Enhanced Heat Transfer in Biporous Wicks in the Thin Liquid Film Evaporation and Boiling Regimes

Abstract

Biporous media consisting of microscale pin fins separated by microchannels are examined as candidate structures for the evaporator wick of a vapor chamber heat pipe. The structures are fabricated out of silicon using standard lithography and etching techniques. Pores which separate microscale pin fins are used to generate high capillary suction, while larger microchannels are used to reduce overall flow resistance. The heat transfer coefficient is found to depend on the area coverage of a liquid film with thickness on the order of a few microns near the meniscus of the triple phase contact line. We manipulate the area coverage and film thickness by varying the surface area-to-volume ratio through the use of microstructuring. Experiments are conducted for a heater area of 1 cm2 with the wick in a vertical orientation. Results are presented for structures with approximately same porosities, fixed microchannel widths w ≈ 30 μm and w ≈ 60 μm, and pin fin diameters ranging from d = 3–29 μm. The competing effects of increase in surface area due to microstructuring and the suppression of evaporation due to reduction in pore scale are explored. In some samples, a transition from evaporative heat transfer to nucleate boiling is observed. While it is difficult to identify when the transition occurs, one can identify regimes where evaporation dominates over nucleate boiling and vice versa. Heat transfer coefficients of 20.7 (±2.4) W/cm2-K are attained at heat fluxes of 119.6 (±4.2) W/cm2 until the wick dries out in the evaporation dominated regime. In the nucleate boiling dominated regime, heat fluxes of 277.0 (±9.7) W/cm2 can be dissipated by wicks with heaters of area 1 cm2, while heat fluxes up to 733.1 (±103.4) W/cm2 can be dissipated by wicks with smaller heaters intended to simulate local hot-spots.