A Study on Enhancement of Critical Heat Flux in Pool Boiling by Marangoni Effect

本研究的目的是开发一种有效的方法来大幅提高池沸腾的临界热流密度(CHF)。目前的研究重点是使用正混合物的方法，即挥发性成分较少的二元水溶液具有较低的表面张力。在正混合物的沸腾中，由于表面张力的梯度，可能会在液-汽界面处引起液体运动：这就是众所周知的Marangoni效应。这种方法简单、无源，不需要有源设备，因此具有潜在的吸引力。然而，Marangoni效应能够提高CHF的原因还不是很清楚。本研究选择异丙醇-水混合物作为正极混合物，用直径12 mm的水平圆盘在5kPa到100kPa下测量CHF。结果表明，在整个压力范围内，该混合物的CHF是水的1.7~2.2倍。为了阐明混合气提高CHF的机理，用尖端直径小于5μm的高精度电导探头测量了靠近受热面的汽液结构，发现对于混合气的沸腾，水蒸气的生成在受热面的中心区域被抑制，在受热面的外围被激活。因此，在受热面中心区域形成的混合物的液体宏观层比水的要厚得多。这种较厚的宏观层是提高混合气CHF的原因之一。受热面中心和外围沸腾活化的差异很可能是由大水汽团周围的Marangoni对流引起的。

A purpose of the present study is to develop an effective method to drastically enhance the critical heat flux (CHF) in pool boiling. The present study focused on the method by using positive mixtures; binary aqueous solutions where less volatile component has lower surface tension. In the boiling of the positive mixtures, liquid motion may be induced at the liquid-vapor interface due to surface tension gradient: it is known as the Marangoni effect. This method will be potentially attractive since it is simple and passive requiring no active equipments. However, the reason why the Marangoni effect is able to enhance the CHF has not been clarified.This research selected 2-propanol-water mixture as the positive mixture and measured the CHF with a 12mm diameter horizontal disk at pressures 5kPa to 100kPa. It was found that the CHF of the mixture was 1.7 to 2.2 times greater than that of water in the whole range of pressures. To clarify the mechanism for the CHF enhancement with the mixture, liquid-vapor structures close to the heating surface were measured using high accurate conductance probe of which tip diameter is less than 5μm. It was found that for boiling of the mixture vapor generation was suppressed in the central area and activated in the periphery of the heating surface. As a result, the liquid macrolayer of the mixture formed in the central area of the heating surface was considerably thicker than those of water. This thicker macrolayer is one of the causes to enhance the CHF of the mixture. The difference of boiling activation between the center and periphery of the heating surface is most likely to be caused by the Marangoni convection induced around the large vapor mass.