Pressure drop and heat transfer characteristics of nanofluids in horizontal microtubes under thermally developing flow conditions

• Heat transfer characteristics of nanofluids with TiO2 and Al2O3 nanoparticles were investigated.
• The experiments were performed in microtubes under thermally developing conditions.
• Experimental friction factor coefficients are in good agreement with existing correlations.
• Experimental heat transfer coefficients were predicted within ±15% with existing correlations.
• A consistent enhancement in heat transfer was observed, once Reynolds number goes beyond 1500.

This study presents pressure drop and heat transfer characteristics of water based nanofluids with TiO2 and Al2O3 nanoparticles of various mass fractions in horizontal smooth hypodermic microtubes with an outer diameter of ∼717 μm and an inner diameter of ∼502 μm over a wide variety of Reynolds numbers under hydrodynamically fully developed and thermally developing conditions. For this purpose, TiO2 and Al2O3 nanoparticles of 20 nm average solid diameters were added to deionized water to prepare nanofluids with mass fractions of 0.01–3 wt.%, and prepared nanofluids were characterized by standard methods such as Dynamic Light Scattering (DLS), Scanning Electron Microscopy (SEM), and zeta potential measurements. Experimental friction factor coefficients were predicted within ±10% and are in good agreement with existing analytical predictions, while experimental heat transfer coefficients were predicted within ±15% with existing correlations for single phase flow. Our results show that there is no considerable heat transfer enhancement for Re < 1000. A consistent enhancement in heat transfer was observed (for average heat transfer coefficient up to 25%), once Reynolds number goes beyond 1500. At low Reynolds numbers, flow is mainly laminar. However, at higher Reynolds numbers, flow starts to transition to turbulent flow, when heat transfer enhancement is also observed. Under these conditions, the enhancement in heat transfer increases with mass fraction.