On the Surface Effects of Nanofluids in Cooling-System Materials
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On the Surface Effects of Nanofluids in Cooling-System Materials Gustavo J. Molina, Valentin Soloiu, and Mosfequr Rahman Department of Mechanical Engineering, Georgia Southern University, Statesboro, GA 304588045, USA, U.S.A. Corresponding author e-mail: [email protected] ABSTRACT Nanofluids are nano-size-powder suspensions in liquids that are of interest for their enhanced thermal transport properties. They are studied as promising alternatives as compared to ordinary cooling fluids, but the effects of nanofluids on wall materials are largely unknown. The authors developed an instrument that uses a low-speed jet on material targets to test such effects. The work is presented of the authors’ experimental research on the early interactions of selected nanofluids (2% weight of alumina nanopowders in distilled water, and in solutions of ethylene glycol in water) with aluminum and copper samples as typical cooling-system materials. The observed surface changes (and possible nanoparticle deposition) for test periods as long as 14 hours were assessed by roughness and volumetric-removal wear measurements, and by microscope studies. Comparative roughness measurements indicate that alumina nanofluids in water and ethylene glycol solutions can start surface changes on aluminum surfaces, but show no effects on copper for the same testing conditions. These investigations set a baseline for further research and provide a suitable method for the testing of nanofluids effects in cooling system-materials. INTRODUCTION Nanofluids are suspensions of solid metal (copper and gold), oxides (alumina, silica, titanium dioxide and copper oxide), carbides or nitrides nanoparticles, or of carbon nanotubes or nanofibers (typically up to 5%) in a continuous and saturated cooling fluid (as water, ethanol and ethylene glycol) [1-3]. Nanofluids are predicted to have higher thermal conductivity and heat transfer coefficients than those of the base fluids, because solids have much larger thermal conductivities than those of fluids, and nanoparticles have a much larger surface-to-volume ratio, and larger mobility than larger solid particles. Therefore, nanofluids are promising as coolants for critical-cooling systems, as nuclear systems [4], large engine radiators, and microchips [5]. Thermal and transport properties of nanofluids can be substantially different than those of the base fluids, such as increased thermal conductivity [6] and larger viscosity than those of the base fluids [7], and an abnormal single-phase convective heat-transfer coefficient [7,8]. Recent measurements of nanofluids effective thermal conductivity and viscosity found them to be substantially higher than those of the base fluids [9]. The heat transfer coefficient enhancements appear to go beyond a mere thermal-conductivity effect, because they cannot be predicted by traditional pure fluid correlations. These abnormal thermal properties may be partially explained by the very large surface to volume ratio and high of mobility nanosize particles [1,2,6,8], but a full
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