Addressing the synergistic effect of hydraulic diameter and aspect ratio on experimental flow boiling in microchannels

The effects of hydraulic diameter (Dh) and aspect ratio (AR) on flow boiling heat transfer characteristics in rectangular microchannels under uniform heating are elucidated in this work. The microchannels tested have Dh of 762 µm, 727 µm, and 909 µm. The Dh = 762 µm microchannel has an AR of 20, while the other two have an AR of 10, with each microchannel having a length of 80 mm. While hydrofluoroether HFE-7000 is selected as the working fluid with a saturation temperature of 34 °C. The experiment covered low flow rate conditions with Reynolds number Re ranging from 22.72 to 68.15 and a heat flux range of 0.05 to 16.02 kW/m2. During the experiment the inlet and outlet temperatures and pressures are measured, while high-speed and infrared cameras captured the flow patterns and wall temperature distributions. Results indicate that as heat flux increases, bubble flow, slug flow, transitional flow, and annular flow, sequentially occur in the microchannel. Meanwhile increasing the AR enhances liquid film thickness in both churn and annular flows, in turn increasing thermal resistance. At high AR the occurrence of liquid film fluctuations also increases, causing the film to thin or even rupture in localized areas, exposing the microchannel walls and forming hot spots, therefore resulting in a worse heat transfer performance. Comparing the different flow pattern maps at the front, middle and rear of the channel, it shows that a higher AR shifts the flow regime transition lines towards the lower outlet vapor quality end, whereas a reduced Dh increases the vapor quality necessary for flow regime transitions. The Dh = 727 µm microchannel achieves superior heat transfer performance. A smaller Dh reduces the effective wetting area, thereby diminishing the resistance of conduction and convection, enhancing the microchannel’s heat transfer performance. This is consistent with the heat transfer results and the increase in thermal resistance as Dh decreases for medium and high Re studied. The coefficient of performance (COP) for microchannels is defined as the ratio of effective input heat to pump power, while increasing the AR effectively reduces pressure drop and enhances overall performance, the Dh exerts a more significant impact on COP than the AR.