Effects of swirl intensity on interfacial and wall friction factors of annular flows in a vertical pipe

Abstract

Interfacial and wall friction factors, fi and fw, of air-water swirling annular flows in a vertical pipe of diameter D = 30 mm were measured in three sections of L* (=L/D) = 7.3, 13 and 31, where L is the distance from a swirler. The gas and liquid volumetric fluxes, JG and JL, were 12.5 ≤ JG ≤ 20.0 m/s and 0.071 ≤ JL ≤ 0.213 m/s, respectively. The interfacial swirl number, si, defined by Viθ/Viz, where Viθ and Viz are the azimuthal and axial components of interfacial velocity, was introduced as an indicator of swirl intensity. For z* (=z/D) ≤ 7, where z is the axial coordinate in the test section, the si increased with z* at low JG and JL, and slightly decreased with z* at high JG and JL. The si decreased for z* > 7 and reached zero at z* ≈ 30. The fw estimated using the three-fluid model was larger than that estimated using the two-fluid model since the droplet flow rate was not negligible even in swirling annular flows. The three-fluid model was therefore more appropriate for evaluation of fi and fw than the two-fluid model. The ratio fw∗ increased with s¯i for s¯i > 0.25, where fw∗ is the ratio of fw to that of non-swirling flows and s¯i is the averaged value of si in the section. The ratio fi∗ increased with s¯i for s¯i > 0.18, wherefi∗ is the ratio of fi to that of non-swirling flows. Correlations in terms of s¯i have prospect to evaluate the fi and fw in swirling annular flows.