Characteristics of Steady and Transient Superfluid Transport for the Cooling of Superconducting Rotating Machines

Abstract

The unique properties of superfluid helium (He II) make it a very efficient cooling agent for superconducting rotating machines. Steady and transient transport characteristics and design formulas for the cooling of superconducting windings are enumerated in this article. Several superfluid transport analytical models and useful design equations are discussed: laminar flow; turbulent flow; and pure superfluid flow under steady-state and transient conditions. An effort was made to consolidate all analytical models and experimental results into a common framework. Under conditions of steady He II transport, a dimensionless heat flux number N q , a dimensionless driving force number N ∇T , and a characteristic length where used so that a generalized equation could be derived to describe superfluid transport in any geometry. In the case of transient transport of He II, a dimensionless heat flux number N q * and a dimensionless driving time number N t were used so that a generalized equation could be derived to describe transient superfluid transport in laminar flow and turbulent regimes. Many experimental data were compiled to substantiate the analysis.Large-scale superconducting magnets are expected to play a major role in future energy systems (Aized et al., 1999;Brechna, . Figure 1 shows the rotor and stator assembly in a conceptual design of a superconducting generator. Aized (1999) described the development of a 1000 hp superconducting motor in which the field winding is made of a superconductor that is cooled by means of closed-cycle helium refrigeration. The stator winding is watercooled by embedded cooling tubes. Vajda (1999) demonstrated the design and construction of an experimental superconducting synchronous machine. It has a permanent magnet rotor and a superconducting stator. Magnets of this type permit the construction of lightweight motors and generators that have obvious advantages for marine and airborne use. There is growing interest in using a superfluid as the coolant in superconducting rotating machines like superconducting motors and generators because of its unique and favorable cooling characteristics. Phenomena that tie directly into the safety and reliability of such superconducting windings are characterized by the thermodynamics of quasi-steady heat transfer and transient heat transfer to liquid helium (He II) during superconductor-coolant interaction. Superfluid heat transport presents many advantages: (1) Higher thermal stability due to lower operating temperatures;(2) higher heat conductivity, which reduces the generation of unfavorable vapor bubbles between windings; (3) insensitivity to winding orientation, which allows for flexibility in the geometry of the winding design. These advantages must be balanced against the added refrigeration requirement and the higher cost as compared with other cooling systems. For example, Kovalev (1998) reported two types of high-temperature superconducting alternators cooled by liquid hydrogen.Many investigators have compiled data concerning the cool...