Effect of Al O /Water and CuO/Water Nanofluids on Heat Transfer Enhancement Flows Through Twisted Clockwise–Counter-Clockwise Square Duct

Fluid flow and heat transfer of Al O /water and CuO/water nanofluids with 1–3% volume fraction of nanoparticle in a clockwise–counter-clockwise twisted square duct (TSD) are numerically simulated using the κ- - ϵ turbulence model. Nanofluids flow through a twisted duct with uniform wall temperature boundary conditions, twist ratios of 8.5, 11.5, 16.5 and Reynolds numbers of 100–20,000 are studied numerically. The results show that reducing twist ratios from 16.5 to 8.5 improves heat transfer significantly. In comparison with the results obtained using simple base water, the average percentage increase in Nusselt number is around 0.1–0.5% higher at 1% volume concentration of Al O nanoparticle and 0.5–1% increment in average Nusselt number is observed for 1% concentration of CuO. It was also discovered that increasing the concentration of Al O nanoparticle in normal water from 2–3% reduces heat transfer enhancement, whereas the CuO nanoparticle enhances heat transfer coefficient for the same volume fraction of nanoparticle, and this enhancement is found to be within 0.5%. To compare twisted and straight ducts, an enhancement factor or quantity is calculated based on constant pumping power criteria. The enhancement factor is used to prioritize the twisted square duct selections. Performance evaluation criteria show that the twisted duct performs well up to the Reynolds number of 2000, while beyond the Reynolds of 2000, this twisted duct is not suitable due to strong presence of reverse flow. It is recommended to use twisted clockwise–counter-clockwise square duct for the Reynolds number till 2000. Correlations involving swirl parameters such as twist ratios, percentage of volume fraction of nanoparticles and Reynolds number are also proposed separately for friction factor and Nusselt number.