Direct laser selective texturing to improve stainless steel surface condensation heat transfer without any chemical treatment

Abstract

Common organic modification of metal superhydrophobic surface has these problems such as serious environmental pollution, high thermal resistance, low processing efficiency, and easy to fall off. A chemical-free direct laser selective texturing technology is proposed to enhance condensation heat transfer performance on stainless steel surface. A micro-textured steel plate was processed by nanosecond pulsed laser to obtain a superhydrophilic surface of a square grid groove-convex structure. After heat treatment, the superhydrophilic surface changed to be superhydrophobic. The superhydrophobic surface was then laser selective textured to get a wedge-shaped superhydrophilic-superhydrophobic surface. Surface morphology, chemical composition and three-dimensional profile were analyzed. By comparing the superhydrophobic and superhydrophilic surfaces, the back-surface temperature and the average detached diameter of the condensed droplets was measured. Furthermore, the surface condensation heat transfer coefficient of samples was calculated according to the relevant condensing heat transfer theory. At the same time, samples with different area ratios of laser textured superhydrophilic-superhydrophobic were designed for heat transfer analysis. Condensation results showed that the heat transfer coefficient of the selective-textured surfaces was enhanced compared to the full laser textured superhydrophobic surface. After the condensed droplet grew to the superhydrophobic boundary of the laser selected area, it was restricted to grow and merge quickly and then was removed by self-transport in the wedge-shaped superhydrophilic area. The laser textured patterns showed a smaller detachment diameter than the fully superhydrophobic surface and had enhanced condensation heat transfer coefficient, as laser textured superhydrophilic-superhydrophobic areas are equal.