Gravity-driven bubbly flows

Question

1. What contributions have the authors mentioned in the paper "Gravity-driven bubbly flows" ?

2. Why is the back mixing small in the air lift reactors?

3. Why is the gravity-driven bubbly flow difficult?

4. What is the main challenge for experimentalists dealing with the bubbly flows?

Accepted Answer

An industrial example is formed by the so-called bubble column: a vertical cylinder filled with liquid through which bubbles flow that are introduced at the bottom of the cylinder.. The authors describe the ideas developed over time and identify some open questions.. The authors discuss the experimental findings on the turbulence generated, the stability of the flow, axial dispersion, and the similarities between bubble columns and air lifts.

Accepted Answer

Due to the liquid flowing in the axial direction, the back mixing in the air lift reactors is small compared to that of the bubble column.

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Because the bubble size and its distribution play an important role in the hydrodynamics as well as mass transfer in the gravity-driven bubbly flows, there is a need for experiments that focus on the bubble size distribution.

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Developing new experimental techniques for the heterogeneous regime that can probe the gas characteristics is a big challenge for experimentalists dealing with the bubbly flows.

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As the vertical part of flow lines in oil production can be very long, the effects of the decreasing hydrostatic pressure in the line must be considered.

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By balancing the energy input via the gas flow to the energy dissipation, Joshi & Sharma (1979) predicted the circulation velocity of the liquid as well as the overall gas fraction.

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The authors point out that the observed increased slip velocity is a consequence of the interaction between the bubbles and the turbulence that causes a drag reduction.

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Because multiphase flows show a wide variety of geometrical distribution of the phases, each with its specific features, it is customary to subdivide the multiphase flows further into separate flow regimes.

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The critical superficial gas velocity drops from 5 cm/s for almost spherical bubbles to 3 cm/s for ellipsoidal bubbles with an aspect ratio of 2.

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By inspecting the Auto Correlation Function as well as the Cross Correlation Function of wall pressure fluctuations, they obtained information onA nnu.

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A special sparger consisting of 561 needles was used to generate bubbles with a very narrow size distribution (∼4.6-mm equivalent diameter).

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The freedom of the bubbles in the dense swarms is limited by their neighbors, to which they become tightly coupled via the liquid inbetween them.

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Rietema solved the momentum balances for the gas and liquid phase, with the constraint that the net liquid flow was zero and the assumption that the slip velocity between the bubbles and the liquid was a constant.

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The bubble size distribution (and connected to that the bubble velocity distribution) is of paramount importance for the industrial applications:

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B IRe = 2 4/10 0 00 3/10 00 0 3/10 k B The author, (3)where k B The authoris the kinetic energy density of the flow field around a bubble that can be estimated from potential theory:k B The author= 1 2 αρlCvmv 2 s , (4)with vs the slip velocity of the bubbles and Cvm the virtual mass coefficient, which for bubbles of 5 mm is approximately 2 (Lance & Bataille 1991).

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This low gas fraction, combined with a column diameter of 140 mm, allowed the authors to use Phase Doppler Anemometry [see also Broder & Sommerfeld (1998)], a technique that not only nonintrusively measures the particle velocity but also the size of spherical particles.

Accepted Answer

An obvious candidate is the shear-induced lift force, which is a consequence of nonzero rotation of the liquid flow field in combination with nonzero slip between a gas bubble and the surrounding liquid:FL = −CLρL Vb( vg − vliq ) × (∇ × vliq ). (13)

Gravity-driven bubbly flows

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Most frequently asked questions

1. What contributions have the authors mentioned in the paper "Gravity-driven bubbly flows" ?

2. Why is the back mixing small in the air lift reactors?

3. Why is the gravity-driven bubbly flow difficult?

4. What is the main challenge for experimentalists dealing with the bubbly flows?

5. What is the effect of the decreasing hydrostatic pressure in the line?

6. How did they predict the circulation velocity of the liquid?

7. What is the effect of the interaction between the bubbles and the turbulence?

8. Why is it customary to subdivide the multiphase flows further into separate flow regimes?

9. What is the critical superficial gas velocity for ellipsoidal bubbles?

10. How did they obtain information on the wall pressure fluctuations?

11. How many needles were used to generate bubbles?

12. What is the freedom of the bubbles in dense swarms?

13. What was the assumption that the slip velocity between the bubbles and the liquid was a constant?

14. What is the importance of the bubble size distribution for the industrial applications?

15. What is the kinetic energy density of the flow field around a bubble?

16. What is the effect of phase doppler anemometry?

17. What is the candidate for the shear-induced lift force?

Citations

How volcanoes work: a 25 year perspective

Explosive dynamics of violent Strombolian eruptions: The eruption of Parícutin Volcano 1943-1952 (Mexico)

A Review of X-Ray Flow Visualization With Applications to Multiphase Flows

Agitation, Mixing, and Transfers Induced by Bubbles

An X-ray system for visualizing fluid flows

References

The Dispersion of Matter in Turbulent Flow through a Pipe

Transverse migration of single bubbles in simple shear flows

Turbulence in the liquid phase of a uniform bubbly air-water flow

3-D turbulence structure and phase distribution measurements in bubbly two-phase flows

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

Related Papers (5)

Turbulence in the liquid phase of a uniform bubbly air-water flow

The Motion of High-Reynolds-Number Bubbles in Inhomogeneous Flows

Transverse migration of single bubbles in simple shear flows

Agitation, Mixing, and Transfers Induced by Bubbles

Turbulent Dispersed Multiphase Flow