암모니아의 이중 역할(연료·작동유체)에 기반한 탈탄소 복합 사이클의 멀티 스케일 열역학적 고찰 = Multiscale Thermodynamic Study of a Decarbonization Combined Cycle Based on the Dual Role (Fuel and Working Fluid) of Ammonia

다국어 초록 (Multilingual Abstract)

Multiscale Thermodynamic Study of a Decarbonization Combined Cycle Based on the Dual Role (Fuel and Working Fluid) of Ammonia Yerim An Advisor Prof. Jungsoo Park, Ph.D Department of Mechanical Engineering, Graduate School of Chosun University As the global demand for carbon-neutral energy systems intensifies, ammonia (NH₃) has emerged as a promising carbon-free energy carrier owing to its zero direct CO₂ emissions during combustion and its suitability as a working fluid in Rankine-cycle-based waste heat recovery systems. This study proposes an integrated combined power generation system in which ammonia simultaneously serves as the fuel for a Brayton-cycle gas turbine and as the working fluid for a bottoming Rankine cycle. A multiscale numerical analysis framework comprising zero-dimensional (0D) thermodynamic modeling, one-dimensional (1D) GT- SUITE simulation, and three-dimensional (3D) CFD analysis using CONVERGE was constructed to evaluate system performance with the Rankine high-side pressure (PR) and heat exchanger tube count (Nt) as primary design variables. The 0D analysis showed that integrating the bottoming Rankine cycle improved total thermal efficiency from 38.08% to a maximum of 43.24% at PR = 55 bar, with the combustor identified as the dominant source of irreversibility accounting for 68–72% of total exergy destruction. The 1D analysis, incorporating actual pressure losses and heat losses, yielded a Brayton net output approximately 52% lower than the 0D prediction, with the turbine back- pressure of 1.45 bar confirmed as the primary cause. Despite this quantitative gap, qualitative performance trends with respect to PR and Nt were consistent between the two models, with the maximum combined cycle thermal efficiency reaching 31.98% at Nt = 100 and PR = 55 bar. CFD analysis of the shell-and-tube WHR evaporator revealed that flow maldistribution was present across all pressure conditions. At PR = 25 bar, the hot gas outlet temperature exceeded the coolant outlet temperature, confirming that a stable temperature driving force was maintained throughout the heat exchanger, with inter-tube coolant outlet temperature deviations of approximately 38.5 K. At PR = 35 bar, the coolant outlet temperature exceeded the hot gas outlet temperature by 9.6 K. While such an outlet temperature relationship is structurally permissible in counter-flow heat exchangers, it indicates that the local gas temperature within the evaporator begins to approach the NH₃ saturation temperature, reducing the effective thermal driving force in the post-evaporation region. The inter-tube deviation (≈38.3 K) remained comparable to the 25 bar condition. At high-pressure conditions (PR ≥ 45 bar), the coolant outlet temperature exceeded the hot gas outlet temperature by 35.6 K at 45 bar and 34.2 K at 55 bar, signifying that an internal pinch point violation occurred within the evaporator — where the local gas temperature fell below the NH₃ saturation temperature (Tsat(45 bar) ≈ 360 K, Tsat(55 bar) ≈ 367 K), causing a loss of effective thermal driving force for evaporation. Inter-tube coolant outlet temperature deviations expanded sharply to approximately 78 K at 45 bar and 74 K at 55 bar, nearly double those observed at lower pressures. This abrupt amplification of inter-tube deviation in the 35→45 bar transition was attributed not to intensified flow maldistribution itself, but to the pressure-dependent reduction in NH₃ latent heat, which renders the coolant phase state increasingly sensitive to non-uniform flow distribution under high-pressure operation. These findings demonstrate that the thermodynamically optimal operating point identified by 0D analysis does not necessarily correspond to stable evaporator performance, and that component-level CFD analysis is indispensable for reliable system design, particularly regarding baffle geometry optimization and expander inlet condition assurance.

목차 (Table of Contents)

• 제 1장 서 론 1
• 제 1절. 연구 동기 및 목적 1
• 제 2절. 연구 개요 7
• 제 2장 연구 배경 및 이론적 고찰 8
• 제 1절. 암모니아 기반 발전 시스템 연구 동향 8
• 1. 암모니아 연소 가스터빈 8
• 2. 발전 폐열 회수 및 ORC 적용 사례 12
• 3. 연료 작동유체 통합 개념 연구 동향 16
• 제 2절. 암모니아의 열역학적 특성 19
• 제 3장 연구방법 및 절차 22
• 제 1절. 시스템 구성 및 해석 개요 22
• 1. 암모니아 기반 복합 발전 시스템 개요 및 구성 22
• 2. 해석 절차 및 다중 스케일 해석 흐름도 26
• 제 2절. 0D 열역학 모델 29
• 1. 브레이튼 사이클 모델링 33
• 2. 랭킨 사이클 모델링 36
• 3. 성능 평가 지표 42
• 4. 0D 정합성 검증 45
• 제 3절. 1D 시뮬레이션 47
• 1. Reference cycle 모델 구성 및 정합성 검증 47
• 2. 브레이튼 사이클, 랭킨 사이클 모델 구성 49
• 3. GT-SUITE 1D 지배방정식 54
• 4. 해석조건 57
• 제 4절 CFD 시뮬레이션 58
• 1. 열교환기 모델 구성 58
• 2. Heat transfer model 63
• 3. Turbulence model (Standard k-ε) 64
• 제 4장 결과 및 고찰 66
• 제 1절. 0D 해석 결과 66
• 1. 브레이튼 사이클 해석 결과 66
• 2. 랭킨 사이클 해석 결과 69
• 3. 복합 사이클 통합 성능 72
• 4. 구성요소별 엑서지 파괴 분석 74
• 5. 0D 모델 정합성 검증 및 고찰 77
• 제 2절. 1D 해석 결과 79
• 1. 암모니아 랭킨 사이클 열역학적 상태점 분석 79
• 2. 종합 성능 결과 84
• 3. 총 출력 및 랭킨 사이클 출력 분석 88
• 4. 폐열회수량 및 NH3 질량유량 분석 90
• 5. 시스템 열효율 분석 92
• 6. 배기가스 열회수 특성 분석 93
• 7. 에너지 밸런스 분석 94
• 8. 종합 고찰 95
• 9. 0D-1D 모델 비교 및 고찰 96
• 제 3절. CFD 해석 결과 98
• 1. 저압 조건 온도 분포 특성 98
• 2. 고압 조건 온도 분포 특성 103
• 3. 셸측 유동 불균일 및 튜브 벽면 온도 편차 109
• 4. 배플 유무에 따른 열유동 특성 비교 112
• 5. CFD 해석 결과 종합 고찰 116
• 제 5장 결론 119
• 제 6장 제언 124
• REFERENCE 125