Press Room - Fluid-structure interaction at Sulzer Innotec

Fluid-structure interaction at Sulzer Innotec

Several industrial processes involve mixing of two or more components which have different physical properties. The static mixers of Sulzer Chemtech homogenize the flow within the mixer without the use of any moving part. Differences in concentration, temperature and viscosity are equalized in a continuous process over a flow cross-section.

The Sulzer SMX mixers are designed for the mixing of high viscosity liquids under laminar flow conditions. The complex structure of the blades generate layers which drive the mixing process. Special versions of the mixer are designed for applications (e.g. polymer melt blending) where the pressure drop is up to 100 bar or higher. The mechanical forces and stresses in the structure caused by the flow are given by the pressure drop at each blade. The resulting total force is transferred from the mixer structure to the outer pipe e.g. at one end of the mixer. Due to the complexity of the flow materials and different mixer designs, a general mechanical testing under real fluid forces is complicated and expensive. However, mixer optimisation requires consideration of the real fluid forces for the determination of maximum stresses inside the structure.

The coupling of commercial CFD-codes and codes for structural mechanics can be carried out by loose coupling through the MpCCI communication library. This library interpolates the required values such as temperature, force, deformation, etc. from one code to the other, on matching and non-matching grids. The codes are coupled on their outer iterations. A first version of the MpCCI library was used in the CISPAR project. Further development of the library was done in close co-operation between some of the CISPAR partners: GMD, Computational Dynamics, Intes, DaimlerChrysler and Sulzer. Together, we developed the first version of code-coupling between STAR-CD (Computational Dynamics) and PERMAS (Intes) that was industrial use.

The deformations and stresses of the Sulzer Mixer subjected to high-pressure load was investigated by coupling STAR-CD and Permas using MpCCI. The geometry model takes into account all the details of the structure, even welding points. The mixer structure was completely built as a 3D solid model in Unigraphics. The fluid flow domain was generated by Boolean subtraction of the 3D solids of the pipe from the mixer structure. The FEM model was meshed by the specialists at the Mechanical Department using Medina (hexahedral) and/or Patran (tetrahedral). The fluid flow domain was meshed by our colleagues at the Fluid Dynamics Department, using unstructured grid generation with tets and prisms created by GridGen and ICEM/Tetra. The two meshes consist of 35,000 hexahedral elements for the FEM model and 1.7 million elements for the fluid flow model. The FEM model used first-order elements and considered contact conditions between the mixer and the outer pipe. One advantage of the loose coupling approach using MpCCI is that both meshes can be generated independently. The coupling interface for the non-matching grids is very complex and consists of 120,000 triangular elements in the fluid domain and 30'000 quadrilateral elements in the solid domain.

As a first step, the steady-state fluid flow was computed by STAR-CD without any code coupling. The pressure distribution and the streamlines are shown below. As a second step, the fluid forces were transferred from the fluid code to the stress code by coupling the codes.

The computed stresses and deformations are shown in the figures. This method (the one-way-coupling) assumes that the fluid flow topology is not affected by the displacement of the mixer, which is realistic for this kind of mixer. The deformations, stresses and rotational movement agree with our experimental observations. We are also working on the full coupling of the flow and stress computations, requiring STAR-CD’s moving mesh capabilities. The use of STAR-CD, Permas and MpCCI enables a more realistic computation of the forces on the structure and better design and optimisation of the mixer geometry.