Experimental study of the liquid velocity and turbulence in a large-scale air-water counter-current bubble column

Abstract

Measuring the local liquid velocity and turbulence in large-scale bubble columns with optical methods is complex and usually limited to low gas holdups in thin geometries. Comprehensive datasets in large bubble columns are therefore seldom published. Since the importance of Computational Fluid Dynamics (CFD) is increasing for multiphase applications, such data is also important for validating models for dispersed bubbly flows. In the present work the liquid velocity and turbulence in a pilot-scale bubble column is studied and a CFD validation database is generated by completing previous measurements of the gas void fractions and bubble sizes. For this purpose, we used a Particle Shadowgraph Velocimetry (PSV) technique that was intentionally designed to study the fluid dynamics in large-scale facilities. The measurements were realized in the 5.3 m high and 0.24 m diameter counter-current bubble column at Politecnico di Milano. The superficial gas velocity ranged from 0.37 to 1.85 cm/s, the counter current superficial liquid velocity from 0 to 9.2 cm/s. All operation points are in the so-called pseudo-homogeneous flow regime, in which the integral gas holdup (ranging from 1.02 to 7.55%) increases linear with the superficial gas velocity but an inhomogeneous flow is present. The dominant frequencies of the bubbly flow, shear rates, and turbulence levels are increasing with increasing superficial gas velocity. With increasing superficial liquid velocities, the dominant frequencies are decreasing, the averaged liquid velocities are shifted downwards, but the overall turbulence levels remain constant. In order to investigate the smaller scales at which the bubble-induced turbulence is expected, a filtering process is proposed. As a result, the filtered turbulence levels of all operation points fall on a linear trend line when plotted over the local void fraction, which is the same result obtained in other studies in small, homogenous tabletop columns. The now available database for CFD validation contains the averaged liquid velocities, basic turbulence information, local void fractions, and the bubble sizes at two different heights. The data will be in particular useful to validate the capabilities of models to upscale bubbly flows from tabletop to the pilot-scale bubble columns.