竖直微通道表面活性剂水溶液流动沸腾换热特性的数值模拟

发布时间：2018-01-16 00:06

本文关键词：竖直微通道表面活性剂水溶液流动沸腾换热特性的数值模拟　出处：《江苏大学》2016年硕士论文　论文类型：学位论文

【摘要】：流动沸腾换热以其良好的换热性能受到研究者们的广泛关注,但用于集成化、微型化的电子器件冷却领域时,由于尺度效应,存在诸多异于常规尺度通道的难题。在水中添加少量表面活性剂,通过改变水溶液的界面润湿性能、表面张力、溶液物化特性等改善流动沸腾的换热行为,强化沸腾换热。因此,研究微通道内表面活性剂水溶液的流动沸腾换热问题,有助于开发沸腾传热强化新技术,实现小空间高热流热量的有效转移,满足电子技术的集成化、微型化发展的实际需求。本文利用计算流体力学软件Fluent对竖直矩形微通道内Alkyl(8-16)Glycoside非离子表面活性剂水溶液的流动沸腾换热特性进行数值研究。基于汽液两相流的VOF模型,通过用户自定义函数对模型进行补充,构建微通道内工质流动沸腾换热模型。重点研究水溶液和纯水在沸腾换热特性,特别是汽泡行为(汽化核心的数量、沸腾换热过程中的汽泡大小、脱离情况)上的差异。主要工作和结论如下:(1)对比分析表面活性剂水溶液与纯水在ONB点、过冷沸腾阶段的换热特性差异。结果发现,表面活性剂水溶液更易进入核态沸腾,ONB点时刻汽泡成核数较多,Tin=368K、v=0.5m/s及qw=200kW/m2的条件下,0.2mm×20mm竖直矩形微通道内,300ppm的水溶液ONB点比纯水出现时间早3.8ms;以距离入口10~15mm区域为例,加热壁面成核数量是纯水的2倍。水溶液过冷沸腾时间周期较长,加热壁面温度及出口汽相体积份额变化较大,距离入口10~15mm区域内,汽泡直径较小,脱离直径也较小,汽泡总数大于纯水。同时,微通道内表面活性剂水溶液所需过热度较小,加热壁面邻近区域温度较低,可以保证加热壁面在较低温度下实现热量转移,有利于器件的可靠运行。(2)考察浓度对表面活性剂水溶液在ONB点、过冷沸腾阶段换热特性的影响规律。相同流速条件下,100ppm~300ppm浓度范围内,随浓度的增大,表面活性剂水溶液ONB点出现时刻相同。但在v=0.3~0.7m/s范围内,随水溶液浓度的增加,加热壁面在ONB点的成核数量增多。浓度对不同工况下表面活性剂水溶液的过冷沸腾周期影响不同,但总体来说,随浓度的增大,微通道内汽泡数量增加,小直径汽泡所占比例增大。浓度对过冷沸腾加热壁面过热度的影响不大。(3)比较表面活性剂水溶液与纯水在饱和沸腾阶段的流动沸腾换热效果,研究浓度对水溶液饱和沸腾换热效果的影响。结果发现,表面活性剂水溶液优于纯水,Tin=368K、v=0.5m/s及qw=200kW/m2的条件下,距离微通道入口15~20mm区域,300ppm的表面活性剂水溶液对应的加热壁面Nu数小范围波动,纯水对应的加热壁面Nu数则出现突降,波动范围增大。但同一流速条件下,沿流动方向,水溶液加热壁面Nu均值随浓度的增加而增大,表明增大水溶液浓度,换热效果增强。(4)比较表面活性剂水溶液与纯水在饱和沸腾阶段的流动沸腾换热可靠性,探究浓度对水溶液饱和沸腾换热可靠性的影响。表面活性剂水溶液更易维持在稳定的饱和沸腾状态,换热较可靠,Tin=368K、v=0.7m/s及qw=200kW/m2的条件下,饱和沸腾时,300ppm的水溶液在微通道内的过热程度较轻;纯水在微通道内的局部过热温度是水溶液的1.5倍,水溶液不易出现加热壁面的严重过热现象。但水溶液浓度对微通道内的流动沸腾换热可靠性的影响较为复杂,有待进一步探索。
[Abstract]:Flow boiling heat transfer with good heat transfer performance has attracted extensive attention of researchers, but for integrated, miniaturized electronic device cooling field, because of scale effect, there are many problems different from conventional scale channels. Adding a small amount of surfactant in water, wetting properties, through changing the water solution the surface tension, the thermal behavior in solution properties to improve flow boiling, enhanced boiling heat transfer. Therefore, the problem of boiling heat transfer in microchannels surface active agent water solution flow boiling heat transfer enhancement contribute to the development of new technology, to realize the effective transfer of small space of high heat flux heat, meet the integration electronic technology, the actual needs of miniaturization. In this paper, using CFD software Fluent on the vertical rectangular micro channel Alkyl (8-16) on boiling heat transfer characteristics of Glycoside nonionic surfactant solution flow Numerical study of gas-liquid two-phase flow VOF model. Based on the model through the user-defined function, construction of micro channel of refrigerant flow boiling heat transfer model. Focus on water solution and pure water in the boiling heat transfer characteristics, especially the bubble behavior (number of nucleation sites of boiling heat transfer in the process of steam bubble size, detachment) of the differences. The main work and conclusions are as follows: (1) comparative analysis of water solution and pure water surface active agent in ONB, subcooled boiling heat transfer characteristics of phase difference. The results showed that the surfactant aqueous solution more easily into the nucleate boiling point ONB bubble nucleation the number of Tin=368K, v=0.5m/s and qw=200kW/m2 under the condition of 0.2mm * 20MM vertical rectangular microchannels, ONB aqueous solution of 300ppm appeared earlier than 3.8ms in pure water; from the entrance of 10~15mm area as an example, the heating wall nucleation number is 2 times of water soluble in water. The longer the time period of boiling liquid cooling, heating wall temperature and outlet vapor volume fraction varies greatly, from the entrance in the 10~15mm region, the bubble diameter, bubble detachment diameter is smaller, the total number of more than pure water. At the same time, the micro channel in the surfactant solution of the overheating is small and the heating wall adjacent area the temperature is low, can ensure the heating wall under the low temperature heat transfer, a reliable operation for devices. (2) the effects of concentration of surfactant in aqueous solution at ONB, subcooled boiling phase influence heat transfer characteristics. The same flow conditions, the concentration range of 100ppm~300ppm, with the concentration increase of surfactant in aqueous solution ONB time. But in the range of v=0.3~0.7m/s, with the increase of the concentration of the aqueous solution, the heating wall increased in the nucleus as the number of ONB. Concentration of aqueous solution of surfactants under different conditions Subcooled boiling cycle is different, but overall, with the increase of concentration, increase in the number of micro channel bubble, bubble diameter small proportion increases. The concentration has little effect on subcooled boiling heating surface superheat. (3) comparison of flow boiling heat transfer effect of surfactant in aqueous solution and in water the saturated boiling stage, concentration of aqueous solution of saturated boiling heat transfer effect. Results show that the surfactant solution is better than that of pure water, Tin=368K, v=0.5m/s and qw=200kW/m2, from the micro channel entrance area of 15~20mm, the heating wall Nu 300ppm surfactant aqueous solution corresponds to the number of small range pure water, heating wall surface corresponding to the Nu number is suddenly increased. But the fluctuation range of the same flow conditions, along the direction of flow of water solution heating wall mean Nu increased, showed that increasing the concentration of aqueous solution, the effect of heat transfer Enhanced. (4) comparison of surfactant in aqueous solution and pure water in the saturated boiling heat transfer reliability stage flow into a saturated aqueous solution concentration on the boiling heat transfer. The reliability of the surfactant solution is more likely to maintain a stable state in the saturated boiling heat transfer, more reliable, Tin=368K, v=0.7m/s and qw=200kW/m2 conditions under saturated boiling, heating of 300ppm water solution in the microchannel is light; the water in the micro channel of the local overheating temperature is 1.5 times of water solution, water solution is not easy to appear the heated wall seriously overheating phenomenon. But the water soluble liquid concentration on the micro channel in the boiling heat transfer reliability the more complex and needs further exploration.

【学位授予单位】：江苏大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：TK124

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