레이저 동시 계측을 이용한 Puffed 화염의 구조와 특성에 관한 실험적 연구

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 기계항공공학부, 2019. 2. 윤영빈.열 방출량과 화염면 사이에는 일련의 관계가 존재하기 때문에 연소불안정에 대한 예측을 위해서는 화염 구조에 관한 연구가 필요하다. 연소불안정은 이를 구성하는 세 가지 섭동이 양성 피드백 루프를 형성하였을 때 발생 및 증폭하기 때문에 각각의 세 가지 섭동에 대해 자세히 고려할 필요가 있다. 본 연구에서는 연소불안정에 관한 연구의 일환으로 puffed 화염이라 불리는 특이 화염 구조에 관한 연구를 수행하였다. 이전 연구에서, 김 등[1]은 특정 주파수 및 입력 속도 섭동 영역에서 puffed 화염이 발생하는 것을 확인하였다. 따라서 본 연구에서는 이러한 puffed 화염에 관한 발생 원인과 응답 특성에 관해 연구하였다. 그에 따라, OH-PLIF 와 PIV 동시 계측을 수행하였다. 실험은 확산 화염의 한 종류인, 연료 분출 속도와 산화제 분출 속도가 서로 동일한 버크 슈만 화염에 관해 진행하였다. 메탄과 수소의 혼합기가 연료로 사용되었고 산화제로는 상온 공기가 사용되었다. 연소불안정 현상을 모사하기 위한 외부 음향 가진은 100 Hz 부터 180 Hz 까지 20 Hz 간격으로 인가되었다. 또한 입력 속도 섭동 진폭은 0.1에서 0.5 까지 0.1의 간격으로 적용되었다. 레이저 동시 계측에 의해 속도장, strain rate장, OH 분포는 동시에 계측되었다. 동시 계측의 결과로 화염의 응답 특성과 유동의 거동은 같은 시간대에 대해 분석되었다. OH-PLIF 계측을 이용하여 화염 구조를 확인하였으며 이 외의 다른 정량적 계측은 PIV 계측을 이용하여 수행하였다. 동일한 외부 음향 가진 주파수 영역에서는 특정 입력 속도 이상에서 flame puffed 현상이 발생되었다. 외부 음향 가진 주파수가 증가하는 경우에는 화염을 분리하기 위해 더 큰 입력 속도 섭동이 필요하였다. 이는 화염이 저역 필터 라는 것을 다시 한 번 알 수 있게 하였다. 또한 flame puffed 현상은 외부 음향 가진 주파수에 따라 주기적으로 발현됨을 확인할 수 있었다. PIV와 OH-PLIF의 동시 계측을 통해 높은 strain rate 와 산화제의 유입이 화염의 절단 과정에 있어 주요한 원인이 됨을 파악하였다. 실험 결과에 따라, 높은 strain rate가 작용하면 화염면이 섭동하고 화염목이 더욱 좁아지게 된다. 또한 flame puffed 과정이 진행됨에 따라, 점점 더 높은 strain rate 가 화염면에 작용하는 것을 확인하였다. Flame puffed 과정의 마지막 단계에서 일반적이지 않은 급격한 변화를 보이는 유동이 계측되었고 이로 인해 더욱 높은 strain rate 가 화염면에 작용하였다. 이와 동시에, 화염으로의 산화제의 유입은 화염 절단 현상의 또 다른 원인이라 할 수 있었다. Flame puffed 과정이 진행되면서 주위 공기가 점차 화염 중간 영역으로 침입하였으며 이로 인해 결국엔 화염이 완전히 분리되고 산화제가 해당 영역 중간으로 완벽하게 침투된다. Flame puffed 현상이 발생하면 화염의 응답 특성은 음향 가진을 인가하지 않은 경우와 완전히 달라진다. 화염의 길이와 면적은 보다 짧아지게 되나 화염의 길이 및 면적 섭동은 큰 폭으로 증가한다. 즉, flame puffed 현상이 발생하는 경우에 화염은 보다 불안정한 상태가 된다고 할 수 있다.To predict combustion instability, the study of the flame surface is necessary because there is a relationship between the heat release rate and the flame surface. Combustion instability is occurred and amplified when the three components of the perturbation constitute a positive feedback. Thus each of three components should be considered carefully. In order to predict the combustion instability, in this paper, the unique flame structure, called as the puffed flame, was investigated. In the previous work, Kim et al. [1] discovered the puffed flame structure at the specific forcing frequency and the velocity perturbation region. In this paper, therefore, we revealed the causes and the dynamic characteristics of the puffed flame. Thus, simultaneous OH-PLIF and PIV measurements were conducted. In case of the flame, the burke-schumann flame, a special case of the non-premixed flame, was considered. The mixture of the methane and hydrogen was used as the fuel, and the air was used as the oxidizer. The acoustic forcing was applied for the several frequency regionfrom 100 Hz to 180 Hz with 20 Hz steps. Also incoming velocity perturbation amplitude was varied from 0.1 to 0.5 with 0.1 steps. The velocity, strain rate, and the distribution of the OH radical were measured at the same time due to the simultaneous laser diagnostics. From these results, the response characteristics and the flow behavior were found at the exact same time. Flame structure was captured from the OH-PLIF measurement and the other quantitative results such as velocity vector and strain rate field were measured from the PIV. In the same forcing frequency region, the flame puffed phenomenon was occurred over the specific velocity perturbation amplitude. If the forcing frequency became higher, the more velocity perturbation amplitude was necessary for the flame puffed. With this results, we could realize that the flame acts as a low pass filter. Also, it was revealed that the flame puffed phenomenon was periodic process which was following the external acoustic excitation wave. With the simultaneous PIV and OH-PLIF results, we could find out the relative high strain rate and the oxidizer entrainment played important roles for the puffed process. High strain rate make flame wrinkle and flame throat more thinner. As the process proceeded, more higher strain rate was applied to the flame surface as well. At the final stage of the flame puffed phenomenon, highly irregular flow was generated, so that the strain rate became much higher. Meanwhile, oxidizer entrainment could be another reason for the puffed flame. During the process, surrounding air was continuously invaded into the middle of the flame. At last, flame was separated into two regions due to the entrainment of the surrounding air. When the flame puffed phenomenon was occurred, the dynamic characteristics became different from the non-excitation flame case. The flame length became more smaller and the flame surface area also decreased. However, flame length and the surface area perturbation were increased. In other words, when the flame puffed, we could say that the flame became more unstable.Contents Chapter 1 INTRODUCTION 1 1.1 Combustion instability 1 1.2 Strain rate 4 1.3 Previous research 8 Chapter 2 EXPERIMENTAL APPARATUS AND METHODS 12 2.1 Burke-schumann flame combustor 12 2.2 Test condition 15 2.3 Particle image velocimetry 20 2.4 OH Planar laser induced fluorescence 23 2.5 Simultaneous laser diagnostics 26 Chapter 3 Results and Discussion 28 3.1 OH-PLIF images for the puffed flame 28 3.1.1 OH-PLIF images for the various forcing frequencies 28 3.1.2 OH-PLIF images for the various velocity perturbation amplitudes 32 3.1.3 Averaged OH-PLIF images 35 3.2 Results of the simultaneous laser diagnostics 37 3.2.1 Radial velocity generation 37 3.2.2 The beginning of the flame surface wrinkling 41 3.2.3 Strain rate distribution near the critical region 43 3.2.4 Comparison of the strain rate distribution 49 3.2.5 The effect of the strain rate to the puffed flame throat 52 3.2.6 The maximum and the minimum strain rate 55 3.2.7 Surrounding air entrainment 61 3.3 Dynamic characteristics of the puffed flame 64 3.3.1 Flame length and surface area 65 3.3.2 Flame length and surface area perturbation 69 Chapter 4 CONCLUSION 71 Appendix A Theoretical understanding of resonance frequency 74 Bibliography 77 Abstract in Korean 83Maste

비예혼합화염의 음향 가진에 따른 화염응답특성

학위논문(박사) -- 서울대학교대학원 : 공과대학 기계항공공학부, 2022.2. 윤영빈.연소불안정의 발생 원인과 메커니즘은 현재까지 정확하게 규명되지 않았으나 반응물의 유동 섭동, 열방출량 섭동, 연소실의 음향학적 경계에 의한 섭동의 상호작용에 의해 발생 유무가 결정된다고 알려져 있다. 위의 세 가지 섭동이 양성 피드백 루프 (positive feedback loop)를 형성하면 연소불안정이 발생할 확률이 높아지며, 음성 피드백 루프(negative feedback loop)를 형성하면 그 확률이 낮아지는 것으로 알려져 있다. 따라서 연소불안정 발생 저감을 위해 연소불안정 발생 조건 및 인자를 파악하는 것은 필수적이다. 본 연구에서는 연소불안정 현상의 인자 중 열 방출량 섭동과 속도 섭동의 상호관계에 대한 연구를 수행하였으며, 특정 음향 가진 조건에서 화염이 끊기는 현상인 pinch-off 화염에 대해 OH PLIF과 PIV 레이저 동시계측을 통해 발생 메커니즘을 실험적으로 규명하였으며, pinch-off 화염과 nonpinch-off 화염의 NOx 배출 특성, 연료와 공기의 유동경계층에 대한 유동 특성 분석을 수행하였다. 음향 가진 발생을 위해서 스피커를 활용하였으며, 화염구조 분석을 위해서 OH* 자발광과 OH-PLIF 레이저 계측기법을 활용하였으며, 유동장 특성분석을 위해 OH-PLIF와 PIV 동시계측을 활용하였다. 열 방출량 계측을 위해서 광전자증폭관(Photo Multiplier Tube, PMT)를 활용하였으며 이를 통해 화염전달함수를 계측하였다. 연소불안정 예측을 위해 음향 가진에 따른 비예혼합화염과 예혼합화염의 응답특성 및 동특성 비교 연구를 수행하였다. 다른 연소 반응을 가진 두 화염은 음향 가진에 따라 동적거동 특성이 상이하다. 비예혼합화염은 화염 면에서 음향학적 파동이 투영되며, 펄럭이는 동적 거동 특성이며 화염 끝 단이 열린 화염형상이다. 반면에 단일 노즐인 예혼합화염은 코니컬 화염(conical flame)의 형태로 수직으로 크게 섭동한다. 비예혼합화염은 가진주파수 증가에 따라서 화염 면의 모듈레이션(modulation) 개수가 증가할 뿐 화염 구조는 크게 변하지 않지만, 예혼합화염은 가진주파수와 속도섭동강도에 따라 다양한 화염 구조가 나타난다. 음향 가진 시 두 화염의 열 방출량 측정을 통해 화염전달함수를 분석한 결과 비예혼합화염은 비선형적인 결과를 나타내었으며, 예혼합화염은 선형적인 결과를 나타내었다. 화염길이와 스트롤 수(Strouhal number)를 도입하여 열방출량과 화염구조의 상관관계 분석을 수행하였으며, 수치해석적 연구와 비교하였다. 비예혼합화염은 20%이상의 속도섭동에서 수치해석결과와 일치하며, 비선형성을 검증하였으며, 예혼합화염은 일부 스트롤 수에서 수치해석 결과와 다르다. 이는 스트롤 수를 계산할 때, 화염 면 곡률, 화염 전파 속도, 화염 끝단 형상 등을 고려한 스트롤 수에 도입해야 함을 확인하였다. 벅-슈만 화염의 한가지 케이스인 비예혼화염에서 음향 가진 시 속도섭동강도와 가진주파수에 따른 다양한 화염 구조를 관찰하였다. 일정 가진주파수와 속도섭동강도 범위에서는 화염이 끊기는 pinch-off 화염이 나타남을 확인하였다. pinch-off 화염은 화염이 끊기는 현상으로 정의하며 노즐에 부착된 화염은 메인(main) 화염과 떨어져 나간 화염을 포켓(pocket) 화염으로 정의한다. Pinch-off 화염의 메커니즘 규명을 위해 OH PLIF&PIV 동시계측을 수행하였다. 가진 주파수와 속도섭동강도에 따른 화염 구조를 맵핑(mapping)하여 3가지 동적 거동으로 구별하였다. 낮은 주파수 범위에서는 화염이 상-하로 크게 섭동하는 flickering 화염이다. 중간 주파수 범위에서는 화염이 끊기는 pinch-off 화염, 그리고 높은 주파수 범위에서는 화염 면의 모듈레이션이 생기는 wrinkled 화염으로 구별하였다. 비반응장 유동에서 음향 가진 시 조수 유동(tidal flow)에 의한 double dipole vortex을 미 산란(Mie scattering)으로 확인하였다. 반응장 유동의 화염이 pinch-off 시 vortical structure에 의한 공기 유입이 화염 변형을 야기함을 확인하였으며, 화염 목 부분에 강한 strain rate을 관찰하였다. 이에 따라 pinch-off flame은 vortical structure에 의한 외부 공기 유입과 강한 strain rate에 의한 상호작용임을 확인하였다. pinch-off 화염의 떨어져나간 포켓 화염을 고려한 질소산화물(NOx), 일산화탄소(CO) 배출 특성 분석을 수행하였다. 질소산화물은 속도섭동강도 증가에 따라 배출양은 감소한다. 이는 속도섭동강도가 증가하면 연료와 산화제 혼합도가 증가하기 때문이며 혼합이 잘 이루어져 완전연소하면 질소산화물 배출이 적어진다. 반면에 일산화탄소는 속도섭동강도 증가에 따라 배출이 증가하는 특성을 나타내었지만, 그 배출량이 매우 작음을 확인하였다. 배출특성의 지표인 EINOx(Emission Index of NOx)와 화염체류시간(flame residence time) 분석을 위해서 pinch-off 화염의 높이를 주화염과 포켓화염으로 세분화하여 정의하였다. 세분화한 화염길이와 화염체류시간 분석 결과, 화염체류시간이 감소함에 따라 질소산화물 배출이 저감되는 경향은 일치하지만, 가진주파수에 따른 경향을 따르지는 않았다. 따라서 화염체류시간만으로 질소산화물 배출 특성을 분석하는데에 그 한계점이 있음을 밝혔다. 스트롤 수와 EINOx의 상관관계 분석 결과 스트롤 수가 다름에도 EINOx 배출이 같은 경향을 확인하였다. 이는 화염길이는 주화염 또는 포켓화염 어느 것을 선택하여도 경향성 분석에 무관함을 의미한다. 이러한 경향성 검증을 위해 화염체류시간으로 정규화한 EINOx가 1/2-power를 잘 따르는 것을 확인하였다. Pinch-off와 nonpinch-off조건에서 strain rate과 shear stress 상관관계 분석을 수행하였다. 연료와 공기 속도가 같아 이론적으로 shear stress가 없는 조건을 기준으로 연료와 공기 속도를 각각 변화시키며 shear stress를 생성하였다. 연료와 공기속도의 유동경계층 분석을 위해서 OH* 자발광과 PIV 동시계측을 수행하였다. 다양한 연료와 공기속도에 따라 pinch-off의 매핑을 수행하여 물리적인 경계를 확인하였다. 연료 속도 증가에 따른 pinch-off 조건에서 shear 효과가 증가함에 따라 strain rate가 기준 데이터에 비해 약 80% 증가하였으며, shear stress는 15% 증가하였다. 연료속도를 더 증가시켜 nonpinch-off 조건에서 계측한 결과 shear 효과는 더 증가하였지만 strain rate은 기존데이터와 비교했을 때 50% 감소하였고 shear stress는 3.3배 증가하였다. 공기속도 증가에 따른 nonpinch-off 조건에서 strain rate과 shear stress의 상관관계를 분석하여 경향성을 재검증하였다. 결과적으로 strain rate이 주요하게 영향을 미칠 때 shear stress는 감소되는 경향이며, shear stress는 pinch-off를 제어할 수 있는 파라미터로서 활용가능성을 확인하였다.Although the cause and mechanism of combustion instability have not been elucidated yet, it is known that the presence or absence of combustion instability is determined by the interaction of reactant flow perturbation, heat release perturbation, and perturbation due to the acoustic boundary of the combustion chamber. When these three perturbations form a positive feedback loop, the probability of combustion instability increases, and when a negative feedback loop is formed, the probability decreases. Therefore, to reduce the appearance of combustion instability, it is essential to identify the conditions required for and the factors influencing combustion instability. In this study, the correlation between heat emission perturbation and velocity perturbation was investigated among the factors governing combustion instability. For pinch-off flames, a phenomenon in which flames are separated under specific acoustic excitation conditions, the mechanism of combustion instability was investigated by simultaneous OH-planar laser-induced fluorescence (PLIF) and particle image velocity (PIV) laser measurements In addition, the nitrogen oxide (NOx) emission and the flow characteristics were analyzed. OH* chemiluminescence and OH-PLIF laser measurements were used for flame structure analysis, and simultaneous OH-PLIF and PIV measurements were used for flow field characterization. A photomultiplier tube (PMT) was used to measure the heat release needed to calculate the flame transfer function (FTF). The flow boundary layer between the fuel and air was also analyzed. To predict combustion instability, we conducted a comparative study of the response characteristics and dynamic characteristics of non-premixed and premixed flames generated by acoustic excitation. Two flames with different combustion reactions have different dynamic behavior characteristics, depending on acoustic excitation. Non-premixed flames show acoustically created waves projected from the flame surface, with a flapping dynamic behavior, and are flame-shaped with an open flame tip. On the other hand, the premixed flame from the single nozzle fluctuates vertically with a conical shape. For the non-premixed flame, the number of modulations on the flame surface increases with increasing excitation frequency, but the flame structure does not change significantly. The flame transfer function analysis by measuring the heat release rate of both flames during acoustic excitation revealed that the non-premixed and premixed flames showed nonlinear and linear results, respectively. By introducing the flame height and the Strouhal number (St number), correlation analysis between heat release and flame structure was performed and the results were compared with those of numerical studies. For non-premixed flames, the nonlinearity was verified by the numerical analysis results in the velocity perturbation of 20% or more. The numerical analysis and the premixed flame results were consistent but showed a locally different tendency. For premixed flames, the Strouhal number calculation does not consider the flame surface curvature, flame propagation speed, and flame tip shape. A more accurate Strouhal number analysis will be possible if these factors are included in the analysis. Various flame structure analyses were conducted in terms of the velocity perturbation intensity and the excitation frequency during acoustic excitation in a non-premixed flame. The non-premixed flame is one example of the Buck-Schumann flame (B-S flame). A pinch-off flame is defined as a phenomenon in which the flame is cut off; the flame attached to the nozzle is defined as the main flame, and the separated flame is defined as the pocket flame. It was confirmed that a pinch-off flame appears in a constant range of excitation frequencies and velocity perturbation intensities. Simultaneous OH PLIF and PIV measurements were performed to investigate the mechanism of the pinch-off flame. By mapping the flame structure in terms of excitation frequency and velocity perturbation intensity, it was classified into three dynamic behaviors. We observed a flickering flame with a large perturbation in the vertical direction in the low-frequency range, a pinch-off flame in the mid-frequency range, and a wrinkled flame with a modulated surface in the high-frequency range. The double dipole vortex caused by the tidal flow during acoustic excitation in the non-reactive field flow was confirmed by Mie scattering analysis. The inflow of air by the vortical structure was found to cause the flame deformation when the flame in the reaction field was pinched off, and a strong strain rate was observed in the flame neck. Accordingly, it was confirmed that the pinch-off flame was an interaction between the inflow of external air by the vortical structure and the high strain rate. NOx and carbon monoxide (CO) emission characteristics were analyzed considering the pocket flame separated from the pinch-off flame. With increasing velocity perturbation intensity, the mixing intensity of the fuel and the oxidizer increases and thus, the amount of NOx emitted decreases. With good mixing of the fuel and oxidizer and complete combustion, NOx emissions are reduced. On the other hand, CO emissions increased with increasing velocity perturbation intensity, but it was confirmed that the emissions were very small. The height of the pinch-off flame was subdivided into the main flame and the pocket flame to analyze the two emission characteristics, viz. the emission index of NOx (EINOx) and the flame residence time. The subdivided flame height and flame residence time analysis showed the same trend of reducing NOx emissions as the flame residence time decreased, but it did not follow the trend of the excitation frequency. Therefore, it was concluded that there is a limit to the analysis of NOx emission characteristics when using only the flame residence time. The correlation analysis between Strouhal number and EINOx confirmed that the EINOx value was the same even though the Strouhal number was different. This means that the flame height is independent of the trend analysis whether the main flame or the pocket flame is selected. To verify this tendency, it was confirmed that EINOx normalized by flame residence time followed 1/2-power well. Strain rate and shear stress correlation analyses were performed under pinch-off and non-pinch-off conditions. The fuel and air velocities are the same, and by changing these, the shear stress was generated based on the condition of no theoretical shear stress. Simultaneous measurements of OH* chemiluminescence and PIV was performed for boundary layer flow analysis of fuel and air velocity. The physical boundary was confirmed by performing pinch-off mapping for various fuels and air velocities. As the shear effect increased under the pinch-off condition with increasing fuel velocity, the strain rate increased by ~80% compared to the reference data, and the shear stress increased by 15%. Under the non-pinch-off condition, the shear effect was further increased by further increasing the fuel velocity, but the strain rate was reduced by 50% compared to the previous data, and the shear stress was increased 3.3 times. The tendency was also verified by analyzing the correlation between strain rate and shear stress under non-pinch-off conditions with increasing air velocity. Shear stress tends to decrease when strain rate has a major influence, and the applicability of shear stress, as a parameter to control pinch-off, was confirmed.ABSTRACT i LIST v LIST OF FIGURES x LIST OF TABLES xv NOMENCLATURE xvi CHAPTER 1 INTRODUCTION 1 1.1 Background 1 1.2 Combustion instability 4 1.3 Flame transfer function (FTF) 5 1.4 Acoustic excitation in non-premixed flame 7 1.5 Strain rate and local flame extinction in non-premixed flame 9 1.6 Motivation 10 1.7 Objectives 11 1.8 Outline 12 CHAPTER 2 EXPERIMENTAL AND MEASUREMENT SYSTEMS 13 2.1 Combustor and nozzles 13 2.2 Flame imaging 16 2.2.1. Chemiluminescence Spectroscopy 16 2.2.2. OH planar laser-induced fluorescence (OH PLIF) measurement 19 2.2.3. OH PLIF system 22 2.2.4. High-speed OH PLIF system 23 2.2.5. Particle image velocimetry (PIV) measurement 26 2.2.6. Simultaneous measurement of PIV and OH PLIF system 30 2.3 Flame Transfer Function (FTF) 32 2.4 NOx measurement system 34 CHAPTER 3 COMPARISON OF FLAME RESPONSE CHARACTERISTICS BETWEEN NON-PREMIXED FLAMES AND PREMIXED FLAMES OF UNDER ACOUSTIC EXCITATION 35 3.1 Objectives 35 3.2 Experimental setup and methodology 38 3.3 Flame appearance comparison between the non-premixed flame and the premixed flame 42 3.4 Flame dynamic characteristics under acoustic excitation of nonpremixed flame 44 3.5 Flame dynamic characteristics under acoustic excitation of premixed flame 48 3.6 Comparison of the flame response characteristics between nonpremixed and premixed flames 55 CHAPTER 4 PINCH-OFF PROCESS OF BURKE-SCHUMANN FLAME UNDER ACOUSTIC EXCITATION 63 4.1 Objectives 63 4.2 Experimental setup and methodology 66 4.3 Flame response characteristics under various excitation frequencies. 71 4.4 Flame response characteristics of pinch-off process at 80 Hz 73 4.5 Flame response characteristics at pinch-off boundary 76 4.6 Vortex-flame interaction and strain rate analysis for the pinch-off mechanism 79 CHAPTER 5 NOX EMISSION CHARACTERISTICS OF PINCH-OFF FLAME UNDER ACOUSTIC EXCITATION 86 5.1 Objectives 86 5.2 Experimental setup and methodology 89 5.3 Global appearance characteristics of non-premixed flame under acoustic excitation 93 5.4 Effects of acoustic excitation on EINOx and CO concentration 98 5.5 Effects of velocity perturbation intensity (u'/u) and forcing frequency on main flame (FM) and net hot product (FN) 102 5.6 Effects of Strouhal number and forcing frequency on flame residence time (τres) 104 CHAPTER 6 EFFECTS OF STRAIN RATE AND SHEAR STRESS ON STRUCTURE OF PINCH-OFF AND NON-PINCH-OFF FLAMES 108 6.1 Objectives 108 6.2 Experimental setup and method 110 6.3 Flame response characteristics according to fuel and air bulk Velocity 114 6.4 Characteristics of pinch-off flame with increasing fuel bulk velocity 117 6.5 Characteristics of non-pinch-off flame by increasing fuel bulk velocity 120 6.6 Characteristics of non-pinch-off flame with increasing air bulk velocity 123 CHAPTER 7 CONCLUSION 126 7.1 Conclusions 126 7.1 Limitation and future work 128 REFERENCES 129 ABSTRACT IN KOREAN 145박

Experimental studies of turbulent flames at gas turbine relevant burners and operating conditions

With the increasing demand of using alternative and renewable fuels, it becomes of vital importance to consider the fuel flexibility when designing a new burner for gas turbines. Hydrogen-enriched fuel and ammonia are two of these potential fuels, and they can significantly change the operability range of the gas turbines. Thus, it is necessary to enhance both the fundamental understanding on turbulent combustion of these fuels and their combustion performance in practical combustors. Due to its advantages of in-situ measurement, non-intrusiveness and high spatial and temporal resolution, laser-based diagnostics technology has been regarded as one of the best measurement methods for researching combustion processes and phenomena. In this thesis work, experimental studies have been conducted to investigate the turbulent flames of different fuels at various gas turbine related burners, employing laser diagnostics measurement. The measurement methods include planar laser-induced fluorescence (PLIF) for various species, particle image velocimetry (PIV), laser doppler anemometry (LDA), etc.A newly designed gas turbine model combustor had been developed at the Swedish National Centre of Combustion and Technology, so it was named the CECOST burner. One of the main objectives of this thesis is to improve the premixing effect of the CECOST burner by changing part of its internal configuration and investigate its fuel flexibility by using natural gas and hydrogen-enriched methane mixtures as fuels. The experiment was conducted at an atmospheric rig, and high-speed OH* chemiluminescence imaging, simultaneous OH-/CH2O PLIF, and PIV were employed. The operability range and flame structures were investigated for different fuels at various Reynolds numbers (Re). The operability range was found to be highly sensitive to Re, as well as the fuel. For natural gas/air flames, the lean blowout (LBO) limit was approximately independent of Re, while flashback showed obvious dependence on Re and no flashback was observed for higher Re. For hydrogen-enriched methane/air flames, a comparison of combustion characteristics between pure methane and hydrogen-enriched methane with two mixing ratios, 25% and 50% in volume, was investigated. It was found that the flame stabilized in an M shape for all pure methane/air flames, whereas the flame shape transits to a П shape at a specific equivalence ratio ("ϕ" ) for hydrogen-enriched methane flames. Besides, the flashback events with two different mechanisms, combustion-induced vortex breakdown (CIVB) and boundary-layer flashback, were observed. By statistical analysis, we can get that the CIVB flashback took place only for pure methane flames with M shape, while the boundary-layer flashback happened for all hydrogen-enriched flames with П shape.Aiming to achieve stable combustion in lean conditions, a plasma-assisted flame control system is a potential way to help stabilize the flame. An industrial gas turbine combustor, known as Siemens dry low emission (DLE) burner, was modified to place a high-voltage electrode in the rich-pilot-lean (RPL) section and was used for investigation of a rotating gliding arc (RGA) discharge effect on swirl flames stabilized in the gas turbine combustor. In the unmodified DLE burner configuration, fuel and air are injected into the RPL to hold a premixed flame which can help stabilizing the main flame, but in the modified configuration, only air/O2 was injected into the RPL. The flame emissions were measured by a gas sampling probe and emission analyzer. The CO emission results were used to identify the improvement of the LBO limit with plasma assistance. NOx emissions were slightly increased by the RGA plasma, but still, less than the same main flame with RPL flame assisted. Flame emission spectra were also measured. Ammonia combustion is recently one of the hot research topics due to its promising future of carbon-free emission. To deepen our knowledge on turbulent ammonia flames, a jet burner with a large scale was constructed and used to investigate the flame structure of premixed ammonia/air flames, by employing simultaneous OH-/NH-PLIF and LDA measurements. Most of the studied flames are located in the regime of the distributed reaction zone (DRZ), determined by their Karlovitz numbers that are larger than 100. Results of simultaneous OH-/NH-PLIF show that the NH and OH layer can coexist in a thin boundary and the NH signal appears evolving to the reactants side. In addition, the practical gas turbine combustors are all operated at elevated pressure conditions, but it is not easy to perform an experiment at elevated pressure in the lab. A co-axial jet burner was installed and studied in the high-pressure combustion rig (HPCR) at Lund University to investigate the characteristics of methane/air inverse diffusion flames (IDF) at elevated pressure (up to 5 bar). The flame structure and its lift-off height influenced by pressure increasing were discussed. More wrinkles and larger curvature of the flame front were found in the inner flame structure at higher pressure

Structure and Dynamics of a Reacting Jet Injected into a Vitiated Crossflow in a Staged Combustion System

Secondary injection of the fuel, also referred to as staged combustion, is being studied by gas turbine manufacturers as a means of increasing the power output of the gas turbine systems with minimal contribution to NOx emission. A reacting jet issuing into a high pressure vitiated cross flow operating at gas turbine relevant conditions was investigated as a means of secondary injection. In this application rapid mixing and chemical reaction in the near field of jet injection is desirable. In this work, advanced diagnostic measurements were performed on an experimental representation of such a system, with a transverse jet injection into a swirling vitiated crossflow. High repetition rate simultaneous particle image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) were performed at five measurement planes perpendicular to the jet axis. Transverse jets composed of premixed natural gas and H2 diluted with N2 were injected through a tube protruding into the crossflow. The influence of the nature of vitiated crossflow, swirling or uniform, on to the reacting jets and its corresponding influence on flame stabilization mechanism was investigated. The vitiated crossflow is produced by a low swirl burner (LSB) that imparted a swirling component to the crossflow and a bluff-body burner produced a uniform vitiated crossflow. The crossflow exhibits considerable swirl at the location of the transverse jet injection. The PIV measurements clearly demonstrate the influence of a swirling/ uniform crossflow on the jet. The jet-to-crossflow momentum flux ratio was varied to study the corresponding effect on the flow field. Two momentum flux ratios, J=3 and J=8 were employed to study the effect of momentum flux ratio on the stabilization of reaction fronts. The time averaged flow field shows a steady wake vortex very similar to that seen in the wake of a cylindrical bluff body which helps to stabilize the reaction zone within the wake of the jet. Jet with J = 8 had a deeper penetration into the crossflow as compared to J = 3 jet. Velocity field for a reacting/non-reacting jet in swirling crossflow exhibits higher in-plane velocity gradients as compared to jets in uniform crossflow. The vorticity field is also found to be weaker in case of jets in uniform crossflow as a result there is delay in the formation of the wake vortex structure. The HRR data acquisition also provided temporally resolved information on the transient structure of the wake flow associated with the reacting jet in crossflow. The wake Strouhal number calculation provides a better physical insight into the influence of jet velocity profile and nature of crossflow. A decrease in wake Strouhal number is noticed with an increase in nozzle separation distance. The effect of near-field heat release is also apparent from the wake Strouhal number. It is higher for a reacting jet as compared to that of a non-reacting jet owing to increase in rate of dilatation due to heat release. Based on the experimental data, it can be stated that wake vortices play a significant role in stabilizing a steady reaction zone within the near-wake region of the jet. The time averaged OH-PLIF images show a broad region of OH distribution in the wake of the jet. The measurements provided qualitative as well as quantitative information on the evolution of complex flow structures and transient events such as re-ignition, local extinction and vortex-flame interactions in the turbulent reacting flow. There is a noticeable difference in the flame structure of a H2/N2 flame as compared to a premixed natural gas flame. A thin flame front in the windward side of the jet is apparent for a H2/N2 flame. Due to the higher propensity of strain rate induced extinctions and lower flame speeds of a natural gas flame, a stable reaction zone is seen only in the jet wake. Thus, such high-data-rate measurements provide significantly improved understanding of the complex flow-field and flame stabilization mechanisms in a turbulent reacting flow. Such data-sets are critical for the development of high fidelity turbulence-chemistry interaction models

Experimental Study of Lean Blowout with Hydrogen Addition in a Swirl-stabilized Premixed Combustor

Lean premixed combustion is widely used to achieve a better compromise between nitric oxides emissions and combustion efficiency. However, combustor operation near the lean blowout limit can render the flame unstable and lead to oscillations, flashback, or extinction, thereby limiting the potential range of lean combustion. Recent interest in integrated gasification combined cycle plants and syngas combustion requires an improved understanding of the role of hydrogen on the combustion process. Therefore, in present study, combustion of pure methane and blended methane-hydrogen has been conducted in a swirl stabilized premixed combustor. The measurement techniques implemented mainly include particle image velocimetry, CH*/OH* chemiluminescence imaging, planar laser-induced fluorescence imaging of OH radical. By investigating the flow field, heat release, flow-flame interaction, and flame structure properties, the fundamental controlling processes that limit lean and hydrogen-enriched premixed combustion with and without confinement have been analyzed and discussed. As equivalence ratio decreases, for unconfined flames, the reduced flame speed leads flame shrinking toward internal recirculation zone (IRZ) and getting more interacted with inner shear layer, where turbulence level and vorticity are higher. The flame fronts therefore experience higher hydrodynamic stretch rate, resulting in local extinction, and breaks along the flame fronts. Those breaks, in turn, entrain the unburnt fuel air mixture into IRZ passing through the shear layer with the local vortex effect, further leading to reaction within IRZ. In methane-only flames, the width of IRZ decreases, causing flames to straddle the boundary of the IRZ and to be unstable. High speed imaging shows that periodic flame rotating with local extinction and re-light events are evident, resulting in high RMS of heat release rate, and therefore a shorter extinction time scale. With hydrogen addition, flames remain in relatively axisymmetric burning structure and stable with the aid of low minimum ignition energy and high molecular diffusivity associated with hydrogen, leading to lower heat release fluctuation and a longer extinction time scale. For confined flames, however, the hydrogen effect on the extinction transient is completely opposite due to spiraling columnar burning structure, in comparison of a relatively stable conical shape in methane flames

A study of lean premixed swirl-stabilized combustion of gaseous alternative fuels.

The burner was constructed of 1.5&inches; (3.8 cm) schedule 40 steel pipe. Fuel injectors were placed 40 cm upstream of the burner to ensure that the fuel and air were fully premixed prior to combustion. The fuel air mixture entered the combustion chamber in the annulus around a centerbody which contained 6 swirl vanes to impart an out of plane motion to the flow. The flow expanded into the combustion chamber which had an 8.1 cm inside diameter, and was exhausted into the ambient at the end of the combustion chamber. Pollutant emissions were measured using an electro-chemical gas analyzer and water-cooled stainless steel and expansion-cooled quartz probes. Flame extinction was studied by visual observation of the flame. Combustion related noise was recorded using a condenser microphone and digitized by a high speed data acquisition card. (Abstract shortened by UMI.)The effects of utilizing gaseous fuels with different compositions was studied for a lean premixed swirl stabilized burner typical of those used in land-based gas turbine engines. The experiments were performed at atmospheric temperature and pressure in a quartz glass combustor. The fuels utilized were binary mixtures containing either methane or propane as the primary component and hydrogen, oxygen, nitrogen or carbon dioxide as the secondary component. The combinations chosen represent constituents of various gaseous alternative fuels. In particular, focus was placed on hydrogen enriched hydrocarbon fuels proposed as a cross-over strategy to the hydrogen energy infrastructure. The operating parameters included fuel composition, total reactant flow rate, and the calculated adiabatic flame temperature. Global flame characteristics such as emissions of oxides of nitrogen (NOx) and carbon monoxide (CO), flame extinction, and combustion noise were studied. The internal characteristics of flames were also studied, including the velocity field and related flow properties, as well as the structures of the reaction zones

Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent flames

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77055/1/AIAA-1998-151-822.pd

Turbulent premixed flames in a model gas turbine combustor: fuel sensitivity and flame dynamics

The demand for energy security and reduction of greenhouse gas emissions has led to a surge of interest in the development of high-efficiency and low-emission gas turbine engines that can run on alternative low carbon content fuels, such as hydrogen-enriched fuel and syngas. However, the combustion characteristics of these fuels can significantly alter the flame characteristics and operability range of existing combustors. Therefore, it is crucial to gain a better understanding of the turbulent combustion characteristics of these fuels from both fundamental and practical perspectives. In this thesis, a combination of numerical and experimental diagnostic methodologies has been employed to investigate how the fuel characteristics can affect the fundamental properties of flames and their structure in gas turbine-like combustors. The aimis to provide a comprehensive understanding of the complex combustion processes associated with these alternative fuels. The propagation of turbulent premixed flames under different density ratio conditions is investigatedusing direct numerical simulation (DNS). The displacement speed, which characterizes the self-propagation of an isosurface defined based on a reaction progress variable in a turbulent premixed flame, has gained significant interest in the scientific community for flame modeling purposes. In this thesis, a set of new transport equations for dilation and curvature-induced flame stretch rate is derived. Based on the set of evolution equations for displacement speed that takes into account the effects of curvature, normal diffusion, and reactions, this thesis analyzes the thermal expansion effect on the correlation between these quantities. The results reveal four scenarios of flame self-acceleration. The findings provide valuable insights into the understanding of the complex dynamics of turbulent premixed flames. A newly improved gas turbine model combustor, known as CeCOST burner, is the focus of an experimentalcampaign that involves laser-based diagnostics techniques, including simultaneous OH-/CH2O planar laser induced fluorescence (PLIF), simultaneous OH-PLIF, particle imaging velocimetry (PIV), and phosphor thermometry for surface temperature measurements. High-speed OH* chemiluminescence and exhaust gas measurements are also utilized. The results of the study reveal that hydrogen-enrichment can significantly extend the operation of methane/air to ultra-lean mixtures, resulting in low NOx emissions. The structures of the flame and the flow show significant variations with hydrogen-enrichment. Isolated flame pockets are identified in lean hydrogen-enriched methane/air flames, as well as in syngas flames where a substantial amount of hydrogen is present. The vortex breakdown structure is found to be strongly coupled with the location of the reaction zones. Furthermore, it is observed that pilot flames can enhance flame stabilization by producing hot gas and radicals that aid in anchoring the flames in the outer recirculation zone of the combustor. The findings of this study provide valuable insights into the combustion characteristics of methane/air, hydrogen-enriched methane/air, and syngas/air flames in the CeCOST burner, as well as the influence of pilot flames on flame and flow structures. These insights contribute to the development of more efficient and environmentally friendly gas turbine combustor designs