Study On Effect Of Surface Morphology On Boiling Heat Transfer In Water Jacket Of Internal Combustion Engine

Recently,the internal combustion(IC)engine requires more compacted structure due to the increasing serious energy and environmental problems.The cylinder head stands higher thermal load so that its inner water jacket should have improved heat transfer capacity.The convective heat transfer could no longer satisfy the strict cooling requirements.Boiling heat transfer has gained more attention owing to its effective heat transfer characteristics.Many experimental and simulation researches focused on the effect of flow parameters or coolants on boiling and established some boiling models in water jacket,but few were concerned about surface morphology.Setting the morphology on the heating surface could promote bubble nucleation to enhance boiling effectively,which makes it meaningful for improving the heat transfer capability in engine cooling systems.In this paper,the mass transfer model of VOF model was modified and the boiling model applicable to engine conditions was established with high accuracy.Different surface morphology were set on the cooling passage to investigate the law and mechanism of enhanced boiling based on the heat flux,velocity and void fraction.The main content and conclusions are as follows:(1)Based on the rectangular channel in Liu's boiling test bench,the geometric and mesh model of the water jacket simulated passage were established.Sample set of mass transfer time relaxation paremeter?which improves the simulation accuracy was obtained with the trial and error method.Prediction model of?suitable for different conditions was developed using SVR method and the VOF model after correcting?was further validated by other experimental literatures.Results show that the heat flux increases with decreasing?under same superheat.MSE and R~2 of test set for?prediction model are respectively 0.02 and 0.997,which makes the error between the predicted and experimental heat flux in other papers less than 20%.(2)Variation rules of?,heat flux and void fraction with flow parameters under typical engine conditions were studied using the VOF model after correcting?.At low superheat,the heat flux increases when the velocity and subcooling are increased,but?increases with the decreasing velocity,subcooling and pressure which will accelerate the interphase mass transfer rate.When the superheat increased,decreasing velocity,subcooling and pressure lead to higher growth rate of heat flux,and?under different conditions gradually approach.The contribution rate of velocity on heat flux is 48.93%,which of pressure on heat flux is only 23.5%.Average void fraction at any positions of heating surface increases when velocity,subcooling and pressure decrease.It will gradually increase along the flow direction at high velocity,but the maximum void fraction appears on the front section of heating surface at low velocity.(3)Boiling simulations were conducted in the cooling passage with specific surface morphology to analyze the strengthening mechanism for boiling heat transfer.Calculated heat flux of cylindrical cavity surface is 10%-22%higher than that of plain surface under different conditions.Velocity in the corner and land side of the cavity is quite slow,which benefits to bubble nucleation and growth.Violent disturbance and low near-wall velocity above cavity surface will promote both convective and boiling heat transfer.According to the response surface,heat flux increases firstly and then decreases with increasing cavity spacing along the flow direction or cavity diameter.It also increases with the increasing cavity depth or decreasing cavity spacing normal to the flow direction.The optimal cylindrical cavity surface was obtained with the ASA algorithm,which improves heat flux by 24%compared with original surface.(4)The cylindrical pillar shows the best heat transfer capability,which increases the heat flux up to 26.7%.The widest low velocity regions around pillars and the lowest near-wall velocity effectively promote bubble nucleation and growth to enhance boiling.For cavity morphology,the cylindrical cavity shows better boiling heat transfer capability due to its lower near-wall velocity and narrower regions in cavity.The hemispherical cavity has greater convective heat transfer capability than truncated-cone cavity for its stronger disturbance resulted from the vortex,but its boiling capability is weaker because the smoother surface and higher velocity in the cavity will suppress bubble nucleation.