(452c) Investigation of Bubble Induced Turbulence Model By Direct Numerical Simulation

Gas-liquid
multiphase flow is commonly encountered in bubble columns, stirred tanks and
other reactors. The complexity of multiphase turbulent flow increases the
difficulty of design and scale-up of the reactors. CFD simulation is an
effective tool to analyze and predict multiphase flow in reactors, which is
widely used for aided design of industrial reactors. The development of
suitable models and numerical methods is hence a significant task.

Gas-liquid flow
can be numerically investigated by various methods. For small-scale simulation
of detailed turbulence and bubbles, direct numerical simulation (DNS) can be
employed. For reactor scale simulation, the Euler-Euler two-fluid model is commonly
used, coupled with the Reynolds-averaged Navier-Stokes (RANS) method. The two-fluid model is
preferable for industrial multiphase flow simulation due to its simplicity. For
gas-liquid flow, the presence of bubbles will affect the structure and
intensity of local turbulence. In turn, the turbulence also affects the bubble
behavior. Therefore, suitable models should be developed taking the bubble
induced turbulence into account, based on the two-fluid model.

There are two ways
to introduce the effect of bubbles into liquid turbulence. One way is additional
viscosity method, in which an additional viscosity due to bubble motion is
added to calculate the effect viscosity [1]. The other way is an additional
source term method, in which additional source terms are added to the turbulence
kinetic and energy dissipation equations [2]. Both methods are widely used for
gas-liquid simulations in stirred tanks and other reactors [3,4]. However,
these models include empirical parameters, which are taken on various values in
the literature.

In this work,
bubble induced turbulence models are investigated by direct numerical
simulations. First, gas-liquid flow in a periodic vertical channel is predicted
by direct numerical simulation using a front tracking method (Fig.1) [5]. The
flow field and other quantities of interest, at every temporal and spatial location,
are given by the DNS results. The results are then used to find the various averages
and statistical quantities in the model equations. Based on the DNS results and
statistical analysis, a bubble induced turbulence model is developed and
validated. The model will be used to simulate practice gas-liquid flow in industrial
reactors in future works.

Acknowledgements: Financial
support from the Li Foundation, the National Natural Science Foundation of
China (21427814, 91434126) and National Key Research and Development Program (2016YFB0301701)
are gratefully acknowledged.

*Corresponding author. Tel:
+86-10-62554558; E-mail: chaoyang@ipe.ac.cn (C. Yang)

Fig.1 Deformable bubbles and vorticity by DNS

Reference

1. Sato Y., Sadatomi M., Sekoguchi K.,
Momentum and Heat Transfer in Two-Phase Bubble Flow-
= 1 \* ROMAN I Theory, Int. J. Multiphase Flow, 1981, 7:
167-177

2. Rzehak R., Krepper E., CFD
modeling of bubble-induced turbulence, Int. J. Multiphase Flow, 2013, 55:
138-155

3. Islam A., Adoo N., Bergstrom D., Prediction of Momo-Disperse
Gas-Liquid turbulent flow in a vertical pipe, Int. J. Multiphase Flow, 2016,
85: 236-244

4. Wang W., Mao
Z.-S., Numerical Simulation of Gas-Liquid Flow in a Stirred Tank with a Rushton
Impeller, Chinese J. Chem. Eng., 2002, 10(4): 385-395

5. Tryggvason G., Scardovelli R., Zaleski S., Direct
Numerical Simulations of Gas-Liquid Multiphase Flow, Cambridge University Press,
2011