Course 2 Find a plan to improve the productivity of the mixing tank on the existing production line! (Part 1)

What is CFD ?

CFD is an abbreviation for Computational Fluid Dynamics.
The behavior of a fluid can be predicted by discretizing the continuous equation that governs fluid motion and the basic equation of the Navier-Stokes equation, approximating it with an algebraic equation, and numerically solving it with a computer. With the birth of this CFD, it has become possible to visualize the mixing state and flow pattern in the mixing tank.

In CFD, various physical phenomena are modeled (formulated) as needed and discretized into algebraic equations, but since assumptions and approximate values are used in modeling and discretization, errors will always occur. In addition, there are errors due to the nature of the computer. Therefore, it is important to keep in mind that numerical solutions include these errors.

The turbulence model is an example of a well-known model for modeling in CFD.
Turbulence occurs when the Reynolds number, which is the ratio of inertial force to viscous force, is large. Therefore, turbulence must be considered when solving high Reynolds number flows in CFD. Various turbulence models such as k-ε model and LES (Large Eddy Simulation) exist by modeling various kinds of eddy motions that make up turbulence. Therefore, the selection of this model is extremely important when solving fluid motions using software such as CFD.

By the way, what are the advantages of conducting a simulation?
The biggest advantage of numerical simulation, especially fluid simulation, is that you can visualize invisible phenomena. Furthermore, there are merits such as verification in unrealistic extreme conditions in experiments, reduction of labor for prototypes and experiments, and analysis under actual conditions.

However, fluid simulation has not only advantages but also disadvantages.
In fluid simulation, it is not possible to calculate unless the phenomenon is formulated, and since the assumptions and approximate values are used when formulating, it is not possible to know if it will be the same as the actual value unless it is calculated is.

For example, in the case of a gas entrainment phenomenon from the liquid surface to the liquid, it is difficult to formulate the mechanism such as how much gas is entrained from the liquid surface under what conditions and what is the diameter of bubbles entrapped in the liquid. Therefore, analysis with general-purpose CFD software becomes difficult.

Even if it is possible to simulate the entrainment of gas from the liquid surface, the consistency between the calculation results and the actual phenomenon could be shown for the whole system by using overall evaluation factor such as kLa (gas-liquid mass transfer coefficient), and difficult to evaluate for the local condition.

Therefore, even if the gas entrainment conditions and entrained bubble diameters are analyzed by simulation, the final evaluation will be the overall evaluation, so verification by a scale-down test is the most effective and reliable means.

That is why the fluid simulation should be used after conducting experiments to verify the consistency with actual phenomena and understanding the disadvantages. However, it goes without saying that it can be a very effective tool if you understand and use it.