Journal of Mechanical Science and Technology 31 (11) (2017) 5539~5544 www.springerlink.com/content/1738-494x(Print)/1976-3824(Online) DOI 10.1007/s12206-017-1048-6 Experimental study of steady state laminar forced heat transfer of horizontal annulus tube with non-Newtonian nanofluid † Ally Javadpour 1 , Mohammad Najafi 1,* and Kourosh Javaherdeh 2 1 Department of Mechanical and Aerospace Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 2 Faculty of Mechanical Engineering, University of Guilan, Guilan, Iran (Manuscript Received February 17, 2017; Revised May 19, 2017; Accepted July 8, 2017) ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Abstract The forced convection steady heat transfer under laminar flow regime in a horizontal annular tube was experimentally investigated in this paper using a non-newtonian, pseudo-plastic nanofluid with an aqueous carboxymethyl cellulose (CMC-0.2 % wt) as the base fluid, and copper oxide (CuO) nanoparticles in 0.5 % wt, 1.0 % wt and 1.5 % volume fractions and Reynolds number from 460 to 1280 and different range of heat flux. The mean and local heat transfer coefficients of the nanofluid were recorded higher than the base fluid at all the applied Reynolds numbers. For instance, at Re = 460, for the nanofluid of 0.5 % vol. concentration, heat transfer coefficient was im- proved by 14.7 % compared to the base fluid, and by increasing nanofluid concentration to two/three times, heat transfer was improved by 4.4 % and 5.2 %, respectively. The maximum mean heat transfer coefficient was observed for the 1.5 % nanofluid at Re = 1280. Keywords: Non-Newtonian; Laminar flow; Nanofluid; Annulus; Forced heat transfer ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1. Introduction Today, with daily advancements in complex industrial and medical systems, designers still face challenges for the un- wanted heat generated while these systems are operating on one hand and the maintenance of the working temperature in the allowable range for optimum performance on the other hand. This encourages heat transfer researchers to be continu- ously looking for new methods for creating heat exchangers with better performance and lower volume. In recent years, many methods including implementation of corrugated tubes, micro and nano-channels and expanded surfaces were utilized for enhancing heat transfer but obstacles such as complex production process and high price, directed researchers to another determining factor in heat exchanger performance [1]. Researchers realized that, given their unfavorable thermal characteristics compared to solid materials, traditional fluids such as water, oil and ethylene glycol is one of the causes for low efficiency of heat exchangers. To solve this problem, a new type of fluid, known as nanofluid, was introduced. Nanofluids are colloidal mixtures consisting of a base fluid and scattered nanoparticles. The presence of nanoparticles results in improved heat transfer properties in such fluids in comparison with common fluids. Adding micro-scale solid particles to common fluids, Ahuja (1975) [2] studied the effect they have on heat transfer in laminar flow regime, but it was Choi [3] who coined the name "Nanofluid" for the two-phase mixture of the base fluid and suspended nanoparticles and paved the way for extended research in this field. The following can be mentioned among the experimental researches that have been performed in the field of forced convection heat transfer: Hojjat et al. [4, 5] studied the laminar flow inside a horizon- tal annular tube using CMC-water with 0.5 % wt as the base fluid and TiO 2 , Al 2 O 3 and CuO nanoparticles, their results suggest the nanofluid thermal entrance length to be extended as the concentration is increased. Heris et al. [6, 7], studied an annular tube such that satu- rated fluid was flowing in the space between the two tubes while CuO/water and Al 2 O 3 /water fluids were flowing in the inner tube in two distinct experiments. Their results show the maximum heat transfer to be obtained for 2.5 to 3.0 % vol. of nanoparticles. Zarringhalam et al. [8] studied the forced turbulent convec- tion in two uniaxial tubes for different volume ratios of the CuO/water nanofluid and pressure drop along the tube. They reported a 57 % improvement in heat transfer in comparison to the base fluid at 2 % volume fraction. He et al. [9] addressed the forced heat transfer inside a hori- zontal tube with smooth surface, covered with micro-fins in * Corresponding author. Tel.: +98 2144865154 E-mail address: mohammad36najafi@gmail.com † Recommended by Associate Editor Youngsuk Nam © KSME & Springer 2017