Heat transfer and friction factor enhancement in a square channel having integral inclined discrete ribs on two opposite walls

The influence of a gap provided in integral inclined ribs on heat transfer and friction factor enhancement is investigated. Experiments are conducted to obtain heat transfer and friction factor characteristics in a square channel with two opposite in-line ribbed walls for Reynolds numbers from 5000 to 40000. The test section of square channel composed of integral inclined ribs with a gap and has a length-tohydraulic diameter ratio (L/Dh) of 20. The rib pitch-to-height ratio (p/e) is 10, the rib height-to-hydraulic diameter ratio (e/Dh) is 0.060 and rib attack angle (α) varies in the range of 300 to 900 (4 steps). The relative gap position (d/W) and relative gap width (g/e) is varied in the range of 1/5–2/3 (5 steps) and 0.5–2.0 (4 steps), respectively. The enhancement in heat transfer and friction factor of this roughened duct was compared with smooth duct and duct roughened with continuous inclined ribs (with no gap) under similar flow condition. Presence of inclined ribs with a gap yields about 4-fold enhancements in Nusselt number and about 8-fold increase in the friction factor compared with smooth duct and about 1.3 times and 1.4 times higher than the case of continuous ribs (without gaps) for the entire range of parameters investigated. Ribs with relative gap width of 1.0 at relative gap position of 1/3 and attack angle of 60° provides maximum heat transfer and friction factor enhancement.