A model of breakup of a rising bubble in a turbulent flow (original) (raw)

Development and validation of models for bubble coalescence and breakup

2014

A generalized model for bubble coalescence and breakup has been developed, which is based on a comprehensive survey of existing theories and models. One important feature of the model is that all important mechanisms leading to bubble coalescence and breakup in a turbulent gas-liquid flow are considered. The new model is tested extensively in a 1D Test Solver and a 3D CFD code ANSYS CFX for the case of vertical gas-liquid pipe flow under adiabatic conditions, respectively. Two kinds of extensions of the standard multi-fluid model, i.e. the discrete population model and the inhomogeneous MUSIG (multiple-size group) model, are available in the two solvers, respectively. These extensions with suitable closure models such as those for coalescence and breakup are able to predict the evolution of bubble size distribution in dispersed flows and to overcome the mono-dispersed flow limitation of the standard multi-fluid model. Besides, I would like to extend my gratitude to all support staff at the Institute of Safety Research for their assistance, especially the secretaries, Claudia Losinski, Petra Vetter, Annett Richter and special thanks to the computer administrator Torsten Berger. I am most thankful to the German Federal Ministry of Economics and Technology for funding my research work through the program of competence maintenance in nuclear technology. Finally, a great thanks to my husband Wenxing, for his love and continuous support, and my children Ye and Lei.

Experiments on breakup of bubbles in a turbulent flow

Aiche Journal, 2017

The breakup of air bubbles in a turbulent water flow is studied experimentally. Water flows from a nozzle array, generating intense turbulence, and then flows downward through a cell. The velocity field is measured by PIV, and the local dissipation rate is estimated using a largeeddy PIV technique. Bubbles (1.8 to 5 mm) are injected in the bottom of the cell and rise toward the region of intense turbulence, where they break. The time spent by bubbles in various zones without breaking and the number of breakups are evaluated, providing information about the breakup frequency. The number of daughter bubbles and their size distribution are determined. The number of daughters depends on a Weber number σ ρε 3 / 5 3 / 2 2 D ′ , where ε is the turbulent energy dissipation rate, D' is the mother particle size, ρ and σ are the liquid density and surface tension. The daughter size distribution is a function of their number.

Bubble breakup in a turbulent shear layer

Journal of Physics: Conference Series

The breakup of a millimetre size buoyantly rising bubble encountering a horizontal plane turbulent jet is experimentally investigated using high-speed shadowgraphy and acoustic techniques. The bubble diameter to jet height ratio is 0.75 and the jet height based Reynolds number is 4000. The high-speed imaging was recorded at 7 kHz simultaneous with hydrophone output at 100 kHz. Bubble breakup events were seen to produce simple binary divisions into products of similar size as well as three products where at least one was much smaller than the other products. Coalescence of products was also observed. In almost all cases time-frequency analysis of the acoustic emissions enabled the products to be identified and sized.

Numerical study of a bubble plume generated by bubble entrainment from an impinging jet

Nuclear Engineering and Design, 2011

The current paper presents the prediction results of a bubbly flow under plunging jet conditions using multiphase mono-and poly-dispersed approaches. The models consider interfacial momentum transfer terms arising from drag, lift, and turbulent dispersion force for the different bubble sizes. The turbulence is modeled by an extended k-ε model which accounts for bubble induced turbulence. Furthermore in case of a poly-dispersed air-water flow the bubble size distribution, bubble break-up and coalescence processes as well as different gas velocities in dependency on the bubble diameter are taken into account using the Inhomogeneous MUSIG model. This model is a generalized inhomogeneous multiple size group model based on the Eulerian modeling framework which was developed in the framework of a cooperative work between ANSYS-CFX and Forschungszentrum Dresden-Rossendorf (FZD). The latter is now implemented into the CFD code CFX.

Considerations on bubble fragmentation models

Journal of Fluid Mechanics, 2010

In this paper we describe the restrictions that the probability density function (p.d.f.) of the size of particles resulting from the rupture of a drop or bubble must satisfy. Using conservation of volume, we show that when a particle of diameter, Do, breaks into exactly two fragments of sizes D and D 2 = (DQ -D 3 ) 1/3 respectively, the resulting p.d.f, f(D;D 0 ), must satisfy a symmetry relation given by D\ f(D;D 0 ) = D 2 f(D 2 ;D 0 ), which does not depend on the nature of the underlying fragmentation process. In general, for an arbitrary number of resulting particles, m(Do), we determine that the daughter p.d.f. should satisfy the conservation of volume condition given by m(Do) J 0 0 (D/Do) 3 /(D;Do)dD = 1. A detailed analysis of some contemporary fragmentation models shows that they may not exhibit the required conservation of volume condition if they are not adequately formulated. Furthermore, we also analyse several models proposed in the literature for the breakup frequency of drops or bubbles based on different principies, g(e, Do). Although, most of the models are formulated in terms of the particle size Do and the dissipation rate of turbulent kinetic energy, e, and apparently provide different results, we show here that they are nearly identical when expressed in dimensionless form in terms of the Weber number, g *(We t ) = g(e, D 0 ) Do /3 e~1 /3 , with We t ~pe 2/3 D 0 5/3 /CT, where p is the density of the continuous phase and a the surface tensión.

On the turbulence modelling of bubble plumes

Chemical Engineering Science, 2021

h i g h l i g h t s An enhanced turbulence model has been developed and validated, which is generally applicable to bubble plumes. The model accounts for the extra turbulence agitation introduced by bubble wakes. The model also considers the turbulence modulations due to bubble-induced stratification and free surface damping effects. The implications of each physical process in turbulence modelling are investigated.

Bubble Induced Turbulence in Bubble Plumes

In bubbly flow, bubbles and the surrounding fluid interact through both force and turbulence coupling. The effects of flow turbulence on bubble trajectory are reflected in turbulent dispersion. Bubbles will introduce extra turbulence into the fluid through wake effects (so-called pseudo turbulence). The single phase k-epsilon model does not incorporate the bubble-induced turbulence [1]. This study aimed to develop and implement a model to account for these effects. The gas-stirred-ladle experiments of [10] and [1] were employed for validation. The model framework combines a volume of fluid (VOF) and discrete phase model (DPM). VOF is used to capture the fountain shape formed by the bubble plume reaching the surface, while DPM is a parcel-based Lagrangian approach to track bubbles.

Modeling of Vertical Bubble-Driven Flows

Industrial & Engineering Chemistry Research, 1997

Bubbly flows comprise a large number of different flow situations, e.g., dispersed pipe flows, flows in multiphase agitated tanks, flows in multiphase fixed-and fluidized-bed reactors, and typical bubble-column and loop-reactor flows. This paper focuses on bubble-driven flows. These are flow situations were the bubble movement itself is the main source of momentum to the flow field and are often characterized by low superficial liquid velocities, relatively high superficial gas velocities, and no mechanical support of the flow (e.g., agitation). Only vertical flow situations are considered. An overview of the verified forces acting on bubbles is given, and examples of both classical and more recent modeling approaches are shown. This include gravity, buoyancy, centrifugal forces, conventional Magnus and Saffman forces, form and friction drag, and added mass as well as turbulent migration and other instability mechanisms. Special emphasis is placed on mechanisms creating bubble movement in the radial direction. Important literature on the subject with regard to the use of computational fluid dynamics to model gas-driven bubbly flows is reviewed, and the various approaches are evaluated, i.e., dynamic vs steady-state descriptions and Euler/Lagrange vs Euler/Euler formulations. Results from steady-state Euler/ Euler simulations are given and discussed, and the demand for amplified modeling including more accurate and stable numerical solution schemes and algorithms is stressed.