An investigation of the thermal performance of cylindrical heat pipes using nanofluids Maryam Shafahi a , Vincenzo Bianco b , Kambiz Vafai a, * , Oronzio Manca b a Department of Mechanical Engineering, University of California Riverside, Riverside, CA 92521, USA b Dipartimento di Ingegneria Aerospaziale e Meccanica, Seconda Università degli Studi di Napoli, Via Roma 29, 81031 Aversa (CE), Italy article info Article history: Received 28 July 2009 Received in revised form 30 August 2009 Accepted 30 August 2009 Available online 23 October 2009 Keywords: Heat pipe Thermal performance Nanofluids abstract In this work, a two-dimensional analysis is used to study the thermal performance of a cylindrical heat pipe utilizing nanofluids. Three of the most common nanoparticles, namely Al 2 O 3 , CuO, and TiO 2 are con- sidered as the working fluid. A substantial change in the heat pipe thermal resistance, temperature dis- tribution, and maximum capillary heat transfer of the heat pipe is observed when using a nanofluid. The nanoparticles within the liquid enhance the thermal performance of the heat pipe by reducing the ther- mal resistance while enhancing the maximum heat load it can carry. The existence of an optimum mass concentration for nanoparticles in maximizing the heat transfer limit is established. The effect of particle size on the thermal performance of the heat pipe is also investigated. It is found that smaller particles have a more pronounced effect on the temperature gradient along the heat pipe. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Heat pipes and their applications in thermal management have been studied for decades. They constitute an efficient, compact tool to dissipate substantial amount of heat from various engineering systems including electronic components. Heat pipes are able to dissipate substantial amount of heat with a relatively small tem- perature drop along the heat pipe while providing a self-pumping ability due to an embedded porous material in their structure. A limiting factor for the heat transfer capability of a heat pipe is re- lated to the working fluid transport properties. In order to over- come this limitation, the thermophysical properties of the fluid can be improved. An innovative way to enhance liquid thermal conductivity is the dispersion of highly conductive solid nanopar- ticles within the base fluid. This new generation of conductive flu- ids with nanoparticles are referred to as nanofluids [1]. The nanoparticles within the fluid change the other thermophysical properties such as density and viscosity. A number of investiga- tions have been conducted to study boiling and natural and forced convection using nanofluids [2]. Due to the useful features of a nanofluid, various research groups [3–14] have tried to engage it within a heat pipe and study the subsequent thermal enhancement experimentally. Different nanoparticles such as silver [3,8,11,13], CuO [5,9], diamond [6,12], titanium [4,7], nickel oxide [10], and gold [14] have been utilized within the heat pipe working fluid. The improved thermal performance is observed through a reduction in thermal resistance [3,6–9,11,13,14], a drop in the temperature gradient along the heat pipe [3,12,13], an increase in the heat pipe efficiency [4], and an enhancement in the overall heat transfer coefficient [10]. In some studies [5,9], the existence of an optimum amount of nanoparticle mass concentration providing the highest thermal performance has been established. Liu et al. [9] have shown that the heat pipe operating pressure has a significant effect on the thermal perfor- mance. In another study, Riehl [10] has observed that a higher heat transfer coefficient can be seen when using nanoparticles in water under low heat input conditions. Tsaia [14] investigated the influ- ence of particle size on the heat pipe thermal performance. Almost all of the research work on the use of nanoparticles in heat pipes is experimental. To the best of authors’ knowledge; there is a lack of information in modeling heat pipe characteristics in the presence of a nanofluid. The focus of the present work is to model the influence of a nanofluid on the thermal performance of a heat pipe. A comprehensive analytical model proposed by Zhu and Vafai [15] is modified to include the effects of nanofluids within a cylindrical heat pipe. A common range of concentration for differ- ent nanoparticles, namely Al 2 O 3 , CuO, and TiO 2 in water is consid- ered as the operational fluid within the heat pipe under various heat inputs. The temperature profile, thermal resistance, and the maximum heat transfer limits are investigated. A significantly higher thermal performance is observed due to a reduction in the thermal resistance as well as the end to end temperature gradient. Furthermore, the maximum capillary limit heat transfer increases when using a nanofluid as the working fluid. To the best of our knowledge, there is no study on the effect of a nanofluid on the 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.09.019 * Corresponding author. Tel.: +1 951 827 2135; fax: +1 951 827 2899. E-mail address: vafai@engr.ucr.edu (K. Vafai). International Journal of Heat and Mass Transfer 53 (2010) 376–383 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt