극저온 냉동기용 얇은 두께의 피스톤 벨로우즈의 피로 특성에 관한 연구 = A Study on Fatigue Properties of Thin-Walled Piston Bellows for Cryogenic Cooler

다국어 초록 (Multilingual Abstract)

Cryogenic temperatures generally refer to temperatures below 120 K (− 153 °C), which corresponds to the liquefaction temperature of natural gas. A cryocooler is a mechanical refrigerator designed to achieve such cryogenic conditions. Among the five commonly used cryocooler technologies, the Stirling cryocooler and the Gifford-McMahon (G-M) cryocooler are widely applied across various commercial sectors. A Stirling-type cryocooler typically consists of a compressor, regenerator, displacer and expander all of which use piston-cylinder mechanisms that perform gas compression and expansion driven by an external actuation system. The compressor can be categorized into two types : rotary compressors which convert rotary motion into linear piston motion via a crank mechanism and linear compressors which directly utilize a linear motor. However rotary compressors require substantial manufacturing time and cost, and they also demand lubrication due to stringent geometric tolerances - such as cylindricity, roundness, and surface roughness - on the order of only a few micrometers. On the other hand, linear compressors suffer from limited reliability and tend to exhibit higher power consumption, larger size, increased cost and pronounced external vibration. To overcome these limitations and enhance system reliability, the authors are developing a novel pressure generator for a Stirling-type cryocooler. Key factors in designing and applying cryogenic refrigeration systems include performance, efficiency, reliability and cost with the relative importance of each factor depending on the specific application. The proposed pressure generator in this work employs opposed angular piston bellows to minimize vertical vibration. This configuration simplifies the overall design while addressing the inherent drawbacks of both rotary and linear compressors. Nevertheless, axial vibration remains, for which a connecting bellows assembly is incorporated to mitigate the remaining vibration. The connecting bellows and piston bellows which are the core parts of the pressure generator are the parts that have the greatest impact on the performance and reliability of the pressure generator and it is very important to evaluate and predict the fatigue life. In this study, the fatigue life of the connecting bellows and piston bellows used in the pressure generator was evaluated. Dedicated fatigue testing machines were designed and fabricated to reproduce actual operating conditions and fatigue tests were performed on prototype bellows. In addition, a commercial finite element analysis program (ANSYS) was used to numerically evaluate stress distributions and to identify the locations and magnitudes of maximum stress. The major findings of this study are summarized as follows: (1) Fatigue tests of the connecting and piston bellows produced S–N curves for each component. The connecting bellows exhibited infinite fatigue life at axial displacement of 1.00mm or less while the piston bellows achieved infinite life at angular displacement of 18mm or less. (2) Based on the actual axial displacement of 0.2mm in the connecting bellows, the safety factor was calculated to be 5. For the piston bellows operating at an angular displacement of 15mm, the safety factor was 1.2. (3) In the connecting bellows fatigue tests 75% of the observed cracks initiated at the first crest whereas in the piston bellows 100% of cracks initiated at the first root. (4) Numerical analysis showed that the maximum stress in the connecting bellows occurred at the first root during compression and the piston bellows also exhibited maximum stress at the first root during compression. (5) For the piston bellows the locations of crack initiation and maximum stress coincided - both occurring at the first root in experiments and simulations. However in the connecting bellows crack initiation during testing occurred at the first crest while numerical analysis predicted maximum stress at the first root. This discrepancy is attributed to structural weaknesses at the crest caused by welding-induced residual stresses and local reductions in crest thickness. The results of this study suggest that the proposed bellows-type compressor has the potential to serve as an alternative to conventional rotary and linear compressor configurations when applied as a pressure generator in Stirling cryocoolers.

목차 (Table of Contents)

• 제 1장 서론 1
• 1.1 연구 배경 1
• 1.2 연구 목적 6
• 1.3 연구 범위 및 내용 7
• 제 2장 이론적 배경 8
• 2.1 극저온에 대한 기초적 배경 8
• 2.1.1 극저온이란 8
• 2.1.2 극저온 냉매 11
• 2.1.3 극저온 냉동기의 분류 및 생성 방법 13
• 2.2 스터링 냉동기 19
• 2.2.1 스터링 냉동기의 작동원리 및 종류 19
• 2.2.2 스터링 냉동기의 기술개발 현황 22
• 2.3 피로해석 32
• 2.3.1 피로하중 32
• 2.3.2 S-N 접근법 34
• 2.4 벨로우즈의 형상 및 제작 방법 36
• 제 3장 시험방법 및 절차 40
• 3.1 인장시험 40
• 3.1.1 재료 및 인장시험편 40
• 3.1.2 시험방법 및 절차 42
• 3.2 커넥팅 벨로우즈의 피로시험 44
• 3.2.1 커넥팅 벨로우즈 제작 44
• 3.2.2 피로시험장치 설계 및 제작 46
• 3.2.3 시험방법 및 절차 49
• 3.3 피스톤 벨로우즈의 피로시험 50
• 3.3.1 피스톤 벨로우즈 제작 50
• 3.3.2 피로시험장치 설계 및 제작 53
• 3.3.3 시험방법 및 절차 58
• 3.4 수치해석 59
• 3.4.1 커넥팅 벨로우즈의 수치해석 59
• 3.4.2 피스톤 벨로우즈의 수치해석 60
• 제 4 장 결과 및 고찰 61
• 4.1 인장시험 결과 61
• 4.2 커넥팅 벨로우즈 피로시험 및 수치해석 결과 63
• 4.2.1 피로시험 결과 63
• 4.2.2 수치해석 결과 67
• 4.2.3 시험 결과와 수치해석 결과의 비교 70
• 4.3 피스톤 벨로우즈 피로시험 및 수치해석 결과 72
• 4.3.1 피로시험 결과 72
• 4.3.2 수치해석 결과 77
• 4.3.3 시험 결과와 수치해석 결과의 비교 81
• 4.3.4 파단면 분석 82
• 제 5 장 결 론 85
• 참고 문헌 87
• 감사의 글 95