Aerothermal performance of square duct enhanced by twisted tape with inclined ribs and slots

Abstract To reduce the pressure drop penalties with substantial heat transfer enhancements for the channel flows enhanced by a twisted tape insert, the conjugate inclined ribs and slots are newly devised to modify the twisted tape. The simulated flow and pressure fields generated by the inclined rib-slot element along a twisted tape are correlatively examined with the measured Nusselt numbers and Fanning friction factors. The tilted transverse ribs convert the flow re-circulation cell behind a vertical rib to the solenoidal vortex and trip the separated flows. The inclined slots enable the mass, momentum and energy exchanges between the two axial swirls along the two spiral pathways segregated by the twisted tape. These flow mechanisms induced by the twisted tape and the inclined ribs and slots act synergistically to improve the heat transfer performance with the lessened rib-induced form drags. The average Nusselt numbers are raised to 4.2–3.8 time of the Dittus-Boelter values at the expenses of the Fanning friction factor augmentations to 32.5–40.2 time of the Blasius levels at the range of Reynolds number between 10,000 and 50,000. Compared with the tubular flows enhanced by the ribbed twist tape, the heat transfer enhancements caused by the rib-slot element along a twisted tape in a square duct are similarly retained but the reductions in Fanning friction factor are substantial. The resultant aerothermal performance improvements achieved by the inclined rib-slot element lighten the prospective applications to heat exchangers involving swirl generators. To assist these applications, the empirical correlations that calculate the average Nusselt number, Fanning friction factor and relative heat transfer power of a square duct enhanced by the twisted tape with the inclined slots and ribs are devised.