Nanofluid Heat Transfer in Laminar Pipe Flow With Twisted Tape

A comparative study with respect to the scaling of heat transfer of Newtonian titania nanofluid with concentrations of 5.0vol.% and 10.0vol.% is undertaken. For that purpose, experiments in circular flow pipe with inserted twisted tape having a pitch to diameter ratio of nearly 6 under constant heat flux are carried out in a Reynolds number range from 300 to 1300. The experimental data are ordered according to three different scaling approaches based on Reynolds number, on Reynolds and Prandtl numbers, and based on Reynolds and Prandtl numbers and nanoparticle concentration. It is found that, despite its relevance with respect to the flow dynamics, the first scaling is not sufficient to capture the heat transfer features properly. While the second approach is already based on the necessary and sufficient similarity numbers, the third approach does not provide deeper insight into the flow physics. It is concluded that description of laminar nanofluid pipe flow with inserted twisted tape based on a combination of Reynolds and Prandtl numbers is sufficient because two-phase flow effects like Brownian and thermophoretic diffusion are of minor importance. This finding is in agreement with most recent experimental results for the entrance region of laminar flow in empty pipes.