Abstract:

This work numerically investigated bubbly flow with phase change in a horizontal serpentine tube.A volume of fluid (VOF)multiphase model was used to study unsteady laminar flow in the tube.The phase change between liquid and vapor phases was modeled by using heat and mass sourses in the governing equations, and was numerically implemented by using UDFs.The model was first validated with experiments in terms of flow pattern and then used for this study.The results showed that the mixing effect of secondary flow at bends caused a quick recovery of temperature at the top wall after the passage of bubble when the bubble entry frequency was extremely low, and effectively increased the temperature at the top wall by bringing hot fluid from the bottom to the top when the bubble entry frequency was relatively large.As a single bubble passed the straight sections of the tube, it absorbed the heat for evaporation and cooled down the top wall.The top wall recovered its temperature slowly becasue the main heat input was from the local heat flux.However, in the bend sections, the top wall temperature recovered quickly after the passage of bubble due to secondary flow caused by centrifugal force.Secondary flow quickly brought hot fluid from other regions to heat the top wall, causing a quick recovery of temperature.When a series of bubbles passed the tube, the entire top region of the tube was effectively cooled.The variation in temperature during the passage of bubble was smaller in the straight sections compared to the bend sections.This was because the temperature level was lower in the straight sections but higher in the bend sections because secondary flow effectively brought hot fluid from the bottom regions of the tube to the top regions.The series of bubbles effectively enhanced heat transfer of the serpentine tubes relatively to single phase liquid flow.