Wall shear stress measurements in vertical air-water annular two-phase flow

Abstract

Measurements of wall shear stress are reported for annular two-phase air-water flow in a vertical 23 m long, 0.0318 m dia tube (the LOTUS rig at Harwell). The shear stress was measured using a hot-film probe system calibrated using both single-phase and zero-entrainment two-phase flows. The measurements were made for both equilibrium annular flow and also for the “unidirectional deposition” region following film removal. The equilibrium measurements were analysed for both average and time-varying shear stress. The average values were in good agreement with the values calculated from pressure gradient and the fluctuating values showed characteristic peak frequencies corresponding to disturbance waves. The measurements in the unidirectional deposition region showed that commonly used assumptions for the effect of droplet deposition on shear stress grossly overpredicted the effect. Better agreement was obtained by taking account of the effect of the droplets on mixing length using the correlation of Owen & Hewitt and by accounting for the deposition effect by using an analogy with condensation.