Experimental study of Al2O3/water nanofluid turbulent heat transfer enhancement in the horizontal double pipes fitted with modified twisted tapes

In this study, fluid flow of the Al2O3 nanofluid in a horizontal double pipe heat exchanger fitted with modified twisted tapes were experimentally studied under turbulent flow conditions. The experiments with different geometrical progression ratio (GPR) of twists as the new modified twisted tapes and different nanofluid concentration were performed under similar operation condition. Pitch length of the proposed twisted tapes and consequently the twist ratios changed along the twists with respect to the geometrical progression ratio (GPR) whether reducer (RGPR < 1) or increaser (IGPR > 1). Regarding the experimental data, utilization of RGPR twists together with nanofluids tends to increase heat transfer and friction factor by 12% to 52% and 5% to 28% as compared with the tube with typical twisted tapes (GPR = 1) and nanofluid. Contrarily, performances were weakened by using for IGPR twists 0.6 to 0.92 and 0.75 to 0.95. The thermal performances of the heat exchanger with nanofluid and modified twisted tapes were evaluated for the assessment of overall improvement in thermal behavior. Generalized correlations were developed for the estimation of Nusselt number, friction factor and thermal performance factor under turbulent flow conditions. Satisfactory agreement between the present correlations and obtained experimental data validate the proposed correlations. (C) 2014 Elsevier Ltd. All rights reserved.