104 research outputs found

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

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    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 1.5ร—\times 10โˆ’310^{-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

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    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 โ€“ 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 โ€œas builtโ€ followed by improvements including, in particular, the use of elastic membranes to supress Gedeon streaming. The prototype achieved a maximum temperature difference of 40ยบC, minimum cold temperature of -7.5ยบC, 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

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    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 ยตm 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

    Effects of errorless learning on the acquisition of velopharyngeal movement control

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    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 participantsโ€™ 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 ยฉ 2012 Acoustical Society of Americapublished_or_final_versio

    Active Control of Thermoacoustical Instabilities.

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    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

    ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์˜ ์Œํ–ฅ ๊ฐ€์ง„์— ๋”ฐ๋ฅธ ํ™”์—ผ์‘๋‹ตํŠน์„ฑ

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    ํ•™์œ„๋…ผ๋ฌธ(๋ฐ•์‚ฌ) -- ์„œ์šธ๋Œ€ํ•™๊ต๋Œ€ํ•™์› : ๊ณต๊ณผ๋Œ€ํ•™ ๊ธฐ๊ณ„ํ•ญ๊ณต๊ณตํ•™๋ถ€, 2022.2. ์œค์˜๋นˆ.์—ฐ์†Œ๋ถˆ์•ˆ์ •์˜ ๋ฐœ์ƒ ์›์ธ๊ณผ ๋ฉ”์ปค๋‹ˆ์ฆ˜์€ ํ˜„์žฌ๊นŒ์ง€ ์ •ํ™•ํ•˜๊ฒŒ ๊ทœ๋ช…๋˜์ง€ ์•Š์•˜์œผ๋‚˜ ๋ฐ˜์‘๋ฌผ์˜ ์œ ๋™ ์„ญ๋™, ์—ด๋ฐฉ์ถœ๋Ÿ‰ ์„ญ๋™, ์—ฐ์†Œ์‹ค์˜ ์Œํ–ฅํ•™์  ๊ฒฝ๊ณ„์— ์˜ํ•œ ์„ญ๋™์˜ ์ƒํ˜ธ์ž‘์šฉ์— ์˜ํ•ด ๋ฐœ์ƒ ์œ ๋ฌด๊ฐ€ ๊ฒฐ์ •๋œ๋‹ค๊ณ  ์•Œ๋ ค์ ธ ์žˆ๋‹ค. ์œ„์˜ ์„ธ ๊ฐ€์ง€ ์„ญ๋™์ด ์–‘์„ฑ ํ”ผ๋“œ๋ฐฑ ๋ฃจํ”„ (positive feedback loop)๋ฅผ ํ˜•์„ฑํ•˜๋ฉด ์—ฐ์†Œ๋ถˆ์•ˆ์ •์ด ๋ฐœ์ƒํ•  ํ™•๋ฅ ์ด ๋†’์•„์ง€๋ฉฐ, ์Œ์„ฑ ํ”ผ๋“œ๋ฐฑ ๋ฃจํ”„(negative feedback loop)๋ฅผ ํ˜•์„ฑํ•˜๋ฉด ๊ทธ ํ™•๋ฅ ์ด ๋‚ฎ์•„์ง€๋Š” ๊ฒƒ์œผ๋กœ ์•Œ๋ ค์ ธ ์žˆ๋‹ค. ๋”ฐ๋ผ์„œ ์—ฐ์†Œ๋ถˆ์•ˆ์ • ๋ฐœ์ƒ ์ €๊ฐ์„ ์œ„ํ•ด ์—ฐ์†Œ๋ถˆ์•ˆ์ • ๋ฐœ์ƒ ์กฐ๊ฑด ๋ฐ ์ธ์ž๋ฅผ ํŒŒ์•…ํ•˜๋Š” ๊ฒƒ์€ ํ•„์ˆ˜์ ์ด๋‹ค. ๋ณธ ์—ฐ๊ตฌ์—์„œ๋Š” ์—ฐ์†Œ๋ถˆ์•ˆ์ • ํ˜„์ƒ์˜ ์ธ์ž ์ค‘ ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ์„ญ๋™๊ณผ ์†๋„ ์„ญ๋™์˜ ์ƒํ˜ธ๊ด€๊ณ„์— ๋Œ€ํ•œ ์—ฐ๊ตฌ๋ฅผ ์ˆ˜ํ–‰ํ•˜์˜€์œผ๋ฉฐ, ํŠน์ • ์Œํ–ฅ ๊ฐ€์ง„ ์กฐ๊ฑด์—์„œ ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” ํ˜„์ƒ์ธ pinch-off ํ™”์—ผ์— ๋Œ€ํ•ด OH PLIF๊ณผ PIV ๋ ˆ์ด์ € ๋™์‹œ๊ณ„์ธก์„ ํ†ตํ•ด ๋ฐœ์ƒ ๋ฉ”์ปค๋‹ˆ์ฆ˜์„ ์‹คํ—˜์ ์œผ๋กœ ๊ทœ๋ช…ํ•˜์˜€์œผ๋ฉฐ, pinch-off ํ™”์—ผ๊ณผ nonpinch-off ํ™”์—ผ์˜ NOx ๋ฐฐ์ถœ ํŠน์„ฑ, ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์˜ ์œ ๋™๊ฒฝ๊ณ„์ธต์— ๋Œ€ํ•œ ์œ ๋™ ํŠน์„ฑ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์Œํ–ฅ ๊ฐ€์ง„ ๋ฐœ์ƒ์„ ์œ„ํ•ด์„œ ์Šคํ”ผ์ปค๋ฅผ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ, ํ™”์—ผ๊ตฌ์กฐ ๋ถ„์„์„ ์œ„ํ•ด์„œ OH* ์ž๋ฐœ๊ด‘๊ณผ OH-PLIF ๋ ˆ์ด์ € ๊ณ„์ธก๊ธฐ๋ฒ•์„ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ, ์œ ๋™์žฅ ํŠน์„ฑ๋ถ„์„์„ ์œ„ํ•ด OH-PLIF์™€ PIV ๋™์‹œ๊ณ„์ธก์„ ํ™œ์šฉํ•˜์˜€๋‹ค. ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ๊ณ„์ธก์„ ์œ„ํ•ด์„œ ๊ด‘์ „์ž์ฆํญ๊ด€(Photo Multiplier Tube, PMT)๋ฅผ ํ™œ์šฉํ•˜์˜€์œผ๋ฉฐ ์ด๋ฅผ ํ†ตํ•ด ํ™”์—ผ์ „๋‹ฌํ•จ์ˆ˜๋ฅผ ๊ณ„์ธกํ•˜์˜€๋‹ค. ์—ฐ์†Œ๋ถˆ์•ˆ์ • ์˜ˆ์ธก์„ ์œ„ํ•ด ์Œํ–ฅ ๊ฐ€์ง„์— ๋”ฐ๋ฅธ ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ๊ณผ ์˜ˆํ˜ผํ•ฉํ™”์—ผ์˜ ์‘๋‹ตํŠน์„ฑ ๋ฐ ๋™ํŠน์„ฑ ๋น„๊ต ์—ฐ๊ตฌ๋ฅผ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋‹ค๋ฅธ ์—ฐ์†Œ ๋ฐ˜์‘์„ ๊ฐ€์ง„ ๋‘ ํ™”์—ผ์€ ์Œํ–ฅ ๊ฐ€์ง„์— ๋”ฐ๋ผ ๋™์ ๊ฑฐ๋™ ํŠน์„ฑ์ด ์ƒ์ดํ•˜๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ํ™”์—ผ ๋ฉด์—์„œ ์Œํ–ฅํ•™์  ํŒŒ๋™์ด ํˆฌ์˜๋˜๋ฉฐ, ํŽ„๋Ÿญ์ด๋Š” ๋™์  ๊ฑฐ๋™ ํŠน์„ฑ์ด๋ฉฐ ํ™”์—ผ ๋ ๋‹จ์ด ์—ด๋ฆฐ ํ™”์—ผํ˜•์ƒ์ด๋‹ค. ๋ฐ˜๋ฉด์— ๋‹จ์ผ ๋…ธ์ฆ์ธ ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์ฝ”๋‹ˆ์ปฌ ํ™”์—ผ(conical flame)์˜ ํ˜•ํƒœ๋กœ ์ˆ˜์ง์œผ๋กœ ํฌ๊ฒŒ ์„ญ๋™ํ•œ๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜ ์ฆ๊ฐ€์— ๋”ฐ๋ผ์„œ ํ™”์—ผ ๋ฉด์˜ ๋ชจ๋“ˆ๋ ˆ์ด์…˜(modulation) ๊ฐœ์ˆ˜๊ฐ€ ์ฆ๊ฐ€ํ•  ๋ฟ ํ™”์—ผ ๊ตฌ์กฐ๋Š” ํฌ๊ฒŒ ๋ณ€ํ•˜์ง€ ์•Š์ง€๋งŒ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„์— ๋”ฐ๋ผ ๋‹ค์–‘ํ•œ ํ™”์—ผ ๊ตฌ์กฐ๊ฐ€ ๋‚˜ํƒ€๋‚œ๋‹ค. ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ๋‘ ํ™”์—ผ์˜ ์—ด ๋ฐฉ์ถœ๋Ÿ‰ ์ธก์ •์„ ํ†ตํ•ด ํ™”์—ผ์ „๋‹ฌํ•จ์ˆ˜๋ฅผ ๋ถ„์„ํ•œ ๊ฒฐ๊ณผ ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ๋น„์„ ํ˜•์ ์ธ ๊ฒฐ๊ณผ๋ฅผ ๋‚˜ํƒ€๋‚ด์—ˆ์œผ๋ฉฐ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์„ ํ˜•์ ์ธ ๊ฒฐ๊ณผ๋ฅผ ๋‚˜ํƒ€๋‚ด์—ˆ๋‹ค. ํ™”์—ผ๊ธธ์ด์™€ ์ŠคํŠธ๋กค ์ˆ˜(Strouhal number)๋ฅผ ๋„์ž…ํ•˜์—ฌ ์—ด๋ฐฉ์ถœ๋Ÿ‰๊ณผ ํ™”์—ผ๊ตฌ์กฐ์˜ ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€์œผ๋ฉฐ, ์ˆ˜์น˜ํ•ด์„์  ์—ฐ๊ตฌ์™€ ๋น„๊ตํ•˜์˜€๋‹ค. ๋น„์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ 20%์ด์ƒ์˜ ์†๋„์„ญ๋™์—์„œ ์ˆ˜์น˜ํ•ด์„๊ฒฐ๊ณผ์™€ ์ผ์น˜ํ•˜๋ฉฐ, ๋น„์„ ํ˜•์„ฑ์„ ๊ฒ€์ฆํ•˜์˜€์œผ๋ฉฐ, ์˜ˆํ˜ผํ•ฉํ™”์—ผ์€ ์ผ๋ถ€ ์ŠคํŠธ๋กค ์ˆ˜์—์„œ ์ˆ˜์น˜ํ•ด์„ ๊ฒฐ๊ณผ์™€ ๋‹ค๋ฅด๋‹ค. ์ด๋Š” ์ŠคํŠธ๋กค ์ˆ˜๋ฅผ ๊ณ„์‚ฐํ•  ๋•Œ, ํ™”์—ผ ๋ฉด ๊ณก๋ฅ , ํ™”์—ผ ์ „ํŒŒ ์†๋„, ํ™”์—ผ ๋๋‹จ ํ˜•์ƒ ๋“ฑ์„ ๊ณ ๋ คํ•œ ์ŠคํŠธ๋กค ์ˆ˜์— ๋„์ž…ํ•ด์•ผ ํ•จ์„ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฒ…-์Šˆ๋งŒ ํ™”์—ผ์˜ ํ•œ๊ฐ€์ง€ ์ผ€์ด์Šค์ธ ๋น„์˜ˆํ˜ผํ™”์—ผ์—์„œ ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ์†๋„์„ญ๋™๊ฐ•๋„์™€ ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์— ๋”ฐ๋ฅธ ๋‹ค์–‘ํ•œ ํ™”์—ผ ๊ตฌ์กฐ๋ฅผ ๊ด€์ฐฐํ•˜์˜€๋‹ค. ์ผ์ • ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” pinch-off ํ™”์—ผ์ด ๋‚˜ํƒ€๋‚จ์„ ํ™•์ธํ•˜์˜€๋‹ค. pinch-off ํ™”์—ผ์€ ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” ํ˜„์ƒ์œผ๋กœ ์ •์˜ํ•˜๋ฉฐ ๋…ธ์ฆ์— ๋ถ€์ฐฉ๋œ ํ™”์—ผ์€ ๋ฉ”์ธ(main) ํ™”์—ผ๊ณผ ๋–จ์–ด์ ธ ๋‚˜๊ฐ„ ํ™”์—ผ์„ ํฌ์ผ“(pocket) ํ™”์—ผ์œผ๋กœ ์ •์˜ํ•œ๋‹ค. Pinch-off ํ™”์—ผ์˜ ๋ฉ”์ปค๋‹ˆ์ฆ˜ ๊ทœ๋ช…์„ ์œ„ํ•ด OH PLIF&PIV ๋™์‹œ๊ณ„์ธก์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๊ฐ€์ง„ ์ฃผํŒŒ์ˆ˜์™€ ์†๋„์„ญ๋™๊ฐ•๋„์— ๋”ฐ๋ฅธ ํ™”์—ผ ๊ตฌ์กฐ๋ฅผ ๋งตํ•‘(mapping)ํ•˜์—ฌ 3๊ฐ€์ง€ ๋™์  ๊ฑฐ๋™์œผ๋กœ ๊ตฌ๋ณ„ํ•˜์˜€๋‹ค. ๋‚ฎ์€ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ์ƒ-ํ•˜๋กœ ํฌ๊ฒŒ ์„ญ๋™ํ•˜๋Š” flickering ํ™”์—ผ์ด๋‹ค. ์ค‘๊ฐ„ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ์ด ๋Š๊ธฐ๋Š” pinch-off ํ™”์—ผ, ๊ทธ๋ฆฌ๊ณ  ๋†’์€ ์ฃผํŒŒ์ˆ˜ ๋ฒ”์œ„์—์„œ๋Š” ํ™”์—ผ ๋ฉด์˜ ๋ชจ๋“ˆ๋ ˆ์ด์…˜์ด ์ƒ๊ธฐ๋Š” wrinkled ํ™”์—ผ์œผ๋กœ ๊ตฌ๋ณ„ํ•˜์˜€๋‹ค. ๋น„๋ฐ˜์‘์žฅ ์œ ๋™์—์„œ ์Œํ–ฅ ๊ฐ€์ง„ ์‹œ ์กฐ์ˆ˜ ์œ ๋™(tidal flow)์— ์˜ํ•œ double dipole vortex์„ ๋ฏธ ์‚ฐ๋ž€(Mie scattering)์œผ๋กœ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฐ˜์‘์žฅ ์œ ๋™์˜ ํ™”์—ผ์ด pinch-off ์‹œ vortical structure์— ์˜ํ•œ ๊ณต๊ธฐ ์œ ์ž…์ด ํ™”์—ผ ๋ณ€ํ˜•์„ ์•ผ๊ธฐํ•จ์„ ํ™•์ธํ•˜์˜€์œผ๋ฉฐ, ํ™”์—ผ ๋ชฉ ๋ถ€๋ถ„์— ๊ฐ•ํ•œ strain rate์„ ๊ด€์ฐฐํ•˜์˜€๋‹ค. ์ด์— ๋”ฐ๋ผ pinch-off flame์€ vortical structure์— ์˜ํ•œ ์™ธ๋ถ€ ๊ณต๊ธฐ ์œ ์ž…๊ณผ ๊ฐ•ํ•œ strain rate์— ์˜ํ•œ ์ƒํ˜ธ์ž‘์šฉ์ž„์„ ํ™•์ธํ•˜์˜€๋‹ค. pinch-off ํ™”์—ผ์˜ ๋–จ์–ด์ ธ๋‚˜๊ฐ„ ํฌ์ผ“ ํ™”์—ผ์„ ๊ณ ๋ คํ•œ ์งˆ์†Œ์‚ฐํ™”๋ฌผ(NOx), ์ผ์‚ฐํ™”ํƒ„์†Œ(CO) ๋ฐฐ์ถœ ํŠน์„ฑ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์งˆ์†Œ์‚ฐํ™”๋ฌผ์€ ์†๋„์„ญ๋™๊ฐ•๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ผ ๋ฐฐ์ถœ์–‘์€ ๊ฐ์†Œํ•œ๋‹ค. ์ด๋Š” ์†๋„์„ญ๋™๊ฐ•๋„๊ฐ€ ์ฆ๊ฐ€ํ•˜๋ฉด ์—ฐ๋ฃŒ์™€ ์‚ฐํ™”์ œ ํ˜ผํ•ฉ๋„๊ฐ€ ์ฆ๊ฐ€ํ•˜๊ธฐ ๋•Œ๋ฌธ์ด๋ฉฐ ํ˜ผํ•ฉ์ด ์ž˜ ์ด๋ฃจ์–ด์ ธ ์™„์ „์—ฐ์†Œํ•˜๋ฉด ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ์ด ์ ์–ด์ง„๋‹ค. ๋ฐ˜๋ฉด์— ์ผ์‚ฐํ™”ํƒ„์†Œ๋Š” ์†๋„์„ญ๋™๊ฐ•๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ผ ๋ฐฐ์ถœ์ด ์ฆ๊ฐ€ํ•˜๋Š” ํŠน์„ฑ์„ ๋‚˜ํƒ€๋‚ด์—ˆ์ง€๋งŒ, ๊ทธ ๋ฐฐ์ถœ๋Ÿ‰์ด ๋งค์šฐ ์ž‘์Œ์„ ํ™•์ธํ•˜์˜€๋‹ค. ๋ฐฐ์ถœํŠน์„ฑ์˜ ์ง€ํ‘œ์ธ EINOx(Emission Index of NOx)์™€ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„(flame residence time) ๋ถ„์„์„ ์œ„ํ•ด์„œ pinch-off ํ™”์—ผ์˜ ๋†’์ด๋ฅผ ์ฃผํ™”์—ผ๊ณผ ํฌ์ผ“ํ™”์—ผ์œผ๋กœ ์„ธ๋ถ„ํ™”ํ•˜์—ฌ ์ •์˜ํ•˜์˜€๋‹ค. ์„ธ๋ถ„ํ™”ํ•œ ํ™”์—ผ๊ธธ์ด์™€ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„ ๋ถ„์„ ๊ฒฐ๊ณผ, ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„์ด ๊ฐ์†Œํ•จ์— ๋”ฐ๋ผ ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ์ด ์ €๊ฐ๋˜๋Š” ๊ฒฝํ–ฅ์€ ์ผ์น˜ํ•˜์ง€๋งŒ, ๊ฐ€์ง„์ฃผํŒŒ์ˆ˜์— ๋”ฐ๋ฅธ ๊ฒฝํ–ฅ์„ ๋”ฐ๋ฅด์ง€๋Š” ์•Š์•˜๋‹ค. ๋”ฐ๋ผ์„œ ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„๋งŒ์œผ๋กœ ์งˆ์†Œ์‚ฐํ™”๋ฌผ ๋ฐฐ์ถœ ํŠน์„ฑ์„ ๋ถ„์„ํ•˜๋Š”๋ฐ์— ๊ทธ ํ•œ๊ณ„์ ์ด ์žˆ์Œ์„ ๋ฐํ˜”๋‹ค. ์ŠคํŠธ๋กค ์ˆ˜์™€ EINOx์˜ ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„ ๊ฒฐ๊ณผ ์ŠคํŠธ๋กค ์ˆ˜๊ฐ€ ๋‹ค๋ฆ„์—๋„ EINOx ๋ฐฐ์ถœ์ด ๊ฐ™์€ ๊ฒฝํ–ฅ์„ ํ™•์ธํ•˜์˜€๋‹ค. ์ด๋Š” ํ™”์—ผ๊ธธ์ด๋Š” ์ฃผํ™”์—ผ ๋˜๋Š” ํฌ์ผ“ํ™”์—ผ ์–ด๋Š ๊ฒƒ์„ ์„ ํƒํ•˜์—ฌ๋„ ๊ฒฝํ–ฅ์„ฑ ๋ถ„์„์— ๋ฌด๊ด€ํ•จ์„ ์˜๋ฏธํ•œ๋‹ค. ์ด๋Ÿฌํ•œ ๊ฒฝํ–ฅ์„ฑ ๊ฒ€์ฆ์„ ์œ„ํ•ด ํ™”์—ผ์ฒด๋ฅ˜์‹œ๊ฐ„์œผ๋กœ ์ •๊ทœํ™”ํ•œ EINOx๊ฐ€ 1/2-power๋ฅผ ์ž˜ ๋”ฐ๋ฅด๋Š” ๊ฒƒ์„ ํ™•์ธํ•˜์˜€๋‹ค. Pinch-off์™€ nonpinch-off์กฐ๊ฑด์—์„œ strain rate๊ณผ shear stress ์ƒ๊ด€๊ด€๊ณ„ ๋ถ„์„์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ ์†๋„๊ฐ€ ๊ฐ™์•„ ์ด๋ก ์ ์œผ๋กœ shear stress๊ฐ€ ์—†๋Š” ์กฐ๊ฑด์„ ๊ธฐ์ค€์œผ๋กœ ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ ์†๋„๋ฅผ ๊ฐ๊ฐ ๋ณ€ํ™”์‹œํ‚ค๋ฉฐ shear stress๋ฅผ ์ƒ์„ฑํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์†๋„์˜ ์œ ๋™๊ฒฝ๊ณ„์ธต ๋ถ„์„์„ ์œ„ํ•ด์„œ OH* ์ž๋ฐœ๊ด‘๊ณผ PIV ๋™์‹œ๊ณ„์ธก์„ ์ˆ˜ํ–‰ํ•˜์˜€๋‹ค. ๋‹ค์–‘ํ•œ ์—ฐ๋ฃŒ์™€ ๊ณต๊ธฐ์†๋„์— ๋”ฐ๋ผ pinch-off์˜ ๋งคํ•‘์„ ์ˆ˜ํ–‰ํ•˜์—ฌ ๋ฌผ๋ฆฌ์ ์ธ ๊ฒฝ๊ณ„๋ฅผ ํ™•์ธํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ ์†๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ฅธ pinch-off ์กฐ๊ฑด์—์„œ shear ํšจ๊ณผ๊ฐ€ ์ฆ๊ฐ€ํ•จ์— ๋”ฐ๋ผ strain rate๊ฐ€ ๊ธฐ์ค€ ๋ฐ์ดํ„ฐ์— ๋น„ํ•ด ์•ฝ 80% ์ฆ๊ฐ€ํ•˜์˜€์œผ๋ฉฐ, shear stress๋Š” 15% ์ฆ๊ฐ€ํ•˜์˜€๋‹ค. ์—ฐ๋ฃŒ์†๋„๋ฅผ ๋” ์ฆ๊ฐ€์‹œ์ผœ nonpinch-off ์กฐ๊ฑด์—์„œ ๊ณ„์ธกํ•œ ๊ฒฐ๊ณผ shear ํšจ๊ณผ๋Š” ๋” ์ฆ๊ฐ€ํ•˜์˜€์ง€๋งŒ strain rate์€ ๊ธฐ์กด๋ฐ์ดํ„ฐ์™€ ๋น„๊ตํ–ˆ์„ ๋•Œ 50% ๊ฐ์†Œํ•˜์˜€๊ณ  shear stress๋Š” 3.3๋ฐฐ ์ฆ๊ฐ€ํ•˜์˜€๋‹ค. ๊ณต๊ธฐ์†๋„ ์ฆ๊ฐ€์— ๋”ฐ๋ฅธ nonpinch-off ์กฐ๊ฑด์—์„œ strain rate๊ณผ shear stress์˜ ์ƒ๊ด€๊ด€๊ณ„๋ฅผ ๋ถ„์„ํ•˜์—ฌ ๊ฒฝํ–ฅ์„ฑ์„ ์žฌ๊ฒ€์ฆํ•˜์˜€๋‹ค. ๊ฒฐ๊ณผ์ ์œผ๋กœ strain rate์ด ์ฃผ์š”ํ•˜๊ฒŒ ์˜ํ–ฅ์„ ๋ฏธ์น  ๋•Œ shear stress๋Š” ๊ฐ์†Œ๋˜๋Š” ๊ฒฝํ–ฅ์ด๋ฉฐ, shear stress๋Š” pinch-off๋ฅผ ์ œ์–ดํ•  ์ˆ˜ ์žˆ๋Š” ํŒŒ๋ผ๋ฏธํ„ฐ๋กœ์„œ ํ™œ์šฉ๊ฐ€๋Šฅ์„ฑ์„ ํ™•์ธํ•˜์˜€๋‹ค.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 (ฯ„res) 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 145๋ฐ•

    Impact of Flow Rotation on Flame Dynamics and Hydrodynamic Stability

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    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

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    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
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