An experimental investigation on the effect of gravitational orientation on flow boiling performance in different channel sizes ranges from minichannels to microchannels

Flow boiling experiments were conducted in this work to investigate the effect of gravitational orientation on flow boiling performance in channels with different hydraulic diameters (Dh) ranging from minichannels to microchannels. Four different channel sizes (Dh ~ 100 μm, 300 μm, 500 μm and 1000 μm) were experimented through four different orientations such as horizontal upward facing (HU), horizontal downward facing (HD), vertical up flow (VUF) and vertical down flow (VDF) with mass flux ranging from 100 to 900 kg/m2 s. Critical heat flux (CHF), heat transfer coefficient, pressure drop and flow patterns data were determined for different channel sizes with different orientations and mass fluxes. The experimental results were also compared with the different criteria reported in the literature regarding the existence of gravity force in minichannels and microchannels. From the experimental analysis it was clear that for channel sizes (Dh) of 100 and 300 μm, the gravity force was not so dominant, but in the case of 300 μm channel, VDF reported degraded heat transfer performance. For channel sizes (Dh) of 500 and 1000 μm, the gravity force was dominant. Reversed bubble flow was observed for these channel sizes in VDF orientation trial. The HD orientation also shows a reduction in critical heat flux and heat transfer coefficient for these channel sizes. VUF orientation was found to be the best for all channel specimens irrespective of channel size. The buoyancy assisted bubble movement along the length of the channel will help in easy bubble removal and flushing. The comparison with different criteria shows that only Bond number (Bo) can able to predict the existence of gravity force in different channel size ranges.