Exploring the potential of nanofluids for heat transfer augmentation in dimpled compact channels: Non-intrusive measurements

Potential of the combination of nanofluids and dimpled channels for possible heat transfer enhancement has recently been demonstrated by Li et al. (2014) [36] in the context of small length scale channels through numerical simulations. The present work reports real time, non-intrusive experimental investigation of this concept wherein the convective heat transfer enhancement has been achieved with the coupled effects of Al2O3/water-based dilute nanofluids and surface roughened compact channels. One of the thermally active walls of the channel has been engraved with hemispherical inward dimples of 3 mm diameter with a pitch of 8 mm. Forced convection experiments have been conducted with water and Al2O3/water-based dilute nanofluids as the coolant medium for a range of Reynolds numbers. Real time measurements have been made in a completely non-intrusive manner using a Mach Zehnder interferometer and the images of the convective fields have been recorded in infinite as well as wedge fringe setting modes of the interferometer. Results revealed a clear influence of the combination of nanoparticles and surface protrusions on phenomena such as disruption of thermal boundary layer profiles, periodic flow separation and re-attachments along the principal wall of the channel. The coupled effects of inward dimples and nanoparticles resulted into a clear reduction in the thickness as well as flattening of the thermal boundary layer in comparison with that achieved with the base fluid. An enhancement by ≈66.5% in the average heat transfer coefficient with 0.05% volumetric concentration of alumina nanoparticles was achieved with respect to that of the de-ionized water at a Reynolds number of Re = 350.