Thermoacoustic instabilities arise and sustain due to the coupling of unsteady heat release from the flame and the acoustic field. One potential driving mechanism for these instabilities arise when velocity fluctuations (u') at the fuel injection location causes perturbations in the local equivalence ratio and is convected to the flame location generating an unsteady heat release (q') at a particular convection time delay, τ. Physically, τ is the time for the fuel to convect from injection to the flame. The n-τ Flame Transfer Function (FTF) is commonly used to model this relationship assuming an infinitesimally thin flame with a fixed τ. In practical systems, complex swirling flows, multiple fuel injections points, and recirculation zones create a distribution of τ, which can vary widely making a statistical description more representative. Furthermore, increased flame lengths and higher frequency instabilities with short acoustic wavelengths challenge the 'thin-flame' approximation. The present study outlines a methodology of using distributed convective fuel time delays and heat release rates in a one-dimensional (1-D) linear stability model based on the transfer matrix approach. CFD analyses, with the Flamelet Generated Manifold (FGM) combustion model are performed and probability density functions (PDFs) of the convective time delay and local heat release rates are extracted. These are then used as inputs to the 1-D Thermoacoustic model. Results are compared with the experimental results, and the proposed methodology improves the accuracy of stability predictions of 1-D Thermoacoustic modeling. / Master of Science / Gas turbines that operate with lean, premixed air-fuel mixtures are highly efficient and produce significantly lesser emission of pollutants. However, they are highly susceptible to self-induced thermoacoustic oscillations which can excite larger pressure fluctuation which can damage critical components or catastrophic engine failure. Such a combustion system is considered to be unstable since the oscillation amplitude increases with time. Understanding the non-linear feedback mechanisms driving the system unstable and their cause are naturally of high interest to the industry. Highly resolved, but computationally demanding simulations can predict the stability of the system accurately, but become bottlenecks delaying iterative design improvements. Low order numerical models counter this with quick solutions but use simplified representations of the flame and feedback mechanisms, resulting in unreliable stability predictions. The current study bridges the gap between these methods by modifying the numerical model, allowing it to incorporate a better representation of fluid flow fields and flame structures that are obtained through computationally cheaper simulations. Experiments are conducted to verify the predictions and a technique that can be used to identify regions of the flame that contribute to amplitude growth is introduced. The improved model shows notable improvement in its prediction capabilities compared to existing models.

An experimental facility was designed and benchmarked that could simulate the non-uniformities in the flow and thermal fields exiting real gas-turbine combustors. The design of the combustor simulator required analyses of the flow paths within a real combustor in a gas turbine engine. Modifications were made to an existing wind tunnel facility to allow for the installation of the combustor simulator. The overall performance of the simulator was then benchmarked through measurements of velocity, pressure, temperature, and turbulence using a straight exit test section to provide a baseline set of data. Comparisons of the measured quantities were made between two test cases that included a flow field with and without dilution flow.One of the major findings from this study was that the total pressure profiles exiting the combustor simulator in the near-wall region were different from a turbulent boundary layer. This is significant since many studies consider a turbulent boundary layer as the inlet condition to the turbine. Turbulent integral length scales were found to scale well with the dilution hole diameters and no dominant frequencies were observed in the streamwise velocity energy spectra. Dilution flow resulted in an increase in turbulence levels and mixing causing a reduction in the variation of total pressure and velocity. Adiabatic effectiveness levels were significantly reduced for the case with dilution flow in both the near combustor exit region and along the axial length of the straight exit test section. / Master of Science

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M. Tech. Mechanical Engineering. / Evaluates the effects of blends (vegetable oil-Butanol (BU) alcohol-diesel) on fuel properties, engine performance, combustion, and emission characteristics. Fuel blends investigated were croton oil (CRO), Diesel (D2), 20% CRO-80% D2, 15% CRO-5% BU-80% D2 and 10% CRO-10% BU-80% D2.

No description available.

Dans cette thèse, un modèle numérique a été proposé pour simuler la combustion liquide des carburants conventionnels et non-conventionnels, en particulier le mélange de biodiesel B20. La matrice de test numérique constitue de quatre cas d’écoulement réactifs c.à.d. avec combustion et d’un cinquième avec injection liquide sans combustion (écoulement non-réactif). Les modèles sont calculés à l’aide du logiciel FLUENT™ v.14 en 3D et a l’état stationnaire. Les flammes de diffusion turbulentes sont modélisées en utilisant l’approche de flammelette laminaire stable, avec une fonction de densité de probabilité jointe (PDF). La Validation est effectuée en comparant les mesures expérimentales disponibles avec les résultats obtenus de la CFD. L’aérodynamique de la chambre de combustion, ainsi que les températures de parois extérieures sont captures avec un degré de précision satisfaisant. La validation des principaux produits de combustion, tels que : CO2, H2O et O2, montre des résultats satisfaisants pour tous les cas d'écoulement réactifs, mais certaines incohérences ont été relevées pour les émissions de CO. On pense que le banc d'essai (la géométrie de la chambre de combustion et son état de fonctionnement) n'est pas suffisamment adéquat pour la combustion de combustibles liquides. D’autre part, et d’un point de vue numérique, l’approche de flammelette laminaire stable a été trouvé raisonnablement hors mesure de saisir les effets profonds du non-équilibre chimique qui sont souvent associés au processus de lente formation d’un polluant, comme le CO. / In this thesis, a CFD model was proposed to simulate the liquid combustion of conventional and non-conventional biodiesel fuels, in particularly the B20 biodiesel blend. The numerical test matrix consists of four reacting flow cases, and one non-reacting liquid fuel injection case. The models are computed using FLUENT™ v.14 in a 3D steady-state fashion. The turbulent non-premixed diffusion flames are modeled using the steady laminar flamelet approach; with a joint presumed Probability density function (PDF) distribution. Validation is achieved by comparing available experimental measurements with the obtained CFD results. Combustor aerodynamics and the outer wall temperatures are captured with a satisfactory degree of accuracy. Validation of the main combustion products, such as: CO2, H2O, and O2, shows satisfactory results for all the reacting flow cases; however, some inconsistencies were found for the CO emissions. It is believed that the test rig (combustor geometry and operating condition) is not sufficiently adequate for burning liquid fuels. On the other hand, from a numerical combustion point of view, the steady laminar flamelet approach was found not reasonably able to capture the deep non-equilibrium effects associated with the slow formation process of a pollutant, such as CO.

Combustion monitoring in internal combustion engine or burners is a difficult task due to the harsh environment for any sensor, therefore optical diagnostics are very attractive for these types of application. Chemiluminescence measurement is one of the most common and most promising ways of implementing optical diagnostics in combustion monitoring applications because the measured signal, emitted naturally with combustion, has potential to be an indirect measure of combustion relevant parameters, such as the equivalence ratio and heat release rate. In hydrocarbon combustion, the most common chemiluminescence emitters are OH*, CH*, C₂* and CO₂*. This thesis focuses on the measurement of OH* and CH* chemiluminescence, whose sensitivities are affected by temperature, pressure, equivalence ratio and stretch rate. To measure OH* and CH* chemiluminescence, an existing constant volume combustion vessel has been refurbished, along with the sub-systems for fuel delivery, ignition, LabView control, data acquisition, and optical detection using a pair of photo-multiplier tubes (PMTs), interference filters and a series of apertures. Modelling accurately the optical setup is essential for the CH* and OH* chemiluminescence measurements in the combustion bomb. To achieve this goal, a narrow field of view system has been selected as it enables the elimination of photons scattered from the internal surfaces. A calibration of the PMTs converts the measurements into the absolute OH* and CH* chemiluminescence in terms of watt. Measurements from a combustion bomb are versatile and accurate since it determines the OH* and CH* chemiluminescence as a function of temperature and pressure from a single experiment. The calculation of the normalised OH* and CH* chemiluminescence (against mass burned rate) was based on a multi-zone combustion model and measured pressure record from the vessel. NIICS (Normalised Intensity Integrated Calculation System) has been created to fetch data from the multi-zone model, the optical model, and experimental measurements, to match them up by interpolation and to normalise the OH* and CH* chemiluminescence. NIICS also allows the user to select data uncorrupted by the noise and heat transfer. The chosen data (in this case, CH*/OH* chemiluminescence ratio) have been fitted using a multi-variate fitting and correlation analysis. This formulation can be used to indicate the local equivalence ratio from premixed methane / air and iso-octane / air flames over the local pressure range 0.5 – 20 bar, the unburned gas temperature range 450 – 600 K, and equivalence ratio range 0.8 – 1.1. The chemical-kinetic mechanisms of the absolute OH* and CH* chemiluminescence have been investigated by studying the influence of the equivalence ratio, unburned gas temperature, and local pressure. It should be pointed out that two confounding observations occur, i.e. a discontinuity in the chemiluminescence along the isentropes, and chemiluminescence continuing after the end of combustion. This led to the further spectroscopic analysis. This study concluded with spectroscopic measurements using an Ocean Optics spectrometer and a Princeton ICCD spectrometer. It was found that the broadband CO₂* is responsible for the two disconcerting observations. In addition, CH* chemiluminescence has been shown to be very faint from premixed laminar methane / air flames; hence the CH*/OH* formula in essence quantifies the CO₂*/OH* ratio as a function of pressure, temperature, and equivalence ratio. The ‘CH* chemiluminescence’ can characterise the background CO₂*, so as to provide a practical way to probe the feasibility of absolute OH* as an indicator of combustion relevant parameters in the future.

The Fluidised Bed Combustion (FBC) technology has been widely used internationally for power generation. This technology has good fuel flexibility and reduced S02 emissions with dry sorbent (Limestone or Dolomite) addition. South Africa has large reserves of coals that are difficult to combust in conventional pulverised fuel fired boilers. These reserves could be potential feedstocks for new build FBC boilers. The chemical composition of these coals is site specific and could have an impact on the combustion performance of the fuel. This necessitates the need for FBC coal tests in the presence of a sorbent. The objectives of this study were to investigate the changes in the production NO" SO" and the combustion efficiency of the three test coals under conditions of fluidised bed combustion, with the same sorbent. Tests with no sorbent were performed to evaluate the coals inherent calcium capabilities of capturing sulphur. Tests with varying ratios of sorbent were performed to evaluate the sorbent's capabilities for further levels of in-bed desulphurisation. The experimental equipment used in this investigation was the Eskom Fluidised Bed Test Facility (FBTF). This facility is a bubbling fluidised bed combustorlgasifier. The investigated bed temperature range was between 800 to 900°C, in intervals of 20°C. The operating pressure was 50kPa (gauge). The three coals were compared at CalS molar ratio of 1. Carbon in ash has shown to decrease with an increase in bed temperature for Coal A, Band C. The best performing coal in terms of least quantity of remaining carbon in ash was Coal A. The NO emissions increased for an increase in bed temperature for Coal A, Band C. The greatest NO emissions were recorded during Coal B tests. The N 20 emissions decreased with an increase in bed temperature for Coal A and B tests. Higher N 20 emissions were observed for Coal B than Coal A tests. In terms of S02 retention Coal C performed the best. The optimal operating bed temperature for S02 retention observed for the three coals was in the region of 800-860°C. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.

The main function of a pyroscrubber in petroleum coke calcining process is to oxidize the carbonaceous contents, including hydrocarbon volatiles, of the exhaust gas from the calcination kiln, so as to leave no more than small traces of unburned volatiles, solid carbon, ashes, or emissions (e.g. CO, NOx and SOx) in the flue gas finally discharged. To maximize the energy recovery and reduce pollutant emission from the pyroscrubber, 3-D computational models are developed using FLUENT to simulate the combustion and thermal-flow phenomena inside the pyroscrubber. The results show the 3-D behavior of the flow, the reaction inside the pyroscrubber, effect of different amounts of air injection with respect to combustion efficiency, energy output and NOx emission. A multistage burning strategy is introduced and studied and results show it successfully cuts emission without compromising energy output. A particle combustion model with the homogeneous gas combustion model is also developed and incorporated to investigate CO emission.

In this thesis, combustion processes in a laboratory-scale methane based low pressure rocket motor (LPRM) is studied experimentally and numerically. Experiments are conducted to measure flame temperatures and chamber temperature and pressure. Single reaction-four species reacting flow of gaseous methane and gaseous oxygen in the combustion chamber is also simulated numerically using a commercial CFD solver based on 2-D, steady-state, viscous, turbulent and compressible flow assumptions. LPRM geometry is simplified to several configurations, i.e. Channel and Combustion Chamber with Nozzle and FWD. Flow in a Bunsen burner is simulated inside Channel geometry in order to validate the reaction model. Grid independence study is also conducted for reacting as well as non-reacting flows. Numerical model is calibrated based on experimental results. Results of the computational model are found in a good agreement with the experimental data after calibrating specific heats of the products. Parametric study is conducted in order to investigate the effects of different mass flow rates and chamber pressures on flow and combustion characteristics of a LPRM to provide insight to future studies.