小管径冷凝管的数学模型

Mathematical model of small diameter condenser pipes

基于环状流流型建立了小管径冷凝管的数学模型.模型考虑了气液界面表面张力的作用,在同一横截面上气相与液相存在压力差.对两相压降的计算,考虑了气液两相间的相互作用,包括摩擦切应力和动量转移切应力.应用该模型,可考察气相与液相压降、液膜厚度、气相与液相平均速度、气液界面切应力以及管内壁换热系数等的沿程变化情况.根据模拟结果可得:两相压降沿管长呈近似线性增加;气相平均速度沿管长先增大后逐渐减小,但变化范围很小,且远大于液相平均速度;动量转移切应力随液膜厚度增加而增大,同摩擦切应力相比不可忽略;管内壁对流换热系数随液膜厚度增加而减小,由于冷凝管的管径很小,即使蒸气冷凝趋于完毕,气液界面接近冷凝管中心线,换热系数仍较大.

Based on the annular flow pattern, the mathematical model of condenser pipes with small diameters was established. The effect of surface tension on the liquid/vapor interface causing the pressure difference between liquid and vapor phases on the same cross section and the interaction between the liquid and va- por phases including the frictional and momentum-transfer shear stresses were considered in the model. The variations of the liquid and vapor phase pressure drops, the thickness of the liquid film, shear stress on the liquid/vapor interface and so on along the pipe length can be obtained by solving the model. Based on the modeling results, the conclusions below can be drawn： the two-phase pressure drop along the pipe length in- creases nearly linearly; the average velocity of vapor phase first increases and then decreases, but the variation range is small and far bigger than that of liquid phase; the momentum-transfer shear stress increases as the thickness of the liquid film increases, and can not be ignored compared to the frictional shear stress; the con- vective heat transfer coefficient at the inner wall of the pipe decreases as the thickness of the liquid film increa- ses, but it is still comparatively bigger when the condensation is almost completed.