Developing mixed convection of a nanofluid in a horizontal tube with uniform heat flux M. Akbari and A. Behzadmehr Mechanical Engineering Department, University of Sistan and Baluchistan, Zahedan, Iran Abstract Purpose – This paper seeks to show the effect of using nanofluid on mixed convection heat transfer in a horizontal tube. Design/methodology/approach – Three-dimensional elliptic governing equation has been solved using finite volume approach. Grid independence test has been performed to find the suitable grids. Obtained numerical results have been validated with the available experimental and numerical results in the literature. Parametric study has been done to see the effects of Reynolds number, Grashof number and volume fraction of the nanoparticles on the hydrodynamic and thermal parameters in a horizontal tube. Findings – The nanoparticles volume fraction does not have a direct effect on the secondary flow and the skin friction coefficient. However, its effect on the entire fluid temperature causes the strength of the secondary flow to reduce. For a given Grashof number, increasing the particles’ concentration augments convective heat transfer coefficient. It does not have a significant effect on the skin friction coefficient at the low Grashof number. However, skin friction coefficient is slightly affected at the higher Grashof numbers. Research limitations/implications – The Grashof number is limited for which the Boussinesq hypothesis for the variation of density with the temperature would be valid. Practical implications – This paper promotes designing heat exchangers, solar collectors, cooling electronic devices. Originality/value – Nanofluid mixed convection in a horizontal tube has been studied and the effects of nanoparticles concentration on the hydrodynamic and thermal parameters have been shown and discussed. Keywords Heat transfer, Convection, Boiler tubes Paper type Research paper Nomenclature C B ¼ Boltzmann constant (1.38 £ 10 223 J/K) C f ¼ Peripherally average skin friction coefficient C p ¼ Specific heat (J/kg K) D ¼ Tube diameter (m) D s ¼ nanoparticle diameter (m) g ¼ Gravitational acceleration (m/s 2 ) Gr ¼ Grashof number ðð¼ gb eff q w D 4 Þ= ðk eff y 2 eff ÞÞ h ¼ Convection heat transfer coefficient (Wm 2 K) k ¼ Thermal conductivity (W/m K) Nu m ¼ Peripherally average Nusselt number P ¼ Pressure (pa) Pe ¼ Peclet number ð¼ 3pD 3 s Dð2ðdP =dZ ÞÞ=ð4C B TÞÞ q w ¼ Uniform heat flux (W/m 2 ) r ¼ Radial direction The current issue and full text archive of this journal is available at www.emeraldinsight.com/0961-5539.htm The authors thank University of Sistan and Baluchistan for its support. Comments of the unknown reviewers are also gratefully acknowledged. HFF 17,6 566 Received 9 January 2006 Revised 22 April 2006 Accepted 6 September 2006 International Journal of Numerical Methods for Heat & Fluid Flow Vol. 17 No. 6, 2007 pp. 566-586 q Emerald Group Publishing Limited 0961-5539 DOI 10.1108/09615530710761216