Department of Mechanical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Langmuir. 2018 Jan 23;34(3):782-788. doi: 10.1021/acs.langmuir.7b02936. Epub 2017 Oct 19.
Boiling is significantly altered by the presence of dissolved salts. In particular, salts whose solubility decreases with temperature have the tendency to crystallize and adhere to the heat transfer surface and adversely affect the thermal performance. Scaling due to the precipitation of such salts poses serious operational and safety challenges in several practical applications, including heat exchangers, pipelines, and desalination. Here, we study the effect of dissolved salts on the dynamics of pool boiling and its impact on the heat transfer coefficient and critical heat flux (CHF). We find that even undersaturated conditions can lead to crystallization and scale buildup on the boiling surface and dramatically lower heat transfer performance. For example, the CHF for a salt solution that is 75% of the saturation concentration is found to be at least 2 times lower than that for deionized water. Using simultaneous high-speed optical and infrared imaging, we determine the interdependence between crystallization-induced scale formation and bubble evolution dynamics, including bubble nucleation, growth, and departure. We find that salt crystallizes in a "coffee-ring" pattern due to evaporation at the contact line of the bubble. On the basis of the role of the microlayer and triple contact line on scale formation, we propose manipulating surface wettability as a means to avoid scale formation and the associated decrease in the heat transfer coefficient. Surfaces with hybrid wettability are demonstrated as a means to mitigate the reduction in the heat transfer coefficient and CHF in the presence of dissolved salts.
溶解盐的存在会显著改变沸腾过程。特别是，那些溶解度随温度降低而降低的盐类，有结晶并附着在传热表面的趋势，从而对热性能产生不利影响。由于这些盐类的沉淀而产生的结垢，在包括热交换器、管道和海水淡化在内的许多实际应用中，都带来了严重的操作和安全挑战。在这里，我们研究了溶解盐对池沸腾动力学的影响，以及它对传热系数和临界热通量（CHF）的影响。我们发现，即使在过饱和度条件下，也会导致沸腾表面结晶和结垢，从而大大降低传热性能。例如，发现盐溶液的 CHF 比去离子水至少低 2 倍，该盐溶液的饱和度为 75%。我们使用同时进行的高速光学和红外成像，确定了结晶诱导结垢和气泡演化动力学之间的相互依赖性，包括气泡成核、生长和脱离。我们发现，盐由于气泡接触线处的蒸发而呈“咖啡环”图案结晶。基于微层和三相接触线在结垢形成中的作用，我们提出通过控制表面润湿性来避免结垢和相关的传热系数下降。我们展示了具有混合润湿性的表面是一种在存在溶解盐的情况下减轻传热系数和 CHF 降低的方法。
