Heat Transfer of Air-Water Mist Flow in Micro Pin-Fin Heat Sinks

• Main Authors: Chiu, Chun-Yi, 邱鈞義
• Other Authors: Liu, Yao-Hsien
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/dutd78

碩士 === 國立交通大學 === 機械工程系所 === 107 === Mist flow is a two-compound flow in which many micron-sized droplets are evenly distributed in the air flow. In addition to the large amount of latent heat absorption from the phase change, a thin liquid film which is effectively can be formed on the heated surface to lower the surface temperature. In this study, the micro fin heat sink was placed in a horizontal flow channel with a width of 30 mm and a height of 3 mm. The wall temperature was set to 40°C and 50°C to investigate the heat transfer and pressure drop of the micro fin heat sink under different volume flow rates of the mist. The volume flow rate of the spray was 0.115 and 0.805 lpm. Reynolds number of the air ranged from 460 to 3500. The micro fin heat sinks in this experiment were basic type and column-removal type. The basic type micro fin is a 90° diamond shape, the side length is 0.75mm, the height is 3mm, the arrangement is staggered array, and the total number of fins is 473. The column-removal micro fins remove 9 columns of micro fins, and the total number of fins is reduced to 314. The main purpose is to reduce the high pressure drop caused by the tight arrangement of micro fins. The study results showed that the effect of the mist flow is better than that of the air cooling. With the rise of wall temperature, the evaporation rate increased and produced the dry regions, reducing the heat transfer. Under different volume flow rates of the mist, there were different heat transfer trends due to changes in Reynolds number and wall temperature. The heat transfer from the mist cooling was 2 to 6 times as high as the air cooling. The results of the column-removal micro fins show that the total heat transfer area is reduced by about 26%, although the heat transfer coefficient of the air flow reduced about 10% to 20%, but the heat transfer of the air flow increased at a high volume flow rate (0.805 lpm). At low Reynolds numbers (520~2170), the mist is sufficient to form a stable liquid film on the surface of the fin, which reduces the generation of the drying zone, and increase the heat transfer. Compared to the basic type, the column-removal micro fin reduced the pressure drop about 50% to 70%, which is very effective in improving the shortcomings of the high pressure drop of the basic micro fin.