Correlations on heat transfer rate and friction factor of a rectangular toothed v-cut twisted tape exhibiting the combined effects of primary and secondary vortex flows

Abstract

It is generally recognized that the heat transfer rate can be improved using active, passive and compound methods. The use of twisted tape inserts in a pipe flow is one of the passive methods. The influence of swirl flow, primary and secondary vortex flow generated due to twist in tape, v-cut on the tape, and the teeth on the v-cut respectively on Nu (heat transfer rate), f (friction factor), and CPE (criterion for performance evaluation) of an inserted toothed v-cut twisted tape is investigated in this study. Experiments were carried out on a double pipe heat exchanger using water as the working medium for a Reynolds number range of 6000–13,000, with the uncertainty analysis of the relevant parameters taken into account. The correlations for Nu and f in terms of factors such as Re (Reynolds number), Pr (Prandtl number), b/c (ratio of depth of v-cut to width of cut), and e/c (ratio of depth of teeth on v-cut to depth of v-cut) are also developed using regression analysis. The results of correlations lie within ±10% for Nu whereas within ±15% for f. The investigations show that the inclusion of teeth on the v-cut develops a secondary vortex flow which further enhances the Nu (though not up to the comparable magnitudes of the primary vortex) and is capable of minimizing the percent decrement of heat transfer rate with increasing depth of cut at higher Re. The friction factor increased up to 7.7 times for e/c ratio equals 0.14 (depth of the tooth to depth of the v-cut) in comparison to the plain tube, while the increase in heat transfer rate for the same was 84% for Re = 6000.