Investigation of thermophysical and optical properties of nanofluids for heat transfer enhancement

Abstract

Nanofluid means a suspension containing evenly dispersed nanometer-sized particles in a base fluid. In the present study, we propose three kinds of nanofluids based on Therminol VP-1 and investigate the optical and thermophysical properties of the nanofluids for the direct absorption solar thermal collector. $Au@SiO_2$ and $Ag@SiO_2$ plasmonic nanofluids which make high absorption performance with localized surface plasmon effect are newly proposed. The graphite nanofluid is also proposed for the broad absorption enhancement in the solar spectrum. The demonstration of the thermal durability and dispersion stability has shown the applicability of proposed nanofluids in high temperature solar thermal collectors. To characterize the optical properties for nanofluids, the measurement system of absorption/scattering has been customized with the integrating sphere and the monochromator. The present study also proposes a determination of the absorption coefficient of nanofluids with an unknown refractive index from reflection and transmission spectra. The graphite nanofluids that can achieve the broad absorption in the solar spectrum are finally selected among the proposed nanofluids and their thermophysical properties such as thermal conductivity and dynamic viscosity are measured to develop the numerical simulation model. Furthermore, the experiment of the static solar thermal collector is conducted as well as a verification of the numerical simulation model using COMSOL. Consequently, the numerical simulation models in real scale are developed and it shows that the energy conversion efficiency is enhanced 38% point by only adding the 0.004 wt. % graphite nanopowder in Therminol VP-1. We finally achieve a 23% point higher energy conversion efficiency than a conventional surface-based type solar thermal collector with the direct absorption solar thermal collector employing graphite nanofluids.