A Combined Active (Piezos) and Passive (Microstructuring) Partial Flow-Boiling Approach for Stable High Heat-Flux Cooling with Dielectric Fluids

Abstract

Controlled but explosive growth in vaporization rates is made feasible by ultrasonic heating of the microlayers associated with micro-scale nucleating bubbles within the microstructured boiling surface/region of a millimeter scale heat exchanger. Such bubbles arise from saturated partial flow-boiling operations of Novec 3M’s 649, HFE (hydrofluoroether)-7000 (3M™ Novec™ 7000 Engineered Fluid Product Information, 2022). Experiments use layers of woven copper mesh to form a microstructured boiling surface/region and its nano/micro-scale amplitude ultrasonic (~1 - 6 MHz) and sonic (< 2 kHz, typically) vibrations – induced by a pair of very thin ultrasonic Piezoelectric-transducers (termed Piezos) that are placed and actuated from outside the heat-sink. The ultrasonic frequencies are for sub-structural micro vibrations whereas the lower sonic frequencies are for suitable resonant structural micro-vibrations that assist in bubble removal and liquid filling processes. The flow and the Piezos’ actuation control allow an approximately 5-fold increase in heat transfer coefficient (HTC) value – going from about 9000 W/m2-°C (no Piezos case) to 50000 W/m2-°C at a representative heat flux of about 25 W/cm2. The partial boiling approach is designed to lead to approximately separated vapor and liquid flows (with 0.4-0.6 range exit quality) out of the 5 cm x 1 cm x 5 mm flow channel’s two exit ports. Further, significant increases to current critical heat flux (CHF) values (~70 W/cm2) are possible and are to be reported elsewhere. The electrical energy consumed for generating nano-/micro-meter amplitude vibrations is small (< 3 %) by design (< 3.5 W for 125 W heat remov