Pablo Hidalgo

Cooling performance of micromachined self-oscillating reed actuators in heat transfer channels with integrated diagnostics

This paper presents heat transfer (HT) performance of small-scale MEMS-enhanced self-powered oscillating actuators for applications in highly-compact high-power air-cooled heat exchangers. Commercial air-cooled heat sinks are typically much larger than the systems they must cool due to large air-side thermal resistance. Our work is applying MEMS technologies to reduce this thermal resistance via the integration of small-scale oscillating actuators into small heat exchangers. Improved HT performance either yields smaller heat sinks or higher heat fluxes. These mm-scale actuators were built using MEMS manufacturing technologies such as laser micromachining, lamination, and/or metal patterning and etching. Conceptually, oscillating reeds inserted into the channels of an air-cooled heat sink induce small-scale motions in low-Reynolds number flows, which increases HT efficacy. Using MEMS-enhanced reed actuators, we experimentally demonstrated local HT enhancement up to 250% in microfabricated channels monitored by integrated temperature sensors.

A novel conduction-convection based cooling solution for 3D stacked electronics

The present investigation focuses on the design and thermal parametric study of a unique liquid interface thermal management solution for a 3D chip stack that is embedded within a cavity, in a radial heat sink cooled by an array of synthetic jet actuators. The heat sink module was previously reported by the authors, who achieved an overall heat transfer coefficient of ~70 W/m2.K. The radial heat sink exploits enhanced, small-scale heat transfer that is affected by a central array of synthetic jet actuators. This approach is very effective due to the short radial thermal path of the cooling air along the fins which couples rapid, time-periodic entrainment and ejection of cool and heated air, respectively to increase the local heat transfer coefficient on the air-side. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high power 3D stacked electronics to the heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive and scalable thermal management solution.

Hybrid Liquid Immersion and Synthetic Jet Heat Sink for Cooling 3-D Stacked Electronics

This paper focuses on the design and parametric numerical study of a hybrid heat sink combining a liquid thermal interface with an array of synthetic jet actuators for 3-D chip stack cooling. The air-side heat sink exploits enhanced localized heat transfer achieved via a central array of synthetic jet actuators. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high-power 3-D stacked electronics to the hybrid heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive, and scalable thermal management solution for 3-D stacked die structures. It is shown that this heat sink with a footprint of 76-mm square × 51-mm height can dissipate a total of 41 W of heat/power from the stack for a 44°C average chip temperature rise above ambient (an Rja of ~ 1.06 K/W obtained passively).

Small-Scale Vorticity Induced by a Self-Oscillating Fluttering Reed for Heat Transfer Augmentation in Air Cooled Heat Sinks

Heat transfer in a high aspect ratio, rectangular mm-scale channel that models a segment of a high-performance, air-cooled heat-sink is enhanced by deliberate formation of unsteady small-scale vortical motions. These small-scale motions are induced by self-fluttering, cantilevered planar thin-film reeds that are placed along the channel’s centerline. Heat transfer is enhanced by significant increases in both the local heat transfer coefficient at the fins surfaces, and in the mixing between the thermal boundary layers and the cooler core flow. The present investigation characterizes the thermal performance enhancement by reed actuation compared to the base flow (in the absence of the reeds) in terms of increased power dissipation over a range of flow rates, along with the associated fluid power. It is shown that because the cooling flow rate that is needed to sustain a given heat flux at a given surface temperature is almost two times higher than in the presence of the reeds, the reeds lead to a four-fold increase in thermal performance (as measured by the ratio of power dissipated to fluid power). The thermal effectiveness of the reeds is tested in a multi-channel heat sink, and it is shown that the improvement in heat transfer coefficient of the base flow is similar to that of the single channel.Copyright

Direct Actuation of Small-Scale Motions for Enhanced Heat Transfer in Heated Channels

Flow-effected, enhanced heat transfer in a high aspect ratio rectangular mm-scale channel that models a segment of a high-performance, air-cooled heat-sink is characterized. The present investigation reports a novel approach to enhanced cooling without increasing the channel’s characteristically low Reynolds number. Heat transport that is governed by the local heat transfer from the fin surface and by subsequent mixing with the core flow is significantly increased by deliberate shedding of unsteady small-scale vortices that are induced by the vibration of a miniature, planar piezoelectric reed. The present investigation focuses on the heat transfer and fluid mechanics that are associated with the small-scale motions induced by the reed. High-magnification particle image velocimetry (PIV) is used to characterize the interaction of the induced vortical structures with the channel flow. Performance enhancement by reed actuation is quantified in terms of increased power dissipation over a range of flow rates compared to the baseline flow in the absence of the reed. It is demonstrated that the channel’s coefficient of performance can be increased by a factor of 1.4 while accounting for the power to the reed and the changes in channel pressure drop.Copyright

Direct actuation of small-scale motions for enhanced heat transfer in heated channels

Flow-effected, enhanced heat transfer in a high aspect ratio rectangular mm-scale channel that models a segment of a high-performance, air-cooled heat-sink is characterized. The present investigation reports a novel approach to enhanced cooling without increasing the channels characteristically low Reynolds number. Heat transport that is governed by the local heat transfer from the fin surface and by subsequent mixing with the core flow is significantly increased by deliberate shedding of unsteady small-scale vortices that are induced by the vibration of a miniature, planar self-oscillating reed. The present investigation focuses on the heat transfer and fluid mechanics that are associated with the small-scale motions induced by the reed. Performance enhancement by reed actuation is quantified in terms of increased power dissipation over a range of flow rates compared to the baseline flow in the absence of the reed. It is demonstrated that the channels coefficient of performance can be increased by a factor of 4 while accounting for all the losses associated with the reed actuation.

Novel Immersion Cooling Technique for a 3D Chip Stack

An experimental investigation of a scheme for cooling electronics packaged in a 3D stack arrangement will be presented in this paper. The scheme utilizes immersion cooling of the stacked electronics in an enclosure filled with a dielectric fluid. Convection and conduction within the dielectric fluid drive heat from the 3D stack to the walls of the enclosure from where a ‘synthetic jet /fan air-cooled heat sink’ ultimately dissipates heat to the ambient. Four layers of thick film heaters embedded in FR-4 sheets, each attached to thin copper plates (innovatively stacked in a pyramidal arrangement for conducting heat laterally to the dielectric fluid and simultaneously promoting natural convection in the fluid), were used to simulate a 3D stack of electronics. For a comparative study, several runs were carried out, where the enclosure was filled with dielectric fluid (FC-770), FC-770 in combination with copper wool (with a goal of enhancing heat transfer in FC-770), and water. For a 40 W total power input to the stack, it was observed that the thermal resistance for heat dissipation to ambient from the four heaters varied from 1.67 K/W to 1.96 K/W with FC-770, 1.47 K/W to 1.87 K/W with FC-770 combined with copper wool, and 1.06 K/W to 1.50 K/W with water. The proposed cooling solution is passive and scalable, and is demonstrated to be practicable and effective in cooling 3D stacked electronics.Copyright