Influence of temperature and properties variation on nanofluids in building heating

Theoretical investigations on the application of nanofluids in the hydronic heating system of buildings have been presented in this paper. Realistic fluid velocities and temperatures employed in heat exchange equipment were used to compare the convective heat transfer coefficient and the pumping power requirement of nanofluids of various particle volume concentrations. The base fluid used in the analyses as a basis for comparison was a solution comprised of 60% ethylene glycol and 40% water (by weight). The thermophysical and heat transfer performance of the base fluid have been compared with copper oxide nanoparticles dispersed in the same base fluid with volumetric particle concentration ranging up to 4%. First, the variations of thermophysical properties such as viscosity, thermal conductivity, specific heat and density have been evaluated for different concentrations. Using these properties, several plots have been developed for quantitatively determining the enhancement of convective heat transfer coefficient as a function of flow velocity, Reynolds number and the nanoparticle volumetric concentration. For constant Reynolds number for flow in tubes, the Nusselt number and convective heat transfer coefficient increases as nanoparticle volume concentration increases. As the properties of nanofluids and the base fluid are temperature dependent, we have performed calculations for two fluid temperatures (323 K and 360 K). The results show that a higher convective heat transfer coefficient is achieved at higher temperatures for a nanofluid of equal concentration circulating at equal velocity. Frictional pressure loss and pumping power increases relative to the base fluid with particle volume concentration, but decreases with mean fluid temperature.