Effect of periodic, tapered enlargements and contractions on turbulent heat transfer and fluid flow in a circular duct

Heat transfer coefficients, pressure drop data and fluid flow patterns were determined experimentally for turbulent flow in circular ducts characterized by periodic tapered enlargements and contractions of cross section. Fully developed and entrance region results were obtained. The effects of the taper angle, cycle length and Reynolds number were investigated. The Reynolds number ranged from 6,000 to 70,000, whereas the half taper angle was varied in the range 2 to 14.8 deg. The two half cycle lengths investigated were respectively 0.762 and 1.143 times the diameter of the largest cross section. Flow visualization experiments showed that for the half taper angles of 6.3, 10 and 14.8 deg., flow separation and recirculation occur in the duct, primarily in the diverging sections. For smaller angles, no backflow was detected. The friction factor for the periodic geometries was found to be always larger than that for the straight pipe at the same Reynolds number, increasing rapidly with the taper angle as well as with the cycle length, and decreasing with Re.