Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt Experimental investigation of graphene oxide nanofluid on heat transfer enhancement of pulsating heat pipe Mohammad Alhuyi Nazari a , Roghayeh Ghasempour a, ⁎ , Mohammad Hossein Ahmadi b, ⁎ , Gholamreza Heydarian c , Mohammad Behshad Shafii c a Renewable Energy and Environmental Engineering Dep., University of Tehran, Tehran, Iran b Faculty of Mechanical Engineering, Shahrood University of Technology, Shahrood, Iran c Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran ARTICLE INFO Keywords: Pulsating heat pipe Graphene oxide Nanofluid Thermal performance ABSTRACT Pulsating heat pipes (PHPs) are heat transfer devices which are widely utilized in electronic devices and energy systems. Improvement in thermal performance of PHPs will lead to higher heat transfer capacity and en- hancement in the efficiency of systems which PHPs are applied. In this study, an experimental investigation was performed on the thermal performance of a pulsating heat pipe by applying graphene oxide nanofluid as working fluid. Four concentrations of graphene oxide (0.25, 0.5, 1, and 1.5 g/lit) in water as base fluid were used in the PHP. Results indicate that adding graphene oxide sheets increased thermal conductivity and viscidity of the base fluid. Moreover, utilizing graphene oxide can decrease thermal resistance of PHP up to 42%. In addition, high concentration (1.5 g/lit) of the nanofluid worsen thermal performance of the PHP in comparison with pure water which is attributed to increase in dynamic viscosity of nanofluid. Finally, a regression model is proposed in order to compare effects of heat input and concentration of nanofluid on thermal resistance mathematically. 1. Introduction Heat pipes are two-phase heat transfer devices which are widely used in various applications due to their high effective thermal con- ductivity [1,2]. There are different types of heat pipe which are cate- gorized based on their structure [3,4,5]. Pulsating heat pipes (PHPs) are the most compact of them. Effective thermal conductivity of PHPs is much higher in comparison with metals due to their two-phase heat transfer mechanism. PHPs consist of a capillary tube which is bent into several turns and filled by degrees with a fluid [6]. If two ends of ca- pillary tubes are connected to each other, the PHP is closed loop PHP, while when the ends are separated, the PHP known as open loop PHP. The main parts of PHPs are evaporator, condenser sections. In addition, adiabatic section exists in the cases there is distance between condenser and evaporator. The working fluid in the PHP evaporates by receiving heat in evaporator section. The vapor converts into liquid by heat dis- sipation in the condenser [7]. These two-phase heat transfer mechanism is the main factor of heat transfer in PHPs. PHPs are passive heat transfer devices since the pressure instabilities in tubes is the main reason of fluid motion. Pulsating heat pipes have several applications. For instance, PHPs can be used in heat recovery systems [8] very efficiently compared with other heat transfer devices due to their high heat transfer capability. In addition, PHPs are used in renewable energy systems such as solar desalination systems [9], solar water heater [10] and thermal storage systems [11]. Moreover, pulsating heat pipe has reliable performance in solar combined heat and power generation systems [12]. By applying fluids with specific properties, PHPs are able to work efficiently at very low temperatures [13]. Several parameters affect thermal performance of PHPs such as geometry [14,15], inclination angle, operating condition [16], and working fluid [17]. Ebrahimi et al. [18] experimentally investigated a flat plate PHP with interconnecting channels. Results indicated that interconnecting channels can improve thermal performance of the PHP due to consolidation in flow circulation in the channels. Xue et al. [19] investigated the effect of inclination angle on the thermal performance of a PHP and concluded that lower thermal resistance is obtainable by increasing inclination angle. Sun et al. [16] conducted a study on a PHP filled with water and HFE-7000. It was observed that for evacuation pressure of 0.01 Torr, at 80 w heat input, the PHP filled with water had better thermal performance; while at increased evacuation pressure (over 100 Torr), the PHP filled with HFE-7000 had lower thermal re- sistance [16]. Working fluid has the most important role in thermal performance of PHPs. Fluid with special thermophysical properties such https://doi.org/10.1016/j.icheatmasstransfer.2017.12.006 ⁎ Corresponding authors. E-mail addresses: Ghasempour.r@ut.ac.ir (R. Ghasempour), mhosein.ahmadi@shahroodut.ac.ir (M.H. Ahmadi). International Communications in Heat and Mass Transfer 91 (2018) 90–94 0735-1933/ © 2017 Elsevier Ltd. All rights reserved. T