Numerical Simulation Helps to Model Mixing Processes

Many engineering companies use computational fluid dynamics (CFD) to analyze the flow of fluids through complex machinery, such as turbo compressors. But there are a growing number of applications in other fields such as process engineering, where the flow of fluids through reactors, mixers and other fluid- handling equipment benefits from numerical simulation. Such is the case at Sulzer Innotec (Winterthur, Switzerland), where CFD is applied to a wide range of problems in process machinery used in the production of chemicals, pulp and paper, textiles and surface coating applications. In a variety of process engineering applications, turbulent flow plays a very important role. Mixing, combustion and other reactions are examples. Modelling such applications when multiphase flows--with free surfaces and with surface tension considerations--really take CFD to its limits. CFD developed as a modelling technique in aerospace and turbomachinery applications in the 1970s. The idea is to divide a physical space into a number of cells or control volumes; this division is called the mesh or grid. In each cell, fluid flow is computed with standard Navier-Stokes partial-differential equations; the output of one cell can be the input to the next cell. Besides physical flow, newer CFD programs are able to account for chemical reactions and heat transfer, such as would occur during combustion. Originally, CFD was a complex computer program that required hours of time on high-speed workstations to be completed. Improvements in microprocessor speeds and advanced data-handling techniques have reduced the time necessary to complete a study. Considerable time is also saved by automatic mesh generation. Finally, advanced visualization techniques allow the user to view the numerical data in the form of colors and flow trajectories. The output can also be animated to show changes over time. Static mixer design One of Sulzer Innotec's major application areas is the computation of mixing processes (Fig. 1). Fig. 1: This premixer blends the flow of two fluids with different compositions and temperatures; figure 1 shows the mixing pattern.

We use CFD to optimize new static mixers, to check the design of our NOx removal system (DeNOx) during the planning stages, and to analyze and improve the mixing performance in various types of mixers in existing plants (Fig. 2).

Fig. 2: A CFD visualization shows how the Sulzer SMV static mixer (grey grid) causes different fluids (colors) to blend together.

The advantage of CFD is the insight it gives into the fluid flow and mixing process in the whole domain of interest. Usually, experimentation with actual equipment is too difficult to set up for these applications.

Sulzer Innotec uses CFX-TASCflow, a CFD program from AEA Technology, (headquartered in Harwell England, with North American offices in Pittsburgh, PA and Waterloo, Ontario). Due to our special applications, such as mixing of highly viscous fluids, we are sometimes forced to develop our own software; in combination with TASCflow, we are able to calculate the mixing process in static mixers with highly viscous flows. In the calculation of highly viscous fluids, the numerical diffusion (i.e., the mathematical modelling) overwhelms the physical diffusion. In this case, a special method is necessary to calculate the mixing performance. We have developed a trajectory method with an added diffusion model to compute accurate mixing performance for these applications.

Plants with mixing problems, e.g. premixers of styrene plants, are also analyzed and improved with the help of CFD. In such cases, CFD is an excellent tool as it allows an engineer to investigate a problem anywhere in the world from his or her office desk. The plant is usually in operation and cannot be shut down, and experimental tests are too expensive to run. The main problem for the engineer is to place the boundaries of the computation to include all important effects, choose the appropriate grid size and the degree of detail to be resolved.

After the analysis of the problem, the experience of the mixing experts from Sulzer Chemtech and Sulzer Innotec is necessary to improve the design of the equipment. The new design is then checked with CFD. When the desired mixing performance is achieved, the new design can be implemented in the plant with some certainty that the suggested modifications will improve performance.

With new applications, a thorough test procedure has to be established to achieve the quality and confidence in applying CFD as an everyday tool. This demands good communication between the CFD user and the code developer, because the developer often has little idea of the new problems that the user wants to tackle with the program. Our experience of collaborating with AEA Technology on such validation has always been very positive.

Sulzer Innotec AG
Computing Ltd.
Fluid Dynamics Laboratory
8401 Winterthur, Switzerland
Email: egon.lang@sulzer.ch
Web: www.innotec.ch

AEA Advanced Scientific
554 Parkside Drive
Waterloo, Ontario N2L 5Z4
tel: 519 886 8435

NOTE: AEA Technology International Users Conference 1997
Chicago, Illinois, USA October 6 - 10, 1997
User Presentations, Product Demonstrations,
Software Training, Consulting Clinics
For more information contact cathy.kramer@engsw.aeat.com

By: Egon Lang, Sulzer Innotec, Winterthur, Switzerland