Thermoacoustic Stabilization of a Sequential Combustor with Ultra-low-power Nanosecond Repetitively Pulsed Discharges

This study demonstrates the stabilization of a sequential combustor with Nanosecond Repetitively Pulsed Discharges (NRPD). A constant pressure sequential combustor offers key advantages compared to a conventional combustor, in particular, a higher fuel flexibility and a wider operational range. However, thermoacoustic instabilities remain a barrier to further widen the operational range of these combustors. Passive control strategies to suppress these instabilities, such as Helmholtz dampers, have been used in some industrial systems thanks to their simplicity in terms of implementation. Active control strategies are however not found in practical combustors, mainly due to the lack of robust actuators able to operate in harsh conditions with sufficient control authority. In this study, we demonstrate that thermoacoustic instabilities can be suppressed by using a non-equilibrium plasma produced with NRPD in a lab-scale atmospheric sequential combustor operated at 73.4 kW of thermal power. We employ continuous NRPD forcing to influence the combustion process in the sequential combustor. The two governing parameters are the pulse repetition frequency (PRF) and the plasma generator voltage. We examine the effect of both parameters on the acoustic amplitude, the NO emissions, and the flame centre of mass. We observe that for some operating conditions, with plasma power of 1.1 W, which is about 1.5$\times$ $10^{-3}$ percent of the thermal power of the flames, the combustor can be thermoacoustically stabilized. This finding motivates further research on the optimization of the plasma parameters as a function of the thermoacoustic properties of the combustor where it is applied. This study is a pioneering effort in controlling the thermoacoustic stability of turbulent flames with plasma discharges at such low power compared to the thermal power of the sequential combustor.Comment: 15 Figures, 36 Page

Design and evaluation of two-stage travelling wave thermoacoustic cooler

The overall aim of this research is to investigate the underpinning science behind constructing a practical travelling-wave thermoacoustic refrigerator. At the outset, this was defined as a demonstrator that could be further developed into a means of thermal management of various enclosures – for example weather proof enclosures containing heat generating electronics, popular across the process industries. The practical requirements were set as 400 to 500 W of cooling power at 25 K temperature difference between the inside of the enclosure and the ambient. The initial research addressed issues of coupling the linear motors to such a refrigerator. This included analytical solutions of equations governing the electrodynamic behaviour of the motors, which lead to obtaining preferred acoustic conditions for their optimum performance. Meanwhile, a series of DeltaEC simulations was conducted to investigate possible configurations of the acoustic network that provide the required acoustic impedance matching. The project had the practical limitations of using two existing Q-drive linear motors. As a result, a refrigerator network has been developed which required a compliance and inertance matching a twin-alternator excitation and a two-stage looped-tube travelling-wave refrigerator. The second part of the research was concerned with engineering a practical demonstrator of the above refrigerator concept. DeltaEC simulations have been used to design a practical build and predict its performance characteristics. A prototype, based on helium pressurised at 40 bar and operating frequency of 60 Hz, has been subsequently built and commissioned. A number of experiments have been conducted to evaluate its performance “as built” followed by improvements including, in particular, the use of elastic membranes to supress Gedeon streaming. The prototype achieved a maximum temperature difference of 40ºC, minimum cold temperature of -7.5ºC, maximum COP of 2.05, highest COPR of 21.72% and total cooling power of 283W. Good overall agreement was found between modelling and experiments

HIGH INJECTION PRESSURE DME IGNITION AND COMBUSTION PROCESSES: EXPERIMENT AND SIMULATION

With nearly smokeless combustion, Dimethyl Ether (DME) can be pressurized and used as a liquid fuel for compression-ignition (CI) combustion. However, due to its lower heating value and liquid density compared with diesel fuel, DME has a smaller energy content per unit volume. To obtain an equivalent energy content of diesel, approximately 1.86 times more quantity of DME is required. This can be addressed by a larger nozzle size or higher injection pressure. However, the effect of high injection pressure on DME spray combustion characteristics have not yet been well understood. In order to fill this gap, spray and combustion processes of DME were studied extensively via a series of experiments in a constant-volume and optically accessible combustion vessel. In the current study, a hydraulic electric unit injector (HEUI) with a 180 µm single-hole nozzle was driven by an oil-pressurized fuel injection (FI) system to achieve injection pressure of 1500 bar. The liquid and vapor regions of DME jet were visualized using a hybrid Schlieren/Mie scattering at non-reacting conditions. At reacting conditions, high-speed natural flame luminosity of DME combustion was used to capture the flame intensity, and planar laser-induced fluorescence (PLIF) imaging was used to characterize CH2O evolution. Spray and combustion characteristics of DME were compared with diesel in terms of rate of injection (ROI), liquid/vapor penetration and, ignition delay. Flame lift-off length (LOL), flame structure, and formaldehyde (CH2O) formation of DME were also studied through high-speed imaging. The RANS Converge CFD simulation was validated against the experimental and used as a powerful tool to explore the DME spray characteristics under various conditions. Further insights into DME spray and flame structure were obtained through experimentally validated Large Eddy Simulations (LES) simulations

ReSCon '12, Research Student Conference: Book of Abstracts

The fifth SED Research Student Conference (ReSCon2012) was hosted over three days, 18-20 June 2012, in the Hamilton Centre at Brunel University. The conference consisted of 130 oral and 70 poster presentations, based on the high quality and diverse research being conducted within the School of Engineering and Design by postgraduate research students. The conference is held annually, and ReSCon plays a key role in contributing to research and innovations within the School

Effects of errorless learning on the acquisition of velopharyngeal movement control

Session 1pSC - Speech Communication: Cross-Linguistic Studies of Speech Sound Learning of the Languages of Hong Kong (Poster Session)The implicit motor learning literature suggests a benefit for learning if errors are minimized during practice. This study investigated whether the same principle holds for learning velopharyngeal movement control. Normal speaking participants learned to produce hypernasal speech in either an errorless learning condition (in which the possibility for errors was limited) or an errorful learning condition (in which the possibility for errors was not limited). Nasality level of the participants’ speech was measured by nasometer and reflected by nasalance scores (in %). Errorless learners practiced producing hypernasal speech with a threshold nasalance score of 10% at the beginning, which gradually increased to a threshold of 50% at the end. The same set of threshold targets were presented to errorful learners but in a reversed order. Errors were defined by the proportion of speech with a nasalance score below the threshold. The results showed that, relative to errorful learners, errorless learners displayed fewer errors (50.7% vs. 17.7%) and a higher mean nasalance score (31.3% vs. 46.7%) during the acquisition phase. Furthermore, errorless learners outperformed errorful learners in both retention and novel transfer tests. Acknowledgment: Supported by The University of Hong Kong Strategic Research Theme for Sciences of Learning © 2012 Acoustical Society of Americapublished_or_final_versio

Active Control of Thermoacoustical Instabilities.

This dissertation presents some advances in active control of thermoacoustic instabilities in combustion chambers. Large-size gaseous and liquid fueled swirl stabilized combustors were used during the studies. Active control was implemented using different types of actuators. Proportional (loudspeakers and fuel valves) and discrete actuators (open-close automotive fuel injectors) were investigated. Acoustic and fuel modulation control were successfully applied. In large-scale combustors, flame stabilization techniques such as swirl add three dimensional characteristics to the flow. Moreover, the induced turbulence creates highly nonlinear interactions in the system. Thus, in order to capture these characteristics nonlinear partial differential equations have to be used. Alternatively, the main dynamics of the combustion process can be modeled experimentally. This approach was chosen. Time and frequency domain linear identification techniques were used for this purpose. Several model-based control strategies such as LQG, Hinfinity Disturbance Rejection and Hinfinity Loop-Shaping techniques were tested experimentally with success. A simple controller whose parameters were optimized on-line is also introduced. An evolution algorithm was developed to perform its parameter optimization achieving good convergence to optimal values. The improvements with these proposed control techniques over classical phase-delay control are demonstrated experimentally. A new control configuration was suggested from heat-release visualizations of the flame. In this new configuration, control actuation is directly focused onto the main area of heat-release in the flame front. Consequently, a more efficient actuation is achieved. It is shown that with just a small amount of modulated fuel, phase-delay control can substantially attenuate the pressure oscillations. Finally, during the development of Hinfinity controllers, there were cases where the stability of the resulting controllers restricted the closed-loop performance. A control design strategy to solve the Hinfinity Strong Stabilization problem is then presented. The proposed design strategy pursues to overcome the conservativeness of existing formulations. Examples show its potential for future applications

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

Impact of Flow Rotation on Flame Dynamics and Hydrodynamic Stability

This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities

Summary of Research 1994

The views expressed in this report are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government.This report contains 359 summaries of research projects which were carried out under funding of the Naval Postgraduate School Research Program. A list of recent publications is also included which consists of conference presentations and publications, books, contributions to books, published journal papers, and technical reports. The research was conducted in the areas of Aeronautics and Astronautics, Computer Science, Electrical and Computer Engineering, Mathematics, Mechanical Engineering, Meteorology, National Security Affairs, Oceanography, Operations Research, Physics, and Systems Management. This also includes research by the Command, Control and Communications (C3) Academic Group, Electronic Warfare Academic Group, Space Systems Academic Group, and the Undersea Warfare Academic Group