Experimental study of nanofluid flow and heat transfer over microscale backward- and forward-facing steps A.Sh. Kherbeet a,⇑ , H.A. Mohammed b,⇑ , B.H. Salman c , Hamdi E. Ahmed d , Omer A. Alawi b , M.M. Rashidi e a Mechanical Engineering Department, College of Engineering, Universiti Tenaga Nasional, Jalan IKRAM-UNITEN, 43000 Kajang, Selangor, Malaysia b Department of Environmental Engineering, College of Engineering, Komar University of Science and Technology (KUST), King Mahmud Circle, Sulaymani-Kurdistan Region, Iraq c FABE, Limkokwing University of Creative Technology, Jalan Teknokrat 1/1, 63000 Cyberjaya, Selangor, Malaysia d Department of Mechanical Engineering, University of Anbar, 31001 Anbar, Iraq e Shanghai Key Lab of Vehicle Aerodynamics and Vehicle Thermal Management Systems, 4800 Cao An Rd., Jiading, Shanghai 201804, China article info Article history: Received 9 July 2014 Received in revised form 18 February 2015 Accepted 23 February 2015 Available online 2 March 2015 Keywords: Forced convection Microscale backward-facing step Microscale forward-facing step Experimental Heat transfer Nanofluids abstract This paper investigates experimentally the effects of laminar nanofluid flow over the microscale back- ward-facing step (MBFS) and forward-facing step (MFFS) on the heat transfer characteristics. The experi- ments were implemented on MBFS and MFFS with a step height of 600 lm. Both MBFS and MFFS have the upstream and downstream lengths of 0.1 m and 0.15 m respectively. The Reynolds number ranged of 280–480. The concentrations of SiO 2 nanoparticle valued at 0.005 and 0.01 with a diameter of 30 nm were immersed in a distilled water. The experimental results revealed that the concentration of 0.01 water–SiO 2 nanofluid recorded the highest Nusselt number. The comparison between MBFS and MFFS revealed that the highest Nusselt number is obtained through the use of the MFFS, which is approxi- mately twice that of MBFS. However, the friction factor recorded a higher value for MFFS. The experimen- tal results were in a good agreement with the numerical published results. Ó 2015 Elsevier Inc. All rights reserved. 1. Introduction The flow separation, with subsequent reattachment occur due to a sudden expansion or compression in the geometry, such as the backward-facing step and forward-facing step. This phe- nomenon plays an important role the design of many engineering applications that require heating or cooling operations. As a result, there is the occurrence of a mixture of a significant amount of high and low energy in the reattachment region. This can affect the per- formance of heat transfer in various applications such as in com- bustion chambers, environmental control systems, cooling systems for electronic equipment, high performance heat exchan- gers, cooling passages in turbine blades and chemical processes as well as energy system equipment. The separation of the backward- facing step occurs at the upper sharp corner of the step causing the development of a recirculating region located behind the step. However, there are two recirculation regions for the forward-fac- ing step geometry; the first region is located downstream and adja- cent to the step while the second region is located upstream the step and its location depends on the step height and the ratio of the boundary-layer thickness at the step [1]. Previous research studies investigated the problem of laminar flow over forward-fac- ing step and backward-facing step geometries in forced, natural and mixed convection both numerically and experimentally [2– 9]. Abu-Mulaweh [10] examined the effects of backward-facing and forward-facing steps on turbulent natural convection along a vertical heated flat plate. The results revealed that the maximum Nusselt number for the backward-facing step is approximately twice that of a flat plate, with occurence of the vicinity of the reat- tachment region. However, the maximum Nusselt number for the case of the forward-facing step is two and a half times more than that of the flat plate. One of the techniques used to enhance the rate of heat transfer is nanofluids whereby nanometer-sized particles are immersed in the conventional base fluids [11–13]. Most of the recent studies showed that, the enhancement of heat transfer coefficient can be increased by adding solid metallic or nonmetallic nanoparticles with a high thermal conductivity of the base fluid. [14–17]. These nanoparticles can be for instance Al 2 O 3 , SiO 2 , Cu, CuO, ZnO and TiO 2 [18]. Many researchers have investigated the effects of nanofluids in the enhancement of heat transfer and the fluid flow [19–31]. Abu-Nada was the first to conduct an investigation of thermal behavior and characteristics of nanofluid flow over backward-fac- ing step [32]. He reported that the Nusselt number can be http://dx.doi.org/10.1016/j.expthermflusci.2015.02.023 0894-1777/Ó 2015 Elsevier Inc. All rights reserved. ⇑ Corresponding authors. Tel.: +6 0173895660; fax: +6 07 55 66159. E-mail addresses: akeel.fa7@gmail.com (A.Sh. Kherbeet), Hussein.dash@yahoo. com (H.A. Mohammed). Experimental Thermal and Fluid Science 65 (2015) 13–21 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs