Experimental investigation into flow boiling heat transfer in ribbed micro-channel with porous-decorated sidewalls

Micro-channel heat sinks with porous structures have attracted significant attention due to their capability in promoting nucleation and postponing local dry-out on heat transfer surfaces. However, research focusing on flow boiling in micro-channels with porous-decorated sidewalls remains limited, and the effects of porous coating thickness on the flow boiling heat transfer and bubble dynamics remain unclear. In this study, ribbed micro- channels with porous-decorated sidewalls (PDS-RMC) at two distinct thicknesses of porous-decorated side- walls (i.e. 100 mu m and 200 mu m) were fabricated. The flow boiling heat transfer performance and pressure characteristics, as well as the flow regimes, within the PDS-RMCs were experimentally investigated and compared with those of the conventional smooth-ribbed micro-channel (SRMC) and porous-ribbed micro- channel (PRMC). Based on the visualization of bubble behavior within the micro-channels, the influence and mechanism of the PDS on the bubble dynamics and flow regime transitions in the micro-channels were investigated. The results indicated that a slip velocity between the bubbles and the near-wall fluid is generated within the PDS-RMCs due to the capillary pressure acting on the near-wall fluid, thereby reducing the resistance to fluid motion within the porous region. When the fluid within the channel is subcooled, the pressure drop in the 100 mu m PDS-RMC is 75 % of that observed in the SRMC. After the onset of flow boiling, the growth rate of bubble slugs in the 100 mu m PDS-RMC is 27.9 % higher than in the SRMC. The average bubble velocity in the 100 mu m PDS-RMC is approximately two times that in the SRMC. A large amount of vaporization nuclei for bubble nucleation are generated on the porous coatings of the 100 mu m PDS-RMC without significantly increasing the resistance to bubble expelling from the porous structure into the flow channel, thus enhancing the heat transfer capacity of the micro-channel. Compared to the SRMC, the heat transfer coefficient in 100 mu m PDS-RMC has increased by 60.6 %, while those in the 200 mu m PDS-RMC and the PRMC have been improved by 15.8 % and 11.3 %, respectively.