Jeheon Jung

Investigation of on-board hybrid pulse tube cryocooler for high temperature superconducting rotor

This paper discusses the practical realization of an advanced cooling method for superconducting rotor; on-board cryocooler. A hybrid pulse tube cryocooler was designed and tested in the rotating system. A Stirling cryocooler precooled the regenerator of the pulse tube cryocooler to produce lower temperature at the cold end of the pulse tube cryocooler. Based on adiabatic model and loss mechanism analysis, the pulse tube cryocooler was designed and fabricated. The hybrid cryocooler was aligned in the test rotor to have a concentric centerline with the rotation axis. Even if the performance of the rotating hybrid pulse tube cryocooler was not satisfactory, achieving 55 K, the feasibility of on-board cryocooler method for superconducting rotor was confirmed.

Modified Roebuck compression device for cryogenic refrigeration system of superconducting rotating machine

Abstract Cryogenic refrigeration system installed on superconducting rotor has the merit that it can eliminate cryogen transfer between rapidly revolving rotor and stationary part and, therefore, the loss generated by it. Nevertheless, such an on-board cryogenic refrigeration system has not been realized yet because of the absence of compression device that can work reliably on rapidly revolving rotor. This paper presents the idea of modified Roebuck compression device as a potential on-board compression device. Although it has the disadvantage that it requires large radial space and external cooling and heating, a modified Roebuck compression device of 14 stages with diameter of 0.80 m mounted on a rotor revolving at 3600 rpm can produce substantially high compression ratio. J–T neon refrigeration system equipped with such a compression device can produce low temperature of 30 K and be useful for cooling superconducting rotor windings made of high temperature superconductor.

Tandem GM Type‐Pulse Tube Refrigerator with Novel Rotary Valve and Bypass Valve Mechanism

A single‐stage tandem pulse tube refrigerator has been fabricated, which is driven by two pulsating pressures of opposite phase. The refrigerator is designed as the first stage machine of a 4 K pulse tube refrigerator using a recuperator as the second stage regenerator. In this tandem configuration, two pressure pulsations are generated by a single GM‐cryocooler compressor. Two identical pulse tube refrigerators are operated with a novel rotary valve that can supply high and low pressures simultaneously. The pressure measurement at the regenerator inlets of the pulse tube refrigerators confirmed the proper pressure wave of two pulsating pressures in opposite phase. Detrimental DC‐flow characteristic of double‐inlet GM‐type pulse tube refrigerator has been suppressed by the newly designed bypass valve mechanism. This new bypass valve set has one more extra degree of freedom than usual ones to eliminate DC‐flow easily. This paper describes a novel feature of the bypass valve set and its performance in the e...

Chemically Etched Cryogenic Micro Structure Heat Exchanger

A tandem 4 K pulse tube refrigerator requires a recuperator to be fully accomplished. The recuperator should be a counter-flow heat exchanger which has micro heat-transfer structure like a regenerator of cryogenic refrigerators. However, the technology of such a heat exchanger is not well established yet. Hence, the development of a counter-flow compact heat exchanger with micro structure is demanded first in order to proceed to the recuperative 4 K pulse tube refrigerator. This paper describes the design, fabrication and preliminary performance test of such a heat exchanger. The stainless steel micro structure with approximately 0.1 mm characteristic length has been created by chemical etching. The parallel V-shape double-sided micro channels (chevron stucture) in the heat exchanger enable the flow to be three-dimensionally well mixed so that the heat transfer at low Reynolds number can be enhanced. The etched plates are stacked and bonded through a vacuum brazing process to compose a plate-type heat exchanger. The chemically etched micro-structure heat exchanger has thermal effectiveness of 97%.Copyright

Rotating Cryocooler for Superconducting Motor

A single‐stage coaxial pulse tube refrigerator has been designed for HTS (High Temperature Superconductor) motor application. This paper discusses a practical realization of an advanced cooling method for superconducting rotor, on‐board cryocooler. When a cryocooler is considered to be mounted on the superconducting rotor, the following two factors must be satisfied for practical application. First, the on‐board cryocooler should not disturb the high‐speed revolution of the rotor. Second, at the same time, the high‐speed revolution of the rotor should not deteriorate the cooling performance of the cryocooler. These mutual technical demands restrict the type of cryocooler suitable for high‐speed rotating environment. We select a Stirling‐type coaxial pulse tube cryocooler and incorporate it on the 1800‐rpm superconducting motor mock‐up. The pulse tube cryocooler is designed with an adiabatic model and a various loss mechanism analysis. The no‐load temperature is approximately 100 K with less than 150 W ele...

BUFFERED ROTARY VALVE SYSTEM OF GM-TYPE PULSE TUBE REFRIGERATOR

The buffered rotary valve system was invented to make a valved refrigeration system more efficient. This paper introduces its basic design and operation process, and describes the experimental performance results. The mass flow rate was reduced and the coldhead temperature at no load condition became lower than that of the conventional rotary valve system even though the improvement was not so much as we expected. The experimental results are discussed with the effect of the orifice reservoir of double inlet PTR (Pulse Tube Refrigerator), and the prospect of buffered rotary valve system is suggested.

Cryogenic Heat Exchanger With Photo-Etched Mini-Perforated Plates Allowing Flow-Bypass

A perforated plate heat exchanger can be made very compact by the reduced perforation size. Hence, it fits to the field where high effectiveness but small size is demanded, i.e., cryogenic applications. The compactness of a perforated plate heat exchanger can even reach that of a cryogenic regenerator. We are developing a compact perforated plate heat exchanger for replacing a low-temperature regenerator. In the compact perforated plate heat exchanger, small perforations (diameter: ∼ 150 μm) were patterned by photo-etching on thin copper plate (75 μm thick). Spacers were made of Kapton film (25 μm thick) or stainless steel plate (30 μm thick). Kapton film spacer is for an experimental heat exchanger which requires convenient disassembly, and stainless steel spacer is for the final product which will be diffusion-bonded. In this type of heat exchanger, stream-to-channel convection heat transfer coefficient is so large that conduction along the plate can rather restrict overall heat transfer rate between cold and hot streams. In order to avoid such conduction-restricted overall heat transfer, 14 strip-shaped channels were formed spirally to reduce effective conduction length and enhance lateral conduction. Transverse bypass among cold or hot stream channels were also adopted to relieve flow mal-distribution. Thermal performance of the heat exchanger was both analyzed and experimented between liquid nitrogen temperature and room temperature. The result shows quite good agreement.© 2007 ASME