Research Of Cooling Heat Transfer Characteristic And Mechanism Of Jet Impingement

For the steel plate,the cooling rate of steel plate in thickness direction is uneven,and the residual stress inside the steel plate will cause the shape problem,and finally influencing the product’s performance and good cooling uniformity.The advanced cooling system after plate rolling with jet impingement has a higher cooling rate.Aiming at the heat transfer mechanism of slit jet cooling process in medium slab,this paper combined with National Natural Science Foundation "No.51404058",as well as the Foundamental Research Funds for the Central Universities "No.150704005".The main research contents and results are as follows:(1)Relative static conditions of steel plate,an experimental study and numerical analysis of impinging heat transfer of a single slit jet were carried out.Combining the fluid characteristics such as velocity distribution and heat transfer data,the cooling characteristics of a single slit jet,especially the wetting state and boiling bubbles in the forward and reverse regions outside the impact zone,were studied.The influence of nozzle inclination and slit width on heat transfer were studied.The distribution of flow field and pressure and the relationship between heat transfer area distribution and fluid distribution were obtained when slit jet impinged on high temperature steel plate.The result in this part is helpful for the further understanding of heat transfer efficiency.(2)Relative static conditions of steel plate,the heat transfer characteristics in thickness direction of steel plate were experimentally studied according to the process parameters such as cooling temperature and flow rate.The basic heat transfer parameters such as heat transfer coefficient,maximum heat flux’(MHF)and time to reach maximum heat flux(tMHF)are obtained.The heat transfer capacity under different conditions is characterized by heat flux,which provides theoretical support for the ultra-fast cooling process.(3)Relative motion of steel plate and jet nozzle,the inclination angles and velocities,the rule of heat transfer of single slit jet impingement were studied.The heat transfer parameters such as jet structure distribution,heat transfer zone distribution,heat flux,width of transition boiling zone and transition boiling time were obtained.The uniformity,synchronization and the speed of entering the nucleate boiling under different experimental conditions were characterized by fluctuation of maximum heat flux,shape of wet front and width of transition boiling zone,respectively.The smaller the width of transition boiling zone is,the faster the nucleate boiling is.The width of transition boiling zone increases with the decrease of inclination angle and the increase of velocity.The results are helpful for the optimization of mathematical model and cooling system.(4)In the moving state,the heat transfer characteristic of multi-beam slit jet impingement under different cooling modes,nozzle spacing and moving speed were studied.The experimental data of heat flux on steel plate surface under different experimental conditions and fluctuation of maximum heat flux were obtained.The maximum heat flux fluctuation on steel plate surface was the smallest when nozzle flow rate was V1>V2>V3.The uniformity of heat transfer is good.The smaller the distance between adjacent nozzles,the better the heat transfer effect.The higher the moving speed of steel plate,the better the uniformity of heat transfer on the surface of steel plate.(5)The heat transfer in the thickness direction of steel plate was studied experimentally and numerically.The heat transfer characteristics of the steel plate under the parameters of cooling temperature,flow rate,nozzle spacing and plate thickness were studied.The heat transfer capability of the jet impingement in the thickness direction were characterized by the temperature drop curves,temperature drop rate and core-surface temperature difference at the surface,1/4H and center of the steel plate.With the increase of cooling temperature and flow rate,the decrease of the distance between nozzles and the movement speed lead to the increase of temperature drop rate and the increase of heart-surface temperature difference.