A COMBINED ACTIVE (PIEZOS) AND PASSIVE (MICROSTRUCTURING) ENHANCED MICRO-NUCLEATION RATE FLOW-BOILING APPROACH FOR STABLE HIGH HEAT-FLUX COOLING

Document Type

Conference Proceeding

Publication Date

2023

Department

Department of Mechanical Engineering-Engineering Mechanics

Abstract

Controlled but explosive growth in micro-scale nucleation rates during flow-boiling of Novec 3M's 649, HFE (hydrofluoroether) -7000/7100 (electronics and environment-friendly liquids from 3M, Inc) is described here. These include follow-up experimental results in support of the earlier ones. Experiments use meshed copper as boiling surface/region and small low-energy consumption nano/micro-vibrations coming from a pair of very thin ultrasonic Piezoelectric-transducers (termed Piezos) that are placed and actuated from outside the mini-channel heat-sink. The flow-loop control and Piezos actuation further ensure that the millimeter-scale rectangular cross-section flow channel, when heated from underneath its bottom boling-region (a microstructured region formed by bonding together some thin and woven square-mesh copper sheets) allows a 4.8-fold increase in heat transfer coefficient (HTC) value − going from about 14000 W/m2-°C (no Piezos case) to 67000 W/m2-°C at a representative heat-flux of 30 W/cm2. Also, there is an all-liquid flow at the inlet, a plug-slug flow regime by the exit, nucleate boiling-surface/region within the microstructure at the bottom, and separated vapor and liquid flows (with 0.4-0.6 range exit quality) out of the flow-channel. This approach leads to no vapor-compressibility-induced system-level flow instabilities. Further, significant increases to current values (~80 W/cm2) of critical heat flux (CHF) are possible and are being reported elsewhere. The electrical energy consumed for generating nano/micro-m amplitude vibrations is small by design (within 5-10 W for 100-500 W heat removal cases). The Piezos are driven at high ultrasonic frequencies (~1 - 6 MHz), modulated by lower frequencies (< 2 kHz for efficient bubble-removal rates), for add-on acoustothermal superheating (with increased vapor generation) of the liquid surrounding the thermally nucleating bubbles generated by the passive (no Piezos) approach.

Publication Title

8th Thermal and Fluids Engineering Conference (TFEC)

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