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Microscale study of nucleation process in boiling of low-surface-tension liquids.

机译：低表面张力液体沸腾过程中成核过程的微观研究。

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摘要

A novel MEMS device has been developed to study some of the fundamental issues surrounding the physics of the nucleation process intrinsic to boiling heat transfer. The study was focused on boiling of FC-72 liquid.; Over the past 50 years, scientists have developed several competing mechanistic models to predict the boiling heat transfer coefficient. Although the developed models are intended to predict the heat transfer coefficient at macroscales, their fundamental assumptions lie on complex microscale sub-processes that remain to be experimentally verified. Two main unresolved issues regarding these sub-processes are: (1) bubble growth dynamics and the relative importance of different mechanisms of heat transfer into the bubble and (2) vapor/liquid/surface thermal interactions and the bubble's role in heat transfer enhancement during the nucleation process.; The developed device generates bubbles from an artificial nucleation site centered within a radially distributed temperature sensor array (with 22--40 mum spatial resolution) while the surface temperature data and images of the bubbles are recorded. The temperature data enabled numerical calculation of the surface heat flux. Using the test results, the microlayer contribution to the bubble growth was determined to increase from 11.6% to 22% when surface temperature was increased from 80°C to 97°C. It was determined that the transient conduction process occurs predominantly at the bubble/surface contact area, and before the bubble departure, contrary to what has been commonly assumed in classical boiling models. For the first time, the convection heat transfer outside the contact area (often known as microconvection) and transient conduction within the contact area were differentiated. The microconvection heat flux was found to be relatively close to that of the equivalent natural convection produced by the same geometry, but becomes significantly stronger than natural convection at higher surface temperatures.; Test results under saturation conditions showed that when surface temperature is increased from 80°C to 97°C, the contribution of the different mechanisms of heat transfer within a circular area of diameter equal to that of the bubble changes from: (1) 28.8% to 16.3% for microlayer, (2) 45.3% to 32.1% for transient conduction, and (3) 25.8% to 51.6% for microconvection.
机译：已经开发出一种新颖的MEMS器件来研究围绕沸腾传热固有的成核过程的物理原理的一些基本问题。该研究集中于FC-72液体的沸腾。在过去的50年中，科学家们开发了几种相互竞争的力学模型来预测沸腾传热系数。尽管开发的模型旨在预测宏观尺度上的传热系数，但它们的基本假设基于复杂的微观尺度子过程，仍有待实验验证。与这些子过程有关的两个主要未解决问题是：（1）气泡的生长动力学以及热传递到气泡中的不同机制的相对重要性，（2）汽/液/表面热相互作用以及气泡在热交换过程中对热传递的促进作用成核过程。所开发的设备从中心在径向分布的温度传感器阵列（空间分辨率为22--40微米）内的人工成核位置生成气泡，同时记录表面温度数据和气泡图像。温度数据可以对表面热通量进行数值计算。使用测试结果，当表面温度从80°C升高到97°C时，微层对气泡增长的贡献被确定为从11.6％增加到22％。已确定，瞬态传导过程主要发生在气泡/表面接触区域以及气泡离开之前，这与传统沸腾模型通常假定的相反。首次区分了接触区域外的对流传热（通常称为微对流）和接触区域内的瞬态传导。发现微对流热通量与由相同几何形状产生的等效自然对流的热通量相​​对接近，但在更高的表面温度下变得明显强于自然对流。饱和条件下的测试结果表明，当表面温度从80°C升高到97°C时，在与气泡相等的圆形区域内，不同传热机制的贡献从以下方面变化：（1）28.8％微层为16.3％，瞬态传导为（2）45.3％至32.1％，微对流为（3）25.8％至51.6％。

著录项

  • 作者

    Moghaddam, Saeed.;

  • 作者单位

    University of Maryland, College Park.;

  • 授予单位 University of Maryland, College Park.;
  • 学科 Engineering Mechanical.
  • 学位 Ph.D.
  • 年度 2006
  • 页码 217 p.
  • 总页数 217
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 机械、仪表工业;
  • 关键词

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