Subcooled flow boiling has been investigated for horizontal mini and micro channels of which hydraulic diameters are 1mm and 150μm, respectively for high heat flux cooling in electronics. The heating surface is 1mm in width and 10mm in length for the mini channel. Eleven micro grooving are made on the copper heating block of 5.25mm×5.25mm. Aqueous solutions of ethanol, 10% and 50% in mass concentration, are used as boiling liquid for the micro channel. Microbubble emission boiling (MEB) of water is generated at liquid subcooling of 40K in the mini channel as same cases of conventional macro channels and the maximum heat flux obtained is a 10MW/m2 at liquid velocity of 1m/s (1000kg/m2s). However, the boiling turns to film boiling at low liquid velocity, 0.3m/s (300kg/m2s) for an example. In subcooled boiling of aqueous solutions, the heat flux becomes small for the lower ethanol concentration. The critical heat fluxes are well agreed with the existing theories and the maximum heat fluxes are higher than CHF. However, no micro bubble emission boiling is observed in subcooled flow boiling of mini channels and the CHF is considerably smaller than the existing theories. It is difficult to generate MEB for micro channels with heating surface of large thermal capacity because the coalescing bubbles formed on the heating surface are filled up in the channel and the liquid vapor exchange is disturbed.
Subcooled flow boiling of water has been investigated for the horizontal multi-microchannel of which hydraulic diameter is 150μm for unit channel. Eleven rectangular microchannels are made on a top of copper heating block of 5.25mm × 5.25mm. The outlet of the channel is opened to the atmospheric surroundings and the maximum pressure in the channel is lower than 25mmHg. The boiling test is performed under the nearly atmospheric condition. The experimental results are discussed compared with subcooled boiling of water in a microchannel of 155μm in hydraulic diameter with Platinum film microheater of 2000μm in length and 200μm in width obtained by Ping Cheng and his co-workers. According to the authors’ previous experiments on subcooled flow boiling in mini and conventional channels, the critical heat flux decreases with decreasing of the hydraulic diameter of the channel. The boiling in the microchannel turns to film boiling after reaching CHF without microbubble emission boiling (MEB) regardless of liquid subcooling and liquid velocity even if the boiling condition is the same as MEB in the minichannels. In the high heat flux region, whole of the microchannels is completely covered with large coalescing bubbles. The results are much different from those of experiments with Platinum film microheater, which have 14.41 MW/m2 of heat flux in MEB. It is difficult to introduce liquid–vapor exchange including MEB for the large capacitance heat sink in microchannel boiling.
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