Thermal performance and operational attributes of the startup characteristics of flat-shaped heat pipes using nanofluids Karim Alizad a , Kambiz Vafai a,⇑ , Maryam Shafahi b a Department of Mechanical Engineering, University of California, Riverside, CA 92521, USA b Department of Mechanical Engineering, California State Polytechnic University, Pomona, CA, USA article info Article history: Received 5 July 2011 Received in revised form 26 August 2011 Accepted 26 August 2011 Available online 21 September 2011 Keywords: Heat pipe Nanofluid Thermal performance Flat-shaped Disk-shaped abstract Thermal performance, transient behavior and operational start-up characteristics of flat-shaped heat pipes using nanofluids are analyzed in this work. Three different primary nanofluids namely, CuO, Al 2 O 3 , and TiO 2 were utilized in our analysis. A comprehensive analytical model, which accounts in detail the heat transfer characteristics within the pipe wall and the wick within the condensation and evapo- ration sections, was utilized. The results illustrate enhancement in the heat pipe performance while achieving a reduction in the thermal resistance for both flat-plate and disk-shaped heat pipes throughout the transient process. It was shown that a higher concentration of nanoparticles increases the thermal performance of either the flat-plate or disk-shaped heat pipes. We have also established that for the same heat load a smaller size flat-shaped heat pipe can be utilized when using nanofluids. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Heat pipes are used extensively in various applications, for achieving high rates of heat transfer utilizing evaporation and condensation processes [1–13]. Heat pipes have been used in spacecrafts, computers, solar systems, heat and ventilating air con- ditioning systems and many other applications [14]. The great majority of research presented in the heat pipe area addresses a cylindrical shaped geometry [1,13,15–23]. However, it has been clearly demonstrated that the flat-shaped heat pipes [2–7,9–12] have an advantage in terms of heat removal capability and geomet- rical adaptation for many applications such as electronic cooling, spacecraft thermal control and commercial thermal applications [2–7,10,12,24]. Most heat pipe analysis is based on the steady state operation of the device. However, in a number of applications start-up characteristics are quite important. Improper start-up can cause damage to the heat pipe. As such it is important to analyze the start-up process for the heat pipes [14]. Several transient models for the start-up of the heat pipes have been presented in the literature such as the ones by Tournier and El-Genk [22] and Chang and Colwell [25]. However, these works were all based on a regular cylindrical based geometry. The only work related to start-up process for a flat-shaped heat pipe was presented by Zhu and Vafai [12]. They had established a compre- hensive analytical solution of the startup process for asymmetri- cal flat-plate and disk-shaped heat pipes. The thermophysical properties of a liquid, specifically, the ther- mal conductivity and heat capacity can significantly affect the heat transfer process in the liquids. Both of these properties can be aug- mented by dispersing the liquid with solid nanoparticles. The new liquid which now has better characteristics in transferring heat is called nanofluid [26,27]. The other properties of this liquid such as density and viscosity also change as a function of concentration of nanoparticles [1,2,28,29]. Several papers have looked at the effect of using nanofluids as a working fluid in a regular cylindrical-shaped heat pipes [1,8,15–21,23,30,31]. These works have shown enhancement in the performance of heat pipes due to a decrease in the thermal resistance when using nanofluids as a working liquid [16,17,19, 20,23,31]. Thermal resistances is also affected when using differ- ent nanoparticles such as silver [15–17], CuO [8,31], nickel oxide [32], diamond [18,19], and gold [23]. The use of nanofluids results in an increase in the efficiency of the heat pipe [21] while result- ing in a decline in the gradient of temperature along the heat pipe [16–18] and an improvement of the overall heat transfer coeffi- cient [32]. Most of the research works that have investigated the use of nanofluids in heat pipes are experimental [8,15–21,23,31]. Shafahi et al. [1,2,30] utilized analytical models to investigate the thermal performance, liquid pressure, liquid velocity profile, temperature distribution of the heat pipe wall, temperature 0017-9310/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2011.08.050 ⇑ Corresponding author. E-mail address: vafai@engr.ucr.edu (K. Vafai). International Journal of Heat and Mass Transfer 55 (2012) 140–155 Contents lists available at SciVerse ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt