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Three flow regimes can be distinguished (ordered in decreasing pressure): the continuum regime, the transitional or rarefied regime and the molecular regime.
Modelling of the continuum flow regime is covered by Computational Fluid Dynamics.
The fluidum is regarded as a continuum and the discrete character of molecules is not taken into account.
For the molecular regime network modeling can be applied. This is the equivalent of electric network modelling applied to flows.
It has always been a problem to model the transitional regime. Transitional flow conditions appear in a wide range of applications where vacuum is applied or small dimensions prevail. Some examples: the design of precision instruments (newest generation lithography machines), physical and chemical vapour deposition processes (fabrication of solar cells or coating of metals) or space technology (plume from a rocket nozzle).
TNO has developed in cooperation with the Delft University of Technology a tool to model the transitional regime. This tool is based on the Direct Simulation Monte Carlo (DSMC) method. In this model flow properties are determined by simulating the movement and collisions of molecules. A mixture of different gases can be modeled as well as chemical reactions between gases and on surfaces. The DSMC model is implemented in the parallelized numerical 3D flow solver CVD-X (developed in-house by TNO). The DSMC model is also valid in the molecular and continuum regime.
The results of a DSMC calculation in a practical case are presented where diffusion of one gas species from one chamber to another is suppressed using a counter flow with a second gas species.
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