Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng Heat transfer and pressure drop investigation of graphene nanoplatelet- water and titanium dioxide-water nanofluids in a horizontal tube Orhan Keklikcioglu, Toygun Dagdevir, Veysel Ozceyhan ⁎ Department of Mechanical Engineering, Faculty of Engineering, Erciyes University, Kayseri 38039, Turkey HIGHLIGHTS • The thermohydraulic performance of graphene nanoplatelet-water and titanium dioxide-water nanofluids was performed. • The weight fractions of 0.5, 0.75, and 1.00% were chosen. • The investigations were conducted for a range of Reynolds numbers from 2000 to 7300. • Thermodynamically advantageous nanofluid types were revealed. ARTICLEINFO Keywords: Graphene nanoplatelet Titanium dioxide nanoparticle Nanofluid Heat transfer Pressure drop ABSTRACT A continuous increase in energy consumption lead to more effective solutions for efficient use of energy in thermal systems. In order to save energy by improving the thermo-hydraulic performance of heat exchangers, a numerical and experimental study carried out on the thermo-hydraulic performance of Graphene nanoplatelet and Titanium dioxide nanoparticle addition to water flow through a horizontal tube. Graphene nanoplatelet (high price) and Titanium dioxide(low price) nanoparticles, which are frequently used in previous studies, were compared for the first time in order to provide users with a perspective in terms of thermo-hydraulic perfor- mance. The study was experimentally and numerically conducted with nanofluid weight fractions of 0.5%, 0.75% and 1.0% and the flow under turbulent conditions. The highest Nusselt number obtained is 133.61 for the Titanium dioxide-water nanofluid and 173.54 for the Graphene nanoplatelet-water nanofluid with the weight fraction of 1.00%. Moreover, the friction factor results of Graphene nanoplatelet-water and Titanium dioxide- water nanofluids are close to each other at same Re number. 1. Introduction As a result of the global energy crisis, which is one of the most crucial problems due to a continuous increase of energy consumption and shortage of energy resources, researchers have been working to improve the efficiency of thermal systems and decrease the size and energy consumption rates. Heat transfer augmentation with using different methods is gen- erally used to increase the heat transfer rate of thermal equipments. These different methods are grouped mainly under two techniques, passive and active. Nanofluids which were first mentioned in 1995 by Choi and Eastman [1], are also used as a passive technique to enhance the heat transfer in thermal systems such as air conditioning systems, auto- mobile radiators, solar systems, nuclear processes, refrigerators, thermal absorption systems etc. with the improvement of thermo- physical properties of conventional working fluids [2]. Conventional heat transfer fluids, such as water and oil, are widely used to control overheating or to enhance the heat transfer rate of different systems. However, conventional fluids do not perform effectively in high thermal load. Therefore, recently, many researchers have centralized their work on the improvement of high performance heat transfer na- nofluids.Becauseofthementionedreasons,numerousinvestigationson nanofluids have been conducted by researches to determine high heat transfer potentials. Some major contributions have been made in the effectofnanoparticletype,size,andweightorvolumefractionsonheat transfer rate and these contributions have been reported in many arti- cles.Gowdaetal. [3] investigatedtheeffectsofparticlespecies,surface charge,concentration,preparationtechnique,andbasefluidonthermal performance of nanofluids. They reported that the improvement of thermal conductivity of nanofluids is dependent on a uniform and stable dispersion of nanoscale particles in a fluid. Besides the effect of https://doi.org/10.1016/j.applthermaleng.2019.114256 Received 15 March 2019; Received in revised form 9 July 2019; Accepted 13 August 2019 ⁎ Corresponding author. E-mail address: ozceyhan@erciyes.edu.tr (V. Ozceyhan). Applied Thermal Engineering 162 (2019) 114256 Available online 19 August 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved. T