Enhancement of Pool Boiling Heat Transfer by Brazed Aluminum Microporous Coating in Saturated Water
Godinez, Juan C.
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Surface treatment is an effective way to enhance pooling boiling heat transfer. It improves the critical heat flux (CHF) and/or the nucleate boiling heat transfer (NBHT). Surface treatment is developed in this experimental work by adding an aluminum microporous coating onto an aluminum surface. Specifically, a high temperature thermally conductive microporous coating (HTCMC) is developed for an aluminum surface. The coating is manufactured by brazing an aluminum powder onto a plain aluminum surface. The microporous coating enhances the NBHT by creating microscale pores. These pores provide reentrant cavities that enhance boiling. Their properties are controlled by varying the brazing conditions, which are highly dependent on the material properties of the surface, such as the brazing particle size and the brazing environment. Pool boiling tests are conducted on 1x1cm² aluminum block. They are conducted in saturated distilled water at 1 atm. A parametric experimental study is conducted by varying the thickness and particle size of the HTCMC coating. The averaged particle size used in this study were 11, 24, 66 µm and the coating thickness was varied from 76 to 357 µm. A reference pool boiling curve for the plain aluminum surface is also presented. Here, enhancing pool boiling CHF is accomplished by Boehmite treatment of the bare aluminum surface. An aluminum surface can develop different hydroxides when submerged in distilled water. Under controlled experimental conditions defined in this work, the aluminum surface develops a stable nanoscale Boehmite layer which decreases the static contact angle, from 55° to 12°, and consequently increases the wettability. The Boehmite treated plain aluminum surface has a CHF of 1,850 kW/m² and a NBHT coefficient of 55.7 kW/m²K. Compared to the plain aluminum surface, the aluminum HTCMC has improvement of more than double in the NBHT coefficient for all the tests conducted in this work. However, the CHF of aluminum HTCMC has no change compared to that of the plain aluminum surface. The study also shows an increase in the NBHT coefficient as the coating thickness of the aluminum HTCMC is increased regardless of particle size. However, after reaching an optimal coating thickness the NBHT coefficient starts to decrease. This optimal coating thickness for a mean particle diameter of 11, 24, 66 µm were 152, 176, 206 µm, respectively. The highest measured NBHT coefficient, 293.5 kW/m²K, was achieved with particle size of 11 µm with a thickness of 152 µm and enhancement of more than five times over the NBHT of Boehmite treated plain aluminum surface.