| In refrigeration and heat pump systems,because of the presence of compressors,it is inevitable that lubricating oil will be mixed with refrigerants and influence the heat transfer of systems.In this thesis,the effect of POE lubricating oil on the heat transfer characteristics of the ternary non-azeotropic working fluid R447A(mass component68%R32/28.5%R1234ze(E)/3.5%R125)was studied.With the help of experiments,which include the test of the phase separation of oil and refrigerant,the fluid viscosity test,the measurements of the nucleate pool boiling and the working fluid containing oil flow boiling heat transfer in a horizontal tube,the mechanism explanation about the function of the lubricating oil in the whole heat transfer process was presented.(1)In the aspect of the study on the physical properties of R447A/lubricating oil mixture,the phase separation curve of R447A/lubricating oil mixture was obtained by the experimental test.The results showed that under the different oil concentrations,the miscible and immicible phenomena of R447A and oil occoured.In the low oil fraction,lubricating oil and the refrigerant dissolved.With the rising of oil concentration,lubricating oil was seperted from R447A.When the oil concentration continued to increase,R447A dissolved with lubricating oil again.The viscosity tests of fluids showed that the viscosity of lubricating oil was 10 to 20 times of those of the refrigerant.As the oil concentration went up,so did the viscosity of the mixture with oil.The non-linear physical property calculation method was used to calculate the physical properties of the mixture,which could provide physical property data for the boiling heat transfer researches.(2)In the study of R447A/lubricating oil mixture pool boiling,the boiling phenomena and heat transfer characteristics of R447A/lubricating oil were observed and tested.It was found that with the increase of oil concentration,nucleate bubbles generated easily,the superheats decreased,the foaming density enhanced,volumes of bubbles were smaller and heat transfer coefficients of refriferants with oil were improved in comparison with those of oil-free working fluids.Due to the exsiting oil,viscous resistance and surface force of the mixture increased.Then,floating speeds of bubbles became slow and the bubbles were not easy to merge.Based on the heterogeneous nucleation theory and Gibbs free energy,the theoretical study of pool boiling was carried out.The study showed that with the increment of bubble contact angle,the critical radius of nucleate bubble decreased,so did the effective energy required for bubble growth.As the oil concentration rised,the wettability of the surface was enhanced and the contact angle of the bubble was increased thereby,which was conducive to the foaming process and consistent with the foaming phenomena observed in the experiments.(3)The study of oil-free refrigerants flow boiling heat transfer was conducted in order to compare with that of working fluids with oil.The results showed that the heat transfer coefficients of pure working fluids were higher than those of zeotropic mixtures without oil,which was caused by the mass transfer resistance in the latter heat transfer process.The heat transfer coefficient of R32 was higher than that of R1234ze(E).The heat transfer coefficient of R32/R1234ze(E)was similar to that of R447A.In order to predict the flow boiling heat transfer of ternary mixtures well,a dimensionless correction factor of 1/R_t,considering the effect of mass transfer,was proposed and introduced into the prediction model of heat transfer coefficient.The prediction deviation of experimental data and literature data of oil-free fluids was less than 20%by using the improved model.After the modification of properties,the improved model could be used to predict the the boiling heat transfer coefficient of R447A with oil in the horizontal tube and the average absolute deviation of the prediction was 19.98%.(4)The flow pattern of R447A/lubricating oil mixture was observed and it was found that the presence of oil promoted the transformation of intermittent flow to annular flow.Compared with the oil-free flow pattern,a large number of bubbles were entrapped in the liquid-phase region of the refrigerant/oil annular flow with the increasing of local oil concentration.Based on the experimental results,the oil enhancement factor was proposed and applied into the heat transfer coefficient prediction model of ternary mixture without oil.The average prediction deviation of the heat transfer coefficient of R447A/oil was 20.4%.Based on the entropy generation theory,the entropy generation caused by the flow boiling pressure drop and heat transfer of refrigerants with oil in the heat transfer process was deduced and quantified.The results showed that with the increase of the mass flux,the entropy contribution of the pressure drop could offset the positive effect of oil on the heat transfer.From the perspective of heat transfer entropy generation,1%oil concentration was recommended.(5)The flow boiling heat transfer of oil-containing R447A mixture in a horizontal tube was investigated.It was implied that oil can improve the heat transfer cofficient of fluid,especially at low vapor quality,which the nucleate boiling dominated the heat transfer.But at high vapor quality,the heat transfer coefficient of fluid decreased.Then,the effect of lubricating oil on the flow boiling heat transfer of R447A could be divided into three stages:At low vapor quality,POE lubricating oil and the refrigerant were mutually dissolvable.The hydrophilic group of POE lubricating oil could enhance the wettability of the heat transfer surface and active substances and additives of oil could promote the generation of nucleate bubbles.Therefore,the foaming effective was improved and the necleate boiling heat transfer was enhanced.The lubricating oil also caused the increases of the surface tension and wettability of the working fluid,that contributed the formation of annular flow and refraining from stratified flow appearing in the oil-free fluid flow.The annular flow could make the liquid film thin and was favor to the heat transfer of fluid.With the increasing vapor quality,the local oil concentration of mixture rised and phase separation of the mixture occured.Part of the separated lubricating oil adhered to the heat transfer surface and became thermal resistance.The lubricating oil dissolved partly in the refrigerant further increased the surface force and viscosity of the liquid phase working fluid,that caused the bubble volume small.The oil containing liquid phase at the gas-liquid interface had high surface tension that not only blocked the aggregation of bubbles but also increased the evaporating resistance of convective evaporation.That was not benefit to the convective evaporation heat transfer.But the acceleration of vapor and the foam effective of lubrication oil were positive for the heat transfer,the heat transer coefficient of R447A with oil was still growing with the incresing of vapor quality.However,its increasing altuidude was much slower than that of oil-free R447A.At high vapor quality,the proportion of lubricating oil rised sharply in the liquid-phase mixture,oil and the refrigerant are dissolved again and viscosity and surface tension of the liquid-phase mixture increased quickly.Then,the necleate boiling was surpressed,bubbles flowed in the liquid film restrictively and were not easy to merge with mainstream gas.The oil concentration gradient near the gas-liquid interface of convection evaporation became larger and so did mass transfer resistance.The convective evaporation heat transfer was surpressed and the heat transfer characteristic of fluid deteriorated. |
PDF Full Text Request