The Flow And Heat Transfer Analysis And Optimization Of Vehicle Heat Exchanger Based On Thermodynamic Method

Energy supply is the cornerstone and driving force of a country’s long-term development,which also plays a guarantee role in national security construction.The supply and demand relationship of energy is also profoundly affected by the international political and economic situation.At present,China’s energy is increasingly affected by the restriction of resources,and many new challenges and new topics hinder the further development of energy.The energy development strategy advocated by the state is "saving,clean and safe",which is a key development strategy that takes saving as the guide,is based on domestic,is low carbon and environmental protection,and advocates innovation.Heat exchanger is the most commonly applied heat equipment in industrial production.Reducing the irreversible dissipation of energy in the process of internal heat transfer can make the utilization efficiency of energy improved,which is a very effective measure to save energy and protect environment,so the characteristic optimization analysis and calculation of heat exchanger is of great significance for energy conservation and environmental protection.In this paper,based on the field of rail vehicles and the thermodynamic methods,such as the principle of minimum entropy generation,the field synergy theory and entransy dissipation theory,taking computational fluid dynamics(CFD)method as the main research method,combined with wind tunnel test technology,the heat transfer enhancement of cross flow heat exchanger in EMU is simulated and experimentally studied,and the structure of heat exchanger is optimized to explore the physical mechanism of thermodynamic research method in the process of heat transfer enhancement.The specific work of this paper is as follows:(1)Taking the plate fin heat exchanger commonly used in the cooling system of EMU traction converter as the research object,based on the multi-objective optimization method of genetic algorithm and the minimum entropy generation theory of thermodynamics,combined with three-dimensional simulation technology,the structural optimization calculation of plate fin heat exchanger is carried out.This type of heat exchanger is widely used in locomotives and high-speed EMUs.The fluid on both sides of the heat exchanger is air and cooling water respectively.In this paper,the three-dimensional simulation calculation of the finned channel of the fluid is carried out,and the experimental correlation calculation formula of the heat transfer factor and friction factor suitable for this type of heat exchanger is determined;Taking the minimum entropy generation,the maximum heat transfer factor and the minimum friction factor as the optimization objectives,a genetic algorithm program is compiled to obtain the optimal solution,and the influence of the main structural parameters of the fin on the objective function is analyzed.The results show that the multi-objective optimization can obtain multiple alternative optimization solutions,which is more convenient and flexible than the single objective optimization,and the flow heat transfer and irreversibility are considered at the same time.(2)Taking the IGBT air-cooled heat exchanger of EMU as the research object,combined with Creo parametric and ANSYS three-dimensional simulation software,the structure of the heat exchanger fins is changed in the form of corrugation,and different corrugation angles correspond to different ratio of corrugation width to height;Then,the field synergy theory is used to optimize and analyze the synergy relationship between velocity field and temperature field.The results show that the change of fin ripple angle can significantly improve the synergy degree of velocity field and temperature field.The average synergy angle α decreases by 2%and heat transfer coefficient increases by 64.8%compared with the traditional fin at 120° and the effect of strengthening convective heat transfer is obvious;At the same time,the average synergy angle of velocity field and pressure field increases by 29.6%,and the flow resistance increases.The optimization results can not only enhance the effect of convective heat transfer,but also can not increase the flow resistance significantly.At the same time,the synergy angle α of velocity field and temperature field in the field synergy theory is verified,its small changes will have an important impact on heat transfer enhancement.(3)The basic heat transfer process is analyzed by using the entransy theory and the entransy degeneration formula of one-dimensional flat wall steady heat transfer is derived.The thermodynamic model of counter-flow heat exchanger is analyzed,and the formula for calculating its entransy dissipation is derived.It is shown that the entransy dissipation is composed of temperature difference and flow resistance.The entransy dissipation expressions of the NTU in the down-flow、counter-flow and cross-flow heat exchanger are derived,the variation of the entransy dissipation with the NTU is analyzed.The larger the NTU in the down-flow heat exchanger,the greater the heat transfer,and the smaller the entransy dissipation,that is,the irreversibility decreases,which will reach a limit value.When NTU is greater than 2,it will reach a minimum value of 0.5.When C<1,the entransy dissipation decreases with the increase of NTU,and tends to reach the minimum limit value at a certain value.When NTU is fixed,the smaller the C value is,the larger the entransy dissipation is.This is consistent with the three types of heat exchanger.Based on the experimental data of cross-flow heat exchanger,the calculation and analysis of the number of entransy dissipation are carried out.The results show that the proportion of entransy dissipation of temperature difference is the largest.In actual operation,when the water flow is fixed,the flow of air side should be reduced as much as possible to reduce the irreversible loss of heat exchanger,so as to achieve the purpose of energy saving.