Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators

The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y = 4 and 5), three winglet- to duct-height ratios, (R-B = 0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R-p = 2, 2.5, 4 and 5) and a single attack angle of winglet, a = 300. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing RB but decreasing R. The inserted duct at R-B = 0.2, R-p = 2 and Y = 4-provides the highest heat transfer rate and friction factor but the one at R-B = 0.1, R-p = 2 and Y = 4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone. (C) 2014 Elsevier Ltd. All rights reserved.