A novel step-gap design on the top-cover of the microchannel cold plate to improve the flow boiling stability and heat transfer performance

The current study investigates the impact of incorporating the step gap in the cover above the microchannel heat sink subject to two-phase convective boiling. The microchannel heat sink size is 49 mm × 52 mm, with a channel width of 200 μm and a height of 3 mm, with a fin thickness of 200 μm. The effect of the step gap above the microchannels (SGAM) was compared at different heights, one 1 mm, which represents 1/3 fin height, and 3 mm, which means the same fin height with the same sample with no step gap above the microchannels (NSGAM). The working fluid used dielectric fluid HFE-7000 at the mass flux ranges from 30 to 260 kg/m²·s, while the heat flux ranges from 50 to 156 W/cm². Providing a step gap above the microchannels in the cover reduces pressure drop significantly, around 37 ∼ 48% lower than the NSGAM. The experimental results demonstrate that the SGAM configuration reduces wall superheat temperatures by approximately 4 ∼ 6 °C compared to the NSGAM configuration. Flow visualizations indicate that the SGAM design avoids flow reversal, where the pressure drop fluctuations are significantly reduced by more than three times. The experimental outcomes for excessive temperatures and total thermal resistance exhibit a high level of concordance with the analytical results, with a deviation being less than 11%. The spreading resistance comprises the major portion of the total resistance, accounting for approximately 52∼58%. The SGAM samples effectively reduce local wall temperature downstream for step gap heights 1 mm, and 3 mm are 4 ∼ 6 °C, and 2 ∼ 4 °C, respectively. Overall, these findings demonstrate the potential of the step gap above microchannels to improve flow boiling stability and heat transfer performance appreciably while offering a much lower pressure drop penalty.