Research On Key Technology Of Gas Cooler In Carbon Dioxide Automotive Air Conditioning System

For a long time,energy shortages and environmental degradation have been a major problem for the world,especially our country.In recent years,we have been deeply affected by the frequent smog in most parts of the country,especially in the north.It has made us more aware of the urgency of protecting environment.The widespread use of Freon and chlorine-containing hydrofluorocarbon refrigerants is the main cause of the ozone layer and the greenhouse effect.Therefore,the development and use of energy-saving and environmental friendly refrigerants,especially natural working fluids,is a general trend.CO₂ has a greater advantage in terms of environmental friendliness,safety and cooling capacity,which made it the preferred natural refrigerant,especially in the field of automotive air conditioning.The flow and heat transfer characteristics of supercritical carbon dioxide in trefoil tubes were studied by suing theoretical analysis,numerical simulation and experimental method.The new type of shuttle shaped fins were developed,and the flow and heat transfer characteristics of the staggered shuttle shaped fins are studied.A micro-channel gas cooler with shuttle shaped fins in air side was proposed and its structural dimensions were optimized by response surface analysis.The influence of different factors on the flow distribution of the parallel flow micro-channel gas cooler was analyzed.On the basis,the new variable combination depth header and interpolated splitter header,which could improve the flow distribution performance,were developed.The main research contents and conclusions of this paper are as follows:1.The physical properties characteristics of supercritical CO₂ were analyzed.The effects of the inlet Reynolds number,the wall heat flux,the heat pipe diameter,the cooling pressure and the flow direction on the flow and heat transfer characteristics of trefoil tube and microchannel trefoil tube were studied.The results show that the local convective heat transfer coefficient of supercritical carbon dioxide in the tubes changes obviously along the axial direction of the tubes.In addition,the maximum value appears when the mainstream temperature reaches the critical point temperature.The maximum value of local convective heat transfer coefficient appears when the fluid mainstream temperature reaches the critical temperature.The new heat transfer correlations of the supercritical carbon dioxide in the microchannel trefoil bule are fitted.Moreover,the new correlation has high accuracy and reliability.2.A new type of shuttle shaped fin was developed.The flow and heat transfer characteristics of shuttle shapped fin and straight fin were analyzed by using ANSYS Fluent.The effects of half width a,half height b on the flow and heat transfer in shuttled shaped fin flow channel were studied.The comprehensive performance of the shuttled shaped fin with different parameters was compared by using the comprehensive energy efficiency factor.In the study of this paper,the comprehensive energy efficiency of shuttled shaped fin is about 41%⁶2%higher than that of straight fin with the same Reynolds number.3.Based on the response surface analysis,the air side structure of the gas cooler with staggered shuttled shaped fin was optimized.The high quality response surface of heat transfer coefficient,pressure drop and heat exchange is constructed,and the influence of input parameters on output parameters was analyzed.The maximum heat transfer coefficient,the minimum pressure drop and the maximum heat transfer coefficient per pressure drop were optimized by using the method of sampling and MISQP method.Multi-objective optimization of the three optimization goals was carried out by the MOGA method,and the first Pareto front with 251 sample points are obtained.The three optimization goals are divided into two groups,and 20 candidate sample points are obtained respectively by different optimization weights.4.The impact of different factors on flow distribution of the microchannel parallel flow gas cooler with D-shaped header was studied.These factors are the combination depth of the inlet and outlet of the flat tube,the diameter of the flat tube,the length of the flat tube,the length of the flat tube,the length of the flat tube,the Reynolds number.From the perspective of heat transfer,based on the entransy and entransy dissipation,the entransy dissipation nonuniformity of the flat tube and gas cooler were deduced to evaluate the uniformity of fluid distribution.It provides a new method for evaluating the uniformity of fluid distribution.The results show that,in this paper,the microchannel diameter of flat tube,the combination depth of inlet and outlet of flat tube and the header,the inlet Reynolds number of the header have large influence on flow distribution nonuniformity and the nonuniformity of the entransy dissipation of the gas cooler.However,the influence of flat tube length and flat tube spacing is relatively small.For the gas coolers studied in this paper,it is recommended that the diameter of microchannel should not be less than 0.7 mm,and the combined depth of the flat tube and the header should be T=0,the remaining parameters can be selected according to the design needs.5.The new variable combination depth header and interpolated splitter header was proposed.The effects of two types of header on the flow distribution nonuniformity of the gas cooler were studied by numerical simulation.The results show that the flow distribution performance of the gas cooler can be improved by changing the combined depth of the flat tube and the header and interpolating splitter in header.While changing the combination depth of three groups of flat tubes simultaneously can achieve a better flow distribution performance of gas cooler than changing the combination depth of only one or two groups of flat bube.Header with two splitters achieves better flow distribution performance of air cooler that with one splitter.When compared to the depth of all tubes combined with T=0,the two new types of header structures can reduce the total flow distribution nonuniformity of gas cooler by 30%to 50%.6.A set of experimental test model of staggered shuttle shaped fin was designed and manufactured.The flow velocity of key positions in staggered shuttle shaped fin were measured by using Laser Doppler Velocimeter?LDV?.Finally,the possible causes of experimental errors are analyzed.The results show that the numerical simulation results are in good agreement with the experimental results.The relative error is not more than 25%,indicating that the numerical simulation method and the results are accurate and reliable.The research provides support for the application of staggered shuttle shapped fin.