Study On Multifield Coupling Enhanced Heat Transfer Principle In Engine Compartment Of Clean-Gas-Bus

Gas engines,such as Liquefied Petroleum Gas(LPG)engines,have slow flame propagation speeds,high exhaust temperatures,and high thermal loads on parts at equivalent specific combustion.This research examines the multi-field coupling-enhanced heat transfer theory and its application to the engine compartment of a typical LPG city bus.The main research contents and results obtained are as follows.Discrepant direction between the temperature gradient vector and the air velocity vector of high-temperature parts,such as the intercooler and radiator,resulting in poor convective heat transfer in the radiator assembly area.In view of the above problems,this research sought to enhance heat dissipation through optimal matching of air velocity and temperature gradient vector.The matching relationship between the air velocity vector and the temperature gradient vector at the heat trasnfer boundary was explored according to the convective heat transfer enhancement theory.The heat transfer enhancement module thus developed yielded a optimum speed and temperature gradient vector field with an effective cooling air inflow direction that would improve heat dissipation in the radiator assembly area.Improvement in structural design of the radiator assembly area was explored according to the principle of heat transfer enhancement achieved through optimal matching of velocity-temperature gradient vector.Simulations were conducted with position for radiator assembly and grid inclination angles for heat dissipation.Results showed that a reduction in average velocity-temperature gradient angle by 36.79% would increase heat transfer efficiency by 16.80%,compared with that in a typical engine compartment structure.To achieve more effective cooling,this research proposed the homogenization of temperature field in the core flow region for enhancing heat transfer of the engine block area.The optimization model thus developed involved minimizing the temperature gradient in the core flow region and maximizing that at the heat transfer boundary.Simulations were conducted with four air inlet locations and the roof improved structure of the engine compartment.Results showed that the average temperature of the engine block area reduced by 21.53% and the average temperature gradient in the core flow region decreased by 63.42%.Moreover,the coefficient of heat transfer of the engine body and engine cover increased to 5.46 and 3.48 times,respectively.More comprehensive changes in structural design were made according to the heat transfer optimization model.Simulation results showed that the average temperature of the engine block area reduced by 23.20% and the average temperature gradient in the core flow region decreased by 67.26%.Moreover,the coefficient of heat transfer of the engine body and engine cover increased to 5.46 and 4.04 times,respectively.The average velocity-temperature gradient angle of the radiator decreased by 34.32% while heat transfer efficiency of the radiator increased by 14.52%.To validate the simulation results of structure improvements made on the basis of the multi-field coupling-enhanced heat transfer theory,experiments were made comparing the heat transfer performance between the typical and the comprehensive engine compartment of a LPG city bus.The engine was set to work under five conditions.The continuous temperature field was obtained using infrared imaging.Results showed that the engine compartment with comprehensive structural design showed better heat dissipation efficiency,with the temperature of incoming cooling water and exhaust manifold reduced by 10.8% and 25.4%,respectively.In summary,the comprehensive structural design of the engine compartment showed enhanced heat dissipation achieved by better convective heat transfer between engine body and cooling air with a larger temperature gradient,more homogenous air temperature in the core flow area and shorter air flow paths.All these contribute to lead heated air out of the engine compartment timely and efficiently,thus ensuring continuous heat dissipation of high-temperature parts.Both simulation and experimental results are consistent,evidencing the validity of themulti-field coupling-enhanced heat transfer theory.