The Large Eddy Simulation (LES) and SIMPLE-C method coupled with preconditioned conjugate gradient methods have been employed to study the effect of aiding/opposing buoyancy on the turbulent flow field and heat transfer across a square cylinder in a vertical channel. The level of wall-confinement (blockage ratio of 10%, 30% and 50%) was changed with a constant Reynolds number (5000) under various Richardson numbers (-1 to 1). With increasing blockage ratio, the buoyancy effect is becoming weaker oil the Nusselt number for the square cylinder. The turbulent heat transfer past the square cylinder can be improved by increasing the blockage ratio. (c) 2007 Elsevier Ltd. All rights reserved.