Settling or hydrodynamic entrance lengths have been determined for turbulent flow of air in three smooth concentric annuli of diameter ratios 0.306, 0.527, and 0.842. Both square-edged and bellmouth entrances were investigated for Reynolds number ranging from 7000–47,500. Flow separation caused by the abrupt change in area of the square-edged entrance resulted in skewed (distorted) velocity profiles near the entrance. This skewness was modified further downstream and finally fully developed velocity profiles were established. The settling lengths for the annuli with the square-edged entrances were between 25 and 35 dia. With the bellmouth entrance, the velocity profile developed conventionally from the nearly flat profile at the inlet. The settling lengths for the bellmouth entrances were about 10–15 equivalent diameters more than those for the square-edged entrances.

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