A Tree-in-a-Pipe: Design, Analysis and Realization of Superheated Loop Heat Pipes

Loop heat pipes (LHPs) are two-phase heat transfer devices in which a working fluid transfers its latent heat as it cycles between an evaporator and a condenser. In conventional LHPs, vapor-liquid coexistence in the compensation chamber forces the thermal cycle to remain close to the saturation and leads to two constraints – the temperature head condition and the sub-cooling condition. Additionally, the condensate film adds conductive resistance and may cause undesired oscillations in response to heat load steps. Motivated by vascular plants - the transpiration process of which is analogous to the operation of LHPs, we propose a new design of LHP with introductions of 1) nanoporous membranes allowing larger capillary pressures be maintained between the liquid and the vapor, 2) removal of the compensation chamber such that the entire liquid path can become superheated and hence decoupled from the saturation curve, and 3) a regulator maintaining sub-saturated state throughout the loop, eliminating liquid from the vapor path. We model the steady-state operation of both conventional LHPs and our superheated LHPs (SHLHPs), including the local thermodynamic equilibrium between phases to deal with the significant pressure differences across the membrane. Our analytical model shows that SHLHPs could: 1) extend the limitations of conventional LHPs imposed by thermodynamic properties of the working fluid, and 2) provide efficient heat transfer over long distances (>10 meters) and against large accelerations (>10 times gravity). For the successful realization of SHLHPs, two of the major challenges are the potential for increased hydraulic resistance relative to conventional wicks and the increased proneness to dry-out due to boiling along the superheated liquid. We choose a MEMS-based platform for the prototype SHLHP. Using high-doped p-type Si wafer, we electrochemically etch nanoporous Si membranes and integrate them with DRIE channels. The ability of the membrane to hold liquid under tension is tested by equilibrating water-filled device with various relative humidity and observing the cavitation events within individual voids underneath the membrane. Our membrane sustains a cavitation pressure (probability reaches 1/2) of ~ -27MPa at 15ºC. Silicon membranes with desired functionality are further incorporated with patterned glass substrates to form prototype MEMS-based SHLHPs. This MEMS-based SHLHP allows us to demonstrate the use of liquid at negative pressure in technologies. After scale-up, SHLHPs could be particularly valuable in applications such as cooling of avionics and energy management in buildings in which heat must be transferred over large distances and against gravity or acceleration.