Experimental study on flow boiling heat transfer in open copper microchannel heat sinks with different aspect ratios

Changing the aspect ratio of the microchannel can adjust the gas-liquid distribution to improve heat transfer and reduce pressure drop, whereas the majority of research has only been focused on closed microchannels. The effect of aspect ratio on heat transfer and pressure drop characteristics of two-phase flow boiling was experimentally studied in open microchannel heat sinks with an aspect ratio of 0.8 - 4. The test was carried out under the inlet temperature of 80 °C, mass flux of 100–300 kg/m²s, effective heat flux of 0–250 W/cm², and outlet pressure of 100–147 kPa. Flow visualization was conducted on all the heat sinks and a flow pattern map was developed containing bubbly flow, confined bubble flow, slug flow, Type Ⅰ stratified flow, and Type Ⅱ stratified flow. As the aspect ratio increases, the variation of effective heat flux corresponding to the transition from Type Ⅰ stratified flow to Type Ⅱ stratified flow is consistent with that of CHF. At all the mass fluxes, the CHF initially rises and then falls with the aspect ratio, reaching a peak at 2.67. When the mass flux increases from 100 kg/m²s to 200 and 300 kg/m²s, the heat sink with an aspect ratio of 2.67 experiences the largest increase in CHF of 74.6 % and 136.9 %, respectively. Furthermore, the heat sink with a higher aspect ratio results in a lower wall temperature and pressure drop at the same heat flux. The former may be due to the improved heat transfer performance of the smaller bubbles, while the latter could be attributed to the reduced expansion pressure of those. This research could be used as a reference for further investigating the heat transfer properties of open microchannels when the aspect ratio varies over a wider range.