Vapor chambers with hierarchical wick structures: Thermal performance and liquid film boiling visualization

This study unveils the effects of hierarchical wick structure on the heat transfer and boiling dynamics in vapor chambers. Through thermal resistance, boiling visualization, and dynamic thermal response experiments, the heat transfer mechanisms of vapor chamber with copper powder and copper mesh wicks, especially with hierarchical design, are investigated. The sintered powder wicks revealed a complex interplay between wick structure and thermal performance. The uniform coarse-powder wick provided low thermal resistance, due to its high permeability. The upward-coarser powder wick excelled at high heat fluxes due to its strong capillary force, while a thinner, similar wick experienced premature dry-out, leading to a maximum 30.8% increase in thermal resistance under high heat flux, as its reduced thickness could not sustain the required liquid inventory. For copper mesh samples, the upward coarser hierarchical structure with increasing porosity from bottom to top exhibits the lowest thermal resistance, with a slight increase at high heat flux. However, the upward finer hierarchical copper mesh wick experiences a maximum of 26.5% increase in thermal resistance compared to the upward coarser hierarchical wick due to insufficient capillary capability of the coarse-bottom mesh. Open-boiling visualization experiments further reveal that copper powder samples, benefiting from strong capillary-driven transport in their three-dimensional porous networks, sustain high-frequency bubble rupture at frequencies up to 2000 times per second under both high and low liquid levels. The upward-coarser mesh sustained high-frequency boiling even at low liquid levels, as its fine-mesh bottom layer provided both the capillary force to supply liquid and the nucleation sites to initiate boiling, a dual-functionality the other mesh wick lacked. The above findings facilitate the further in-depth understanding of the boiling phenomenon in vapor chamber and provide valuable guidance for optimizing the heat transfer performance of vapor chambers by hierarchical structures under large heat flux.