Heat transfer enhancement by helically twisted tapes inducing co- and counter-swirl flows

An experimental investigation has been conducted for determining heat transfer enhancement by inserting helically twisted tapes, to induce co- and counter-swirl flows, (the tapes are symbolized as co-HTT and C-HTT, respectively). Tape pitch ratio (p/D) was varied between 1.0 and 2.0, while tape width ratio (w/D) and twist ratio (y/w) were fixed at 0.2 and 3.0, respectively. The experiments were performed for fully developed turbulent flow with Reynolds number range (Re) between 6000 and 20,000, under uniform wall heat flux condition. At similar conditions, the use of Co-HTT results in lower Nusselt number and friction factor but higher thermal performance factor than that of C-HTT. Nusselt number and friction factor increase with decreasing pitch ratio, while thermal performance factor possesses opposite trend. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance factor as functions of the Reynolds number (Re), Prandtl number (Pr) and tape pitch (p/D), were developed through a multi-variant linear normal regression. (c) 2013 Elsevier Ltd. All rights reserved.