Bubbly Gas-liquid Flow Research Articles

On the dynamics of instabilities in two-fluid models for bubbly flows

In this paper we look at instabilities in bubbly gas-liquid flows and investigate the emergence and characteristics of phase heterogeneities. We apply a shared-pressure two-fluid model to low bubble loadings and demonstrate the existence of persistent gas fraction instabilities of a characteristic size larger than the applied computational grid. In particular, we investigate the influence of a virtual mass effect on the stability of the two-fluid model and we demonstrate a change in the emergence and the dynamics of the phase heterogeneities. The change is accounted to a difference in the degree of hyperbolicity due to the inclusion of the virtual mass force. Furthermore, the results indicate that an initial instability, concluded as numerical in its character, evolves into a state with a physical character of the heterogeneities. We discuss implications of the existence and dynamics of the heterogeneities and the importance of the numerical behavior for interpretation of the results. In particular, we argue that underlying characteristics of the model cannot and should not be concealed with additional sub-models (such as momentum exchange terms) but must be acknowledged in the analysis of results from the two-fluid model for bubbly flows.

Heat transfer and shear stress in a gas-liquid flow in an inclined flat channel

Results of experimental investigation of a bubbly gas-liquid flow in an inclined flat channel are presented. Themeasurements were carried out in the range of superficial liquid velocities of 0.3–1.1 m/s and with different values of the volumetric gas flow rate ratio. The hydrodynamic structure wasmeasured bymeans of an electrochemical method using miniature shear stress probes. Values of average shear stress and heat transfer coefficient for different orientation of the channel were found. It is shown that in a bubbly gas-liquid flow the shear stress and heat transfer depend substantially on the channel inclination angle.

Local characteristics of the upward bubbly flow in an annular channel

Results of experimental investigation of the bubbly gas-liquid flow in a vertical annular channel are presented. The average and pulsation shear stresses and distributions of local void fraction were measured by the electrochemical method on both channel walls. It is shown that with a rise of gas flow rate ratio the value of wall shear stress increases significantly, and this effect becomes higher at a decrease in superficial liquid velocity. A presence of the gas phase effects significantly shear stress on the inner wall. Relative intensity of shear stress pulsations increases similarly on both channel walls.

Influence of bubbly clusters on the characteristics of the two-phase gas-liquid flow in a flat channel

Results of experimental investigation of a bubbly gas-liquid flow in horizontal and weakly inclined (from −20° to +20°) flat channel are presented. These measurements were carried out within the 0.2–1 m/s range of superficial velocities and volumetric gas flow rate ratio of up to 0.2. The hydrodynamic structure was measured by the electrochemical method with application of wall shear stress and conductivity microprobes. During the experiments signals of shear stress on the upper channel wall and local gas flow rate ratio were recorded completely. After numerical treatment of recorded signals the profiles of local gas flow rate ratio were obtained, average shear stress and its relative mean square pulsations on the upper channel wall were determined. It is shown that under the studied regimes the bubbles are grouped into clusters, and the bubbly flow is presented by alternation of bubbly clusters and single-phase liquid with separate bubbles and without them. Average wall shear stress and absolute shear stress pulsations in the range of bubbly clusters and beyond them were determined. Histograms of probability density distribution were obtained for the wall shear stress on the upper wall. It is shown that average shear stress and absolute pulsations in clusters are significantly higher than those in the flow zone free from bubbles.

Measurement of bubble velocity components in a swirling gas–liquid pipe flow using a local four-sensor conductance probe

This paper presents a novel technique for measuring the local axial, radial and azimuthal velocity components of the gas in bubbly gas–liquid flows using a local four-sensor conductance probe. A mathematical model is presented showing how the velocity vector of a gas bubble can be calculated from seven time intervals taken from the output signals from each of the four conductance sensors located within the probe. The paper goes on to describe the construction of a local four-sensor probe and the associated electronic measurement circuitry. Results are presented showing the distributions of the mean local axial, radial and azimuthal gas velocity components in vertical, bubbly gas–liquid flows, both with and without swirl. These results were obtained using the four-sensor probe in a vertical 80 mm diameter pipe into which a swirl generator could be installed. Additional results are presented showing the local gas volume fraction distribution, also obtained from the probe, in bubbly gas–liquid flows with and without swirl. It was found, as expected, that the presence of swirl caused a significant increase in the magnitude of the measured azimuthal velocity of the gas, particularly at the pipe walls. It was also found that, at a comparatively high water flow rate, the presence of swirl caused the gas bubbles to preferentially accumulate at the centre of the pipe.

Methodology development for the analysis ofvelocity particle image velocimetry images of turbulent, bubblygas-liquid flows

Methods of analysing and interpreting two-dimensional velocityfield data in order to understand the scales of turbulence ofbubbly two-phase flow were developed. Reynolds decompositionand large eddy simulation (LES) decompositions (low-passfiltering), in conjunction with proper orthogonaldecomposition (POD) energy spectra analysis, as well asadjusted convective decomposition (constant convectionvelocity is gas bubble velocity) were applied to analyse thestructure of turbulence. Particle image velocimetry was applied for velocity measurements. Decomposition analysis wasperformed for the local velocity around gas bubbles in a chosenregion. Various decomposition methodologies were applied forinterpretation of the results and it was found that the numberof eddies revealed and vorticity magnitude varied with thedecomposition method used. In particular, LES decomposition was found to perform better at showing smaller eddies.POD indicated the energy changesquantitatively through the spatial energy spectra, while thecomparison of single-phase flow with bubbly two-phase flowoffered an efficient way of decomposing the total velocity.A combination of convective decomposition and POD was applied tothe energy spectra level in order to obtain a novel view of theturbulence energy introduced by the gas bubbles.

A new method for measuring concentration of a fluorescent tracer in bubbly gas–liquid flows

A new experimental model, the two-tracer method (TTM), based on the planar laser-induced fluorescence technique (PLIF), is presented for the measurement of the local concentration of a fluorescent tracer in the liquid phase of a bubbly two-phase system. Light scattering and shading effects due to the bubbles were compensated for using the new model. The TTM results were found to give more accurate predictions of the local concentration than the normal PLIF method in a bubbly two-phase system.

Velocity field in isothermal turbulent bubbly gas-liquid flow through a pipe

Velocity field was measured by laser Doppler velocimetry in isothermal, turbulent bubbly gas-liquid flow through a 26.6 mm inner diameter vertical pipe. The measurements were made about 33 diameters downstream from the pipe entrance, gas injection being just upstream of the entrance. The gas phase radial distribution at the measurement plane exhibited influence of the injection device in that higher gas fraction existed in the central region of the pipe. For comparison, velocity field was also measured in isothermal, turbulent single-phase liquid flow through the same pipe at the same axial plane. Measured were the radial distributions of liquid mean axial and radial velocities, axial and radial turbulent intensities, and axial Reynolds shear stress. The radial distributions of gas bubble mean axial velocity and axial velocity fluctuation intensity were also measured by LDV. A dualsensor fiberoptic probe was used at the same time to measure the radial distributions of gas fraction, bubble mean axial velocity and size slightly downstream of the LDV measurement plane.

Liquid—solid mass transfer in packed beds of Raschig rings with upward two-phase (gas—liquid) flow

The effect of upward two-phase bubbly gas—liquid flow on the rate of liquid—solid mass transfer in fixed beds of Raschig rings was studied by measuring the rate of diffusion-controlled dissolution of a bed of copper Raschig rings in acidified chromate solution. Variables studied were gas (N 2) flow rate, liquid flow rate, physical properties of the solution and Raschig ring diameter. The data were correlated by the following: Sh = 2.867 Sc 0.33 Re g 0.25 Re L 0.35

Traveling waves in multi-phase flows

Equations of motion based on the theory of interacting and interpenetrating continua are used to study pattern forming instabilities in two-dimensional dispersed two-phase flows such as a suspension of particles in a fluidized bed and a bubbly gas-liquid flow. By making use of analytical and numerical bifurcation techniques it is shown that the main instability sequence in a vertically suspended configuration consists of the transition from a homogeneous flow through a one-dimensional concentration wave train to a two-dimensional traveling wave.