| The boiling heat transfer process is a phase change heat transfer process in which a liquid is heated from a liquid to a gaseous state.This process occurs at the interface between the solid and the liquid.The boiling process can be divided into four processes,and the nucleate boiling in the boiling process has a high heat transfer coefficient and efficient heat transfer performance has become the focus of research,so rescue nucleate boiling is particularly important for enhancing heat transfer performance.A single phenomenon does not appear in the process of nucleate boiling.It is difficult to reveal the phenomena and physical mechanisms in the process of boiling based on heat transfer performance in traditional boiling theory.It is necessary to start from a more microscopic bubble kinetic theory to analyze the nucleus.The influence of the bubble growth process and the number of bubbles in the state boiling helps to understand the heat transfer mechanism from a more microscopic state.The main work of this paper includes:building an experimental platform for visualizing the growth characteristics of isolated bubbles in flat heat pipes and studying the fusion phenomenon of bubbles in the tubes.A new type of heat pipe for heating the bottom surface is designed and the characteristics of the bubble generated on the bottom surface structure are studied.The typical image of the bubble in the heat pipe is visualized by a high-speed camera.Based on the time-series image,the growth characteristics of the isolated bubble,the density of the vaporization core on the bottom structure,and the escape frequency of the bubble are recorded and quantitatively analyzed.In addition,the fusion phenomenon between bubbles is also analyzed.And through the analysis of previous research results,a bubble growth model suitable for this experiment was established.The experimental results show that.(1)At the beginning of the nucleation and growth process of a single bubble(about 1/4 of the entire cycle),the diameter change is the largest,and the height change becomes linearly larger.The diameter of the bubble changes little in the later period,and the height suddenly increases at the end of the bubble growth.The volume changes linearly throughout the process.When the bottom structure is the same,when the heat flux density increases,the breakaway frequency of the vapor bubble and the number of vaporization cores also increase,but the breakaway diameter of the vapor bubble decreases accordingly.(2)Under the same working conditions,the bubble separation diameter of the bottom surface of the composite heating is the largest,the separation frequency is also the fastest,and the number of vaporization cores on the bottom surface is also the largest.When the heat flux is1.76×10~5W/m~2,the number of vaporized cores on the bottom of the composite type is about twice that of the bottom of the smooth type.After the bubbles are fused horizontally,there is no sudden increase in height after the fusion of similarly sized bubbles and large and small bubbles,and there is a sudden increase in the large diameter after the fusion.There are two bubble fusions in the prismatic bottom structure,which are horizontal fusion and oblique fusion,while the composite bottom structure only has horizontal fusion.Through the analysis of the heat transfer performance of the three structures,the heat transfer of the composite bottom structure The effect is also the best.(3)When the filling rate is different,the separation frequency of the bubble decreases with the increase of the filling rate,and the separation diameter of the bubble continues to increase,but it has little effect on the density of the vaporization core.As the heat flux increases,the heat exchange effect of the heat pipe weakens.Boiling heat transfer is the highest heat transfer efficiency among common heat transfer methods.It generates vapor bubbles at the interface between the heating surface and the liquid working fluid,and takes away the heat on the heated wall through the growth motion of the vapor bubbles.The dynamics of a bubble is not a process of a single liquid absorbing heat,but a process of continuous heat exchange with the thermal boundary layer of the surrounding liquid.The interaction between the two is the focus of current scientific research. |
