Experimental investigation of heat transfer performance and frictional loss of functionalized GNP-based water coolant in a closed conduit flow† K. H. Solangi, * a Ahmad Amiri, * b M. R. Luhur, c Soheila Ali Akbari Ghavimi, d S. N. Kazi, * a A. Badarudin a and Mohd Nashrul Mohd Zubir a The convective heat transfer coefficient and friction factor for fully developed turbulent flow of trimethylolpropane tris[poly(propylene glycol), amine terminated] ether-treated graphene nanoplatelet (TMP-treated GNP)-based water coolants are experimentally determined at constant velocity flowing through a horizontal copper tube with uniform heat fluxes. The TMP-treated GNP was first analyzed in terms of structure and morphology to confirm the GNP functionalization with TMP. The colloidal stability of TMP-treated GNP-based water coolant shows the high potential of the coolants for using in heat transfer equipment. Then, the experiments were conducted at a Re range of 3900–11 700 at constant velocity flow (1–3ms 1 ) and concentrations of 0.025 wt% to 0.1 wt%. The enhancement in thermal conductivity for TMP-treated GNP-based water coolants was between 20% and 31% compared to the basefluid. The convective heat transfer coefficient for the TMP-treated GNP-based water coolant was found to be up to 107% higher than the basefluid. The Nusselt number increased up to 72% at a heat flux of 23 870 W m 2 . However, the friction factor drop increases simultaneously in the range 4–10%. The results suggest that TMP-treated GNP-based water coolants could function well as working fluids in heat transfer applications and provide good alternatives to conventional working fluids. 1. Introduction Nanouids are suspensions obtained from dispersing different nanoparticles in host uids to enhance thermal properties. 1–3 They have better thermal properties than conventional heat transfer uids. 4–13 Over the past two decades, nanouids have exhibited remarkably improved thermal conductivity, stability, and heat transfer coefficients as well as reduced overall plant consumption and costs. Nanouids have great application potential in several elds. Nanouids are increasingly utilized in different heat exchangers to optimize energy consump- tion. 14–16 Among all nanouids carbon-alotropes based nano- uids have attracted tremendous attention due to its unique thermal, electrical, and mechanical properties, 17–20 which have very high thermal conductivity with 2-D structure for phonon transport and offer an interface contact area with polymer matrix resulting in the improvement of the various properties of the composite. Graphene is an allotrope of carbon atoms which has drawn attention of researchers recently due to its superior properties, such as high elastic modulus, good electrical conductivity, good thermal conductivity, and self-lubricating behaviour. A favourable thermo-physical property of graphene has made it an excellent candidate for use in nanouids. 21 Graphene nanoplatelets (GNPs) are used as ller in epoxy resin, natural rubber and other polymer matrix to enhance their thermal, electrical, and mechanical properties. GNPs are the promising candidate material for the application in thermal management. 22–25 It is evident that nanouids improve thermo-physical prop- erties, such as the thermal diffusivity and the thermal conduc- tivity, provide excellent stability and convective heat transfer coefficients, and only slightly increase the pressure drop and required pumping power. 26–30 Many studies have been conducted to enhance the thermal properties of heat transfer uids by adding highly thermally conductive nanoparticles. 31–33 Recently, a signicant number of studies have been performed on carbon- based nanostructures, including carbon ber, 34 carbon black, 13,35 carbon nanotubes (CNTs), 36 graphite, graphene oxide (GO), gra- phene, and graphene nanoplatelets. 32,37–41 An experimental investigation of the convective heat transfer coefficient for nanouids owing through different types of tubes has been conducted in several studies, and these have considered a Department of Mechanical Engineering, University of Malaya, Kuala Lumpur, Malaysia. E-mail: solangi.quest@gmail.com; salimnewaz@um.edu.my b Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad, Iran. E-mail: ahm.amiri@gmail.com c Department of Mechanical Engineering, Faculty of Engineering, Quaid-e-Awam University of Engineering Science and Technology Nawabshah, Sindh, Pakistan d Department of Chemical Engineering, Faculty of Engineering, University of Missouri- Columbia, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23998b Cite this: RSC Adv. , 2016, 6, 4552 Received 13th November 2015 Accepted 18th December 2015 DOI: 10.1039/c5ra23998b www.rsc.org/advances 4552 | RSC Adv., 2016, 6, 4552–4563 This journal is © The Royal Society of Chemistry 2016 RSC Advances PAPER Published on 23 December 2015. Downloaded by Missouri University of Science and Technology on 14/10/2016 21:22:41. View Article Online View Journal | View Issue