Research On Frosting And Defrosting Of Flat Fin Tube Heat Exchanger

Flat fin tube heat exchanger has high heat exchange efficiency,low air resistance and stable mechanical performance.It is widely used in household air conditioning,environmental test chamber,closed heat source tower and other heat pump refrigeration system.However,in the low temperature and high humidity environment,when the wet air flows through the surface of the flat finned tube heat exchanger,it is easy to phase change.In general,frost will occur,blocking the gas channel of the flat finned tube heat exchanger,reducing the wet air quality flow through the heat exchanger,increasing the heat transfer resistance,so that the efficiency of the heat exchanger is greatly reduced.Mastering the formation process and physical properties of frost layer is an important prerequisite for effectively solving the frost formation problem.Based on the fluid-solid thermal coupling calculation method,this paper takes into account the interaction between wet air,frost layer and finned tube structure,calculates the mass transfer rate of phase transition by using the temperature and water vapor concentration of the surface of the frost layer,and dynamically calculates the density and thermal conductivity of the updated frost layer.The frost layer distribution characteristics and the change of air side heat transfer coefficient during the frosting process of flat finned tube heat exchanger were analyzed.The research results showed that the thickness of frost layer on the finned tube decreased gradually along the direction of air flow,and the higher the inlet velocity and relative humidity was,the faster the frosting rate was.The inlet velocity was 3.7m/s,the relative humidity was80%,and the thickness of frost layer was 1mm at 120min.The fin gap is completely blocked;In the early frost stage,the higher the relative humidity of air,the higher the average heat transfer coefficient of air side of heat exchanger.In the late frosting period,the higher the relative humidity of air,the lower the average heat transfer coefficient of air side.It provides guidance for the design of flat fin tube heat exchanger.After analyzing the frost formation mechanism,it is found that the frost formation process on cold surface usually includes the formation,polymerization and freezing of microdroplets.Therefore,the formation of microdroplets in the fin gap of flat-finned tube heat exchangers at the initial frosting stage.Based on this,the existing theoretical formula was derived in this paper,and the calculation model of the volume fraction of microdroplets growth was obtained.Then,the numerical simulation of the growth process of microdroplets under different temperature,humidity and surface contact Angle was carried out by using computational fluid dynamics software,and the comparative verification was made with the literature values.The growth data of microdroplet at different time were obtained.Through comparison,it is found that the growth rate of microdroplet is slower when the wall contact Angle is larger.When the wall contact Angle is 145°,the diameter of microdroplet is about 0.62mm at 800s.Finally,based on the growth data of microdroplet at the early stage of frosting,this paper applied mechanical vibration boundary conditions on the base tube of flat fin tube,obtained the movement process of microdroplet in the fin gap under the action of sinusoidal periodic mechanical vibration,and built an experimental platform for verification.The results show that mechanical vibration can accelerate the sliding process of microdroplets on the base tube of flat-finned tube heat exchanger in the early stage of frosting.The microdroplets with a volume of 1.1×10^-10m³ move to about 1.7mm in the negative y direction in 80ms under sinusoidal vibration of 50Hz,which provides a reference for the research on mechanical vibration of flat-finned tube heat exchanger for frost suppression.Figure[68]table[10]reference[83]...