Bubbly Flow Structure Research Articles

Bubble behaviour and flow structure of bubble columns

Numerous experimental results concerning the flow structure in bubble columns are presented, including axial and radial components of the bubble velocity, axial components of the liquid velocity and its fluctuations as well as the radial distribution of the gas hold-up. The experiments were conducted in a column of inner diameter 0.29 m and height 4.5 m. The gas distributor was designed in such a manner that uniform distributions of gas and liquid could be achieved in the column. Gas entered through a series of holes of diameter 0.2 mm and merged with liquid which was fed through uniformly distributed holes of diameter 30 mm.

The structure of bubbly flow through venturis

The phase structure of vertical air-water mixture flows through venturis were investigated using area contraction ratios of 3.16 and 7.11 and with variations in angles of convergence and divergence. The flow conditions were predominantly of the bubbly type and covered a range of gas volume fraction at the throat between 0.2 and 0.6 for average mixture velocities of up 32 m/s. Resistivity probe signals indicating void fluctuations were analyzed to yield local void fraction, bubble velocity, bubble detection rate and probability density function of bubble sizes in the flow. Velocity ratios were also obtained to provide information on the overall behaviour of the two concurrent phases. The resistivity probe was shown to give reliable results for bubble flows in a wide range of speeds indicating velocity ratios up to 1.7 in the venturi throat. All flows tended toward a stable and well-mixed bubbly pattern downstream of the venturi exit following a sufficient length. The void and velocity profiles here always appeared to be characterized by a local maximum in the pipe centre, the local maximum close to the wall of some of the inlet flows being eliminated. Bubble coalescence was noted in the convergent passage whilst significant bubble fragmentation in the divergent passage was observed from the results.

Turbulence structure of air-water bubbly flow—I. measuring techniques

This paper, the first in a series describing our work on the turbulence structure of air-water bubbly flow, describes the principles of measurement and specially developed electronic instrumentation for determining various important local parameters, and the rates of turbulent transport of heat and bubbles in air-water two-phase bubbly flow. These instruments indicate the phase distribution, the bubble velocity and its spectrum, the water velocity and the turbulent intensity, and the turbulent dispersion coefficient of bubbles. Brief discussions are also presented on the accuracy of these techniques.

Turbulence structure of air-water bubbly flow—III. transport properties

An experimental study of heat and bubble transport in turbulent air-water bubbly flow was carried out by means of tracer techniques. Helium tracer gas concentration data and temperature distributions were used to extract bubble and heat diffusivity information. The results indicated that the turbulent velocity components of the liquid phase play a predominant role in the turbulent transport process. A systematic increase of diffusivity of heat, ϵ H , with quality and water velocity was observed. An empirical correlation for the diffusivity ratio ϵ H, TP / ϵ H, SP is presented. The Péclet number, u′ c ovbar| dφ, for bubble dispersion can be approximated by 2.0, independent of the flow variables. The bubble-to-heat diffusivity ratio, φ/ ϵ H , approaches unity with increasing quality and water velocity. Momentum transport is also discussed, based on a mixing length theory.

Turbulence structure of air-water bubbly flow—II. local properties

Microstructure was studied experimentally in air-water two-phase bubbly flow flowing upward in a vertical pipe of 60 mm diameter under atmospheric pressure. The results indicate that over a large portion of fully-developed bubbly flow the phases, the velocities of bubbles and water, and the ratio between the velocities of the phases have fairly flat radial profiles. In the wall region a maximum void fraction was observed. Spectra of the velocities of bubbles and water showed a Poisson distribution and a normal distribution function, respectively. The experimental evidence indicated a trend for the turbulent intensity to decrease first with increasing gas flow rate for constant water velocity and to increase again with a further increase in the gas flow rate. This phenomenon was more significant for a higher water velocity.