Enhancing laminar forced convection heat transfer by using Al2O3/PCM nanofluids in a concentric double-tube duct

In this study, a tube with a smaller diameter was inserted in an isoflux heated circular tube to form a concentric double-tube duct. The forced convection heat transfer enhancement efficacy of using concurrent flows of Al2O3/PCM nanofluids through outer annulus/inner tube of this resulting double-tube duct to replace its parent single-tube (with water) was experimentally investigated. The results showed that a high total flow rate combined with a high concurrent flow ratio could effectively increase heat transfer effectiveness. When heating power = 200 W, Reynolds number = 1700, and the concurrent flow ratio = 2.38, for 1% Al2O3-water nanofluid in the outer annulus and 4.63% phase-change nanofluid in the inner tube, the convective heat transfer increased by 32%. Due to the excessive pressure drop, the figures of merit of the investigated objectives are all less than 1.