Studies on the Clarification of Heat Transfer Characteristic for Flow Boiling in Small Channels and its Improvement

点击翻译按钮获取中文摘要

Flow boiling heat transfer in a single small tube is investigated by using FC72 as a test fluid. The heat transfer coefficients were measured in the ranges of heat flux 2-24kW/m^2 and mass velocity 100-400kg/m^2s. Test tube, made of stainless steel, has an inner diameter of 0.51mm and a heated length of 200mm. The tube is located horizontally in a vacuum chamber to reduce the heat loss and to minimize the time to obtain data regarded as that of steady state. In the experiments of strict inlet flow rate regulation, heat transfer characteristics were similar to those observed in normal size tubes. But if the flow rate was fluctuated up to±20% of the total under the weak inlet flow rate regulation, the trend that the heat transfer coefficient was increased with increasing vapor quality in moderate vapor quality region was disappeared and heat transfer deterioration due to partial dryout started at lower vapor quality. Boiling heat transfer characteristics in minichannels could be changed … More considerably by the existence of flow fluctuation caused by the rapid axial growth of elongated bubbles at low vapor quality. The regulation of inlet flow rate seems to be one of key parameters to reduce the scattering in heat transfer data encountered in flow boiling of minichannels.In order to enhance flow boiling heat transfer in narrow channels, the effects of liquid supply method to channels on CHF and heat transfer coefficient were investigated using atmospheric water for various combinations with gap sizes of narrow channels and volumetric flow rates. An auxiliary unheated channel for liquid supply was installed in parallel with the back side of a narrow heated channel. These channels were connected by two sintered metal porous plates. Fine V-shaped grooves were machined on the heat transfer surface of heated channel. High CHF values of 2.0×10^6 W/m^2 were measured under conditions the total flow rate is 3.60 liter/min and gap sizes are 2 mm and 5 mm. In the series of the experiments for total flow rate of 3.60 liter/min, highest heat transfer coefficients as much as 1.1×10^5 W/m^2 for gap size of 2 mm and 7.8× 10^4 W/m^2 for 5 mm were measured.The selection of working fluid for the high-performance thermal systems by applying boiling phenomena was examined. For mixtures of 1-propanol/ water, 2-propanol/ water and water/ ethylene glycol, nucleate boiling heat transfer characteristics were investigated. Alcohol aqueous solutions can be expected heat transfer enhancements due to "Marangoni effect". At low alcohol concentration region, CHF is decreased sharply by the increase in alcohol concentration. After reaching a minimum value, CHF is increased gradually up to the value of the azeotropic concentration with the increase in alcohol concentration at the region of moderate alcohol concentration. Less