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This work investigated the application of fiber-based endoscopes (FBEs) in combustion and flow measurements, especially for multidimensional and quantitative measurements. The use of FBEs offers several unique advantages to greatly reduce the implementation difficulty and cost of optical diagnostics. However, the use of FBEs requires registering the locations and orientations of the FBEs carefully for quantitative measurements, and degrades the spatial resolution of the images transmitted. Hence this work conducted a series of controlled tests to quantify the accuracy of the view registration process and the spatial resolution degradation for FBEs. The results show that, under the conditions tested in this work, the view registration process can be accurate within ±0.5 degree and the FBEs can resolve spatial features on the order of 0.25 mm. The combined effects of such view registration uncertainty and spatial resolution degradation are reflected in the re-projection error, which was shown to be within ±0.5 pixels under typical conditions used in this work. Finally, based on these understanding, experiments were conducted to obtain instantaneous measurements of flame structures at kHz temporal resolution using FBEs, demonstrating the capability of resolving flame features on the order of 0.2~0.3 mm in three-dimensional. / Master of Science

<div>Thermoacoustic instabilities in rocket engines have been studied for decades and models have been attempted, however, the heat release fluctuations and overall response</div><div>is still poorly understood. To understand the heat release mechanism in a rocket combustion chamber the effect of hydrodynamics and chemical kinetics on the mode/s of combustion need to be studied. Using prior simulations of the CVRC, an initial design for a new model rocket combustor was proposed. The new design improved on past experiments by having better control of all important boundary conditions; facilitate higher fidelity pressure and optical measurements with emphasis on quantifying the results and using them to validate simulation models of the design; and allow good control over the characteristic parameters of the injection mechanics. A prior simulation was done on the proposed design to allow fine tuning of the</div><div>design elements. Three distinct modes of self-excited instability were observed in the experiment, two of which transitioned between one another with a sweep in oxidizer</div><div>temperature. A number of configurations and operating conditions were tested, but the primary focus was on three oxidizer rich cases, at different oxidizer temperatures. The two extreme cases were compared to the simulations conducted. At low oxidizer temperatures there was good agreement, where at high oxidizer temperatures there</div><div>was a fairly good agreement in the type of mechanics observed, but there were a few discrepancies. The vortex shedding off of the fuel collar was captured using chemiluminescence measurements and compared quite well with the simulations. It was found that the fuel collar vortex shedding did not directly contribute to the generation of</div><div>instabilities.</div>

Gas turbine engines are an efficient and flexible way of power generation and aircraft propulsion. Even though different combustion systems can be implemented in these engines, more stringent regulations on pollutant emissions have been imposed throughout the years, especially in regard to nitrogen oxides (NOx). A very promising technology to reduce NOx emissions is lean premixed combustion (LPC), however, it is plagued by intense flame dynamics. Thermoacoustic instabilities, lean blow-off and lean instabilities are examples of dynamical phenomena that are detrimental to the gas turbines. In view of this, the present thesis presents the experimental investigation of the response of lean premixed swirl flames to acoustic perturbations at atmospheric and elevated pressures. The results of this investigation may be used to understand the thermoacoustic instabilities and further could be helpful in their prediction. Moreover, this work addresses the effects of non-thermal plasma discharges on the lean blow-off and stability limits of premixed swirl flames at elevated pressures. For the analysis of the flame response to acoustic fluctuations, the flame transfer functions, the flame dynamics, phase-locked velocity fields, and phase-locked measurements of flame curvature are collected through heat release and velocity fluctuations measurements, phase-locked images of the flame, particle image velocimetry, and planar laser-induced fluorescence, respectively. For the analysis of the effects of plasma discharges on the stability limits, electrical measurements and direct imaging of the flame are performed. The results include the development of an empirical relation based on the laminar burning velocity and on the circulation of the acoustically generated vortex to predict the response of the flame to acoustic fluctuations in different operating conditions. Moreover, the results show that the pressure has a strong impact on the response of lean premixed swirl flames to acoustic oscillations and on the flame-plasma interactions. Therefore, extrapolating results obtained at atmospheric conditions to elevated pressures may result in erroneous conclusions. Furthermore, it is shown that non-thermal plasma discharges can effectively extend the stability limits of lean premixed swirl flames at elevated pressures, underlining the potential of these discharges at conditions relevant for gas turbines.

Measurements of soot emission properties are of interest in both fundamental research and combustion-based industries. Laser-induced incandescence of soot particles is a novel technique that allows unobtrusive measurements of both soot volume fraction and particulate size with significant advantages. An apparatus utilizing this technique has been customized and used to provide measurements of soot concentration and particle sizing of a laminar, diffusion methane/air flame at pressures of 10, 20 and 40 atm at 6~mm above the burner. Soot volume fraction measurements correlate well with literature findings at all pressures. Despite similar trends, particle size values are found to be consistently larger than values reported in literature. Discussion on the errors of laser-induced incandescence as well as recommendations for improving the apparatus and results are herein.

Measurements of soot emission properties are of interest in both fundamental research and combustion-based industries. Laser-induced incandescence of soot particles is a novel technique that allows unobtrusive measurements of both soot volume fraction and particulate size with significant advantages. An apparatus utilizing this technique has been customized and used to provide measurements of soot concentration and particle sizing of a laminar, diffusion methane/air flame at pressures of 10, 20 and 40 atm at 6~mm above the burner. Soot volume fraction measurements correlate well with literature findings at all pressures. Despite similar trends, particle size values are found to be consistently larger than values reported in literature. Discussion on the errors of laser-induced incandescence as well as recommendations for improving the apparatus and results are herein.

A novel nonintrusive soot diagnostics methodology was developed, validated and applied for in-situ determination of temperature, volume fraction and refractive index of soot aggregates formed inside flames by using near-infrared emission spectrometry. Research was conducted in three main parts, first one addressing development and validation of a comprehensive &quot / direct&quot / model for simulation of line-of-sight radiative emission from axisymmetric sooty flames by coupling sub-models for radiative transfer, radiative properties and optical constants. Radiative property estimation for soot agglomerates was investigated by experimentally validating discrete dipole approximation against microwave measurements and using it as reference to assess applicability of simpler Rayleigh-Debye-Gans approximation for fractal aggregates (RDG-FA). Comparisons between predictions of two methods for soot-like model aggregates demonstrated that radiative property predictions of RDG-FA are acceptably accurate for relatively small soot aggregates encountered in small-scale flames. Part two concerns experimental investigation of an axisymmetric ethylene/air diffusion flame by Fourier Transform Near-Infrared spectroscopy. Measurement of line-of-sight emission intensity spectra was performed along with analyses on calibration, noise, uncertainty and reproducibility. A noise characterization approach was introduced to account for spatial fluctuations which were found to dominate over spectral noise. Final part focuses on development, evaluation and application of an inversion methodology that inputs spectral emission intensity measurements from optically thin flames, removes noise, identifies soot refractive index from spectral gradients and retrieves soot temperature and volume fraction fields by tomographic reconstruction. Validation with simulated data and favorable application to measurements indicate that proposed methodology is a promising option for nonintrusive soot diagnostics in flames.

Current research in CI combustion aims to reduce PM and NOx emissions by controlling mixture homogeneity. Low CN fuels are suitable due to their auto-ignition resistance, but the in-cylinder mixture stratification level must be carefully visualised and controlled. Numerous diagnostic techniques exist for imaging the in-cylinder hydrocarbon species concentration. Tomographic techniques based on spectroscopic modalities are minimally-intrusive and able to target species of interest even in multi-component fuel blends. The high-speed CST technique applied in this work is based on the NIRAT modality. A number of collimated LASER beams at 1700nm traverse the optically accessible engine combustion chamber and are spectroscopically absorbed by the first overtone of the C-H stretch bond. Non species-specific attenuation mechanisms are suppressed by a DWR scheme utilising a reference wavelength at 1651nm. Ratiometric data is used to tomographically reconstruct the spatially-varying fuel concentration. In this work the first application of NIRAT on a commercial CI engine is presented, using instrumentation capable of imaging 13 frames/CAD at 1200rpm using a 31-beam array. A novel method was developed to experimentally quantify the tomography system’s non-uniform spatial resolution. The method was applied in laboratory experiments involving free-space propane plumes and a map of the spatial resolution was created. The spatial resolution varies between 4mm and 14mm. The mean of 9mm is 72% better than previous estimates in the literature. Regions of poor performance correlated with non-uniformities in the sensitivity matrix, indicating that a regular beam array may contribute towards more accurate and objective reconstructions of unknown concentrations. The characterised tomography system was installed on an optically-accessible Volvo D5 CI engine. The optically-inaccessible CAD region achieved was ±18CAD, a reduction of ±12° from previous works. The vibration-tolerance of the optical access system was verified, concluding that the initial alignment of the beams is the dominant factor that determines beam integrity after prolonged engine operation. The behavior of individual beams was studied, finding strong cycle-to-cycle correlation of the anomalies present. This was exploited to develop a novel, robust analysis algorithm to process the engine data. The algorithm achieved a standard deviation of <10% of the maximum pk-pk magnitude of the transmission signal in the fuel vapour phase. The system was applied to qualitatively visualise the mixing of a 50/50% blend of iso-/n-dodecane in a motored, nitrogen-aspirated engine under a range of operating conditions. A study by simulation of the decomposition of n-dodecane concluded that only 0.492% of the quantity injected is pyrolytically converted during a compression stroke. Spray-phase imaging was not possible due to severe reduction of the optical throughput, lasting for 8-15 CAD for a lean mixture and for 15-30 CAD for a rich mixture. Vapour-phase reconstructions using the enhanced iterative Landweber algorithm were successful in resolving rich fuel pockets consistent with the injection pattern. It was shown that the degree of mixture homogeneity at TDC is dependent upon the initial intake temperature. PLIF was used to cross-validate the NIRAT reconstructions. Localisation of the features reconstructed with NIRAT was excellent, with a maximum angular deviation of ±10°. A swirl motion of the mixture by 1°/CAD was observed using both techniques, confirming the features previously observed in the NIRAT reconstructions. In conclusion, NIRAT has been, for the first time, successfully applied for in-cylinder fuel distribution imaging in a CI engine. The results, created using an original data analysis algorithm, were successfully cross-validated using PLIF. A novel spatial resolution quantification method was formulated and used to characterise the tomography system. The numerous findings and learning points from the individual stages of this work will be used to advance the field of combustion diagnostics as well as contribute towards the development of advanced in-cylinder tomographic imaging systems.

With global emission regulations becoming stringent, development of new combustion technologies that meet future emission regulations is essential. In this vein, this dissertation presents the application of sensitive diagnostic tools to validate and improve chemical kinetic mechanisms that play a fundamental role in the design of new combustion technologies. First, a novel high sensitivity laser-based sensor with a wide frequency tuning range (900 – 1000 cm-1) was developed utilizing pulsed cavity ringdown spectroscopy (CRDS) technique. The novel laser-based sensor was illustrated by measuring trace amounts of multiple combustion intermediates, namely ethylene, propene, allene, and 1-butene in a static cell at ambient conditions. Subsequently, pulsed CRDS technique was utilized to develop an ultra-fast, high sensitivity diagnostic to monitor trace concentrations of ethylene in shock tube pyrolysis experiments. This diagnostic represented the first ever successful application of CRDS technique to transient species measurements in a shock tube. The high sensitivity and fast time response (10μs) diagnostic may be utilized for measuring other key neutrals and radicals which are crucial in the oxidation chemistry of practical fuels. Secondly, a quadrupole mass spectrometer (QMS) was employed to measure relative cation mole fractions in atmospheric and low-pressure (30 Torr) flames of methane/oxygen diluted in argon. Lean, stoichiometric and rich flames were 4 examined to evaluate the dependence of ion chemistry on flame stoichiometry. Spatial distribution of cations was compared with predictions of an existing ion chemistry model. Based on the extensive measurements carried out in this work, modifications were suggested to improve the ion chemistry model to enhance the fidelity of such mechanisms. In-depth understanding of flame ion chemistry is vital to model the interaction of flames with electric fields and thereby pave the way to enable active combustion control for increased efficiency and reduced emissions. Finally, a compact fast time-response time-of-flight mass spectrometer (TOFMS) was coupled to the shock tube through a pin-hole end-wall to enable timeresolved species concentration measurements. This diagnostic tool was demonstrated by investigating the decomposition of 1,3,5-trioxane over a wide range of shock conditions. Reaction rate coefficients were extracted by the best fit to the experimentally measured species time-histories. TOF-MS coupled to the shock tube is an ideal diagnostic tool for developing kinetic mechanisms for future fuels due to its ability to simultaneously measure several species during fuel pyrolysis/oxidation processes.