水平管道过冷沸腾换热的非稳态数值计算

Unsteady numerical calculation of subcooled boiling heat transfer in a horizontal pipe

摘要:

为研究循环式冷却系中涉及的过冷沸腾流动传热特性,基于VOF多相流模型和Lee相变模型对一水平管道中的过冷流动沸腾过程进行非稳态数值计算。考虑到沸腾起始点的影响,在Lee模型的基础上引入Bergles提出的沸腾起始点关联式以对其进行修正。从热边界层发展和沸腾阶段的发展两方面分析水平管道过冷沸腾换热过程中的流动换热特性及其波动规律,总结了不同热流密度工况下相关参数的分布关系以及对流换热系数沿流动方向的分布规律。结果表明:热边界层的发展和沸腾不断加剧使得流场的不稳定性增加,加热区域后部对流换热系数的波动幅值是入口附近的2倍；热流密度的增加使得流动和换热参数沿流动方向的变化速度加快,热流密度为250 kW/m²工况下,热边界层发展所影响区域约为150 kW/m²工况下的60%；热边界层的发展使得加热段前部的对流换热系数呈现前高后低的特点。当受热区域热流密度较大时,换热设备可以通过减小换热通道长度的方式,在提升换热效率同时减小沸腾带来的换热系数波动的影响。

Abstract:

In order to study the characteristics of heat transfer in subcooled flow boiling of circulating cooling system, the unsteady numerical calculation of the subcooled flow boiling process of a horizontal pipe was carried out based on the VOF multiphase model and the Lee phase change model. The correlation of boiling onset proposed by Bergles was introduced for correcting the Lee model, taking into account the influence of the boiling onset. The heat transfer properties of the subcooled flow boiling process in the horizontal pipe and their variation rules were examined from two aspects: the thermal boundary layer development and the boiling development. The distribution regularity of relevant parameters under different heat flux conditions was summarized, as well as the distribution rules of heat transfer coefficient along the flow direction. Results showed that the development of the thermal boundary layer and boiling would enhance the instability of the flow field, and the fluctuation amplitude of the heat transfer coefficient in the rear was twice as large as in the front. Moreover, the increase of the heat flux accelerated the variation of the flow and heat transfer parameters along the flow direction. In the 250 kW/m² condition, the area affected by the thermal boundary development was about 60% that of the 150 kW/m² condition. The heat transfer coefficient in the front of the heating section showed a descending trend with the development of the thermal boundary layer. Therefore, when the heating area is in high flux condition, we can shorten the length of the heat exchanger to increase the heat exchange efficiency and weaken the fluctuation of the heat transfer coefficient caused by flow boiling.