【作者】 管鹏；

【导师】 贾力；

【作者基本信息】 北京交通大学 ， 载运工具运用工程， 2014， 博士

【摘要】 本文以流动沸腾在能源动力、航空航天以及核能等领域的广泛应用为背景,从流体力学和传热学的基本原理出发,通过搭建流动沸腾实验台,分析了关键因素对流动沸腾中汽泡行为和两相流型的影响,并通过建立汽泡脱离数理模型,来揭示汽泡在各种条件下的脱离特性。通过对不同尺寸的通道内,不同实验工况下的流动沸腾进行可视化实验研究,观测汽泡的各种行为,包括汽泡的聚合、胀缩和滑移等,以及整个通道中流型的分布和变化,发现环形通道中过冷流动沸腾的流型不同于常规通道,实验中观测到三种典型流型：泡状流,弥散泡状流和搅拌流,且三种流型所占据的区域长度随热流密度和质量流量的变化而变化。微小通道中的五种典型流型分别为：泡状流,弹状流,搅拌流,细环流和环状流,且通道尺寸对汽泡行为和汽液两相流型影响显著。通过对可视化实验数据的分析,得到了汽泡直径和汽泡附壁直径随汽泡生长时间的变化曲线,结果显示,汽泡由惯性力控制生长的初期阶段大约维持在0-0.3ms之间,在此阶段内,各工况下,汽泡生长速率非常接近,受实验工况的影响较小；约在0.3ms之后,汽泡进入由热扩散控制生长的后期阶段,汽泡的生长速率在此阶段随实验工况展示出明显的不同,汽泡的生长速率受加热壁面热流密度的影响较为显著,随热流密度的增大而增大。基于对汽泡脱离时汽泡附壁直径与汽泡与脱离直径的关系曲线的分析,总结了汽泡附壁直径和汽泡脱离直径的关系的关联式,根据加热壁面特性和实验工况选取适当的经验参数,该关联式能够较好的反映汽泡临近脱离时,汽泡附壁直径和汽泡脱离直径之间的关系。实验观测到同一工况、同一加热壁面、同一沸腾区域(孤立汽泡区)内的不同核化点上,汽泡脱离直径存在差异的现象,对此现象进行了分析,并通过对汽泡的受力分析,结果显示汽泡的附壁直径对汽泡脱离直径影响显著,汽泡附壁直径越大,对应的汽泡脱离直径越大。通过对不同实验工况下,汽泡脱离直径变化规律的分析,发现在其他工况一定的情况下,汽泡脱离直径随热流密度的增大而增大,随流体质量流量的增大而减小。通道尺寸的变化对汽泡脱离直径的影响显著,汽泡脱离直径随通道尺寸的减小而减小。通过对作用在汽泡上的每个作用力的详细分析,并结合汽泡附壁直径和汽泡脱离直径的关系,基于汽泡受力平衡建立了汽泡脱离直径预测模型,并将模型预测结果与本文实验数据对比,吻合较好,误差在±15%以内,与文献中的实验数据的比较,考虑到统计误差等因素,也取得了较好的一致性,数据对比误差保持在了±30%以内。更多还原

【Abstract】 Flow boiling heat transfer was widely used in the fields of energy, aerospace and nuclear energy, in this thesis, based on the fundamental principles of fluid mechanics and heat transfer, the influences of key factors on bubble behaviors and two-phase flow patterns in flow boiling were analyzed by building an experimental system, and the departure characteristics of bubble in various conditions were revealed by making the mathematical models on bubble departure.The various bubble behaviors including bubble coalescence, bubble expansion-shrink and bubble slippage, and the flow pattern as well as its variation along the channel were observed by high-speed digital camera. It was found that the flow patterns in subcooled flow boiling in annular channel were different from those in conventional channels. Three types of flow patterns, namely the isolated bubbly flow, dispersed bubble flow and churn flow, were observed, and the lengths of areas occupied by these flow patterns changed with the variation of the heat flux and mass flux. There were five types of typical flow patterns in microchannel:bubbly flows, slug flow, churn flow, wispy-annular and annular flow, and they were intensely influenced by the channel size.The variation regularities of the bubble diameter and the bubble contact diameter with the bubble growth time were obtained by analyzing the experimental data of the visualization. It was found that in0-0.3ms after bubble nucleation the bubble growth is controlled by inertia force, and after then it is controlled by heat transfer. In the inertia controlled period, the bubble growth rate in various working conditions were very closed, however, the bubble growth rate was obviously influenced by the working condition, it increased with the increase of the heat flux and decreased with the increase of the mass flux.Based on the analysis of the variation curves between the bubble contact diameter and bubble departure diameter at the bubble departure, the experimental correlation between them was established, which can accurately predict the relationship between the bubble contact diameter and bubble departure diameter by selecting proper empirical parameters for heat wall features and operating conditions.In the experiments, the bubble departure diameters differ from each other even in the same heating wall surface and same boiling region (isolated bubble region) under identical operating conditions. This phenomenon was investigated by analyzing the forces acting on the bubble, and eventually it was found that the bubble departure diameter was significantly impacted on by the bubble contact diameter, it increased with the increase of the latter.The variation regularities of the bubble departure diameter with the operating conditions were investigated, it was observed that the bubble departure diameter increased with the increase of the heat flux and decreased with the increase of the mass flow rate. On the other hand, the bubble departure diameter was obviously influenced by the channel size, it decreased with the decrease of the channel size.Based on the force analysis of bubble, integrating the relationship between bubble contact diameter and bubble departure diameter, the prediction model for the bubble departure diameter was established according to the force balance on the bubble. It was found that the model can effectively predict the experimental data in our experiments with an error of±15%, and get a better consistency by comparing the model to the experimental data from existing literatures, the comparison error was less than±30%.更多还原