Simulation, manufacturing and experimental investigation of surface structures to reduce clearance mass flow rates in rarefied gases

• Applicants: Professor Dr.-Ing. Dirk Biermann; Professor Dr.-Ing. Andreas Brümmer
• Subject Area: Hydraulic and Turbo Engines and Piston Engines ; Metal-Cutting and Abrasive Manufacturing Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 513663608

In the course of this project, surface structures will be investigated in order to influence the scattering direction of molecules in rarefied gas flows. As a result, for example, clearance mass flow rates can be reduced, which improves the operating behaviour of vacuum pumps. The focus of the investigation is on the one hand the production of such surface structures in the micro meter range (Institute of Machining Technology) and on the other hand the experimental and theoretical investigation of the rarefied flows through clearances with structured surfaces (Chair of Fluidics). Based on theoretical preliminary investigations, initial structures are to be manufactured and measured in a vacuum test rig so that the influence of the surface structure on the clearance mass flow rate can be determined. Furthermore, a new wall reflection model is being developed for the DSMC method, which maps the reflection properties of the surface structures without resolving the geometry of the surface structures in the mesh of the DSMC simulation. For this purpose, the surface geometry is analysed, abstracted and geometrically stored in the wall reflection model. The determination of the wall reflection within the wall reflection model is then based on the test particle method, with which the direction of scattering of the molecules in the surface structure can be calculated. In order to ensure a targeted and reproducible preparation of the surfaces, tools with geometrically defined cutting edges are developed and manufactured with the help of which the desired surface can be created. The preparations are carried out on a special machine for chip formation analysis, which works according to the principle of planing. Prior to tool grinding, a finite element chip formation model is developed and the preparation of the target surface with varying tool micro-shapes is investigated. The technological focus is on the prevention or targeted adjustment of the burr formation, which was identified as a challenge in simulative preliminary investigations - especially due to the small chip crosssections. In addition, the simulation is used to design the tool shape with the aim of minimizing passive forces so that the desired surface structure can be manufactured as accurately as possible. After the developed tools are manufactured by grinding in the in-house laboratory, they are used to prepare the surface structure. Based on a microscopic digitization of the generated structures and the simulation of the rarefied clearance flows using the DSMC method, an iterative adjustment of the tools to optimize the resulting surfaces is carried out. The experimental and simulative clearance flow investigation of the manufactured surface structures shows their potential with regard to the reduction of clearance mass flow rates in rarefied gas flows.

• DFG Programme: Research Grants