Page 177 - Fundamentals of Magnetic Thermonuclear Reactor Design

158 Fundamentals of Magnetic Thermonuclear Reactor Design

Listed are some of the factors that should be taken into account in the
thermal–hydraulic analysis of a large SC magnet under normal and abnormal
conditions:
l A large number of cooling pipes and channels, which generally form one or
more closed hydraulic loops;
l Potential heat and mass exchange between coolant flows in different chan-
nels (e.g. in CICC);
l Non-uniform transient heat release in solids adjacent to cooling channels
l Presence of valves and pumps in the cooling circuit; and
l Complex interplay of the thermal and hydraulic parameters of the SC mag-
nets and the cryogenic system.
In most cases, a 1D approximation for cooling pipes and superconduct-
ing winding gives an acceptable assessment of longitudinal convective heat
transfer associated with coolant flows and transverse conductive heat trans-
fer through solid walls. For more complex cases, when the 1D approxima-
tion is inadequate and information is needed to understand the non-stationary
temperature distribution over a thick solid, a 2D thermal diffusion model is
applied.
The Venecia thermal–hydraulic model is comprised of basic models (primi-
tives) for the typical components of a magnet system [26]:
l non-stationary fluid flow in the 1D approximation
l volume with merging flows (collectors, manifolds, etc.) in the 0D approxi-
mation (lumped object identified by pressure and enthalpy)
l flow control components (valves) in the 0D approximation
l pressurised flow (pumps, compressors, etc.) in the 0D approximation
l thin solids in the 1D approximation (conductors and pipes)
l massive solids in the 2D approximation

The number of basic models of each type used in a global model depends
only on task specifics and adopted assumptions. The global computational mod-
el of a real magnet with its cooling circuit is built out of the basic models using
thermal and hydraulic shared links in much the same way as ‘Lego’ pieces are
assembled and connected to construct an object.
Thermal diffusion in solids is modelled in 2D approximation taking into
consideration properties of different structural materials and relevant boundary
conditions. A special strategy is applied to integrate the 2D model for thermal
diffusion in solid structures with the 1D model of a forced-flow circuit to pro-
vide quasi-3D modelling of an SC magnet. The modelling approach has been
verified by a comparison with the data obtained in the experiments on the ITER
CSMC [22].
In Venecia models all the thermal–hydraulic and thermodynamic prop-
erties of materials are represented as nonlinear functions of relevant pa-