An Experimental Study Of Boiling And Condensation Co-existing Heat Transfer In A Small And Closed Space

Flat heat pipe is of good temperature leveling ability with reasonable good axial heat transfer capacity,which is mainly through boiling and condensation co-existing phase change heat transfer of working fluid to achieve heat transport in the axial,and improving radial temperature distribution uniformity.This paper mainly studies enhanced boiling and condensation co-existing phase change heat transfer in a small and closed space.As efficient as it might be,the boiling and condensation co-existing phase change heat transfer are as strong as it is expected,due to limited active nucleation sites,large bubble departure diameter and the slow fluid reflux inside a small and closed space where boiling and condensation take place simultaneously.An experimental study is made of enhancing heat transfer of boiling and condensation co-existing phase change heat transfer in a small and closed space by using a self-designed experimental table.High speed camera was used to record the experimental phenomena.Combining the experimental results and the experimental phenomena,the mechanism of enhancing boiling and condensation co-existing phase change heat transfer in a small and closed space by copper foam and thermal conduction bridge inserts was explored.And the influence of filling amounts was also investigated.The heat transfer characteristics and phenomena of boiling and condensation co-existing phase change heat transfer in a small and closed space with copper foam inserts are studied.The influences of copper foam on the heat transfer performance are analyzed.The typical images of boiling and condensation co-existing phase change phenomena are recorded by the high speed camera.The pore density of copper foam used is 20 PPI and the porosity is 0.96.The inserts used include the cut and non-cut copper foam blocks.Experimental results show that the filling ratio for obtaining the largest boiling heat transfer coefficient and that for obtaining the largest condensation heat transfer coefficient are different if the space is filled with non-cut copper foam block,the filling ratio for obtaining the largest boiling heat transfer coefficient is 35.7% and the filling ratio for obtaining the largest condensation heat transfer coefficient is 85.7%.And there is an optimum filling ratio at which both boiling and condensation heat transfer coefficient acquire their maximum value if the space is filled with the cut copper foam block and this ratio is 50%.Compared with the results of the smooth heating surface experiments,although inserting non-cut copper foam block into the space could enhance boiling heat transfer,it weakens condensation and inserting the cut copper foam block can enhance both boiling and condensation heat transfer.Through visual observation,it was found that the number of active nucleation sites increased and the diameter of bubble departure decreased.There was a phenomenon of bubble groups.The increasing number of bubble lead to the number of boiling fluid scouring condensation surface increasing and accelerating the return of condensation fluid.The heat transfer characteristics and phenomena of boiling and condensation co-existing phase change heat transfer in a small and closed space with heat-conduction bridges are studied.The influences of the cross sectional area size of heat-conduction bridge and the space between the heat-conduction bridges on the heat transfer performance are analyzed.The typical images of boiling and condensation co-existing phase change heat transfer are recorded by the high speed camera.The heat-conduction bridges include 2mm×2mm-2mm,3mm×3mm-1mm and 2mm×2mm-1mm.Compared with the results of the smooth heating surface experiments,the heat-conduction bridges enhance boiling and condensation co-existing phase change heat transfer.When the filling ratio of three type heat-conduction bridges was 50%,the boiling,condensation and overall heat transfer coefficient acquired their maximum.So 50% is its optimum filling ratio.In three type heat-conduction bridges,the boiling,condensation and overall heat transfer coefficient of the 2mm×2mm-1mm heat-conduction bridge were the largest and the heat transfer performance was best.And the heat transfer effect of the 2mm×2mm-2mm heat-conduction bridge was the weakest.So the optimum size of a single heat-conduction bridge is 2mm×2mm and the optimum distance among heat-conduction bridges is 1mm.Visualization study proved that the number of active nucleation sites is increased and the diameter of bubble departure is decreased due to the existence of heat-conduction bridges.The increased number of vapor bubbles leads to the times of boiling fluid scouring condensation surface increasing and accelerating the return of condensation fluid.