Extending the limit of phase change heat transfer with piranha pin-fins for ultra-high heat flux dissipation

Abstract

The Microfluidic, Extreme heat flux, CMOS compatible, Heat-eXchanger (MECH-X) microdevice employs the third generation of PPF microstructure (Gen III PPF) in a bi-layer architecture to augment CHF beyond the 1 kW/cm² mark. Experimental results are reported for the MECH-X system at mass fluxes from 680 – 3350 kg/s/m² under system pressures of 239 and 377 kPa with HFE7000 used as a working fluid. The MECH-X system achieved HTCs approaching 500 kW/m²/°C to push CHF values beyond 1.1 kW/cm². The upper limit of the MECH-X system is unknown due to dissipating the 50 [VDC] power supply used for experiments with stable multi-phase flows.

The Piranha Pin Fin (PPF) microstructure has been invented to augment CHF by venting the working fluid from the high-heat flux surface, which is replenished by highly subcooled fluid. Experiments have been performed on the silicon-based PPF heat sink at microgap mass fluxes ranging from 500 – 3700 kg/s/m² under system pressures of 1.4 and 2.8 atm. Heat transfer coefficients up to 75 kW/m²/°C were reported to mitigate chip heat fluxes up to 700 W/cm² using dielectric fluid HFE7000. Analysis of high speed camera microscope images showed that CHF in the PPF system begins with a vapor phase instability as vapors expand and coalesce in an unconfined area of the heat sink. The CHF condition was observed to be temporally dependent on the equilibrium quality of the fluid at the inlet, as well as the mass flux through the heat sink. Results from the first generation heat sink were used to optimize the PPF microstructure, as well as the overall heat sink geometry.

As integrated circuit (IC) architecture continues to shrink in size while maintaining large power throughputs, unprecedented thermal loads and heat fluxes are being generated in advanced electronics. In some cases heat loads are so extreme that conventional single-phase cooling schemes are inadequate to keep heat generating components at an acceptable operating temperature. Overheating of chips causes excess signal noise, reduces chip life, and may even cause joints to crack and desolder leading to catastrophic failure. Phase change heat transfer is a powerful cooling mode which is able to overcome thermal limits of single phase liquid or air cooling, producing heat transfer coefficients (HTCs) several orders of magnitude larger than liquid-cooled systems. The upper limit to phase change heat transfer is known as the critical heat flux (CHF), where vapor crowding at the heated surface moderates the heat transfer. At heat fluxes above the CHF point, surface temperatures surge and HTCs diminish, making the thermal management system ineffective. A primary goal of ultra-high heat flux thermal management is to extend CHF beyond the heat fluxes produced by IC chips, on the order of 1 kW/cm2.