垂直矩形窄通道流动沸腾换热特性实验研究

Experimental Study of the Boiling Heat Exchange Charateristics of a Flow in a Vertical Rectangular Narrow Channel

流动沸腾换热是典型的两相流问题。窄通道与常规通道相比较,其流动沸腾换热系数有较大提高,换热机理也更加复杂。针对截面为250 mm×5 mm的竖直矩形窄缝通道,在低压、入口温度过冷、不同质量流速及加热功率密度的条件下,对水流动沸腾换热特性进行实验研究。通过实验分析可知:入口温度27～60℃、质量流速2.22～3.49 kg/(m2.s)及加热功率密度0～12 kW/m2对饱和沸腾起始点和过冷段长度有重要影响;高的空泡份额和通道结构的限制使汽液两相流动不稳定而影响换热系数,换热系数随着功率的增大而减小,流体进入完全对流沸腾阶段;由于实验段通道顶部结构的限制,干度的增加不会出现干涸点,换热不会得到恶化,换热系数随着功率的增大基本不变。

The flow boiling heat exchange pertains to a typical two phase flow problem.When a narrow channel is compared with a conventional one,the flow boiling heat exchange coefficient of the former is markedly higher than that of the latter and the heat exchange mechanism of the former is much more sophisticated.Under the condition of a low pressure,a supercooling inlet temperature,different mass flow rates and heating power densities,the water flow boiling heat exchange characteristics of a vertical rectangular narrow channel with a cross section of 250 mm×5 mm were experimentally studied.The experimental and analytic results show that the inlet temperature(27-60℃),mass flow rate(2.22-3.49 kg/m2s) and heating power density(0-12 kW/m2) have an important influence on the saturated boiling starting point and the length of the supercooling section.A high void fraction and limitation of the structure of the channel make the steam-liquid two phase flow unstable and affect the heat exchange coefficient.The heat exchange coefficient will decrease with an increase of the power and the fluid enters into its total convection-based boiling stage.Due to the limitation of the top structure of the section under the test,with an increase of the dryness,there will not emerge a dry-out point,the heat exchange will not get worsened and the heat exchange coefficient will be basically constant with an increase of the power.