Theoretical And Experimental Study On Bubble Behavior In Flow Boiling

Flow boiling heat transfer was widely used in the fields of energy, aerospace and nuclear energy, in this thesis, based on the fundamental principles of fluid mechanics and heat transfer, the influences of key factors on bubble behaviors and two-phase flow patterns in flow boiling were analyzed by building an experimental system, and the departure characteristics of bubble in various conditions were revealed by making the mathematical models on bubble departure.The various bubble behaviors including bubble coalescence, bubble expansion-shrink and bubble slippage, and the flow pattern as well as its variation along the channel were observed by high-speed digital camera. It was found that the flow patterns in subcooled flow boiling in annular channel were different from those in conventional channels. Three types of flow patterns, namely the isolated bubbly flow, dispersed bubble flow and churn flow, were observed, and the lengths of areas occupied by these flow patterns changed with the variation of the heat flux and mass flux. There were five types of typical flow patterns in microchannel:bubbly flows, slug flow, churn flow, wispy-annular and annular flow, and they were intensely influenced by the channel size.The variation regularities of the bubble diameter and the bubble contact diameter with the bubble growth time were obtained by analyzing the experimental data of the visualization. It was found that in0-0.3ms after bubble nucleation the bubble growth is controlled by inertia force, and after then it is controlled by heat transfer. In the inertia controlled period, the bubble growth rate in various working conditions were very closed, however, the bubble growth rate was obviously influenced by the working condition, it increased with the increase of the heat flux and decreased with the increase of the mass flux.Based on the analysis of the variation curves between the bubble contact diameter and bubble departure diameter at the bubble departure, the experimental correlation between them was established, which can accurately predict the relationship between the bubble contact diameter and bubble departure diameter by selecting proper empirical parameters for heat wall features and operating conditions.In the experiments, the bubble departure diameters differ from each other even in the same heating wall surface and same boiling region (isolated bubble region) under identical operating conditions. This phenomenon was investigated by analyzing the forces acting on the bubble, and eventually it was found that the bubble departure diameter was significantly impacted on by the bubble contact diameter, it increased with the increase of the latter.The variation regularities of the bubble departure diameter with the operating conditions were investigated, it was observed that the bubble departure diameter increased with the increase of the heat flux and decreased with the increase of the mass flow rate. On the other hand, the bubble departure diameter was obviously influenced by the channel size, it decreased with the decrease of the channel size.Based on the force analysis of bubble, integrating the relationship between bubble contact diameter and bubble departure diameter, the prediction model for the bubble departure diameter was established according to the force balance on the bubble. It was found that the model can effectively predict the experimental data in our experiments with an error of±15%, and get a better consistency by comparing the model to the experimental data from existing literatures, the comparison error was less than±30%.