This work investigates numerically by finite volume method using Low k–ω model, the forced turbulent convection through a channel roughened by seven heated ribs arranged transversely. These ribs of rectangular cross section have a blocking ratio H/h = 10 and pitch ratio λ/h = 3. The modeling the problem parameters are Reynolds number, ranging from 5480 to 68500, and the width of the first rib L1 ranging from 0.5h to 15h. The objective of this study is to look for the width of the first rib L1 which induces the best heat transfer. The flow configurations of identical ribs from the first one generate a large eddy spreading along the top of the two first ribs, blocking the flow of the first cavity. The widening of the first rib may solve this problem. This flow configuration is required in several engineering applications necessitating the flow periodicity starting from the first cavity. The streamlines show that the first rib acts as a forward facing step when L1 > 5h. The effect of the width of the first rib is highlighted by velocity, pressure, turbulent kinetic energy and temperature profiles. Nusselt number distributions confirm that the widening of the heated surface is not recommended for improving heat transfer in spite of flow periodicity in all cavities (roughness d-type). The best improvement in heat transfer of 18%, compared to a smooth wall is obtained for thinnest first rib of L1/h = 0.5. However, this configuration provides a minor heat exchange at the first pitch and the second rib, which might be a disadvantage for the material structure.

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