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

本研究的目的是发展一种有效的方法来大幅提高池沸腾的临界热流密度（CHF）。本研究的重点是通过使用正混合物的方法;二元水溶液中，挥发性较低的组分具有较低的表面张力。在正混合物的沸腾中，由于表面张力梯度，液体运动可能在液体-蒸气界面处引起：这被称为马兰戈尼效应。该方法简单、无源、不需要有源设备，具有潜在的吸引力。本研究选用异丙醇-水混合物作为正混合物，在5 ~ 100 kPa压力下，用直径为12 mm的水平圆盘测量了CHF。发现在整个压力范围内，混合物的CHF比水的CHF大1.7至2.2倍。为了阐明掺混物增强CHF的机理，采用直径小于5μm的高精度电导探针测量了加热表面附近的汽液结构。有人发现，对于沸腾的混合物的蒸汽生成被抑制在中心区域和激活在周边的加热表面。结果，在加热表面的中心区域中形成的混合物的液体宏观层比水的液体宏观层厚得多。该较厚的宏观层是提高混合物的CHF的原因之一。受热面中心和周围沸腾活化的差异很可能是由大蒸汽质量周围的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.