Engineers at ITT Flygt in Solna, Sweden, have devised a systematic approach for the setup and modeling of mixing applications in the sewage treatment industry and for other industrial liquid processes. The applications involve immersible mixers in tanks that are not necessarily cylindrical. An immersible mixer usually consists of an impeller and a motor mounted on a shaft attached to the tank through a flange on the side wall. The leeway for positioning the mixer assembly in the tank is immense, making these mixing simulations non-traditional, with more degrees of freedom than are typically encountered in conventional installations.

“Racetrack” tank showing mixing zone

The software developed by ITT Flygt carries out a number of functions. It accepts input from the user on the tank shape and size, as well as the type of mixer employed and its orientation in the vessel. It then instructs GAMBIT, Fluent’s grid generator, to build a hybrid mesh for the simulation using journal files. Momentum and turbulence sources that represent the time-averaged action of the mixer are then computed and the source terms are applied to the FLUENT CFD code via user-defined functions.

Velocity field in the vicinity of a Flygt large compact mixer

This approach has shortened the lead-time for model creation for sewage mixing tank simulations by a factor of 5. Perhaps just as importantly, this in-house design tool has made CFD mixing simulations accessible to non-CFD experts within ITT Flygt, thereby expanding the use of CFD within the company. It also shows how easy it is to customize CFD applications using a combination of GAMBIT and FLUENT software.

Velocity field in the corresponding momentum source model of the mixer

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