The present paper deals with experiments using air in three helically coiled rectangular ducts of mean diameters 12.7 cm, 17.8 cm, and 22.8 cm, respectively, made of rectangular wave-guide tubing of dimensions 1.27 cm × 0.64 cm. Pressure variations observed around the ducts were qualitatively in agreement with the expectations for secondary flow. The friction factors change gradually with increasing Reynolds numbers over the range 1200–10,000 without exhibiting a sudden transition from laminar flow to turbulence. At all Reynolds numbers, these are higher than those for a straight duct by 20–100 percent. The heat transfer coefficient is also higher than that for straight ducts ranging between 20–300 percent, depending on the Reynolds number. The largest increases are seen in the Reynolds number range 1200–2500.

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