Investigation Of The Impacts Of Surface Characteristics On Boiling Heat Transfer Process

The rapid development of electronic devices has put forward demanding requirements for cooling capacity of the system in recent years.How to increase the heat transfer capacity and improve heat transfer efficiency with a smaller heat transfer area and greater integration has become a hot issue in present.Compared to the traditional cooling methods,such as the air-cooling and water-cooling,boiling heat transfer process has higher heat transfer efficiency.Ideally,the value of heat flux for nucleate pool boiling can be 10~6W/m~2,while using water as the working fluid.Therefore,it is significant to investigate the boiling heat transfer process for theories and practices.Boiling heat transfer performance can be affected by plenty of factors,such as the heated surface characteristics,fluid properties,heating conditions and so on.Thus,when the boiling heat transfer process is investigated by experimental method,factors are often coupled with each other and it is difficult to explore a certain one.In order to solve the problems above,the heated surface characteristics which affect the boiling heat transfer performance are investigated by numerical simulation method in present work.The physical model which used in present work is a square vessel.Nucleate pool boiling process including the critical heat flux(CHF)is investigated by manufacturing heated surfaces with some modified structures(i.e.,heated surfaces with uniformly arranged hemispheres which have different quantities and orientations),transforming the wettability and roughness of the heated surfaces.It's remarkable that the hydrophilic smooth surface(?(28)60?,r=0)is firstly calculated by numerical method in present work and it is regarded as the baseline case.All the calculations of the modified heated surfaces are compared with the results above.In the calculation,heated surface is applied with a constant heat flux,and adiabatic conditions are applied to the lateral walls of the vessel,while the operating pressure is1atm.Then,the correlation between the wall superheat and heat flux is obtained.Particularly,the rapid increase of the wall temperature indicate the occurrence of boiling crisis.The results show that a huge similarity exists between the results which are obtained in present work and the data in the literature.Thus,the reliability of the method is proved.According to the calculation,it is found that the heated surface temperature fluctuates within a narrow range during the nucleate pool boiling process.Compared to the baseline case(?(28)60?,r=0),it is found that heated surfaces with hemispheres have a better heat transfer performance,especially the surface with downward facing hemispheres.Furthermore,the quantity of the hemispheres has significant impacts on the heat transfer process.When the heated surface possesses 16hemispheres,the heat transfer rate and the value of critical heat flux(CHF)are both improved.The hydrophilic surface has a higher value of critical heat flux(CHF),and the hydrophobic surface has a better boiling heat transfer performance.Particularly,the boiling heat transfer rate and critical heat flux(CHF)can be both improved with the superhydrophilic surface(?(28)3?).Roughness surfaces adopted in present work could improve the boiling heat transfer performance.Compared to the baseline case,the value of critical heat flux(CHF)increases by about 1.83 times,using the heated surface with a roughness height of 1nm.This is because that the roughness surface can provide more heat transfer area and increase the number of nucleation sites,which is beneficial to the bubble coalescence and fluid disturbance.In addition,the appearance of the wall roughness can prevent the premature formation of the vapor film,then the higher critical heat flux is realized.