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。本研究的重点是采用正混合法；二元水溶液中挥发性成分较少，表面张力较低。在正混合物的沸腾中，由于表面张力梯度，液体运动可能在液-气界面处引起：这就是众所周知的马兰戈尼效应。由于该方法简单且无源，不需要有源设备，因此具有潜在的吸引力。然而，马兰戈尼效应能够增强CHF的原因尚不清楚。本研究选择2-丙醇-水混合物作为正混合物，在5kPa ~ 100kPa压力下，用直径12mm的水平圆盘测量CHF。在整个压力范围内，混合物的CHF比水的CHF大1.7 ~ 2.2倍。为了明确混合物增强CHF的机理，采用尖端直径小于5μm的高精度电导探针测量了受热面附近的液-气结构。研究发现，混合气沸腾时，蒸汽生成在受热面中心区域受到抑制，而在受热面外围被激活。因此，在受热面中心区域形成的混合物的液体大层比水的大层厚得多。这种较厚的宏观层是提高混合物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诱导的马兰戈尼对流引起的。

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.