Effects of pipe diameter on interfacial and wall friction factors of swirling annular flows in a vertical pipe

Interfacial and wall friction factors, fi and fw, of swirling annular flows in a vertical pipe of 20 mm in diameter D were measured for comparison with those in a vertical pipe of D = 30 mm. A three-fluid model was applied to evaluate the friction factors. Measurements were carried out in two sections, one near the swirler and the other far downstream the swirler. The swirl intensity of the flow was evaluated by the interfacial swirl number si defined by the ratio of the azimuthal to the axial components of interfacial wave velocity. The interfacial swirl numbers were smaller in the smaller pipe. The ratio of fi in swirling annular flows to that in non-swirling annular flows decreased to unity with decreasing si* which is the arithmetic mean value of si for 0 < z* (= z/D) < 3.8, where z is the axial coordinate in the test section, and is higher in D = 20 mm than in D = 30 mm. The ratio of fw in swirling annular flows to that in non-swirling annular flows decreased to less than unity with decreasing si* and is higher in D = 20 mm than in D = 30 mm.