Flow boiling in high-aspect ratio mini- and micro-channels with FC-72 and ethanol: Experimental results and heat transfer correlation assessments

Flow boiling heat transfer in micro-channels using transparent heating was experimentally investigated. Three high-aspect-ratio (In the present study, for a channel cross section with a width Win and a depth din, the channel aspect ratio α   is considered to be high when MAX(Win,din)MIN(Win,din)>10 or MIN(Win,din)MAX(Win,din)<0.1.) rectangular micro-channels were used, the hydraulic diameters of which were 571 μm, 762 μm and 1454 μm. Mass fluxes of 11.2 kg m−2 s−1, 22.4 kg m−2 s−1 and 44.8 kg m−2 s−1 and a wide range of heat fluxes were applied. Visualization results, thermography measurements and heat transfer characteristics of FC-72 and ethanol were obtained. Moreover, ten existing correlations for macro-, mini- and micro-channels were assessed. For ethanol, good predictions of three micro-channel correlations were achieved (MAE = 16.4–18.8%). However, all the correlations showed very high MAE for FC-72 (MAE = 80.7–641.3%). It is found that liquid properties and the unique high-aspect-ratio micro-channel geometry are crucial in developing a more accurate correlation. A modified correlation is proposed for FC-72 and ethanol with a MAE of 23.6%.

► Flow boiling heat transfer in high aspect ratio micro-channels is investigated.
► Transparent heating technique and thermographic tools are used with the unique channel geometry.
► The channel outer surface temperature fluctuates during the recoiling and re-wetting of the channel.
► ONB and CHF increase with increasing mass flux. CHF is higher for ethanol than FC-72.
► Ten heat transfer correlations are assessed and a modified correlation provides a MAE of 23.6%.