Heat Transfer and Friction Factor in a Square Channel With One, Two, or Four Inclined Ribbed Walls

An experimental study was carried out to investigate the heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with 45 deg inclined ribs on one, two, or four walls. Either two opposite walls or all four walls in the channel were heated. Tests were performed for Reynolds numbers (Re) ranging from 7600 to 24,900, the pitch to rib height ratio $(P∕e)$ of 8.0, the rib height to channel hydraulic diameter ratio $(e∕Dh)$ of 0.0667, and the channel aspect ratio of 1.0. The results show that the local Nusselt number and friction factor increase with the number of ribbed walls. With one ribbed wall, the Nusselt numbers on the ribbed side (B) were 50% and 63% greater than those on the adjacent smooth sides (L∕R) and the opposite smooth side (T), respectively. The Nusselt numbers, when the two opposite walls of a four-wall ribbed channel are heated, are found to be 1.49–1.52 times greater than those obtained when all four walls are heated.