| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 100 === Experiments were conducted in this research to investigate size and roughness effect on flow characteristics and heat transfer coefficient of air and CO2 flow in circular micro-tubes.
For smooth microtubes with inside diameter of 86, 308 and 920 ?m, the Liquid Crystal Thermography method was used to measure the tube surface temperature for avoiding the thermocouple wire thermal shunt effect. The experimental results show that the friction coefficient of gas flow in micro tube is the same as that in the conventional larger tubes if the effect of compressibility was well taken into consideration. The conventional heat transfer correlation for laminar and turbulent flow can be well applied for predicting the fully developed gaseous flow heat transfer performance in microtubes. There is no significant size effect for air flow in tubes within this diameter range.
For rough tubes, the internal surfaces are structure helical fin and random roughness. The rough circular tubes were lab made Nickel tube with diameters ranging from 901 to 977 ?m and roughness elements from 5.3 to 44.6 ?m in height. The experimental results indicated that the friction factor was significantly higher than the prediction of conventional correlations for smooth tube both in laminar and turbulent flow. Heat transfer enhancement in laminar flow is slight, but in turbulent flow the heat transfer enhancement was significant and the enhancement increases with the increasing of Re for the random rough tubes.
In order to verify the conclusions of the above experiments, a micro-channel heat exchanger was also designed and tested in this study. It provides an experimental analysis on the heat transfer performance of a flat aluminum tube micro-channel heat exchanger with/without spray cooling. The effects of water spraying rate, air flow rate and relative humidity were investigated. The test results show that the analysis methods for conventional size heat exchanger are still well applied in micro-channel heat exchanger; the spray cooling can increase the heat transfer performance with increasing spraying rate but without penalty of increased flow resistance at low spray conditions.
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