Daniel F. Hanks

Nanoporous evaporative device for advanced electronics thermal management

We report the design, fabrication and modeling of a thin film evaporation device for cooling of high performance electronic systems. The design uses a membrane with pore diameters of ~100 nm to pump liquid via capillarity to dissipate the high heat fluxes. Viscous losses are minimized by using a thin membrane (~200 nm) which is supported by a ridge structure that provides liquid supply channels. As a result, the external pumping requirements are low, enabling an integrated cooling device with a large coefficient of performance. By integrating the cooling solution directly into the substrate, the thermal resistance of the spreader and interface material are removed entirely. Pentane is used as the working fluid based on its dielectric properties, surface tension and latent heat of vaporization. We first developed a model to capture the heat and fluidic transport within the membrane and supporting ridge structure using conservation of mass, momentum and energy. Using the model, we conduct a parametric sweep of the ridge and membrane geometries to elucidate their influence on thermal performance. We then show how the temperature of hot spots can be managed with a customized cooling solution while independently managing the temperature of background heated regions through variation in the membrane porosity over a realizable range of 10 - 50%. This work provides design guidelines for the development of a high performance evaporator device capable of dissipating the extreme heat fluxes (> 1 kW/cm2) required for next generation high power electronic devices.

High heat flux evaporation from nanoporous silicon membranes

We investigated the evaporative cooling performance of a nanoporous membrane based thermal management solution designed for ultra-high heat flux dissipation from high performance integrated circuits. The biporous evaporation device utilizes thermally-connected, mechanically-supported, high capillarity membranes that maximize thin film evaporation and high permeability liquid supply channels that minimize viscous pressure losses. The 600 nm thick membrane was created on a silicon on insulator (SOI) wafer, fusion-bonded to a separate wafer with larger liquid channels. Overall device performance arising from non-uniform heating and evaporation of methanol was captured experimentally. Heat fluxes up to 412 W/cm2 over an area of 0.4×5 mm, at a temperature rise of 24.1 K from the heated substrate to ambient vapor, were obtained. These results are in good agreement with a high-fidelity coupled fluid convection and solid conduction compact model that incorporates non-equilibrium and sub-continuum effects at the liquid-vapor interface. This work provides a proof-of-concept demonstration of our biporous evaporation device. Simulations of the validated model at optimized operating conditions and with improved working fluids, predict heat dissipation in excess of 1 kW/cm2 with a device temperature rise under 30 K, for this scalable cooling approach.

Development and Characterization of a Loop Heat Pipe With a Planar Evaporator and Condenser

We present the development and characterization of an air-cooled loop heat pipe with a planar evaporator and condenser. The condenser is mounted vertically above the evaporator, and impellers are integrated both sides of the condenser with tight clearance. The planar geometry allows for effective convective cooling by increasing the surface area and the convective heat transfer coefficient. To ensure condensation across the area of the condenser, a wicking structure is integrated in the condenser. The evaporator incorporates a multi-layer wicking structure to maintain a thermal gradient between the vapor and liquid regions, which is used to sustain the vapor and liquid pressures necessary for operation. The loop heat pipe was demonstrated to remove 140 W of heat at a temperature difference between the evaporator base and inlet air of 50 °C. This work is the first step towards the development of an air-cooled, multiple-condenser loop heat pipe.Copyright

Integration of a Multiple-Condenser Loop Heat Pipe in a Compact Air-Cooled Heat Sink

We present the characterization of a compact, high performance air-cooled heat sink with an integrated loop heat pipe. In this configuration, heat enters the heat sink at the evaporator base and is transferred within the heat pipe by the latent heat of vaporization of a working fluid. From the condensers, the heat is transferred to the ambient air by an integrated fan. Multiple condensers are used to increase the surface area available for air-cooling, and to ensure the equal and optimal operation of the individual condensers, an additional wick is incorporated into the condensers. We demonstrated with this design (10.2 cm × 10.2 cm × 9 cm), a total thermal resistance of less than 0.1 °C/W while dissipating a heat load of 500 W from a source at 75 °C. Furthermore, constant thermal resistance was observed in the upright as well as sideways orientations. This prototype is a proof-of-concept demonstration of a high performance and efficient air-cooled heat sink design that can be readily integrated for various electronics packaging and data center applications.© 2013 ASME

Scaling the performance of an air-cooled loop heat pipe with the addition of modular condensers

We report the performance of a novel air-cooled loop heat pipe with a planar condenser and planar evaporator, and the performance improvement expected from a multiple-condenser version of the device. The condensers are developed in a modular design, and the heat pipe is configured to allow the installation of multiple, stacked condensers. An analytical model is developed to predict the scaling of convective thermal resistance with the addition of condensers. The result of the model is compared with the thermal resistance measured from a single-condenser prototype (0.18-0.23°C/W), and the effect of condenser count on thermal resistance is discussed. The heat pipe is intended for the integration into a high-performance air-cooled heat sink.

Characterization of a Condenser for a High Performance Multi-Condenser Loop Heat Pipe

We experimentally characterized a condenser design for a multi-condenser loop heat pipe (LHP) capable of dissipating 1000 W. The LHP is designed for integration into a high performance air-cooled heat sink to address thermal management challenges in advanced electronic systems. The multi-layer stack of condensers utilizes a sintered wick design to stabilize the liquid-vapor interface and prevent liquid flooding of the lower condenser layers in the presence of a gravitational head. In addition a liquid subcooler is incorporated to suppress vapor flashing in the liquid return line. We fabricated the condensers using photo-chemically etched Monel frames with Monel sintered wicks with particle sizes up to 44 μm. We characterized the performance of the condensers in a custom experimental flow rig that monitors the pressure and temperatures of the vapor and liquid. The condenser dissipated the required heat load with a subcooling of up to 18°C, while maintaining a stable liquid-vapor interface with a capillary pressure of 6.2 kPa. In the future, we will incorporate the condenser into a loop heat pipe for a high performance air-cooled heat sink.Copyright