Thermal Management

Thermal management is a significant challenge due to the increasing heat generation rates in commercial electronics, energy (solar cells, fuel cells, etc.), space, and defense systems. In many of these applications, thermal management is the bottleneck towards achieving optimal performance.

The DRL seeks to develop novel thermal management solutions to meet the significant demands in these various applications.

Hotspot Thermal Management

Thermal management is a primary design concern for numerous power-dense equipment such as power amplifiers, solar energy convertors, and advanced military avionics [1]. During operation, these devices generate large amounts of waste heat (> 1 kW/cm2) from sub-millimeter areas. These concentrated heat loads are spatially and temporally non-uniform and cause hotspots which are localized regions with extreme heat flux and exceedingly high temperature [2] that can adversely impact device performance and reliability [3].

In Device Research Laboratory, we study phase-change-based thermal management techniques targeted for cooling hotspots. Our approach uses capillary-fed thin-film evaporation. We experimentally characterized hotspot cooling with a silicon microstructured device via thin film evaporation in the absence of nucleate boiling and dissipated ultra high heat fluxes (6.0 kW/cm2) from a 620x640 µm footprint when the hotspot temperature was 290 °C. The average temperature over the entire 1x1 cm2 microstructured evaporative area as well as the local temperatures within a 3 mm radius from the hotspot were significantly lower (< 50 °C) than the hotspot temperature, indicating significant temperature gradient in the vicinity of the hotspot due to reduced thermal conductivity of silicon at high temperature.

Our experimental results show that the capillary-limited dryout heat flux decreases by creating concurrent hotspots over the 1x1 cm2 microstructured area as well as by superposing moderate uniform background heating with the hotspot. Despite the decrease in the dryout heat flux, the total heating power dissipated via thin-film evaporation increases by spatially distributing the hotspots over the microstructured surface and by superposing mild uniform background heating with the hotspot. We also observed that the dryout heat flux is insensitive to the location of the hotspot, i.e., the dryout heat flux remained within the experimental error when the hotspot was created at different locations within the microstructured surface. We attribute this to the two-dimensional fluidic transport within the micropillar wick in our device.

In addition to experimental characterization, in Device Research Laboratory, we are engaged in developing theoretical models. We developed a semi-analytical thermal-fluidic model that compares reasonably well with our experiments. The model captures both the fluidic and thermal transport within the micropillar wick. Our experimental characterization and modeling highlight the promise of thin-film evaporation as a viable thermal management strategy for dissipating highly localized extreme heat fluxes from sub millimeter areas from high performance power electronics and radio frequency devices, where dissipating ultra-high heat fluxes is a significant challenge.

The work was funded by the Office of Naval Research (ONR) with Mark Spector as program manager and the National Research Foundation Singapore through the Singapore MIT Alliance for Research and Technology's LEES IRG research program.

Air Cooled Heat Exchangers (Past project)

One project is developing a high performance air-cooled heat sink in collaboration with Prof. John G. Brisson in Mechanical Engineering, Prof. Jeffrey H. Lang in Electrical Engineering and Computer Science and Lockheed Martin. The Pumped Heat Exchanger (PHUMP) seeks to dissipate 1000 W with using only 33 W of electrical power with a thermal resistance of 0.05 K/W in a 4" x 4" x 4" volume. We are incorporating a multiple-condenser loop heat pipe, a blower with multiple impellers and a low-profile motor to achieve these metrics. A loop heat pipe is an enclosed two-phase system with an inherently high effective thermal conductivity due use of the high energy of vaporization. The working fluid evaporates where the heat pipe is in contact with the heat source and condenses where convective cooling occurs. By integrating the blower impellers along each wall of the condensers, high convective heat transfer is achieved with small air flow velocities and therefore low pumping power. This research is supported by DARPA, under the MACE program.

  1. I. Mudawar, "Assessment of high-heat-flux thermal management schemes," IEEE Trans. Compon. Packag. Technol., vol. 24, pp. 122-141, 2001.
  2. A. Bar-Cohen and P. Wang, "Thermal management of on-chip hot spot," J. Heat Transfer, vol. 134, p. 051017, 2012.
  3. J.-M. Koo, et al., "Integrated microchannel cooling for three-dimensional electronic circuit architectures," J. Heat Transfer, vol. 127, pp. 49-58, 2005.