In this paper, the first of a series, we propose a novel approach, based on Computational Fluid Dynamics (CFD), to understand the complex transverse thermalhydraulic processes in the dual-channel cable-in-conduit conductors (CICC), which are used for the superconducting magnets of the International Thermonuclear Experimental Reactor (ITER). Advanced 2D and 3D CFD, including sophisticated turbulence models, is used to compute the mass flow rate corresponding to an imposed pressure drop in ribroughened pipes, including spirals mimicking the central channel of an ITER CICC and used in several experiments. The results of the calculation are validated against measured data and can be used to deduce the friction factor f H in the central channel, throwing at the same time some light on the role played by the different parameters (Reynolds number, spiral geometry, etc.) in the central channel friction process for an ITER CICC.

Currently, commercially available turbo-drag pumps for high vacuum systems are based on either Gaede- or Holweck-type molecular drag pumping stages used in series downstream of axial bladed stages to extend the maximum compression ratio up to the 10 mbar foreline pressure range. Modern Gaede-type molecular drag stages use a disk-shaped impeller, allowing a very compact design, and the maximum compression ratio is limited by the leakage effect to about 10 per stage. Holweck stages are able to supply a high pumping speed, thanks to the presence of many channels in parallel and a high compression ratio, but this is obtained with the use of a less compact drum-shaped impeller. In this article, a new spiral molecular drag stage design is presented with the advantages of both high compression ratio and pumping speed per stage and very compact design: a stage occupying the very small axial space of one Gaede can supply the same compression ratio and pumping speed of a Holweck stage of the same diameter and peripheral speed in a much smaller axial space. The new spiral drag stage allows the design of very compact, high compression ratio turbo-drag pumps. The comparison of a 700 l/s turbo-drag pump implementing the new spiral molecular drag pump design with the existing Gaede- and Holweck-based products of the same pumping speed is presented, showing the performance advantages of the new design.

Holweck molecular drag pumps are used as high-pressure stages in hybrid turbomolecular vacuum pumps, where they can operate in the transition and the viscous regime. In this article we develop a Navier-Stokes model of a Holweck pump with tapered pumping channels, applying slip-flow boundary conditions, to predict vacuum performances with and without gas flow. The commercial computational fluid dynamic code FLUENT is used to solve the model equations and to predict the pressure profile along the grooves. A specifically designed experiment is presented, whose arrangement provides the boundary conditions as input to the model and whose results are used to validate it.

A three-dimensional model for a Gaede pump, based on the Navier-Stokes equations with no-slip boundary conditions, was introduced by the authors in a previous work. A commercial computational fluid dynamics code was used to obtain the solution in an outlet pressure range corresponding to the viscous laminar regime and incipient transition to molecular flow and the validation against the experimental data showed a good level of accuracy in terms of compression ratio. However, the mechanical power dissipation predicted by the no-slip model shows a trend diverging from the measured data for decreasing pressure, in the transitional flow regime. As most of the mechanical power is dissipated into heat by viscosity, slip-flow boundary conditions are introduced here in order to model the transitional effects on the wall shear stress and improve the accuracy of the Navier-Stokes description at low pressure. The calculation is carried out up to Knudsen numbers KnϷ 1, showing a very good agreement of the Navier-Stokes model, even close to molecular flow conditions, and leaving a residual error acceptable for design purposes. The model is used to analyze the flow fields in the Gaede pump and explain its behavior. The slip model needs just one adjusting parameter, i.e., the momentum accommodation coefficient, but exhibits a small sensitivity to its variations. Hence it can predict the compression ratio and power dissipation of Gaede pumps of any size in their whole operating pressure range, from the laminar viscous regime down to the transition to the molecular flow regime.

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