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Effect of Surface Hydration and Interfusion of Suspended Silica Nanoparticles on Heat Transfer

D. Milanova, X. Wu and R. Kumar
University of Central Florida, US

Keywords:
nanofluid, silica, hydration, heat transfer, critical heat flux

Abstract:
Nanofluid suspensions enhance thermal conductivity and the heat transfer. The authors have previously shown that the Critical Heat Flux (CHF) of NiChrome wire in a silica nanofluid can be increased by 200% compared to conventional fluids. Particle size, concentration and chemistry play a major role in improving the heat transfer characteristics and the critical heat flux. In this paper, we will report the effect of surface chemistry of amorphous silica, the process of surface hydration and interfusion of two particles in determining the CHF enhancement.
CHF is seen to increase as the concentration is increased to 0.5%, then decrease at 1% before increasing again at 2%. The reasons are attributed to an increase in the effective diameter which stays a constant after 0.5% concentration. Beyond this concentration, the number density of the particles increases. This unreported event is strongly correlated to surface chemistry. One of the factors for the discharging of nanoparticles is the dehydration due to particle’s surface shape and structure. At 0.2%, the particle has positive radius of curvature due to strong dispersion, while at 0.5%, the agglomeration results in negative curvature. Interaction between SiOH groups leads to dehydration, i.e., the formation of water molecules around the surface with negative radius of curvature. This process is accompanied by discharging of those sites and formation of immobilized water layer surrounding the uncharged SiOH surface.

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