Numerical Simulation Of Boiling Heat Transfer On Microstructure Surface

With the rapid miniaturization and high-density integration of electronic components and the accompanying increase in power density,conventional air cooling methods or single-phase liquid cooling technologies for heat dissipation of electronic components cannot meet the heat dissipation requirements of smallsized devices.It is well known that the heat transfer with phase change is much better than the heat transfer without phase change.Boiling heat transfer has been widely studied in recent decades as one of the most effective phase change heat transfer.The four kinds of boiling mechanisms of fully developed nucleateboiling,transition boiling and film boiling have different heat transfer principles.partial nucleate boiling,.The physical mechanism of single nucleate boiling is very complicated,because it involves the nucleation,growth,aggregation,and departure of bubbles.The process has different effects on heat transfer in different processes.By analyzing the heat transfer mechanism of each process,theoretical guidance for enhancing heat transfer in the future production process.This paper focuses on the experiment of micro-heater heating the surface which was used as the simulation object.The formation process and the movement law of the bubble on the heat transfer surface structure were studied by numerical simulation.The simulation process achieves the complete process from the nucleation of bubble growth.This numerical simulation investigates the whole process of bubble formation in the boiling process of single cavity and multiple cavities,and shows the influence of external factors on heat transfer.The following results were obtained by numerical simulation:(1)Based on the heat flux history,the bubble life cyclecan be categorized into the following periods: nucleation,growth,detachment,and departure.The maximum heat transfer coefficient at the initial stage of bubble nucleus formation.As the bubble grows,the heat transfer coefficient gradually decreases.The bubble grows up to a certain volume and leaves the heating surface,at this time,the heat transfer coefficient reaches a maximum value.When the bubbles exiting the heating surface gradually grow,the heat transfer coefficient will decrease.The minimum heat transfer coefficient will also be greater than the minimum value of the heat transfer coefficient during bubble formation.When the bubble grows to a certain volume,the bubble will completely escape from the heating cavity,and the heat transfer coefficient at this time reaches a maximum again.After that,new bubbles are generated at this point and the above process is repeated.Processes of multiple bubbles growth as the same as that of a single bubble,and there is a merger of bubbles during the growth of multiple bubbles.When the bubbles are combined,the heat transfer coefficient reaches a maximum value within a short period of time;after the merger,the bubbles gradually grow up,and the heat transfer coefficient gradually decreases during the growth;the grown bubbles are completely combined into one bubble at a certain moment.(2)Effect of superheat on heat transfer.Within a certain range,as the superheat increases,the heat transfer coefficient increases,and the frequency of bubble detachment also increases.In this range,the greater the degree of superheat,the better the heat transfer effect.After the critical value of this superheat degree is exceeded,the greater the degree of superheat,the higher the frequency of escape,and the increase in the heat transfer coefficient.Instead,there is a tendency to decrease.Finding the critical value of superheat degree has guiding significance for enhancing heat transfer in practice.The effect of contact angle on heat transfer is that the greater the contact angle,the greater the value of the heat transfer coefficient and the frequency of bubble detachment.the greater the contact angle,the better the heat transfer.(3)Change the structural parameters of the cavity such as diameter,spacing,height and other boiling process,and found that when the cavity width is 0.08 mm,the bubble leaving frequency is high,and the boiling heat transfer coefficient is good;when the cavity height(cavity width is If the setting is 0.08 mm,the boiling heat transfer effect is also good.In the process close to the optimal structure,the heat transfer effect is getting better and better.In the actual process,it is also a great advantage of numerical calculation that the heat transfer coefficient and the detachment frequency of various structures can be calculated by numerical modeling,and the optimal solution can be selected and applied in practice.(4)Comparing the boiling heat transfer process of the cylindrical cavity,Vshaped cavity and modified V-shaped cavity,it was found that the cylindrical cavity has less heat transfer effect than the other holes,and the improved V-shaped cavity has the best The heat transfer effect.In addition,by changing the contact angle,it is found that the larger the contact angle,the larger the value of the heat transfer coefficient,and the larger the frequency of escape of bubbles,that is,the larger the contact angle,the better the heat transfer effect.