【作者】 张楠；

【导师】 曹海亮；

【作者基本信息】 郑州大学 ， 化工过程机械， 2018， 硕士

【摘要】 由于具有较高的表面换热系数和热流密度,沸腾换热技术在工业生产和高新技术领域中具有重要的应用价值。而泡沫金属因其高孔隙率、复杂的三维网状互通结构,被认为是极具研究价值的强化沸腾换热结构。本文创新性地提出一种微细通道多孔介质复合结构,对其进行了科学合理的可视化沸腾换热实验,系统研究了微细槽道几何参数对其表面强化沸腾换热特性的影响规律,获得了其表面强化换热作用机理,并采用可视化技术分析了其汽泡运动行为。具体研究内容和结论如下:(1)在实验测试范围内,微细通道多孔介质复合结构表面沸腾起始过热度最小值仅为0.5K、最大值也只有1.9K,平均沸腾起始过热度仅为1.4K;各微细通道多孔介质复合结构表面最大热流密度、最大表面换热系数均大于无槽铜泡沫表面,槽深0.9mm槽宽0.6mm表面在过热度为18K时有复合结构表面中的最大热流密度3.1×10~6 W/m~2,是无槽铜泡沫表面临界热流密度的1.4倍,是无槽铜泡沫表面在相同过热度时热流密度的6.3倍,该表面有复合结构表面中的最大换热系数1.8×10~5 W/(m~2·K),是无槽铜泡沫表面最大换热系数的3.3倍。微细通道多孔介质复合结构表面更易较早进入沸腾换热状态,可在较小的过热度下达到较高的热流密度及表面换热系数,大幅提高了临界热流密度和推迟膜态沸腾,强化沸腾换热效果显著。(2)复合结构表面中,槽深0.9mm槽宽0.6mm表面沸腾换热性能最优,槽深0.9mm槽宽0.9mm表面沸腾换热性能最差,槽深、槽宽对沸腾换热强化效果的影响规律十分复杂,但存在普遍性规律,在复合结构强化沸腾换热效果最佳时,随着槽深的增加,对应的最优槽宽逐渐减小。改变槽深、槽宽,复合结构中有效换热面积、泡沫多孔结构体积随之改变,进而影响汽核密度、汽泡逸出、工质补充等情况,存在合理的槽深、槽宽,使微细通道多孔介质复合结构表面的沸腾换热性能最优。(3)汽泡动力学研究结果:对汽泡动态变化的可视化研究结果与实验数据分析结果具有一致性,微细通道多孔介质复合结构表面能够明显提前沸腾起始点,增加汽化核心,推迟沸腾危机点,提高临界热流密度;复合结构中,下层铜基微细通道为毫米级槽道,上层泡沫多孔结构为微米级孔隙,双尺度结构可满足沸腾换热不同阶段对结构尺度的不同要求,使沸腾过程更加稳定有序;在低热流密度情况下,随着过热度的增加,各表面汽泡的跃离直径均有增加;在相同过热度情况下,随着槽深、槽宽的增加,各表面汽泡的跃离直径都略有增加,但是变化不明显;在低热流密度阶段,随着热流密度增加,汽泡与外界热交换加快,生长周期变短,可近似认为汽泡脱离频率随着槽深、槽宽的增加而减小。更多还原

【Abstract】 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.更多还原