Simulation Study On Strengthening The Heat Transfer Of Ammonia Refrigeration Evaporator Pipes By Steel-based Oleophobic Modified Materials

After the refrigeration system is used for a period of time,the refrigeration efficiency decreases.Especially in the special environment of the evaporator,the viscosity and surface tension of the refrigeration oil become larger,and the residual oil film between the evaporator tube wall and the refrigerant increases the thermal resistance of the refrigerant and the outside heat exchange,resulting in the refrigerant and the outside world.The heat exchange is not sufficient,and the power consumption of the compressor increases,thereby increasing the energy consumption of the air conditioning system.Therefore,if you want to reduce the energy consumption of the air conditioning system,it is necessary to weaken and slow down the thickness of the oil film in the evaporator pipeline.The interfacial properties of the oil/ammonia system at different temperatures and oil contents were investigated by dissipative particle dynamics（DPD）.Using the method of combining molecular dynamics（MD）and dissipative particle dynamics（DPD）,the conservative force parameters for refrigerant（liquid ammonia）and refrigeration oil（isotetradecane）in the theoretical equation of DPD are obtained.According to the dispersion and aggregation of the oil/ammonia system,the flow state of each temporal oil-ammonia system was analyzed.Under different conditions of temperature and oil content,the flow model and interaction of the oil/ammonia system are analyzed,and the turning point of the flow regime transition of the two-phase flow is explored.The results show that the oil/ammonia system is divided into two forms:emulsion and layered liquid.30%oil content is the inversion point.Zinc oxide was selected as the material to modify the stainless steel substrate,and the hydrophobicity of zinc oxide was simulated and verified by molecular dynamics（MD）method.Different solid-liquid interaction strengths and interfacial free energy were obtained by changing the potential energy function parameters between different molecules Relationship.Based on the minimum surface energy interface of zinc oxide to construct micro-nano structures,the wettability of different micro-nano structures was analyzed,and the microscopic mechanism of zinc oxide hydrophobicity was explored.The results show that the free energy of solid interface(γ_sv)and the interfacial energy of solid-liquid(γ_sl)of Zn O materials increase with the increase of solid-liquid interaction strength f.The solid-liquid interaction strength f=1.6,which is the turning point of hydrophilic and hydrophobic.Under the condition of certain solid-liquid interaction strength,adding micro-nano structure will increase the hydrophobicityDissipative molecular dynamics was used to verify that the zinc oxide-based modified material could weaken the oil film thickness.Firstly,according to the cohesive energy density theory,the repulsive force parameters of zinc oxide and stainless steel substrates were calculated,and the two substrates were coarse-grained.On this basis,the effects of different surface energies and micro-nanostructure oleophobic effects of zinc oxide substrates in liquid phase were studied.The results showed that with the decrease of surface energy,the oleophobic angle in liquid phase gradually increased,are 148°,168°and 171°,respectively.Polarity affects the liquid film thickness between the refrigerant oil and the bottom layer,but polarity has less effect on oleophobicity than surface energy.The column height of the micro-nano structure has a certain influence on the oleophobic model.When the column height reaches a certain height,the liquid phase molecules will interact with the column in the micro-nano structure,and there is a certain energy barrier in the gap.The electron energy of the molecule is not enough to support the movement of the liquid ammonia molecule to the bottom of the micro-nano structure.The above research content proves that the zinc oxide modified material substrate has better wettability to the refrigeration oil than the stainless steel substrate,increases the contact angle of oil droplets,reduces the resistance between the refrigeration oil and the rigid wall,and thus achieves the effect of weakening the thickness of the oil film.The heat exchange between liquid ammonia and the outside is increased.