The role of vapor venting and liquid feeding on the dryout limit of two-layer evaporator wicks

Abstract

Vapor chambers developed for high-heat-flux operation require advanced evaporator wick designs that can sustain capillary flow when boiling occurs over the heater region. A two-layer evaporator wick integrates a thin base wick layer that is supplied with liquid from a thick cap layer through an array of vertical feeding posts distributed over the heated area. This design allows boiling to occur within the thin base layer, while separating the incoming liquid feeding and outgoing vapor venting pathways. In our prior work, boiling in two-layer wicks was experimentally demonstrated to provide high-heat-flux dissipation over larger heater areas and at low thermal resistance. The current study experimentally explores the effect of two-layer wick design parameters, specifically the dimensions that alter the area available for liquid feeding and vapor venting, on the thermal performance and dryout limit of the wick, using water as the working fluid. Four different two-layer wick designs are fabricated over a 1 cm 2 evaporator area by sintering 180-212 μm copper particles. Increasing the vapor-venting area from 7% to 16% of the total evaporator area yielded a significant increase in the dryout limit, from 315 W/cm 2 to 405 W/cm 2 . Increasing the liquidfeeding area using wider posts increased the dryout limit further. Finally, a parametrically optimized design with fewer but larger posts and vents resulted in better performance compared to a design with denser features. With this two-layer wick design, we demonstrate an extremely high dryout limit of 512 W/cm 2 over the large 1 cm 2 heated area at a thermal resistance of 0.08 K/W.

Coupling droplets/bubbles with a liquid film for enhancing phase-change heat transfer

“…For evaporation/boiling, dynamic bubbles are activated in the wicking structure to promote convection of capillary liquid film, to enlarge the evaporation area near the tri-phase contact lines of the bubbles, and to accelerating vapor escape by bubble bursting at the liquid film surface. Such a hybrid evaporation-nucleate boiling mode in a capillary liquid film, referred to as liquid film boiling heat transfer has demonstrated both high CHF and HTC ( Ćoso et al., 2012 ; Sudhakar et al., 2019a ; Sudhakar et al., 2020 ; Weibel et al., 2010 ; Wen et al., 2018b ).…”

“…Maintaining sufficient liquid supply to wet the heated area and maximizing heat dissipation across the liquid fluid are the key to enhancing both CHF and HTC of evaporation and boiling processes ( Plawsky et al., 2014 ; Sudhakar et al., 2020 ; Wen et al., 2018a ). It is thus desirable to have a surface that has high capillarity and high permeability to spread the liquid over the heated area for large CHF while minimizing the overall thermal resistance across liquid film for high HTC.…”

“…The dynamic behaviors of the bubbles, e.g. , nucleation, growth, escape, and bursting, can frequently disturb liquid film that is confined within the wicking structure, increasing convection heat transfer through the liquid film ( Ćoso et al., 2012 ; Palko et al., 2015 ; Sudhakar et al., 2020 ; Weibel et al., 2010 ; Zhang et al., 2018 ). The bubbles formed in the wicking structure can also increase additional liquid-vapor interfaces for liquid vaporization near the tri-phase contact line of the bubbles and carry heat away directly from the heated substrate by bubble growth, motion, and bursting.…”

Heat dissipation performance of grooved-type and copper foam-type vapor chambers

“…The current research progress of vapor chamber lie in as follows: (1) The thinner vapor chamber [11] puts forward high requirements for the manufacturing process; (2) The compound capillary wicks [12,13] are applied to alleviate the contradiction between the capillary force and flow resistance in vapor chamber; (3) The wick optimization and matching between the evaporating and condensing surface [14].…”