Effect of subchannel flow velocities on the stability of hollow cable conductors

Forced flow, helium cooled compacted twisted strand hollow cable conductors have been proposed for use in a number of large superconducting coils. The twisted strands and the compaction process result in non-uniform void distributions that are expected to affect the stability of the conductor and the helium pump work required. For the design of a large coil for the Large Coil Program (LCP), it was postulated that the flow velocity in the smallest subchannel controls the recovery of the conductor initially driven normal by a fault condition. However, the helium pump work and the resultant refrigeration load are determined by the nominal mass velocity. Consequently, for the design of hollow cable conductors, a relation between the minimum subchannel mass velocity and the nominal mass velocity is needed. Parallel channel models were developed and theoretical relations were derived between subchannel mass velocities and the nominal conductor mass velocity. The results show that the nominal mass velocity is several times greater than the subchannel mass velocities required for conductor recovery. The implications of this work with respect to optimum conductor configurations are discussed; and further research and development work required to better understand subchannel flow effects are suggested.