过冷降膜的换热特性和破断条件

Heat Transfer Characteristics and Breakdown Criteria of Subcooled Falling Films

本文在实验观测的基础上对垂直降膜的换热情性和破断临界现象进行了简化分析.据此整理实验数据,给出正常液膜和接近破断临界时的液膜对流换热努塞尔数综合关系式以及破断准则数综合关系式.

Simplified physical models and analyses were presented in order to describe heat transfer and breakdown of subcooled liquid films falling down the outside of a vertical tube. On the basis of the analogy of heat transfer in the boundary layer, Nusselt number correlations for convection heat transfer of normal and subnormal films (the latter are films approaching breakdown)were obtained by fitting the test data. The film breakdown patterns were observed in our experiments. The thermocapillary force and the flow regimes seemed to play a dominant role in the film breakdown. Simplified physical models based on the analogy of flooding mechanisms in annular two-phase flow were proposed to describe critical phenomena of film breakdown and some important dimensionless groups could be derived. Ma, Nu; was defined as the breakdown criterion number Bd. Its correlations for temporary and permanent breakdown were obtained by fitting the test data. As a first approximation, the simplified physical models are reasonable and the correlations can be used for predicting breakdown conditions. The experiments were performed under the following parameters: r = 0.207-1.181kg/(m·s), Tin = 19-37℃, and the maximum heat flux was up to 8.67 × 104W/m2.