Study of neon heat flux in thermosyphon cooling system for high-temperature superconducting machinery

Abstract

High-temperature superconductors (HTS) applied to rotating machine draw much interest in the shipbuilding industry as the urge to get high-power density and efficient electric propulsion systems. To achieve an optimal performance of HTS field pole magnets in ship propulsion motors, it is crucial to regulate the operating temperature under 40 K and a simple and reliable cooling system is necessary. A thermosyphon (TS) is highly suitable since its operation principle consists in using natural convection without the intervention of mechanical circulation pump. The system composition is simple and light-weight. The TS benefits from a high heat transfer coefficient thanks to latent heat. The available temperature for TS depends on the saturation temperature of the refrigerant, which provides adequate controllable cooling for cryogenic machine application. A TS system using neon enables us to supply cooling temperatures of 28–40 K range. To determine the optimal gas-liquid state operation for HTS motors, a scaled model of a 20 MW motor TS cooling system was designed and constructed. This enabled us to visualize any phenomena and transient state change inside the evaporator part of the rotor. We studied heat transfer capacity under heat load for different neon quantities. Neon heat flux was then calculated to determine the heat transfer area for effective cooling. The boiling curve leads to the refined evaporator design with optimal heat transfer area. Introducing coaxial tubing between condenser and evaporator, possible sustainable liquid-gas circulation has been achieved without interruption under inclined condition required for ship operation. The present results contribute to the next stage HTS ship propulsion motor and superconducting machine design.