Flow uniformizing distribution panel design based on a non-uniform porosity distribution M.K. Choi a , Y.B. Lim a , H.W. Lee b , H. Jung b , J.W. Lee a,n a Department of Mechanical Engineering, Pohang University of Science & Technology, Pohang, Republic of Korea b Agency for Defense Development, Daejeon, Republic of Korea article info Article history: Received 10 July 2013 Received in revised form 1 April 2014 Accepted 13 April 2014 Keywords: Flow uniformity Flow distributor Local porosity distribution Effective porosity Porous media CFD abstract The conditions required for a flow resistance element to uniformize a non-uniform flow in a two- dimensional channel were derived in terms of a non-uniform porosity profile. The validity of this approach was confirmed through a numerical analysis over a wide range of parameter conditions. The proposed equation for the non-uniform porosity distribution gave satisfactory results for a wide variety of velocity profiles at the channel inlet. For sufficiently thick orifices, the equation was valid over a wide range of average porosities, from 0.3 to 0.6. For thin orifices, satisfactory uniformity was obtained only for a mean effective porosity over a narrow range of 0.42–0.48. Two different methods of generating variable porosity, using a constant hole size plus variable blocking plates or using a constant blocking plate size plus variable holes, did not show any appreciable difference in results. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction A uniform flow environment is essential in a variety of engineer- ing systems. A wind tunnel is a typical example where a uniform flow enables the generation of predictable fields of flow velocity, temperature, and concentration around obstacles installed within a channel (Hancock, 1998; Moonen et al., 2007). Also, the overall performance of heat and mass transfer is optimized under uniform flow conditions in a variety of engineering applications, including heat exchangers (Bassiouny and Martin, 1984; Wen and Li, 2004), electronics cooling (Choi et al., 1993a, b; Kim et al., 1995; Wang et al., 2001), various air conditioning systems (Cheng et al., 1998; Luo and Tondeur, 2005; VanGilder and Schmidt, 2005), solar heat collectors (Weitbrecht et al., 2002), and electrostatic precipitators (Sahin and Ward-Smith, 1987), to name a few. Typical process equipment related to uniform flow distributions includes various chemical reactors such as contactors, mixers, burners, extrusion dies, and textile- spinning chimneys (Commenge et al., 2002; Kareeri et al., 2006; Perry et al., 1984). In recent years, uniform flow distributions have been a concern in fuel cells and biological systems (Aricò et al., 2000; Bi et al., 2010; Danilov and Tade, 2009; Huang et al., 2008; Kee et al., 2002; Lee et al., 2009; Li and Sabir, 2005; Wang et al., 2010). Small-scale non-uniformities decay naturally with time due to the action of viscosity, but macroscopic velocity profiles tend toward fully developed profiles in the presence of walls and are inherently non-uniform. Therefore, a uniform velocity profile can be maintained only artificially, such as by installing flow- resistance devices within the flow passage. The non-uniform pressure drop due to the non-uniform resistance and flow velocity redistributes the flow. Perforated plates or diffusers used in electrostatic precipitators (Şahin and Ward-Smith, 1991) and multiple screens used in wind tunnels are typical examples of flow-resistance devices (Hancock, 1998; Moonen et al., 2007). Another example is a successively bifurcating tube network used in fuel cells to supply uniform flow to each terminal exit (Liu et al., 2010, 2012). In its simplest design, a uniformizing baffle has a uniform flow resistance or uniform flow opening (Hancock, 1998). Even when the flow opening or flow resistance is uniform across a non-uniform flow, a uniformizing effect emerges because the pressure drop across a resistance element in a turbulent flow is proportional to the square of the flow velocity. But although the use of a uniform resistance device can reduce the level of non-uniformity, the original velocity Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jweia Journal of Wind Engineering and Industrial Aerodynamics http://dx.doi.org/10.1016/j.jweia.2014.04.003 0167-6105/& 2014 Elsevier Ltd. All rights reserved. Abbreviations: C d , discharge coefficient [dimensionless]; D, hole diameter [m]; f(y), normalized velocity profile ( ¼V/V 0 ); H, channel height [m]; L, channel length [m]; _ m, mass flow rate [kg/s]; N, number of holes [EA]; ΔP, pressure drop through the distribution panel [Pa]; P 0 , pressure at the inlet [Pa]; P 2 , pressure after the distribution panel [Pa]; S, distance between the inlet and the distribution panel [m]; t, panel thickness [m]; V(y), velocity profile across the flow cross-section [m/s]; V 0 , average velocity [m/s]; W, length of the separating plate [m]; β, local porosity ( ¼ D=ðDþWðyÞÞ) [dimensionless]; β n , effective porosity ( ¼ βC d ) [dimensionless]; β 0 , average porosity [dimensionless]; ρ, density [kg/m 3 ] n Corresponding author. J. Wind Eng. Ind. Aerodyn. 130 (2014) 41–47