School of Mechanical Engineering Seminar
Monday, June 12, 2017 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Micro-Gap Flow Obstructers for Enhanced Direct Chip Cooling
Amir Gorodetsky
M Sc. Student of Dr. Herman Haustein
Heat dissipation in modern high-power electronics require increasingly higher performance cooling, which traditional air-flow systems cannot provide and novel liquid-based solutions are pursued instead. The present study identifies several liquid flow patterns which significantly increase heat transfer, and examines them through simulations and experiments. It is well known that traveling vortices, due to generation of perpendicular unsteady or periodic flows, target a major inhibitor of heat transfer – the steady thermal boundary layer. Traditionally, micro-channels have been widely employed for micro-electronics cooling. However, for generating the desired vortices and a lower pressure drop, micro-gaps are more suitable – despite having lower heat transfer surface area, they also have reduced wall friction. The vortex laden flow generation and heat transfer can be further enhanced by inlet and local pulsation using active flow control methods.
Numerical DNS (transient laminar 2D) were conducted in order to converge to desirable experimental conditions. The experimental micro test-sections incorporated several novel aspects including a fully transparent system, a sputter-coated transparent heater, laser drilled and micro-CNC elements.
In addition, several novel measurement techniques were employed: High speed IR thermography, High speed microscopy, µ-PIV and image processing. In order to examine MEMS-based active flow control for mitigating local hotspots, a single slot jet system was also created, employing an electromagnetically actuated micro valve and enabling in-situ characterization.
Two main concepts were examined to achieve the desired wall temperature decrease and overall temperature uniformity in a micro-gap flow: bluff-body vortex generators or the addition of impinging slot-jets. The former is very suitable to micro scale flows as vortex shedding (von Karman street) nominally begins at low Reynolds numbers (Re>50). Present investigations showed that vortex shedding onset was only mildly suppressed by the confinement at the micro-scale (200 micron bluff body in a 600 micron channel), while heat transfer increased three-fold at the cost of a mild pressure drop increase. The latter concept improves heat transfer by imposing perpendicular flows – implemented by a row of slot-jets into the micro-gap cross-flow, a configuration previously only examined with a microchannel incorporating circular jets. A significant increase of heat transfer over a plain gap was found, as well as great improvement in wall temperature uniformity. In cooperation with a partner group performing more advanced 3D simulations an optimal configuration for heat transfer was obtained.
Regarding the active flow control aspects of the study, simulations showed that pulsating inlet provides only marginal gains in heat transfer, however local jet throttling experiments have been demonstrated as a potential promising alternative for future devices.
