Experimental Investigation Of Pool Boiling Heat Transfer Characteristics On Microchannel With Porous Medium Composite Structure

Because of its high surface heat transfer coefficient and heat flux density,boiling heat transfer technology has important application value in the industrial production and high-tech fields.Due to its high porosity and complicated three-dimensional mesh of interworking structure,foam metal is considered to be an enhanced boiling heat transfer structure,which has great research value.In this thesis,a microchannel porous medium composite structure was innovatively proposed,a scientific and reasonable visualized boiling heat transfer experiment has been performed on this composite structure surface.The Influence law of the microchannel's geometric parameters on the enhanced boiling heat transfer characteristics of the composite structure surface had been systematically studied,the mechanism of action of heat transfer enhancement has been obtained and the visualization technology had been used to analyze the heat transfer mechanism.The specific research contents and conclusions are as follows:(1)In the range of experimental tests,the minimum boiling initial superheat of the microchannel with porous medium structure surface is only 0.5K and the maximum value is only 1.9K,the average value of boiling initial superheat is only 1.4K;the maximum heat flux and the maximum heat transfer coefficient of the microchannel with porous medium structure surface are larger than the grooveless copper foam surface,the surface with a groove depth of 0.9 mm and a width of 0.6 mm has the maximum heat flux density 3.1×10~6 W/m~2 in the composite structure surfaces when the superheat is18K,which is 1.4 times critical heat flux of the grooveless copper foam surface,it was also 6.3 times critical heat flux of the grooveless copper foam surface at the same superheat,this surface aslo has the maximum heat transfer coefficient 1.8×10~5W/(m~2·K)in the composite structure surfaces,which is 3.3 times the maximum heat transfer coefficient of the grooveless copper foam surface.The microchannel with porous medium structure surface is easier to enter the boiling heat transfer state earlier,it can achieve a higher heat flux density and heat transfer coefficient with a smaller degree of superheat,it drastically increases the critical heat flux and delays the the arrival of film boiling,and the effect of strengthening boiling heat transfer is remarkable.(2)Among the composite structure surfaces,the best boiling heat transfer performance is belong to the surface with the groove depth of 0.9mm and the groove width of 0.6mm,the worst boiling heat transfer performance is belong to the surface with the groove depth of 0.9mm and the groove width of 0.9mm,the effects of the depth and width of the microchannel on the enhanced boiling heat transfer performance is very complicated,but there is a universal law,the optimal groove width decreases with the increase of the groove depth when the enhanced boiling heat transfer performance of the composite structures is the best.By changing the depth and width of the groove,the effective heat transfer area and the volume of the foam porous structure of the composite structure will be changed accordingly,which will affect the density of the vaporization nucleus,the escape of bubbles,the replenishment of the working medium,etc..The boiling heat transfer performance of the microchannel porous media composite structure surface will be the best when the depth and width of groove are reasonable.(3)The research results of bubble dynamics:the visualization study about the bubble dynamics are consistent with the analysis of the experimental data.The onset of the nucleate boiling can be significantly advanced by the microchannel porous media composite structure surface,the vaporization cores can be increased,the departure from nucleate boiling can be postponed,the critical heat flux density can be improved;In the composite structure,the lower layer copper based microchannel is a millimeter-level channel,and the upper-layer foam porous structure has a micron-sized pore.The dual scale structure can meet the different requirements for the structural scale in different stages of boiling heat transfer,so that the boiling process is more stable and orderly.Under the condition of a lower heat flux density,the departure diameter of the bubble has increased slightly with the increase of superheat on each surfaces;Under the condition of the same superheat,the departure diameter of the bubble has increased slightly with the increase of the depth and width of the groove on each surfaces,but the change is not obvious.In the stage of a lower heat flux,when the heat flux increases,the heat exchange between the bubble and the outside environment is accelerated,the growth cycle of the bubble becomes shorter,it can be roughly considered that the bubble detachment frequency decreases with the increase of the depth and width of the groove.