Wettability on Nanoparticle Modified Surface: For Thermal Engineering

摘要

In many thermal engineering applications, manipulation of the wetting behavior of liquids is used as a heat transfer enhancement strategy. Hydrophilicity, implying better wettability of liquid on solid surface, is preferred in some processes of air-conditioning and power-generation systems, such as dehumidification, evaporative cooled condensers, and pool boiling at high heat flux. Recent research in nanofluid boiling has revealed a manufacturing technique that is promising for air-conditioning and refrigeration applications. Nanofluid boiling on a solid surface induces deposition of the particles on the boiling surface, and these surfaces exhibit enhanced surface wettability. The properties of the deposited nanoporous layer are affected by the nanofluid boiling parameters such as nanoparticle concentration, nanoparticle type and size, solvent liquid type, boiling surface roughness, heat flux, boiling deposition duration and so on. In this thesis, an investigation of the effect of the nanofluid boiling conditions on the resulting wetting behavior of the treated surface is presented. Understanding how the fabrication process influences the wettability enhancement will guide the design of a surface treatment technique to achieve super-hydrophilicity. Experimental results show that boiling duration positively affects wettability, but little additional enhancement occurs for durations beyond 10 minutes of NBND. Surface wettability change by NBND is independent of boiling heat flux if the particle concentration is 1 wt.%, while at a low nanoparticle concentration of 0.01 wt.%, heat flux has some random influence. The overall systematic trend observed in the experimental study is that, the higher the nanoparticle concentration, the higher the wettability after NBND process, and at the same time the rougher the surface. The goal is to obtain superhydrophilic surface, which is achieved at high particle concentration (1%wt) NBND. Microscopic analysis gives evidence of particle deposition after NBND. Nano-micro structures were studied using SEM. The images show the growth of "nano-grass" like pseudoboehmite on aluminum surfaces after NBND using alumina nanofluids. Surface roughness factor was obtained from AFM scan and contact angle measurements independently, but show good agreement. If the coating was to be applied on fins to enhance their wettability, the height of a droplet on the fin surface would be a parameter that would affect the optimization of fin spacing. The higher the wettability, the lower the height of a droplet of fixed volume, so close fin spacing could be for dehumidification. Also, wetting experiments on rough surfaces with a porous coating by NBND at high nanoparticle concentration reveals the involvement of imbibition effect. This suggests the Hemi-wicking mode of wetting. In this thesis, the solid fraction ϕ is determined by contact angle data analysis and confirmed by linear fitting of data. Durability of the treated surface under dry conditions was studied by exposure to air. Air-borne contamination reduces surface wettability, but the NBND treated surface remained more hydrophilic than the untreated surfaces. Eventually, the surface treatment loss its ability to enhance wettability. A possible solution is recommendation as a future work. Overall, this study provides an understanding of wettability changes by nanofluid boiling nanoparticle deposition, and provides a guidance to the wettability treatment for thermal engineering applications.