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.

Design of micropillar wicks for thin-film evaporation

“…One phase-change based cooling strategy that has received significant interest is thin-film evaporation [7,8], which has been used in heat pipes [9,10] and can be used in other closed-loop configurations for high heat flux thermal management [6]. Thin-film evaporation relies on the phase-change of a thin liquid film (≈few microns) from the liquid-vapor interface of an extended meniscus near the three phase contact line [11].…”

Biphilic nanoporous surfaces enabled exceptional drag reduction and capillary evaporation enhancement

“…4(b)). Compared to the biporous wicks, 20 the CHF on the biphilic nanoporous coatings is 141.7% higher. The biphilic nonporous coatings can enhance the overall liquid supply and delay CHF conditions in two aspects: hydrophobic areas can effectively reduce the flow resistance while hydrophilic functional groups induce capillarity-enhanced liquid spreading.…”

“…6 We also compared our results with an evaporation study on biporous wicks with similar sample size and working conditions (i.e., 1 cm 2 heating area and 1.5 cm 2 wick area in vertical direction with 10 mm away from the water level to the center of heating area). 20 In the high heat flux region, the peak HTC of the present 80 nm biphilic MEMs is 13.4 W/(cm 2 ÁK), resulting in 22.9% higher than 10.9 W/(cm 2 ÁK) based on the heating area, which is 50% smaller than the wick area.…”

“…4(e)) and 94.5% over the biporous wick. 20 However, HTC downturn shows up with the further increase of coating thickness. For example, 1600 nm-thick biphilic nanoporous coatings even underperform 80 nm-thick coatings (Figs.…”

“…19 To achieve superior fluid flow and heat transfer rate during nucleate boiling and evaporation, hybrid surfaces consisting of both hydrophilic and hydrophobic features are highly desirable because the composite surface wettability is superior to liquid spreading by generating favorable capillarity and maintaining low friction. 20,21 Although these composite surfaces with microscale hydrophilic and hydrophobic areas have been developed to enhance capillary flow 22,23 and boiling heat transfer, 22,24 they can neither impact the nanoscopic slip length nor effectively reduce the pinning forces governed by the contact line. 25 CNT-enabled hydrophobichydrophilic nanoporous surfaces have been shown to substantially enhance nucleate boiling in our previous study by tuning surface wettability at nanoscale.…”