Biomimetic Structures by Leaf Vein Growth Mechanism for Pool Boiling Heat Transfer Enhancements

Abstract

Novel biomimetic structures are designed and synthesized to enable liquid water flowing from the periphery to the center of a heating surface, and promote local liquid wicking and active nucleation site density, which enhance the critical heat flux (CHF) and heat transfer coefficient (HTC) of pool boiling heat transfer. The biomimetic structures are designed by mimicking leaf vein growth from an initial point of petiole to an architecture that supplies water and nutrition to the whole leaf lamina. The in-plane flow channels are developed to distribute liquid toward bubble nucleation sites on a heating surface. The in-plane flow channels are then fabricated by: 1) machining a solid biomimetic groove structure at 250 µm height on plain copper surfaces, 2) depositing ~70 nm diameter copper nanowires (CuNWs) of height 25 µm on the solid biomimetic groove structure, and 3) sintering ~25 µm diameter copper powder to create porous biomimetic structures at various heights. The experimental pool boiling results on different structures show that the sintered porous biomimetic structure at 1.0 mm height yields the highest CHF of 343.1 W/cm2, an increase of 206% over that on a polished plain copper surface, which is due to the biomimetic channels providing easier access of liquid water flow toward the dry-out spots as indicated by a liquid inflow factor. The biomimetic structure of CuNWs has dramatically increased CHF and heat transfer coefficient (HTC) than that of a plain surface and a solid biomimetic structure. A theoretical analysis of the liquid thin film beneath hovering bubbles reveals that the population density of vapor stems in the liquid thin film increases with a decrease of the vapor stem diameter as heat flux increases. Moreover, the porous biomimetic structures take advantage of active nucleation sites and their wicking effect to delay the hydrodynamic instability of the liquid thin film, thus increasing the pool boiling heat transfer.