Vacuum systems with high gas throughputs (big nuclear fusion machines are a typical example), are very often operated in the transitional flow range with Knudsen numbers around unity. The vacuum flows in this regime are difficult to predict accurately as the standard formulae (viscous, free molecular) do not apply. There are results for given duct cross-sections (Direct Simulation Monte Carlo results, solutions of the kinetic equations), but these approaches do require substantial computational efforts.
In order to have a more pragmatic approach, the semi-empirical flow code ITERVAC was developed in Forschungszentrum Karlsruhe, based on a generalized flow function which is applicable to all prismatic cross-sections in a wide range of Kn numbers. It includes a network module to represent complex vacuum system flow patterns. The main advantage of this code is that it provides the vacuum designer immediately with the information how the system performance reacts upon an implemented design change. It is already under regular use for the optimisation of the ITER large vacuum systems.
Because of the importance which is given to the area of transitional flows, a comprehensive programme for code benchmark and validation has been initiated. Unfortunately, it was found that literature data for the transitional flow range are scarce and, if at all, do only exist for relatively simple geometries and also not seem to be very consistent. Therefore, it was decided to set up the dedicated test facility TRANSFLOW to provide parametric sets of well defined data points in transitional flow which can be used for benchmark. The size of the facility was chosen such that it allows to investigate 1:1 scale ducts (up to DN 600) and to have gas inlet and outlet via large domes in order to have well defined conditions there. The duct conductance is derived from an adjusted high precision isothermal flow and the measured stationary pressure differential across the duct. The typical pressures inside the test rig vary from 100 Pa and 10-6 Pa, depending on the channel length.
This paper describes the facility, assesses the experimental challenges and presents first results.
|