The influence of bottom wall heating on the mean and turbulent flow behavior in the near wall region during mixed convection Ahmed Elatar, Kamran Siddiqui * Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada article info Article history: Received 10 May 2013 Received in revised form 11 October 2013 Accepted 2 November 2013 Available online 15 December 2013 Keywords: Channel flow Wall heating Low Reynolds number Mixed convection Particle image velocimetry Near wall region abstract An experimental work is reported that studied the effect of mixed convection on the mean and turbulent flow structure in the near wall region inside a horizontal square channel heated from below at low Reynolds numbers (Re) and high Grashof numbers (Gr). The Gr/Re 2 values ranged from 9 to 106 indi- cating that natural convection was dominant over forced convection for all studied cases. Velocity fields were measured using particle image velocimetry (PIV) in the vertical mid plane and two horizontal planes close to the heated wall. The results show that the bottom wall heating altered the mean velocity field and induced turbulence. Both mean and turbulent velocity magnitudes showed partial dependency on the Gr/Re 2 ratio. In the higher range of Gr/Re 2 , mean streamwise velocity showed larger magnitude whereas, in the lower range of Gr/Re 2 , streamwise and spanwise turbulent velocities have larger mag- nitudes. The streamwise and spanwise turbulent velocity magnitudes were also found to be largest close to the heated wall. It was observed that in the vicinity of the heated wall, the warm fluid converges along the streaks which initiate the rising plumes while the falling parcels of cooler fluid disperse in the spanwise plane. Ó 2013 Elsevier Masson SAS. All rights reserved. 1. Introduction Buoyancy is known for its influence on the hydraulic and thermal behavior of the flow. Mixed convection heat transfer is the process where buoyancy through natural convection coexists with forced convection. Mixed convection can occur at both low and high Rey- nolds number flows. Grashof number (Gr) to Reynolds number (Re) ratio (Gr/Re 2 ) determines the relative contribution of natural and forced convection modes in the mixed convection regime [1]. The Grashof number is the ratio between buoyancy forces and viscous forces while, Reynolds number is the ratio between the inertial forces and viscous forces. Hence, the Gr/Re 2 represents the ratio between the buoyancy and inertial forces. Low Reynolds number mixed con- vection can be found in several industrial applications such as: electronic cooling, chemical and nuclear reactors, food process in- dustry and biomedical applications. Mixed convection heat transfer has been studied extensively over the past few decades. However, majority of the previous work was focused on evaluating overall heat transfer behavior by quantifying the Nusselt number (Nu). Gajusingh and Siddiqui [2] experimentally studied the effect of wall heating on the flow characteristics in the near wall region inside a square channel. They found that the buoyancy generates turbulence for originally laminar flow while for originally turbulent flow, buoyancy dampens turbulence. They argued that for turbulent flow, turbulence is dampened due to working against buoyancy. Mahaney et al. [3] studied mixed convection inside rectangular duct with bottom heated wall and adiabatic side and top walls. They found that the secondary flow induced by natural convection was in the form of vortices due to the interactions between ascending plumes near the side walls and falling of cooler fluid. Nusselt number was found to increase downstream of the channel due to buoyancy-driven secondary flow enhancement. Osborne and Incropera [4,5] experimentally investigated the effect of buoyancy on convection heat transfer inside horizontal channels with heated top and bottom walls. For turbulent flow regime, they observed forced convection near the top wall and mixed convection near the bottom wall [4]. For laminar flow, they observed the presence of thermally stable boundary layer at the top blocking the thermal plumes originating from the bottom wall to reach the top wall [5]. Wang et al. [6] identified different flow patterns for mixed convection along the heated test section of a horizontal square channel with a bottom heated wall through flow visualization. They found that the flow passes through four different flow regimes along the channel heated section: laminar forced convection, laminar mixed convection, transient mixed convection and turbu- lent free convection. Ozsunar et al. [7] experimentally studied * Corresponding author. Tel.: þ1 519 661 2111x88234; fax: þ1 519 661 3020. E-mail address: ksiddiqui@eng.uwo.ca (K. Siddiqui). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ e see front matter Ó 2013 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.ijthermalsci.2013.11.001 International Journal of Thermal Sciences 77 (2014) 233e243