Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure
Bubble growth on a smooth metallic surface at atmospheric and sub-atmospheric pressure
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大气压和负压下光滑金属表面上的气泡生长
DOI：
10.1016/j.ijheatmasstransfer.2023.124103
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发表时间：
2023
影响因子：
5.2
通讯作者：
Mahmoud M
中科院分区：
文献类型：
--
作者：
Mahmoud M
气泡生长速率是建立精确的机理核态沸腾换热模型所需的重要参数之一。这对于理解流体动力和气泡脱离的机理也是非常重要的。本文介绍了常压和亚大气压下去离子水在纯铜表面饱和池沸腾过程中气泡生长的实验研究。这些测量是使用带有显微镜镜头的高速、高分辨率相机进行的。讨论了气泡生长的机理，并利用测得的气泡生长曲线对微层蒸发机理进行了评价和讨论。微层蒸发对单个气泡生长的贡献约为70%，而表面潜热传递(蒸发)对总传热率的贡献约为30%。其余70%是由其他机制造成的，即传导和对流。这些数值仅基于对气泡生长曲线的分析，并与使用集成传感器或红外热像仪进行局部传热测量的一些研究人员的结果一致。这些详细的测量技术不能用于当前研究中测试的厚铜块，这也得到了许多研究人员在文献中的测试，是工业使用表面的代表。研究还发现，常压下气泡的脱离机制是由于表面张力和浮力之间的静态平衡，而在亚大气压下，气泡的脱离机制是浮力和液体惯性力之间的平衡。压力对动态接触角的特性没有显著影响，动态接触角也是通过气泡的瞬时图像测量的。并得出结论：当两种力相等时，应进行力平衡，才能准确预测偏离直径，此时发生的时间小于偏离时间，动态接触角约为45。在大多数气泡离开模型中，研究人员建议在气泡形成接触角为900的颈部时进行平衡(低估了表面张力)。对一种常用的均相生长模型的分析表明，要使均相气泡生长模型适用于核沸腾，必须考虑到在气泡生长过程中过热度随时间变化这一事实。
Bubble growth rate is one of the most important parameters required for the development of accurate mechanistic nucleate boiling heat transfer models. It is also very important for understanding the hydrodynamic forces and the mechanism of bubble departure. This paper presents an experimental study on bubble growth measurements in saturated pool boiling of deionized water on a plain copper surface at atmospheric and sub-atmospheric pressure. The measurements were conducted using a high-speed, high-resolution camera with a microscopic lens. The mechanisms of bubble growth are discussed, while the microlayer evaporation mechanism has been evaluated and discussed using the measured bubble growth curve. The estimated contribution of microlayer evaporation to a single bubble growth is about 70 %, while the contribution of latent heat transfer (evaporation) to the total heat transfer rate from the surface is about 30 %. The remaining 70 % is due to other mechanisms, i.e. conduction and convection. These values were obtained based on the analysis of the bubble growth curve only and agreed with some researchers who conducted local heat transfer measurements using integrated sensors or infrared thermography. These detailed measurement techniques cannot be used with the thick copper block tested in the current study, which was also tested by many researchers in literature and is representative of industrially used surfaces. It was also found that the bubble departure mechanism at atmospheric pressure is due to a static balance between surface tension and buoyancy forces while at sub-atmospheric pressure, it was between buoyancy and liquid inertia forces. The pressure did not have a significant effect on the characteristics of the dynamic contact angle, which was also measured from the instantaneous images of the bubble. It was concluded also that the force balance required for the accurate prediction of departure diameter should be conducted when the two forces are equal, which occurred at time less than the departure time and dynamic contact angle of about 45. In most bubble departure models, researchers recommended the balance to be conducted at the moment of departure when the bubble forms a neck with contact angle of 900(underestimation to the surface tension force). The analysis of one of the commonly used homogeneous growth models indicated that for homogeneous bubble growth models to be applicable in nucleate boiling, an allowance must be made for the fact that the degree of superheat varies with time during a bubble growth period.