LAUSR

Abstract This article presents an experimental study of convective heat transfer enhancement using combined techniques. One of these techniques was the use of twisted tape generating swirls along the whole tube length inserted in a micro-fin tube that effectively combined the features of extended surfaces, turbulators and artificial roughness. Nanofluids were used for improving the thermo-physical properties of the fluid. In this study, fluid flow of an Ag-water nanofluid in a micro-fin tube into which non-uniform twisted tapes were inserted was examined under turbulent flow. The effects of the twist ratios of non-uniform twisted-tapes (y/W in three series, 3.0 > 2.8 > 2.6, 3.0 > 2.6 > 2.2 and 3.0 > 2.4 > 1.8), tube and tape arrangements (counter and co-current arrangements), nanofluid concentrations (ϕ of 0.007, 0.016 and 0.03% by volume) were determined. The Nusselt numbers, friction factors and the thermal performance parameters of these devices were obtained and compared with the use of micro-fin tubes alone with a base fluid (water). Utilization of micro-fin tubes into which twisted tapes were inserted and through which nanofluids were allowed to flow tended to increase heat transfer rates over that of the base fluid. The experimental results revealed that heat transfer, friction loss and thermal performance factors increased with decreasing twist ratios for non-uniform twisted tapes and increasing nanofluid concentrations. Micro-fin tubes into which non-uniform twisted tapes were inserted in a counter-current arrangement gave higher heat transfer enhancement than the ones in a co-current arrangement. The optimum condition was found using the micro-fin tube with a non-uniform twisted-tape in a counter-current-arrangement with twist ratios in a series of 3.0 > 2.4 > 1.8 with a Ag-water nanofluid at a concentration of 0.3% by volume up to 112.5% for the heat transfer rate and 1.62 for thermal performance. Furthermore, the dependence of twist ratios, tape arrangement, nanofluid concentration and Reynolds number for enhancement of thermal performance and increased pump power due to higher pressure losses were analysed over the range of parameters considered.