Vapor chambers with transformative evaporator wick designs capable of passively dissipating high heat fluxes over large areas, while maintaining low thermal resistances, can meet the thermal management needs of next-generation power semiconductor devices. Our prior work proposed a two-layer evaporator wick structure to enhance the performance of vapor chambers operating at high heat fluxes. The current study experimentally characterizes the capillary-fed boiling heat transfer behavior in such a two-layer evaporator wick, compared to a homogeneous (single-layer) wick, over a 1 cm2 evaporator area. The two-layer design comprises a thin base wick layer that is fed with liquid from a thick cap wick layer above using an array of vertical posts. The two-layer wick is fabricated using a sequence of sintering and laser-machining steps to form the base wick layer (200 mm), array of liquid-feeding posts, and cap wick layer (800 mm) using 90–106 mm copper particles. A test facility is constructed to replicate the conditions that exist at the evaporator of a vapor chamber; the novel facility design uses a physical restriction to prevent flooding of the wicks during testing. Two-layer wicks having 5 x 5 and 10 x 10 arrays of liquid feeding posts are characterized, along with a 200 mm-thick single-layer evaporator wick. The 10 x 10 array provides a >400% enhancement in the dryout heat flux compared to the single-layer wick. High-speed visualizations are used to identify the characteristic regimes of boiling operations for the wicks. The single-layer wick exhibits a partial dryout mode of operation, where a dry spot formed in the center of the heated evaporator area causes an increase in the thermal resistance with heat flux. In contrast, the distributed feeding provided by the two-layer wicks mitigates the development of this partial dryout regime and maintains a constant low resistance (~0.1 K/W) during capillary-fed boiling until a complete dryout event occurs. This study demonstrates the significant enhancement in dryout heat flux offered by the liquid-feeding approach realized in the two-layer evaporator wicks characterized here.
Evaporator wick, Two-layer wick, Dryou, t Capillary-fed boiling, Vapor chamber, Thermal resistance, High-heat-flux dissipation
Date of this Version
S. Sudhakar, J. A. Weibel, and S. V. Garimella, “Experimental Investigation of Boiling Regimes in a Capillary-Fed Two-Layer Evaporator Wick,” International Journal of Heat and Mass Transfer, Vol. 135, pp. 1335-1345, 2019.