Turbulent unsteady flow and heat transfer in channels with periodically mounted square bars Alvaro Valencia * , Marcela Cid Department of Mechanical Engineering, Universidad de Chile, Casilla 2777, Santiago, Chile Received 16 March 2001; received in revised form 15 August 2001 Abstract A numerical investigation was conducted to analyze the unsteady turbulent flowfield and heat transfer character- istics in a channel with streamwise periodically mounted square bars arranged side-by-side to the approaching flow. The transverse separation distance between the bars is varied, whereas the bar height to channel height (d =H ) are 0.152 and 0.2, the Reynolds number Re based on channel height is 2  10 4 and the periodicity length is 2H. The channel walls are subjected to a constant wall temperature. The k–e turbulence model was used in conjunction with the Reynolds- averaged momentum and energy equations for the simulations. A finite volume technique is applied with a fine grid and time resolution. Complex periodic vortex shedding develops in the channel due the interaction between the two streamwise periodically mounted square bars. Results show that the unsteady flow behavior, pressure drop and heat transfer are strongly dependent of the transverse separation distance of the bars. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Numerical simulation; Turbulence; Heat transfer; Channels; Vortex generator 1. Introduction The arrangement of bluff bodies as turbulence pro- moters in channels to periodically disrupt the flow is a widely adopted technique for heat transfer enhance- ment. The bluff bodies investigated have mainly con- sisted of cylinders, flat plates and rectangular bars. The optimal design requires however of a thorough under- standing of the influence of the interaction between unsteady vortex structures on heat transfer and flow loss. Most previous studies were related to a single bluff body immersed in freestreams, while there are less per- tinent studies to flow passing square bars in confined ducts with different arrangements. Bosch and Rodi [1] presented the results of numerical simulations of vortex shedding past a free-standing square bar at Re d ¼ 22000, obtained with different tur- bulence models. Using wall functions, the standard k–e model was compared with a modification suggested by Kato and Launder [2] (hereafter Kato–Launder model). The Kato–Launder model reduces the excessive pro- duction of turbulent kinetic energy in the stagnation region of the bar due to an unrealistic simulation of the normal turbulent stresses in eddy-viscosity models, and therefore the vortex shedding around the bar is stronger. In terms of engineering parameters such as Strouhal number, lift and drag coefficients, the predictions of the Kato–Launder model and the standard k–e were close to each other, however a detailed comparison in terms of velocity profiles reveals Kato–Launder model to have closer agreement with experiments. Bosch et al. [3] reported experiments on the flow past a square bar placed near a wall. Visualization studies were carried out for various gap distances between bar and wall. Over the range of the dimensionless gap dis- tances c=d ¼ 0:35–0:5, the fraction of time with periodic shedding motion increases from zero to one. Below this range, the shedding motion is completely suppressed and, above it, regular shedding occurs at all times. At c=d ¼ 0:375 the bistable behavior of the flow is strong, and here shedding occurs only at a relatively small * Corresponding author. Tel.: +56-2-6784466; fax: +56-2- 6988453. E-mail address: alvalenc@cec.uchile.cl (A. Valencia). International Journal of Heat and Mass Transfer 45 (2002) 1661–1673 www.elsevier.com/locate/ijhmt 0017-9310/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0017-9310(01)00267-8