| Ever since the establishment of classical pool boiling curve that was proposed by Nukiyama in the1930s,the boiling heat transfer(BHT)has attracted considerable attention not only from academia but also from industry such as thermal nuclear power reactors,microelectronic device,radars and avionics.Compared to the heat conduction and heat convection,the BHT has a higher heat transfer coefficient(HTC)due to the large latent heat transfer in the process of changing phase and mass exchange from liquid to vapor with a small increment in the temperature.Therefore,the research on BHT enhancement has become a hot topic.It is found that passive enhancement of pool boiling using surface modification techniques is a common way as it does not require external power assistance considering economics and equipment operation stability,but the effect of geometric variables on the BHT has been rarely investigated,and limited to the analysis of heat transfer characteristics without corresponding mechanism.In this paper,visualization-based nucleate pool boiling experiments are carried out on 16 groups of square micropillar array surfaces(SMAS)and plain surface using deionized water as working fluid at atmospheric pressure,the quantitative measurements of the bubble dynamics are obtained using a high-speed camera at different working conditions.At the same time,the computational fluid dynamics(CFD)method is used to compare and analyze the experimental behavior mechanism and morphological characteristics of bubbles.The experimental results show that the boiling curve on the plain surface is consistent with the result of the Rohsenow correlation.The bubble departure diameter and frequency under different heat flux have the same trend as the Cole correlation.The nucleation site density approach to the Benjamin correlation.The obtained results show that the SMAS,with square micropillar in the range of 0.2-0.8 mm height and width,and the spacing between the micropillar is equal to the micropillar width,has a considerably BHT characteristics,which enhanced the HTC by 32-203%compared to the plain surface.Generally,the BHT performance intensifies with the increase in the square micropillar height or width when the heat flux is less than 500 k W/m~2,but the tendency reverses at high heat flux levels.Combining the high-speed visualization and numerical simulation results reveals that the higher micropillar has larger bubble departure diameter and faster frequency at low heat flux,because higher micropillar has larger heat transfer area and conducive to induce capillary flow and aids in separating the paths of departing bubble and replenishment liquid,while the distribution of nucleation site density is independent of micropillar height.However,the larger area of bubble coverage brought by the increase of nucleation site density and bubble diameter at high heat flux restrains bubble departure with higher micropillar.The effect of micropillar width change on the BHT performance is similar to that of the height change,the main mechanism for wider micropillar at low heat flux is the larger bubble departure diameter and for narrow micropillar at high heat flux is the faster bubble departure frequency.Furthermore,in order to elaborate the effect of geometric parameter of SMAS on BHT performance,the dimensionless correlation of different sizes of SMAS in pool boiling were fitted based on the Rohsenow model,and the constantwhich is related to the combination of heating surface and working medium,and the constant8)which is the index of bubble Reynolds number were modified.As a result,the optimization of different size design guideline for SMAS is presented. |
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