Discover how ANSYS Fluent and the Population Balance Model can be used to simulate complex bubble dynamics in multiphase stirred tank systems.

Challenges

Simulations of gas–liquid systems with bubble aggregation and breakage are essential in a wide range of industrial applications where mass transfer, reaction kinetics, or phase distribution are driven by bubble behavior. These include fermentation reactors, wastewater treatment tanks, flotation cells, chemical absorption towers, and aerated reactors in the pharmaceutical and food industries.  In such systems, the size and distribution of bubbles directly affect the gas–liquid interfacial area, which controls the rate of oxygen transfer, solute absorption, or reaction efficiency. Accurately predicting how bubbles evolve under different hydrodynamic conditions enables better design and scale-up of reactors, ensuring both process efficiency and product quality.

This type of simulation addresses the key challenge of capturing the dynamic and non-uniform nature of bubble populations in stirred environments, where bubbles continuously coalesce (aggregate) or split (break) due to turbulence. Traditional multiphase models often assume a fixed bubble size, which overlooks critical effects on mass transfer and energy dissipation. By using a Population Balance Model (PBM), engineers can account for the evolution of the bubble size distribution in space and time. This added realism is crucial for optimizing reactor performance, reducing experimental costs, and understanding scale-dependent phenomena that are otherwise difficult to measure directly in large-scale systems.

Engineering Solution

Method

We’ll walk through setting up a multiphase simulation in ANSYS Fluent using the Population Balance Model (PBM) to capture bubble aggregation and breakage in a Stirred Tank Reactor filled with water. Air bubbles are introduced near the bottom, and agitation is applied via an impeller at different radial speeds. We use the Liao model to describe the physics of bubble interactions. This setup is relevant for chemical, biochemical, and wastewater processes involving gas–liquid mass transfer.

Key Features:

• Eulerian multiphase model (water + air)
• PBM with aggregation and breakage
• Multiple Reference Frame (MRF) on the impellers/baffles
• This model allows us to study of angular velocity effect on bubble size distribution

Multiphase:

• Model: Eulerian
• Phases: Primary = Water, Secondary = Air
• Bubble size distribution: Population balance (PB) model, with discrete method consisting of 10 bins (1 mm to about 11 mm bubble sizes) for air phase. The range of sizes can be estimated with Ansys Fluent's size calculator.
• Drag Law: Schiller-Naumann model, with surface tension effects,

Aggregation and Breakage Model for bubbles

Fluent has different tools to model the breakage or agglomeration of bubbles. Here, we use the Liao aggregation models, which consider the effects of turbulence, eddy capture, velocity gradient, body forces, and wake.

The Liao breakage model is based on the assumption that there is a threshold shear stress that can break bubbles. Below this value, bubbles would deform. The rate of bubble breakage is also a function of the surface tension and diameter. 

A detailed description of these models can be found in Ansys Fluent theory manual. These models capture turbulence-induced interactions, making them suitable for this application.

Results

Faster angular velocities induce mixing, which will in turn increase collision frequency between bubbles. However, the increased shear stress may also lead to bubble breakage. Simulation can help us identify which regime is more predominant as a function of angular impeller speed, flow rates, fluid properties, and tank geometry. Increasing angular speed of the impeller can be observed to facilitate the mixing of bubbles and facilitate some degree of coalescence, but that may be size-dependent. Here are the resulting velocity and diameter profiles at three different angular speeds:  

To illustrate the expected paths of bubbles, we can implement one-way discrete phase method (DPM) with particles of the same sizes and density as the air bubbles themselves, as shown below (note that bubbles have been enlarged 5x for easy visualization). 

We can also inject air as small 1-mm bubbles to track the degree of coalescence at 30 rad/s. Bubbles tend to grow in the vicinity of the impeller as they come off, as shown below.

Simulating bubble dynamics in a CSTR using ANSYS Fluent’s PBM allows us to explore the complex interactions of aggregation and breakage under varying mixing conditions. This approach provides critical insight into optimizing reactor performance in multiphase systems.

To see this application of Ansys Fluent for in action, watch our video tutorial below. The video provides a walkthrough from initial setup to results analysis.

Benefits

ANSYS Fluent is highly effective for modeling bubble flow in tanks because it offers robust multiphase flow solvers like Eulerian-Eulerian and Mixture models. These can capture key physical phenomena such as bubble rise, coalescence, and breakup, allowing for detailed prediction of gas holdup, liquid recirculation, and phase interactions in stirred or aerated systems.

Fluent's built-in Population Balance Model (PBM) is particularly useful for simulating bubble size distribution, which can further influence mass transfer, surface area, and reactor efficiency. PBM can include nucleation, coalescence, and breakup mechanisms, and can be customized further with user-defined functions (UDFs) to account for temperature effects, variable properties, or reaction kinetics.

The ability to model realistic tank geometries—including baffles, impellers, and spargers—lets users assess design impacts on bubble behavior and mixing. Fluent also supports species and heat transfer, enabling prediction of interphase mass transfer (like oxygen dissolution) and thermal gradients. Together, these features make Ansys Fluent a powerful tool for optimizing bubble-driven processes in reactors, bioreactors, and other industrial tanks.

Ozen Engineering Expertise

Ozen Engineering Inc. utilizes its extensive consulting expertise in CFD, FEA, thermal, optics, photonics, and electromagnetic simulations to deliver outstanding results on engineering projects. We tackle complex challenges, including multiphase flows and erosion modeling, using Ansys software. Our team specializes in expert consulting and training in engineering simulations with Ansys, particularly Ansys Icepak AEDT, helping clients maximize its potential through scripting and automation.

We deliver customized engineering solutions in thermal management, fluid dynamics, and electromagnetic simulations. Our consulting, training, and support optimize performance and reliability across new and existing systems. Learn more at https://ozeninc.com.

Suggested blogs by Ozen Engineering

• Simulation of Steady Vortex in a Stirred Tank using Ansys Fluent Software Parametric Workflow (Link)
• Flow transition in vertical gas liquid columns using Ansys Fluent (Link)