    ## Mixing plants

Stirrer are used in many different industries. To procedurally optimize your stirrer, you need detailed knowledge of for example the mixing times. By means of high-resolution 3D CFD flow simulations, the mixing times can be calculated in a short period of time and with little effort. The simulation graphics are clearly illustrated and  enable to analyse the data.
Therefore, CFD simulations are a reliable Tool for all thinkable configurations.

• reproducible results
• no time-consuming measurements
• direct identification of the problem areas

We will give you secific information regarding to your situation.

### Best practice example: The simulation of a continuous stirrer tank for mixing water and ethanol

To simulate the situation, the geometry of the stirrer tank has to be defined as a first step. As an illustration, you can see the grid view of the tank with approx. 470.000 grid cells on the left.

• Geometry oriented towards a stirrer tank with
h = 1,4 m und d = 1,2 m
• stirrer with h = 0,3 m und d = 0,6 m
• installation height of the stirrer to 0,4 m
• Total volume flow rate of 0,05 m3/s (1 to 1)

Assuming a transient flow in the tank, we determine the mixing quality. The mixing quality is a measure for the uniformity of a mixture and is calculated by the coefficient of variation sigma.
100 % mixing quality correspond to a coefficient of variation of 0 and represent the perfect mixed state, which is not accessible in reality. The reason for that is that the concentration at the outlet is affected by local fluctuations.  The coefficient of variation describes the intensity of these fluctuations, which is demonstrated with the ethanol concentration alpha below. As used in the following example, it is usual to accept sigma = 5 % in practice, which equals a mixing quality of 95 %.

Demands: alpha = 50 % (0,5) and simultaneous sigma = 5 % (0,05)  at the outlet

The video on the right shows the time dependent mixing behavior of water and ethanol, which depends on the rotational speed as well as the geometry of the stirrer.

On the left, you can see the whole construction, whereas the focus on the right is on the concentration array at the outlet.
The scala and the colouring on the right simulation graphic demonstrate, that the percentage of ethanol at the outlet fluctuates between 40 % and 60 %.
The ideal situation would be a temporally constant and exhaustive percentage of 50 % ethanol.

The simulation process is followed by the evaluation of the simulation results. The gained knowledge form the basis of a joint elaboration of optimization possibilities with the client. In this case, the shovel blades of the stirrer were modified to balance the fluctuations in concentration at the outlet of the stirrer tank.