Thermal performance of nanofluid in ducts with double forward-facing steps Hussein Togun a,b, *, G. Ahmadi c , Tuqa Abdulrazzaq d , Ahmed Jassim Shkarah b,e , S.N. Kazi a , A. Badarudin a , M.R. Safaei f a Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Mechanical Engineering, University of Thi-Qar, 64001 Nassiriya, Iraq c Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY 13699-5725, USA d Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia (UPM), 43400 Selangor, Malaysia e Department of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, 75450 Melaka, Malaysia f Young Researchers and Elite Club, Mashhad Branch, Islamic Azad University, Mashhad, Iran 1. Introduction Flow separations and recirculation flows occur due to sudden changes in the passage geometry. Such flows are found in a variety of applications, such as power plants, combustion furnaces, nuclear reactors, heat exchangers and cooling of electronic devices. Attempts to enhance the heat transfer rate in thermal systems have been reported in a number of studies over the past decades. Typical examples are introducing flow separation over a forward or backward-facing step, sudden expansion, and ribs channels. Separation due to a sudden contraction or a forward-facing step (FFS) in the passage not only developed in practical engineering applications but is also occurs in nature, such as lakes and rivers. Generally, the phenomena of separation and reattachment flow are addressed in numerous experimental and numerical studies [1–11]. Shakouchi and Kajino [12] presented an experimental study of heat transfer and fluid flow over single and double forward-facing steps using the laser Doppler anemometry technique. The effect of step height on heat transfer and flow characteristics showed a greater enhancement of heat transfer with the double forward-step compared to the single step. Yilmaz and Oztop [13] numerically studied airflow and heat transfer over a double forward-facing step using a standard k-e turbulence model. They insulated the top wall and steps while the bottom wall before the step was heated. The obtained results indicated that the second step could serve as a control device for heating and fluid flow. Recently, Tinney and Ukeiley [14] used particle image velocimetry to investigate turbulent oil flows over a double backward-facing step. They observed large turbulence at the central region of the backward step. Laminar and turbulent convection flows over a vertical forward-facing step were numerically and empirically studied by Abu-Mulaweh et al. [15] Journal of the Taiwan Institute of Chemical Engineers 47 (2015) 28–42 A R T I C L E I N F O Article history: Received 29 June 2014 Received in revised form 30 September 2014 Accepted 5 October 2014 Available online 23 November 2014 Keywords: Separation flow Nanofluid flow Double forward-facing step Heat transfer A B S T R A C T The turbulent heat transfer to nanofluid flow over double forward-facing steps was investigated numerically. The duct geometry and computational mesh were developed with ANSYS 14 ICEM. Two- dimensional governing equations were discretized and integrated using finite volume technique. The k-e turbulence model was used in the analysis. Al 2 O 3 and CuO nanoparticles at volume fractions varying from 1% to 4% with water as the base fluid were employed for turbulent flow in a passage with a double forward-facing step. The effects of volume fraction and step height were compared with the base fluid thermal performance. The obtained results showed an increase in the Nusselt number with the increase in volume fraction of nanofluid, Reynolds number, and step height. A higher local Nusselt number value was found at the second step compared to the first step for all cases. Velocity contours were developed to visualize the recirculation regions before and after the first and second steps. The results also demonstrated enhanced heat transfer with the increase of nanoparticle concentration, and the largest thermal enhancement factor occurred for the highest nanoparticle volume fraction (4%) of Al 2 O 3 considered in this investigation. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. * Corresponding author at: University of Malaya, Mechanical Department, Street University, 50603 Kuala Lumpur, Malaysia. Tel.:+60172530157/+964 7829318426. E-mail addresses: htokan_2004@yahoo.com, htokan_phd@siswa.um.edu.my (H. Togun). Contents lists available at ScienceDirect Journal of the Taiwan Institute of Chemical Engineers jou r nal h o mep age: w ww.els evier .co m/lo c ate/jtic e http://dx.doi.org/10.1016/j.jtice.2014.10.009 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.