Research On Thermal Characteristics Of Diesel Engine Cooling System Based On Subcooled Boiling Heat Transfer

The increase of diesel engine intensity rises higher requirement for cooling efficiency.Efficient heat transfer methods,reasonable cooling structures and advanced control strategies are important means to improve the efficiency of engine cooling system.As an efficient heat transfer method the subcooled boiling heat transfer has high potential in the engine cooling system.Therefore,the research on the thermal characteristics of engine cooling systems based on subcooled boiling heat transfer has high scientific and engineering significances.Based on the mechanism of subcooled boiling,a multiphase flow model is developed including submodels of drag force,lift force,virtual mass force,wall lubrication force,turbulent dispersion force,bubble departure diameter,bubble departure frequency,nucleation sites density,interfacial heat transfer coefficient,interfacial area concentration and bubble diameter.A fluid-structure coupling model of diesel engine is established to simulate the subcooled boiling phenomenon and verified by temperature measurement of the fire face of diesel engine cylinder head.Simulation results show the coolant flow dominates the temperature of the cooling channel,and asymmetric local structure makes the difference in cooling performance of symmetric positions.Sufficient connection of cooling channels reduces the local overheat,and properly increasing the inlet/outlet number benefits the gas distribution,the uniformity of coolant flow and the heat exchange.For the inconsistency of the bubble diameter and the gas distribution,the bubble diameter submodel is improved by considering the bubble diameter as a synthesis of the basic departure diameter,the growth due to heat transfer,collision and separation.The three bubble diameter components are calculated by estimating the bubble location,interfacial area concentration equation,phase interaction force as well as gas viscous force based on C language programming global search.After validation of the bubble diameter model,combining with the engine coolant flow path prediction,the bubble formation process in the engine cooling channel is analysed.An experimental study on the influence of the cooling system on the thermal characteristics and emissions of the diesel engine is carried out.Seventy-two cases are designed consisting of 3 speeds,4 fuel injections and 6 coolant flows.To investigate the effect of coolant flow on the thermal characteristics of the diesel engine under different intake air,9 cases are carried out including 3 air intake and 3 coolant flows.The effects of coolant flow under various speed,fuel injection,air intake on the temperature difference of the coolant flow,the cooling power,the temperature difference between the intake and exhaust,the engine power,the gas temperature,the gas pressure,and the CO,CO₂,HC,NO_x,O₂ and smoke emissions at different rotating speeds,fuel injection and air intake are discussed.To investigate thermal change in a working cycle of diesel engine,the dynamic coupling between cooling and in-cylinder combustion is simulated.The global reaction mechanism is used for simplifing the combustion process to construct a 1D heat balance equation and the multiphase flow method is employed to simulate the coolant flow considerating the wall boiling and the interphase forces.The information exchange between in-cylinder temperature,heat transfer coefficient,and wall temperature is achieved through the programming approach to coordinate the 1D combustion and the 3D cooling.The proposed modeling process is verified against the measured power,in-cylinder pressure and fire surface temperature of cylinder head.Then the effects of different cooling conditions on the cyclic engine performances are discussed.Based on the transient temperature field of the cylinder head,the cylinder head stress,strain and fatigue life are evaluated.