The experimental analysis on the influence of different conditions in microchannel on critical heat flux and boiling starting point

In order to explore the mechanism of boiling heat transfer in microchannels and the appearance of CHF and ONB under different conditions, a visualized experimental research has been conducted on the flow and heat transfer characteristics in microchannels. Combined with the behaviour of bubbles, the occurrence of ONB and CHF in the microchannels with different shapes, different flow velocities and different inflow directions has been compared and analyzed. Results indicate that, after the heat flux exceeds 20 W/cm 2 , the four microchannels all enter the nucleate boiling stage (ONB), with little difference in order. The appearance of ONB for the droplet pin fin is the earliest ( q e = 41.51 W/cm 2 ), with non-closed pin fin with flow direction B ( q e = 56.65 W/cm 2 ) as the latest. Accordingly, the channel with droplet fin ( q e = 106.2 W/cm 2 ) pin fins appear CHF first, and the non-closed droplet shape (flow direction A: q e = 141.6 W/cm 2 ; flow direction B: no CHF) is the latest. While for the same channel, with the increase of mass flow, the convective heat transfer coefficient becomes higher and ONB and CHF will be delayed. For the different layout, the more favorable flow arrangement (flow direction B) is helpful to delay ONB and CHF. A comprehensive factors η is introduced to evaluate the boiling flow and heat transfer performance of the microchannels with different situations. At higher heat flux density (58 W/cm 2 -150 W/cm 2 ), flow direction B perform best. Under the discussed condition, the heat flux value as 109.91 W/cm 2 , 106.22 W/cm 2 , and 143.05 W/cm 2 are the limit values for the CHF, in the square, droplet and non-closed droplet A pin fin arrays. Where, the CHF has not appeared in non-closed droplet B, whose limit value is more than 145 W/cm 2 . In general, the results illustrate that the opening structure is helpful for boiling heat transfer. The novel design can prolong the duration of bubble flow in nucleate boiling, which plays a key role in delaying CHF.