3D engineered capillary evaporators for enhanced critical heat flux and heat transfer coefficient in pool boiling and vapor chamber

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Nasersharifi, Yahya
Hwang, Gisuk

A major challenge in pool-boiling heat transfer lies in the limit of the Critical Heat Flux (CHF) and Heat Transfer Coefficient (HTC), which is caused by the poor counterflow of liquid and vapor over a heated surface, especially at high heat fluxes. To overcome this challenge, we favorably controlled liquid and vapor flow for efficient phase separation using 3D engineered wicks, i.e., monolayer, columnar, and mushroom post wicks, thereby improving the CHF and HTC. The monolayer wicks in the pool boiling test with and without a mushroom post structure enhanced the CHF by 20% and 87%, respectively, compared with a plain surface. It was found that the CHF enhancement was attributed to the pitch distance, 3.5 mm, which effectively reduced hydrodynamic instability (Rayleigh–Taylor) wavelength. A further reduction of the pitch distance of 1 mm resulted in a 250% improvement in the CHF, in agreement with theory. The columnar and mushroom posts with monolayers increased HTC by tenfold compared with the plain surface. This increase was related to the reduced conduction path through the thin monolayer wick in the controlled vapor region, and through the columnar and mushroom post wicks (a vapor chamber-like environment in pool boiling). The wicks also improved the HTC and CHF in the vapor chamber where a similar technical challenge remains, i.e., surface dryout. The wick performances were tested using a face-down test setup to focus only on the capillary-driven liquid supply. The effects of individual design parameters such as particle size and distance, and the number of wick layers were investigated separately. The wicks were fabricated using 30–200 μm copper particles, while the post and phase-separating sections used 200 μm particles for all samples. The results showed that 60 μm particles resulted in the minimal wick superheat for the monolayer wicks. Additionally, it was found that the two- and three-layer wicks substantially decreased the wick superheat compared with the monolayer wick.

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Thesis (Ph.D.)-- Wichita State University, College of Engineering, Department of Mechanical Engineering