The development of ever more compact electronic circuits has brought the demands for thermal management to unprecedented levels. From CPUs and power amplifiers to lasers and solar cells, all of these devices are prevented from reaching inherent electrical operating limits because of internal heat generation and low temperature requirements of the transistor junctions. Thermal management solutions are needed to efficiently dissipate these concentrated heat loads and evaporation in the thin film regime is one promising technique.
Our proposed concept offers several advantages. The device architecture leverages nanoporous membranes to decouple the heat dissipation from the pressure drop of the device. With nanopores of diameter (~100 nm), high capillary pressures can be generated to achieve high mass flow rates necessary for evaporation with low liquid inlet to outlet pressure drops. By relying on capillarity as the main pumping mechanism, the power need to supply the manifold channels can be less than 10 W, suggesting an evaporator thermo-fluidic CoP greater than 100 is possible. This approach offers increased reliability, reduced complexity in fabrication to create a monolithic device, and the flexibility to be integrated with advanced solid-state thermal solutions in the future. The fluidic delivery to the evaporative region from the inlet microchannels is self-regulated by the capillarity of the membrane and eliminates the need for additional fluidic valves and active control on chip.
D. F. Hanks, Z. Lu, S. Narayanan, K. R. Bagnall, R. Raj, R. Xiao, R. Enright, E. N. Wang. "Nanoporous Evaporative Device for Advanced Electronics Thermal Management", in proceedings of IEEE ITherm, Orlando, FL, May 2014 (pp. 290-296).
We investigated enhancements in air-cooled heat exchangers by integrating a high performance blower into a loop heat pipe (LHP) with a single evaporator and multiple condensers which serve as fins to increase the surface area available to convection. The low thermal resistance of the LHP leads to a nearly isothermal surface temperature which improves convective heat transfer compared with a solid conducting heat sink. Traditional LHP design does not allow for stacking of components or parallel condensation pathways due to gravity-induced liquid pressure which are prone to flood some condenser layers. The multi-condenser LHP utilizes a sintered wick design in the condenser to stabilize the liquid-vapor interface and prevent liquid flooding of the lower condenser layers in the presence of a gravitational head.
While the vertical stacking of multiple condensers in a heat pipe increases the flexibility of geometries for these high performance heat exchangers, it also introduces new challenges to the loop heat pipe design. When multiple condensers are introduced, the gravitational pressure head resulting from a 10 cm column of water (1.0 kPa) is sufficient to flood the lower condenser layers inhibiting their capacity to condense vapor. Likewise if the heat pipe were inverted, there would be flooding of upper layers. A sintered wick was thus incorporated into the condenser to stabilize the liquid-vapor interface using capillary forces. The stability of the vapor-liquid interface at a wick surface depends on a differential pressure sustained between the vapor and liquid regions. In the LHP, this requirement is achieved using by reducing the liquid pressure in the two-phase compensation chamber.
Daniel F. Hanks, Teresa B. Peters, John G. Brisson, Evelyn N. Wang, H.A. Kariya, T.B. Peters, M. Cleary, D.F. Hanks, W.L. Staats, J.G. Brisson, E.N. Wang
Characterization of a Condenser for a High Performance Multi-Condenser Loop Heat Pipe
Development and Characterization of an Air-Cooled Loop Heat Pipe with a Wick in the Condenser
Daniel F. Hanks, Teresa B. Peters, John G. Brisson, Evelyn N. Wang,
H.A. Kariya, T.B. Peters, M. Cleary, D.F. Hanks, W.L. Staats, J.G. Brisson, E.N. Wang