Mixer Module

Model Fluid Flow in Mixers and Stirred Tank Reactors

The Mixer Module, an add-on to the CFD Module, is specifically designed for the modeling and simulation of mixers and stirred tank reactors, encompassing both batch and continuous processes. It includes a wide range of capabilities for modeling and simulating mixers and reactors in the pharmaceutical, fine chemicals, and food industries.

The module includes functionality for modeling laminar and turbulent flow as well as single-phase and multiphase flow. It also includes functionality for the surface tracking of liquid surfaces in tanks equipped with rotating impellers.

Typical simulation results include the velocity and pressure fields, mixing efficiency, maximum shear rate, impeller power and torque, solute concentration or residence time, and temperature fields, and any other derived quantity from the velocity, concentration, and temperature fields.

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A mixer model with particle trajectories in the Rainbow color table.

Fluid Mixing Simulations

In pharmaceutical, fine chemical, and food product processes, the quality, reproducibility, and uniformity of the products is of utmost importance. One way of making sure that these product requirements are met is to perform simulations in order to design and optimize the operation of the mixing process and the mixer or reactor itself. Models and simulations are particularly useful when they can first be validated by a pilot process and then be used for scale-up computations. Once validated, such models may be used to avoid the costs involved in building and running pilot-scale processes and instead go directly from lab-scale production to full-scale production.

Features and Functionality in the Mixer Module

Discover capabilities for performing a variety of fluid mixing simulations.

A close-up view of the Rotating Domain settings and a mixer model in the Graphics window.

Laminar and Turbulent Flow in Rotating Machinery

The Mixer Module contains flexible and robust modeling interfaces for describing fluid flow in tanks with rotating impellers. The Fluid Flow interfaces account for laminar and turbulent flows involving incompressible, weakly compressible, or fully compressible fluids. In addition, there is a wide range of non-Newtonian fluid models available in the Laminar Flow interface.

Available for use when modeling rotating impellers are the Fluid Flow interfaces for turbulent flow, which include all of the Reynolds-averaged Navier–Stokes (RANS) turbulence models and non-Newtonian fluid models in the CFD Module.

A close-up view of the Nonisothermal Flow settings and a mixer model in the Graphics window.

Nonisothermal Flow

The Mixer Module features Nonisothermal Flow interfaces for the modeling of temperature and flow fields, including the effects of buoyancy and temperature-dependent properties. Also included in the Nonisothermal Flow interfaces is the ability to model heat transfer in fluids and solids, known as conjugate heat transfer. The module includes Rotating Machinery, Nonisothermal Flow interfaces that can be used with turbulence models for both laminar and turbulent flow.

A close-up view of the Model Builder with the Fluid Properties node highlighted and a mixer model in the Graphics window.

Algebraic Turbulence Models

The zero-equation RANS turbulence models available for rotating machinery are listed below:

Algebraic yPlus:

  • Zero-equation model based on the local wall distance
  • Resolves the flow all the way down to the wall
  • Solves for the wall distance in viscous units


  • Zero-equation model by Agonafer et al. (1996)
  • Resolves the flow all the way down to the wall
  • Solves for the tangential velocity (in viscous units) along the wall
A close-up view of the Model Builder with a loaded part node highlighted and a set of impellers in the Graphics window.

Part Library

The Mixer Module includes a Part Library with the most common impeller and tank geometries. These parts can be used to create a model of a mixer or stirred tank reactor. The geometry parts are fully parameterized, and both dimensions and configurations can be changed. Impeller blades can be cut or folded, and their corners or edges can be rounded or sharpened. There are six different types of axial impellers, four types of radial impellers, and two types of impellers for fluids with very high viscosity. There are three types of tanks: cone-bottom, dished-bottom, and flat-bottom tanks. All three can be with or without baffles.

A close-up view of the Reactions settings and a mixer model in the Graphics window.

Reacting and Multiphase Flow

Mixers and stirred tank reactors are subjected to both temperature and composition variations, which impact the local density and viscosity. These effects can be modeled with the Reacting Flow interfaces, which automatically couple the Fluid Flow interfaces with the Transport of Concentrated Species interface or the Transport of Diluted Species interface so that the transport and reactions of chemical species can be accounted for. Reacting flow in mixer and tank reactors with rotating impellers can be investigated in both the laminar and turbulent flow regimes.

In addition, the Mixer Module includes modeling interfaces for separated multiphase flow with surface tracking as well as for dispersed multiphase flow models, for which the local mass or volume fraction of each phase is resolved.

A close-up view of the Frozen Rotor settings and a mixer model in the Graphics window.

Study Types

A time-dependent study for fluid flow in rotating machinery uses a sliding mesh approach to account for rotation with time. COMSOL® defines a rotating domain that encompasses the impeller and a stationary domain outside of this where the tank walls and baffles are situated.

The Mixer Module also features a Frozen Rotor study that simulates rotating flow by assuming that the topology of the system relative to the rotating reference frame is fixed, or frozen. This significantly reduces the computational cost required to simulate a pseudo steady-state condition. The Frozen Rotor study is equivalent to solving the stationary Navier–Stokes equations, where the centrifugal and Coriolis forces have been added to the rotating domains. The Frozen Rotor study is also frequently used to obtain initial conditions for a time-dependent study with a rotating domain.

A close-up view of the Turbulent Flow, k-ε settings and a mixer model in the Graphics window.

Transport-Equation Turbulence Models

The RANS turbulence models that involve transport of turbulence quantities, which are available for rotating machinery, are listed below:

  • k-ε turbulence model with realizability constraints
  • Realizable k-ε turbulence model
  • k-ω turbulence model
  • Shear-stress transport (SST) model:
    • Menter SST two-equation model from 2003 with realizability constraints
  • Low-Reynolds (Re)-number k-ε turbulence model:
    • Abe–Kondoh–Nagano (AKN) model with realizability constraints
    • Resolves the flow all the way down to the wall
  • v2-f turbulence model:
    • Captures turbulence anisotropy
    • Suitable for swirling flows in, for example, cyclones
  • Spalart–Allmaras model:
    • Rotational correction version

Expand the Mixer Module's Modeling Capabilities

The Mixer Module, an add-on to the CFD Module, can be used in conjunction with any other add-on module in the COMSOL® product suite. For instance, it can be combined with the:

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