Experimental Study On The Heat Transfer Characteristics Of Multi-jet Impingement On Pin-fin Structured Surfaces

With the ever-increasing heat flux in a variety of electric and electronic devices,traditional air-cooling technologies cannot satisfy with heat dissipation demand anymore.Due to its excellent heat transfer performance and the adaptability to deal with large heat transfer area,jet impingement cooling has become one of the most promising pumped liquid cooling technologies.In this thesis,a mechanically pumped closed-loop liquid cooling experimental system was set up to study single and two-phase heat transfer of jet impingement cooling.For a 10 mm×20 mm target cooling surface,multi-jet visual test sections with one-side outlet or effusion holes were designed and fabricated respectively and the heat transfer characteristics of smooth surface and pin-fin structures were tested under different conditions.With the help of high speed CCD,the impingement boiling heat transfer mechanisms at different flow rate and jet direction were also investigated.For a practical 40 mm×40 mm target cooling surface,a multijet evaporator with one-side outlet was designed and optimized.In addition,the thermal performance of the evaporator under different possible working conditions such as variable flow rate,variable heat flux,variable placement directions,as well as the combination of two evaporators in series were studied.The following results and conclusions were obtained :(1)With the increasing of flow rate,the single phase heat transfer coefficient increases,while the boiling heat transfer coefficient may decrease because of the decreasing degree of phase change,and the ONB and CHF are also delayed;(2)For the multi-jet with effusion holes,areas under effusion holes have the weakest heat transfer performance,where nucleation is generated and bubbles are accumulated with the increasing heat flux;while for the multijet with one-side outlet,the heat transfer performance increases along the crossflow direction,indicating the dominant role of crossflow,and the nucleation will be first trigged in the upstream areas,where crossflow is much slighter and the thermal transfer performance is dominated by jet impingement;(3)The effusion holes type can increase the CHF,and its heat transfer uniformity is much better than that of one-side outlet type,and this advantage becomes more obvious for high flow rate;(4)Pin-fin surface can significantly improve the thermal performance because it not only increases the heat transfer surface area but also provides more nucleation sites;the role of effusion holes on heat transfer enhancement however attenuates because the pin-fin enhanced surface dominates the enhancement;(5)The placement direction of test section has little influence on average heat transfer coefficient in both the single-phase and nucleate boiling developing regimes;it however strongly affects the temperature uniformity of heating surface,with the better uniformity obtained in the cases of downward-jet and upward-crossflow;(6)When two multi-jet evaporators with one-side outlet are used in series,the second evaporator will boil at first and have lower heat transfer limits.