池沸腾孤立气泡生长过程中微液层蒸发影响的实验和模拟耦合分析

doi:10.11949/0438-1157.20201396

摘要/Abstract

摘要：

微液层蒸发是沸腾过程中重要的换热机理。本文旨在通过单个气泡池沸腾实验中测得的气泡动态参数探究孤立气泡生长过程中加热表面的换热机理。首先通过沸腾池和加热表面的严格设计实现了单个气泡沸腾。进一步通过对孤立气泡生长时序图像的处理，得到了气泡在一个生长周期内气泡直径、纵横比以及气泡根部基圆半径的变化。对比发现，气泡生长速率与气泡根部基圆半径随时间的变化呈现显著正相关，而与大液层区域的变化相关程度较低，这表明微液层蒸发直接影响气泡体积变化，在孤立气泡沸腾过程中起主导作用。在此基础上进一步建立了加热表面换热过程的数值模型，基于实验中测得的气泡动态参数对气泡底层的微液层厚度进行了预测；通过多次迭代计算并匹配气泡生长速率和加热棒的温度发现，当表面过热度为4.82 K时，气泡底层微液层厚度约为3.43 μm，与相关文献中的微液层厚度测量值基本一致，进一步证实了微液层蒸发在孤立气泡沸腾换热过程中的重要性。本研究揭示了孤立气泡池沸腾过程中近壁面处的换热机制，为进一步的孤立气泡沸腾传热过程数值模拟奠定了理论基础。

关键词: 相变, 蒸发, 气泡, 气泡生长速率, 基圆半径变化, 微液层厚度预测

Abstract:

The evaporation of the micro-liquid layer is an important heat exchange mechanism in the boiling process. This paper aims to investigate the heat transfer mechanism in boiling surface with an isolated bubble via the bubble dynamics observed in the boiling experiment. The single-bubble pool boiling was realized and the sequential images of the growth of the single bubble under different input heat fluxes were obtained first. The bubble sizes and radii of the root adhesive to the heating surface during the bubble period were further measured. Through the comparison between the measured bubble growth rate and the variation of bubble root radius, it is found that, the bubble growth rate is remarkably correlated with the variation of bubble root radius, whereas its correlation with the evolution of the macro-liquid layer region is relatively lower. This indicates that it is the evaporation of the micro-liquid layer underneath the growing bubble, rather than the macro-liquid layer at the vicinity of the bubble that plays a dominant role in the phase change process of single-bubble boiling. A numerical model of boiling heat transfer was established by matching the observed variation of bubble growth rates and measured temperatures of discrete points on the basis of micro-liquid layer evaporation. The heated surface was divided into three parts with different heat transfer mechanisms: dry area, micro-liquid layer region and natural convection region. Through multiple iterative computations for matching the measured data of bubble growth and the temperature underneath the heating surface, the micro-liquid layer thickness can be predicted as 3.43 μm when the surface superheat was 4.82 K. The predicted micro-liquid layer thickness is in a favorable agreement with the available references, which further confirms the dominant role of micro-liquid layer evaporation during the phase change of isolated-bubble boiling. This work provides theoretical basis for the future numerical simulation of isolated-bubble boiling heat transfer.

Key words: phase change, evaporation, bubble, bubble growth rate, evolution of bubble root radius, prediction of micro-liquid layer thickness

中图分类号:

TK 124

潘丰, 王超杰, 母立众, 贺缨. 池沸腾孤立气泡生长过程中微液层蒸发影响的实验和模拟耦合分析[J]. 化工学报, 2021, 72(5): 2514-2527.

PAN Feng, WANG Chaojie, MU Lizhong, HE Ying. Analysis of the influence of microlayer evaporation on single-bubble pool boiling by coupling the experimental observations and numerical simulations[J]. CIESC Journal, 2021, 72(5): 2514-2527.

图/表 14

图1 气泡底部微液层与大液层区域示意图

Fig.1 Schematic diagram of the microlayer and macrolayer underneath a vapor bubble

图2 单气泡池沸腾实验装置

Fig.2 Schematic diagram of the experimental apparatus for single bubble boiling system

图3 气泡轮廓提取流程以及所提取到的气泡轮廓与原始图像的对比

Fig.3 The extracting procedure of bubble outlines and the comparison of the obtained bubble outlines with the original images

图4 基于微元法的气泡体积计算示意图

Fig.4 The measurement of the bubble volume from the bubble growing images

图5 气泡生长过程中不同阶段的气泡形状及底部基圆半径的变化

Fig.5 The variation of vapor-liquid interface and bubble root during the bubble growing period

图6 气泡生长过程中气泡高度、直径、体积、接触角以及浮力和表面张力随时间的变化(ΔT = 4.82 K)

Fig.6 The variations of bubble height, diameter, volume, contact angle, buoyancy and surface tension with time during the bubble growth period

图7 实验中测得的气泡生长速率、气泡底部基圆半径和大液层区域随时间的变化

Fig.7 Comparison of the bubble growth rates with the bubble root radii and macrolayer regions measured in the experiment

图8 计算区域和沸腾换热机理示意图

Fig.8 Schematics of the calculated domain and the heat transfer mechanisms on the boiling surface

图9 加热表面传热计算与实验测量参数间的迭代匹配策略

Fig.9 The matching strategy between the simulation of boiling surface heat transfer with the measured bubble dynamics

图10 不同网格数量下热电偶处的过热度变化

Fig.10 The local superheat at the position of thermal couple TTC in simulations with different mesh sizes

图11 气泡底层基圆半径、干区和微液层区域半径变化的模拟结果

Fig.11 The simulated variations of bubble root, dry area and microlayer region radii with time

图12 气泡生长速率的模拟结果和实验结果的对比

Fig.12 The comparison of the simulated and experimental bubble growth rate

图13 加热表面及加热表面以下2.5 mm处过热度随时间变化的模拟结果(a)和一个周期内的加热表面过热度随时间的变化(b)

Fig.13 The simulated variations of the mean superheat on the heated surface and the local superheat at the position of 2.5 mm below the surface (a) and the mean superheat of heated surface within one bubble period (b)

图14 气泡生长过程中不同时刻加热表面过热度分布的模拟结果及气泡界面位置的实验结果

Fig.14 The superheat distributions of the heated surface in the simulation and the positions of bubble interface of the experiment at different moments

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