Study On Velocity-temperature Field Coupling Enhanced Heat Transfer In Engine Compartment With Inverted Transverse Engine

With the extensive application of the high power density engine and vehicle new technologies and new systems,the heat transfer load in engine compartment is increasing and the space are more crowded.This often leads to problems such as inadequate cooling and local high temperature baking,which affect the operation reliability,safety,fuel economy and emissions of one vehicle.In order to solve the above problems,the CFD method is an important technology to analyze and improve the air flow field in the engine compartment to enhance heat transfer.Due to the current improvement of the flow field ignoring the coupling effect of air velocity field and temperature field on the heat transfer,the heat transfer performance of engine compartment sink and cooling air will be restricted.The inverted layout transverse engine,called inverted transverse engine,has been widely used in commercial vehicles with regard to the advantages of adequate combustion,gravity center low and so on.However,the cooling air that flow to the engine rear exhaust manifold was limited in this structure,leading to heat dissipation difficulty and the local high temperature,which are technical difficultiesof the inverted transverse engine in the vehicles thermal management.This paper studies on an engine compartment with inverted transverse engine.The enhanced heat transfer method for engine compartment based on velocity-temperature field coupling was carried out,and the specific work was as follows:(1)The engine compartment heat dissipation characteristics were tested.Based on the analysis of the structural characteristics and cooling principle of the inverted engine compartment,referring to the standard GB/T12542-2009 "road test method of automobile thermal balance ability",an experiment scheme of the real vehicle engine compartment heat dissipation characteristics were designed under multi-conditions.The temperature in 16 points and radiator hot flow inlet temperature were monitored,and by the comparison analysis of the experiments data in all conditions,the bad cooling condition was determined,and all the experiments data and conclusions were the prerequisites for the CFD simulation analysis of the engine compartment cooling characteristics.(2)The CFD analysis of flow and heat dissipation characteristics in engine compartment was studied.Firstly,by the retention of the pressure gradient in the momentum equation and enclosing turbulent stress with turbulent mixing length theory,an modified wall function(MWF)was proposed for boundary calculation,and its calculation reliability was testified by the numeral calculations for the classic later steps separation flow and international MIRA back separation flow.Then,the CFD simulation of flow and heat transfer in the engine compartment was carried out,whose boundary layers were calculated with the MWF and CFD results were validated by the engine compartment experiment data.At last,the air flow and heat transfer of the bad cooling condition was analyzed to find the causes of inadequate cooling and local high temperature.(3)The study of engine compartment enhanced heat transfer principle was conducted on the basis of velocity-temperature field coupling method.Based on the radiator heat dissipation principle and distributed parameter method,the radiator cooling power model was established,and the coupling influence mechanism of the velocity-temperature field in radiators’ windward was analyzed by the undimensionalization method to obtain the field coupling coefficient evaluating the coupling influence.In the light of the maxima condition of the field coupling coefficient,the "big-small" symmetrical coupling model for air velocity and cold-hot fluid temperature difference in the radiator windward was proposed,and then radiator windward velocity distribution was optimized and verified by simulation.Meanwhile,based on convective heat transfer field synergy theory,the fluid velocity vector and temperature gradient vector 0° angle principle for enhancing heat transfer of the non-radiator high temperature parts was clarified.Then,according to the 0° angle principle and the temperature distribution feature of natural convection,the radial optimization direction of the coming air velocity of the non-radiator high temperature part was proposed for heat transfer enhancement,whose effectiveness was verified by simulations.(4)The structure improvements for velocity-temperature field coupling enhanced heat transfer in the front of engine compartment were researched.Firstly,for the radiator’s inadequate cooling in the front of engine compartment,based on the "big-small" symmetrical coupling principle for air velocity and cold-hot fluid temperature difference and the optimized velocity distribution discipline,the radiator windward flow field was further analyzed for obtaining its distribution irrationality.And then,flow field in radiator windward side and its influence on heat transfer performance were researched under different conditions,such as the layout of the front grille bars,grille outline,layout of radiator-fan-engine and the radiator shroud.In order to improve the windward velocity distribution with velocity-temperature field coupling model and increase the cooling air flow,the improved structure for velocity-temperature field coupling enhanced heat transfer was determined.(5)The structure for velocity-temperature field coupling enhanced heat transfer in the rear end of engine compartment was improved.The radial optimization direction of coming air flow was applied to analyze its irrationality for the coming air velocity direction of the exhaust manifold in original engine compartment,which was based on the 0° angle principle of velocity vector and temperature gradient vector.And then,five structural improvements were designed to increase cooling air flow,improve the coming air velocity direction and eliminate reflux.By the analysis of the flow field and its cooling characteristics in the rear end engine compartment,the "radiator~fan" shroud composite structure for velocity-temperature field coupling enhanced heat transfer was proposed.(6)The comprehensive improved structures for velocity-temperature field coupling enhanced heat transfer in the whole engine compartment were studied.Based on the improved structures for the flow field enhanced heat transfer in the front and the rear end of the engine compartment,one comprehensive improved structure was obtained.The flow field heat transfer comparative analysis of the comprehensive improved structure with original engine compartment was executed,which illustrated that,the inlet temperature of the radiator hot fluid was reduced to 88℃.Through increasing the exhaust manifold cooling air flow rate and optimizing the coming air velocity direction,the heat transfer performance of exhaust manifold was finally improved.The average surface temperature was reduced to 486℃,and local high temperature disappeared.The heat dissipation of other parts were good,and area with air temperature arrange 62℃~73℃ in the whole engine compartment was significantly reduced,the engine compartment heat transfer was comprehensively improved.