Research On Bubble Behaviors In A Narrow Rectangular Channel During Subcooled Flow Boiling

The narrow rectangular channels are widely used in high technology field due to its compact structure and higher heat transfer efficiency,which promotes the research on flow and heat transfer in narrow channels towards deep and detailed development direction.Bubble behaviors in a narrow rectangular channel under subcooled flow boiling were experimentally studied by adopting the high speed digital camera technology in present work.The experiment is processed in closed loop and using deionized water as working fluid.The image sequences of bubble nucleation, growth, departure and sliding under different experimental conditions by adjusting the thermal parameters such as system pressure, heat flux, inlet subcooling and mass flux were recorded.In order to simulate the ocean conditions, inclination experiments with various angles were also carried out.After the processing of image data and theoretical analysis, the bubble evolution characteristics and mechanism were summarized,and models to predict the bubble growth and departure were proposed.Typical bubbles are selected as the research object. It is found that the system pressure has significant effect on the characteristics of bubble size and sliding distance.Under low pressure, the bubbles are large and have complete life cycle with short sliding distance.However, under higher pressure, the bubbles experience short period of growth at the nucleate, and then depart from the site by sliding upward along the heated wall. The bubble maximum diameter or sliding bubble diameter increases with increasing the heat flux, or decreases with increasing the inlet subcooling or mass flux. But when the density of nucleation sites increases, the convective heat transferring will be enhanced and the proportion of heat transfer by nucleation decreases. The bubble maximum diameter or sliding bubble diameter are decided by the combined action of heat flux and bubble density.Under certain working condition of low degree of subcooling at low pressure, the nucleation frequency will increase obviously,which leads the bubble diameter smaller.According to the analysis on dimensionless bubble growth curves obtained under different system pressure, sections model and exponential model of bubble growth were proposed. Considering the influence of channel size and thermal parameters on bubble growth, a new model for bubble growth was established based on the Zuber model and including the effect of Ja, Bo, Re, dimensionless temperature 6.The characteristics of forces on bubble at low and higher system pressure are different.Under low pressure, the buoyancy is dominant, and with increasing the inclination angle, the bubble would depart from the heating wall easily. However under higher pressure,the surface tension is dominant, and inclination angle has limited effect on bubble detachment and lift-off. When the bubble size is large, surface tension could be ignored, but wall lubrication force must be considered.Bubble detachment time and diameter will also be affected by the experimental thermal parameters, and quasi-steady drag is the main factor promoting bubble departing from nucleation site.