Comparative Evaluation of Subcooled Pool Boiling Performance Using Novec 7000 and Water in a Taper Microgap Boiling Chamber

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A comparative experimental investigation of subcooled pool boiling heat transfer performance was conducted using 3M Novec 7000 (HFE-7000) near atmospheric pressure (1 atm, Tsat = 34 °C) and de-ionized water at subatmospheric pressure (12 kPa, Tsat ≈ 49 °C) in a compact boiling chamber with submerged condensation. The chamber incorporates an enhanced finned-tube condenser and dual-taper microgap manifolds over microchannel-enhanced copper substrates. Four open microchannels were tested. For Novec 7000, the highest critical heat flux (CHF) of 51.4 W/cm2 was achieved in the 762 μm channel and taper microgap at 20 °C coolant, indicating a 38% enhancement over the no-taper baseline. At a coolant inlet temperature of 40 °C, the same configuration reached approximately 66 W/cm2, the highest value reported for HFE-7000 on any surface area above 1 cm2. At 12 kPa, the 381 μm microchannel without taper achieved 185.3 W/cm2. Increasing the coolant inlet temperature from 20 °C to 45 °C in the 381 μm taper configuration raised water CHF to 195.3 W/cm2, a 56% increase. The water-to-Novec CHF ratio ranged from 3.6× to 5.0× across configurations.