Subcooled Flow Boiling in Mini and Micro Channel: Contribution Toward High Heat Flux Cooling Technology for Electronics

Abstract

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.

Experimental investigation of transition process from nucleate boiling to microbubble emission boiling under transient heating modes

“…Both the maximum heat flux of MEB and CHF was reduced with an increase in the heater size due to higher downstream liquid temperature. Nomura et al investigated MEB in a copper multimicrochannels with the same subchannel hydraulic diameter of 0.15 mm. Visualization results showed that the microchannels were almost occupied by coalesced bubbles and MEB did not appear even at a liquid subcooling as high as 80 K. However, Wang and Cheng realized MEB in a microchannel with hydraulic diameter of 0.155 mm.…”

Boiling Heat Transfer Enhancement on Micro- and Nanoscales

“…Actually the heat transfer enhancement in a microchannel is not a trivial [2]. Suzuki and others [3] investigated microbubble emission boiling (MEB) in miniand microchannels as a heat transfer enhancement method. The MEB allows to increase the CHF in subcooled boiling, but occur of the boiling crisis depends on a flow velocity and liquid subcooling.…”