenomenon.A falling film evaporation experiment was conducted with R134 a to examine the accuracy of the model.The calculation of the rivulet's width has good agreement with the experiments.Moreover,the theoretical analysis shows that the thermal-capillary effect will intensify the shrinkage of the rivulet,but the apparent contact angle change in the heated refrigerant is mainly caused by the evaporation of the micro-film.In this paper,a method is presented to calculate the micro-layer evaporation in 3-D parallelepiped cells in the framework of the VOF method.This method divides the micro-layer into two parts in the cells based on the difference in thickness of the liquid film.The heat conduction of two liquid films is calculated respectively and integrated.The method is validated by the analytical solution of micro-layer-evaporation of the circle triple-phase line and the falling film flow process.Furthermore,the heat conduction proportion between micro-layer and heated surface is calculated.The result shows that the micro-layer evaporation can hold at least half proportion in falling film evaporation.Therefore,prolong the length of the three-phase line may be a better way to improve the heat transfer efficiency of falling film evaporation of refrigerant.