Heat transfer enhancement by multiple twisted tape inserts and TiO2/water nanofluid

The aim of this present work is to enhance thermal performance characteristics in a heat exchanger tube by studying: (i) multiple twisted tapes in different arrangements; (ii) TiO2 nanoparticles with different concentrations as the working fluid. The tube inserted the multiple twisted tapes showed superior thermal performance factor when compared with plain tube or the tube inserted a single twisted tape, due to continuous multiple swirling flow and multi-longitudinal vortices flow along the test tube. The higher number of twisted tape inserts led to an enhancement of thermal performance that resulted from increasing contact surface area, residence time, swirl intensity and fluid mixing with multi-longitudinal vortices flow. Moreover, arrangement of twisted tapes in counter current was superior energy saving devices for the practical use, particularly at low Reynolds number. This was especially the case for quadruple counter tapes in the cross directions (CC-QTs) where heat transfer enhancement with relatively low friction loss penalty was deserved. The use of CC-QTs led to the highest thermal performance factor up to 1.45. Using water with TiO2 nanoparticle as a working fluid yielded a higher thermal performance than using pure water. The tube inserted CC-QTs with TiO2/water nanofluid at concentration of 0.21% by volume provided the highest thermal performance factor 1.59, where heat transfer rate and friction factor increased to 3.52 times and 11.7 times of those in the plain tube with water as the working fluid. (C) 2014 Elsevier Ltd. All rights reserved.