Heat transfer-friction characteristic comparison in rectangular ducts with slit and solid ribs mounted on one wall

A comparison of fully developed heat transfer and friction characteristics has been made in rectangular ducts with one wall roughened by slit and solid ribs. The effects of rib void fraction and flow Reynolds number are examined. The rib height-to-duct hydraulic diameter and pitch-to-height ratios are fixed at H/D-e = 0.167 and P-i/H = 10, respectively. To understand the mechanisms of the heat transfer enhancement, smoke-wire flow visualization and measurements of mean velocity and turbulence intensity are conducted in the slit and solid-ribbed ducts. In addition, by separately measuring the floor and rib heat transfer, two contributive factors of heat transfer promotion, namely, the fin effect and the enhanced turbulence effect, have been isolated. Because of the greater turbulence-mixing effects the slit-ribbed geometry displays higher floor heat transfer than the solid-ribbed geometry. In addition, the fin effects for the slit rib are greater than that for the solid rib. The pressure drop across the slit ribs is lower than that across the solid ribs due to less duct blockage. Furthermore, slit ribs with larger void fractions in a lower flow Reynolds number range provide better thermal performance under a constant friction power constraint.