Liquid turbulence structure in two-phase bubbly flow in a large diameter vertical pipe

The liquid turbulence structure of air-water bubbly flow in a 200mm diameter vertical pipe was experimentally investigated. A dual optical probe is used to measure the bubble characteristics, while the liquid turbulence characteristics are measured using single and cross hot-film anemometry. Experiments were performed at liquid superficial velocities in the range of 0.2 to 0.68m/s for gas superficial velocities in the range of 0 to 0.18m/s, corresponding to an area averaged void fraction up to 15A%. In general, the bubbles tend to migrate toward the pipe centerline forming a core-peak void fraction profile, which is different than the wall-peak void profile usually observed in small diameter pipes. The introduction of the bubbles augment the liquid turbulence for most flow conditions; however, a turbulence suppression is observed near the pipe wall for very low void fraction flows. The balance between the lift and the turbulent dispersion forces is studied to specify the main flow parameters that affect the radial migration of the bubbles. A non-dimensional map between these parameters and the force ratio is proposed to characterize the flow conditions under which a wall or core peak void profile occurs in bubbly flows. The available correlations for the turbulence characteristics in small diameter pipes are examined and evaluated against the current data. The integral length scale is correlated using the inter-distance between the bubbles as a characteristic length scale. The effect of the bubbles on the liquid turbulence energy spectra is analyzed, where a significant turbulence energy is observed within the length scale range comparable to the bubble diameter. The effect of the bubbles on the liquid turbulence kinetic energy is investigated from the global (non-local) and spectral (local) point of views. A model for the spectral bubble production is introduced and discussed to explain the turbulence suppression phenomena.