Convective heat transfer coefficient and pressure drop of water-ethylene glycol mixture with graphene nanoplatelets C. Selvam a , T. Balaji a , D. Mohan Lal a,⇑ , Sivasankaran Harish b a Refrigeration and Air-Conditioning Division, Department of Mechanical Engineering, Anna University, Chennai 600 025, Tamil Nadu, India b International Institute for Carbon-Neutral Energy Research Kyushu University (WPI-I 2 CNER), 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan article info Article history: Received 24 May 2016 Received in revised form 8 August 2016 Accepted 8 August 2016 Available online 8 August 2016 Keywords: Convective heat transfer Graphene Nanofluid Laminar Turbulent Pressure drop abstract In the present work, we report the convective heat transfer coefficient and pressure drop of water- ethylene glycol mixture seeded with graphene nanoplatelets under laminar, transition and turbulent flow regions. Sodium deoxycholate was used as the surfactant to prepare stable nanofluid dispersions. Thermophysical properties of nanofluids were measured experimentally. Experimental investigations on the convective heat transfer coefficient and pressure drop were performed in a tube-in-tube counter flow heat exchanger using nanofluid as the hot fluid and chilled water as the cold fluid. The effects of nanofluid inlet temperature on the convective heat transfer coefficient and pressure drop were investi- gated for different mass flow rates. The enhancement of convective heat transfer coefficient was found to increase with respect to Reynolds number, graphene loading and inlet temperature. The maximum enhancement of convective heat transfer coefficient is observed to be 170% at 0.5 vol% in the turbulent region. The pressure drop increment of the nanofluid is predominant in the laminar region as compared to turbulent region. The enhancement of pressure drop is moderate in the turbulent region which favours these nanofluids to be used in the thermal systems for different engineering applications. Ó 2016 Elsevier Inc. All rights reserved. 1. Introduction Energy transport plays a vital role globally and occupies a very predominant position in various fields such as mechanical, electri- cal, chemical, transportation, nuclear and oil industries. Heat trans- fer can be achieved by means of conduction and convection posing both advantages and disadvantages. Conduction is not always pos- sible in variety of applications which leads to the pathway of emerging trends in convective heat transfer using fluids. Varieties of conventional fluids such as water, ethylene glycol, oil, and air are being used to transfer the heat based on the areas of applications. However, the performance of heat transfer fluids becomes chal- lenging and highly critical where large amount of heat is to be transferred. Several explorations were performed over the years to enhance the thermal conductivity of conventional heat transfer fluids by seeding the high thermal conductivity nanosized parti- cles. These explorations lead to the emergence of new class of heat transfer fluid termed as nanofluids coined by Choi and Eastman [1]. This fluid contains the dispersion of nano sized highly thermal conductive particle to the basefluids providing enhanced thermal conductivity, excellent stability and enhanced heat transfer coeffi- cient which is the major deciding factor for convection studies. Numerous convection experiments were done on metals (Al, Cu, Ag, Au and Ti) and metal oxides (Al 2 O 3 , CuO, TiO 2 and ZnO) based nanofluids and researchers reported that the heat transfer coeffi- cient enhances due to the high thermal conductivity of nano parti- cles [2–5]. However these studies face serious challenges due to simultaneous increase in viscosity of nanofluids for higher volume fractions due to higher density of nanoparticles. These challenges are overcome by the carbon based nanostructures having higher thermal conductivity than metal and metal oxides which leads to the enhancement of heat transfer coefficient at lower volume frac- tions itself. Similarly, the bulk density of carbon nano structures are low compared to metal and metal oxides that only a limited increase in viscosity is experienced as the volume fraction increases. Investigations on convective heat transfer characteristics of car- bon nanostructure based nanofluids are limited as compared with other nanofluids. Experiments on thermal conductivity and heat transfer coefficient of nanofluids with carbon nanostructures such as carbon nanotubes (CNTs) [6–14] and graphene nanoplatelets (GnP) [15–25] reveal the high heat transfer characteristics of nano- material based nanofluids as compared with other nanofluids. Lack of agreement existing between the reported experimental results http://dx.doi.org/10.1016/j.expthermflusci.2016.08.013 0894-1777/Ó 2016 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail address: mohanlal@annauniv.edu (D. Mohan Lal). Experimental Thermal and Fluid Science 80 (2017) 67–76 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs