Development of One-Dimensional Calculation Model for Microchannel Steam/Methane Reformers based on Catalyst Effectiveness Factor Correlations

학위논문(석사) -- 서울대학교대학원 : 공과대학 기계공학부, 2023. 8. 김찬중.수소의 운반 및 저장에 소요되는 비용 및 위험성을 줄이기 위한 분산형 수소생산시스템에 관한 연구가 활발하게 이뤄지고 있다. 하지만 소형화로 인해 발생되는 효율 감소 및 안정성의 문제로 인해 최적화가 요구되고 이를 위한 높은 정확도의 해석이 필수적이다. 정확한 공정 예측을 위해 전산유체역학에 기반한 해석모델 개발이 주로 이뤄졌으나 개질 과정 예측을 위해 많은 격자가 배치되어 해석에 소요되는 비용이 상당히 높다는 한계점이 있다. 본 연구에서는 분산 발전용 연료전지의 개질시스템에 적합한 저압 개질 조건(1~3bar)에서의 소형 마이크로채널 수증기/메탄 개질기를 효율적으로 해석할 수 있는 간략화된 1차원 해석모델을 개발하였다. 촉매유효도 상관식을 도입하여 다공성 촉매층 내부의 복잡한 반응/확산 문제를 간단하게 계산하였고 개질기 내부의 온도 및 농도는 에너지 보존과 화학종 보존을 고려해 계산하는 방식을 취하였다. 제시된 모델은 개질 과정을 상세히 고려한 CFD 해석모델과의 비교를 통해 신뢰성을 입증하고 파라미터 연구를 진행하여 다양한 공정 변수가 개질 과정에 미치는 영향을 조사하였다. 추가적으로 각 영역에 대한 개별적인 결과를 계산할 수 있는 1차원 상세모델을 개발하여 제시된 간략화모델의 한계점을 극복하고 정확도를 향상시키기 위한 연구가 진행되었다. 개발된 2개의 1차원모델은 CFD 해석모델과의 해석 시간을 비교하여 실효성을 증명하였다.Distributed hydrogen production systems are being actively studied to reduce the cost and risk of hydrogen transportation and storage. However, miniaturization of these systems can lead to decreased efficiency and stability, which requires optimization. High-accuracy analysis is essential for this. In this study, a simplified 1D analysis model was developed that can efficiently analyze a small microchannel steam/methane reformer under low-pressure reforming conditions (1-3 bar) suitable for the reforming system of distributed power generation fuel cells. The catalyst efficiency correlation was introduced to simply calculate the complex reaction/diffusion problem inside the porous catalyst layer, and the temperature and concentration inside the reformer were calculated by energy conservation and mass conservation. The presented model was validated through comparison with a CFD analysis model that considered the reforming process in detail, and a parametric study was conducted to investigate the influence of various operating and design conditions on the reforming process. In addition, a 1D detailed model that can calculate individual results for each region was developed to overcome the limitations of the presented simplified model and improve accuracy. The two 1D models developed were proved to be effective by comparing the CPU time with the CFD analysis model. The results of this study suggest that the 1D analysis model is a promising tool for the optimization of distributed hydrogen production systems.목 차 요 약 i 목 차 ii List of Tables iii List of Figures iv Nomenclatures vi 제 1 장 서론 01 1.1 연구 배경 01 1.2 연구 목표 06 제 2 장 1차원 간략화모델 08 2.1 모델 설명 08 2.2 모델 검증 22 2.3 파라미터 연구 30 2.4 결론 43 제 3 장 1차원 상세모델 45 3.1 모델 설명 45 3.2 모델 검증 52 3.3 모델 비교 57 제 4 장 결론 65 Appendix 67 References 69 Abstract 72 List of Tables Table 2.1 Reaction rate constants 12 Table 2.2 Adsorption coefficient 12 Table 2.3 Equilibrium costants 12 Table 2.4 Physical properties 21 Table 2.5 Operating conditions 21 Table 3.1 Summary of methane conversion ratio from parametric studies of varying inlet velocity and convective heat transfer coefficient 58 Table 3.2 Summary of hydrogen production from parametric studies of varying operating conditions 64 List of Figures Figure 1.1 Steam/methane reforming process in microchannel reformer. 05 Figure 2.1 Schematics of an examplary microchannel steam/methane reformer considered in this study: (a) physical domain and (b) its discretized calculation grid[23] 08 Figure 2.2 Schematics of the simplified 1-D model 13 Figure 2.3 Comparison of temperature along the axial coordinate between calculation results of simplified 1D model and CFD simulation results at standard operating conditions[23] 23 Figure 2.4 Comparison of mole fractions along the axial coordinate between calculation results of simplified 1D model and CFD simulation results at standard operating conditions[23] 23 Figure 2.5 Comparison of reaction rate along the axial coordinate between calculation results of simplified 1D model and CFD simulation results at standard operating conditions[23] 25 Figure 2.6 Effect of inlet velocity on methane conversion ratio along the axial coordinate at standard operating conditions[23] 28 Figure 2.7 Effect of heat transfer coefficient on methane conversion ratio along the axial coordinate at standard operating conditions[23] 28 Figure 2.8 Effect of operating (a) pressure, (b) inlet temperature and (c) S/C ratio on methane conversion ratio along the axial coordinate at standard operating conditions 31 Figure 2.9 Effect of operating (a) pressure, (b) inlet temperature and (c) S/C ratio on temperature along the axial coordinate at standard operating conditions 32 Figure 2.10 Effect of operating (a) pressure, (b) inlet temperature and (c) S/C ratio on hydrogen production at standard operating conditions 33 Figure 2.11 Effect of (a) channel height, (b) washcoat thickness and (c) reformer length on methane conversion ratio along the axial coordinate at standard operating conditions 37 Figure 2.12 Effect of (a) channel height, (b) washcoat thickness and (c) reformer length on temperature along the axial coordinate at standard operating conditions 38 Figure 2.13 Effect of (a) channel height, (b) washcoat thickness and (c) reformer length on hydrogen production at standard operating conditions 39 Figure 3.1 Schematics of an examplary microchannel steam/methane reformer considered in this study: (a) physical domain and (b) its discretized calculation grid[23] 45 Figure 3.2 Schematics of the detailed 1-D model 47 Figure 3.3 Comparison of (a) temperature and (b) mole fractions along the axial coordinate between calculation results of detailed 1D model and CFD simulation results at standard operating conditions 54 Figure 3.4 Effect of (a) inlet velocity and (b) heat transfer coefficient on methane conversion ratio along the axial coordinate at standard operating conditions 56 Figure 3.5 Validation of detailed 1D calculation model: reaction rate along the axial coordinate at standard operating conditions 59 Figure 3.6 Comparison of (a) temperature and (b) mole fractions along the axial coordinate between simplified 1D model calculation results and detailed 1D model calculation results at standard conditions 62 Figure A.1 CFD analysis model for microchannel type steam/methane reformer 67 Nomenclature Di,eff effectiveness diffusivity in the catalyst layer, m2/s Dij binary diffusivity of i j species pair, m2/s DK,i Knudsen diffusivity of species i, m2/s Dm,i effective diffusivity in the flow channel, m2/s dpore mean pore diameter, m Hch channel height, m h convective heat transfer coefficient, W/m2-K h _i total enthalpy of species i, J/mol h _(f,i) formation enthalpy of species i, J/mol Keq,j equilibrium constants for reaction j Ki adsorption coefficient of species i Kv viscous flow permeability, m 2 keff effective thermal conductivity, W/m-K Kj reaction rate constant for reaction, j Lch channel length, m Mi molecular mass of species i, kg/kmol Mnom,CH4 nominal methane diffusion rate, mol/s Ni molar flow rate of species i, mol/s pi partial pressure of species i, bar pt total pressure, bar Rg universal gas constant, 8.314 kJ/kmol-K Rnom,j nominal reaction rate of reaction j, mol/s Rj reaction rate of reaction j, mol/s rj reaction rate of reaction j, kmol/kgcat-h SC steam to carbon (S/C) ratio Si source for species i, mol/s T temperature, twc washcoat layer thickness, m tsub substrate layer thickness, m x coordinate, m xi mole fraction of species i Greek letters ρ_cat apparent catalyst density, kgcat/m 3 β_CH4 methane conversion ε porosity φ_j effective Thiele modulus for reaction j φ_j^* modified Thiele modulus for reaction j η_j effectiveness factor for reaction j τ tortuosity Subscripts and superscripts 0 inlet value at the washcoat surface c cell value ch channel cat catalyst eff effective i species index j reaction index k grid index ref reference s solid t total석

Streptomyces exfoliatus SMF13의 세포분화와 관련된 단백질분해효소들의 특성 및 역할

학위논문(박사)--서울大學校 大學院 :생명과학부,2003.Docto

低分子量 β-lactamase 沮害物質 生産菌株의 探索

학위논문(석사)--서울대학교 대학원 :미생물학과,1998.Maste

Statistical Characteristics of Fractal Dimension in Turbulent Premixed Flame

With the introduction of Fractal notation, various fields of engineering adopted fractal notation to express characteristics of geometry involved and one of the most frequently applied areas was turbulence. With research on turbulence regarding the surface as fractal geometry, attempts to analyze turbulent premised flame as fractal geometry also attracted attention as a tool for modeling, for the flame surface can be viewed as fractal geometry. Experiments focused on disclosure of flame characteristics by measuring fractal parameters were done by researchers. But robust principle or theory cant be extracted. Only reported modeling efforts using fractal dimension is flame speed model by Gouldin. This model gives good predictions of flame speed in unstrained case but not in highly strained flame condition. In this research, approaches regarding fractal dimension of flame as one representative value is pointed out as a reason for the absence of robust model. And as an extort to establish robust modeling, Presents methods treating fractal dimension as statistical variable. From this approach flame characteristics reported by experiments such as Da effect on flame structure can be seen quantitatively and shows possibility of flame modeling using fractal parameters with statistical method. From this result more quantitative model can be derived

A Study of Thermal and Chemical Quenching of Premixed Flame by Flame-Surface Interaction

Incomplete combustion due to quenching in a narrow confinement has been a major problem for realization of a reliable micro combustion device. In most micro combustors, effects of flows are absent in the quenching because the flow is laminar and no severe stretch is present. In such circumstance, quenching is caused either by heat loss or by removal of active radicals to the wall surface of the confinement. An experimental investigation was carried out to investigate the relative significance of these two causes of quenching of a premixed flame. A premixed jet burner with a rectangular cross section at the exit was built. At the burner exit, the flame stands between two walls with adjustable distance. The gap between the two walls at which quenching occurs was measured at different wall surface conditions. The results were analyzed to estimate the relative significance of heat loss to the wall and the removal of radicals at the surface. The measurements indicated that the quenching distance was independent of the wall surface characteristics such as oxygen vacancy, grain boundary, or impurities at low temperature. At high temperature, however, the surface characteristics strongly affect the quenching distance, implying that radical removal at the wall plays a significant role in the quenching process. Incomplete combustion due to quenching in a narrow confinement has been a major problem for realization of a reliable micro combustion device. In most micro combustors, effects of flows are absent in the quenching because the flow is laminar and no severe stretch is present. In such circumstance, quenching is caused either by heat loss or by removal of active radicals to the wall surface of the confinement. An experimental investigation was carried out to investigate the relative significance of these two causes of quenching of a premixed flame. A premixed jet burner with a rectangular cross section at the exit was built. At the burner exit, the flame stands between two walls with adjustable distance. The gap between the two walls at which quenching occurs was measured at different wall surface conditions. The results were analyzed to estimate the relative significance of heat loss to the wall and the removal of radicals at the surface. The measurements indicated that the quenching distance was independent of the wall surface characteristics such as oxygen vacancy, grain boundary, or impurities at low temperature. At high temperature, however, the surface characteristics strongly affect the quenching distance, implying that radical removal at the wall plays a significant role in the quenching process

Analysis of Heat Loss Effect of Combustion in Closed Vessel

Interests and importance of down-scale combustor is increasing with the emerging need for miniaturized power source which is now a bottleneck of micro system development. But in down scaled combustor increased heat loss compared to thermal energy generation inhibits the usability and application of the device, so as a preliminary work of down scaled combustor fabrication. Modeling tool for the device should be established, in this study modeling approach of closed vessel combustion phenomena that can express heat loss effect and resulting quenching is proposed and the result is compared with experiment data. From this model heat loss effect following combustor scale down can be further understood, and further more design parameter and analysis tool can be obtained

Radiographic Changes of Mandibular Cortical Bone in Bisphosphonate Drug Holiday

학위논문(석사)--아주대학교 임상치의학대학원 :임상치의학과,2022. 8목적: 비스포스포네이트의 잔류 효과에 대한 연구는 지금까지 많지 않았다. 이 연구의 목적은 비스포스포네이트 약물 휴지기 동안 파노라마 방사선 상의 하악 피질골 두께를 관찰함으로써 잔류 효과를 조사해보고자 함이다. 재료 및 방법: 본 후향적 연구에서는, 2010년부터 2021년까지 아주대학교병원에서 MRONJ로 진단되어 부골 적출술을 시행 받은 36명의 환자를 대상으로 하였다. 모든 환자들은 비스포스포네이트 처방의와의 협의진료 하에 약물을 중단하도록 교육되었다. 파노라마 방사선 사진은 약물 휴지기 시작 (T0), 12 개월 이후 (T1), 18 개월 이후 (T2)에 촬영되었다. Ledgerton 등의 방법에 의하여 지표값을 계산하였다 (panoramic mandibular index와 mental index). 시간의 따른 변화를 비교하기 위하여 paired t-test를 시행하였다. 결과: T0와 T1의 지표값 차이는 없었으나 (Paired t-test, P>0.05), T0와 T2의 지표값 차이는 통계적으로 유의미하였다 (Paired t-test, P<0.05). 결론: 비스포스포네이트 약물 중단 18개월 이후부터 파노라마 방사선 영상에서 하악 피질골의 두께 감소가 관찰되었다.I. Introduction 1 II. Materials and methords 4 A. Study sample 4 B. Panoramic mandibular index and mental index 5 1. Panoramic mandibular index 5 2. Mental index 5 C. Statistical analysis 8 III. Results 9 IV. Discussion 15 V. Conclusion 18 References 19MasterObjectives: There have been few studies so far on residual effect of bisphosphonate. This study investigated the radiographic changes of mandibular cortical thickness in bisphosphonate drug holiday. Materials and Methods: This retrospective study includes 36 patients diagnosed as MRONJ at Ajou University Dental Hospital in 2010-2021. All patients were educated to stop taking bisphosphonate under consultation with the prescribing physicians. Panoramic radiograph study was done at the starting point of discontinuation (T0), 12 months after (T1) and 18 months after (T2) discontinuation of bisphosphonate, respectively. Mental index and panoramic mandibular index were calculated using Ledgerton’s method. Paired t-test was used for analyzing statistical difference over time. Results: The difference in indices (mental index and panoramic mandibular index) of T0 and T1 was not statistically significant (Paired t-test, P>0.05). However, the difference in indices (mental index and panoramic mandibular index) of T1 and T2 was statistically significant (Paired t-test, P<0.05). Conclusion: The cortical thickness of mandible decreased in the late stage (after 18 months) on panoramic radiograph

Design and Development of Micro Combustor (I)

Combustion phenomena in a sub-millimeter scale combustor have been investigated. To evaluate scale effect on flame propagation characteristics, a cylindrical combustion chamber with variable depth was built in-house. The combustor was charged with premixed gas of hydrogen and air and ignited electronically. A piezo electric pressure transducer recorded transient pressure after the ignition. Measurements were made at different test conditions specified with chamber depth and initial pressure as parameters. Visual observation was made through a quartz glass window on top side of the combustion chamber using high speed digital video camera. From the pressure data, available work was estimated and compared with energy input required for stable ignition. The preliminary results suggested that the net thermal energy release is sufficient to generate power and enables a combustor of the size in the present study to be used as the energy source of a micro power devices