Thesis (Ph.D.) - University of Queensland, 2005. / Includes bibliography.

Laminar flame speed of a novel liquid bio-fuel has been determined experimentally using a closed spherical combustion vessel of 29 L equipped with two pairs of fused silica windows for optical access at atmospheric pressure and elevated temperature conditions. Schlieren technique was used to visualize and record the temporal evolution of the outwardly spherical flame front, and an in-house developed Matlab code was employed to process the flame front images and calculate its area by applying several image processing techniques. The test conditions consisted of varying the fuel-air mixture equivalence ratio at atmospheric standard pressure and different initial temperatures. Validation of the present method was achieved by measuring and comparing the flame speed of methane/air and n-heptane/air mixture with their published counterparts. Experimental results revealed comparable laminar flame speed of the novel liquid biofuel (1, 3- dimethoxyoctane) to heavy liquid hydrocarbons such as n-heptane and isooctane, especially at stoichiometric and fuel rich conditions. Additionally, the flammability limits of this novel fuel showed similarities with those of gaseous hydrocarbons fuels (e.g. methane, ethane) but higher than those of liquid hydrocarbons (e.g. diesel, gasoline). / February 2016

En physique des explosifs, la postcombustion désigne la phase de combustion qui intervient après la fin de la détonation lorsque l’explosif considéré est initialement déficient en oxydant. Les produits de détonation, qui apparaissent sous la forme d’une boule de feu, peuvent alors à leur tour être oxydés, ce qui permet de libérer une quantité supplémentaire d’énergie dans l’écoulement et d’augmenter le souffle. Ce phénomène complexe est piloté par l’interaction entre des ondes de chocs, une zone de mélange turbulente créée par des instabilités hydrodynamiques de type Rayleigh-Taylor et Richtmyer-Meshkov, et une flamme de diffusion. Compte tenu de son effet significatif sur la performance d’une explosif, une bonne compréhension de la postcombustion est nécessaire afin de pouvoir la modéliser et déterminer avec précision les effets d’une charge donnée. A cette fin, des travaux, à la fois numériques et expérimentaux, ont été menés afin de mieux comprendre le processus de mélange intervenant dans les boules de feu puis le phénomène dans son ensemble. Afin de contourner les difficultés liées à la caractérisation des produits de détonation, cette étude s’est concentrée sur l’explosion de capacités sphériques sous pression qui permet de produire un écoulement similaire à celui provoqué par une détonation sphérique. Les résultats obtenus sont semblables à ceux de la littérature sur la postcombustion des explosifs et apportent un éclairage nouveau sur l’influence de certains paramètres tels que la masse de l’explosif ou les propriétés des perturbations initiant les instabilités.

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Made available in DSpace on 2014-06-11T19:35:41Z (GMT). No. of bitstreams: 0 Previous issue date: 2005-11Bitstream added on 2014-06-13T20:27:54Z : No. of bitstreams: 1 botura_ca_dr_guara.pdf: 4170323 bytes, checksum: 21316bd955d294e528bc1edcbfa2bd8b (MD5) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Universidade Estadual Paulista (UNESP) / Este trabalho tem a finalidade de investigar a incineração de resíduos sólidos na presença de ondas acústicas para incrementar o processo de combustão. Para tanto foi projetado e construído um forno rotativo para incineração de resíduo sólido industrial. Um combustor do tipo sintonizável foi desenvolvido e acoplado ao forno rotativo para indução de oscilações acústicas, além de outros acessórios utilizados no processo de combustão (alimentador de resíduos, ejetor, sonda para análise de gases). Os resultados obtidos mostram que a presença do campo acústico melhora o processo de combustão. Estes resultados foram avaliados principalmente através da análise de gases de combustão, permitindo uma redução da quantidade de combustível utilizado. / This work has the objective of investigating the incineration of solid wastes with acoustics oscillations to improve the combustion process. A rotary kiln was designed and built for the research. A tunable combustor was developed and connected to the rotary kiln for induction of the acoustics oscillations. Accessories were also built and used in the combustion process (feeder of waste, air ejector, probe for gas analysis). The results show that the presence of the acoustic field improves the combustion process. These results had been evaluated mainly through the analysis of gas combustion, allowing a reduction of the amount of used fuel.

An alarming number of compression ignition (CI) engines in the transport, mining and heavy engineering environments have been failing due to combustion irregularities within their combustion chambers. It has been found that diesel fuels containing contaminants or diesel fuels with poor lubricity characteristics lead to stickiness of diesel injector needles, which badly affects injector spray patterns resulting in the phenomenon of “cold combustion”. This study has been undertaken to develop a technique for detecting and preventing the damage resulting from this deviation in the combustion of a diesel engine. The technique has been formulated with a view to being as non intrusive as possible, so as not to require major modification of an existing test engine to accommodate the technique. The practice of monitoring individual cylinder exhaust gas temperatures (EGTs) proved to be an effective way of determining whether potentially destructive combustion abnormalities were taking place within the diesel engine. By recording these temperatures at certain stages during the engine’s operation, taking their average, and comparing each one to this average it is also possible to isolate the location of the combustion abnormality. This method proved to be most effective at full loads and maximum fuel delivery where combustion temperatures are highest and the effects of poor combustion are most noticeable and potentially damaging. The second goal was to develop a small, portable electronic device that makes use of the monitoring technique developed and provides a visual and audible alarm to notify a vehicle operator or technician of a combustion fault within a diesel engine. A Combustion Monitoring System (CMS) prototype was developed and tested on a small naturally aspirated engine at the University of Pretoria’s engine testing facilities. The prototype met its primary goal of detecting simulated combustion abnormalities under a variety of test conditions. It is envisaged that the monitoring techniques applied in developing the CMS unit may eventually be incorporated into the powerful processing abilities of the modern diesel Engine Control Unit (ECU). In its current form the CMS prototype is a useful tool in sensing combustion related malfunctions within a diesel engine and preventing damage from occurring. / Dissertation (MEng)--University of Pretoria, 2009. / Mechanical and Aeronautical Engineering / unrestricted

Modern vehicles have to ful ll new CO2 emission and additionally customer comfort requirements to stay competitive. A major impact to the fuel consumption of an internal combustion engine (ICE) has the starting period. An ICE equipped with a preconditioning system which heats up the ICE much faster than a common ICE. This procedure of preconditioning is called peak heating. The main benet of preconditioning of an ICE is less fuel consumption. Recently the only way to obtain a fast heating up of an ICE is the injection of a higher amount of fuel during the starting period. This heat up procedure can be changed if a heat reservoir is available to the ICE during the starting period. In this case the additional injection of fuel is redundant and therefore the consumption during the starting period can be reduced. The major advantages of this strategy are achieved in cold ambient conditions. During this project di erent preconditioning strategies and di erent points of interaction in the coolant circuit of an ICE have been investigated. The preconditioning concepts have been evaluated according to their heating up performance and their implementation into the engine compartment. The results obtained by this project highlight that a system layout which enables a preheating of the cylinder block by a heat reservoir located in a bypass-line to the heater core is the most e ective point of interaction. The best results have been achieved with a coolant ow of 10 l/min at a temperature of 90 C. Furthermore, this project points out that the implementation of a preconditioning system into the oil cooler will achieve similar results. This strategy of preconditioning the engine oil reduces the internal frictions of the ICE which leads to a decreasing consumption. This solution is much more energy e cient and technically easier to implement into a modern vehicle with its limited space. An additional side e ect of the preconditioning of the oil is a longer service life of the ICE.

This work concerns the prediction of potentially damaging thermoacoustic oscillations in gas turbine combustion systems by computational means. A framework is laid out to predict numerically the frequency and stability of thermoacoustic oscillations, with focus on the high frequency screech instability of afterburners. A hybrid numerical method is used that includes separate calculations of the mean flow and of the perturbed field due to the acoustic oscillations. This modularity supports the choice of models that are the most appropriate for combustion and for acoustic wave propagation, which are the processes that make up the feedback mechanism that can lead to the establishment of an instability. This gives flexibility, improved accuracy and more insight into the physics of the thermoacoustic system at a potentially reduced computational cost. The mechanism leading to screech involves the formation of vortices induced by acoustic transverse modes at the afterburner flameholder. These vortices trap fresh reactants that burn after a certain time delay, therefore feeding energy into the oscillation. Within a linear approximation, the effect of small amplitude acoustic fluctuations on the flame is studied by perturbing harmonically the transverse velocity at the flameholder lip over a range of frequencies using forced combustion CFD calculations. The response in heat release rate, which is a thermoacoustic source of sound, is represented by a flame transfer function (FTF). It is argued that for the investigation of screech oscillations, this FTF must be multi-dimensional because of the transverse nature of the acoustic oscillation. For fully premixed flames, the main contributor to heat release rate fluctuations is the variation in flame surface area. This information is used to develop a novel flame model that represents the multi-dimensional, frequency dependent response of the flame to velocity perturbations. Compared to FTFs, which require computationally expensive forced calculations, this model has the advantage of providing the frequency dependent flame response as part of the acoustic calculation. After verification and validation of each of the tools used for the acoustic and combustion simulations, this flame model is used in the analysis of a simplified afterburner, where a high frequency, radial and longitudinal resonant mode was computed. Convective modes, which are important in the prediction of the frequency of thermoacoustic oscillations are predicted as a result of the interaction between the acoustic wave and the flame.

Due to concerns about pollutant emission combustion systems are increasingly being designed to operate in a lean premixed mode. However, the reduction in emissions offered by lean premixed combustion can be offset by its susceptibility to instabilities and ignition and extinction problems. These instabilities, caused by the coupling of unsteady heat release and pressure fluctuations can cause significant damage to combustion devices. One method of avoiding these problems whilst still operating a globally lean system is to employ a stratified premixed mode where areas of richer mixture are used to enhance the stability of the flame. In this thesis a computational modelling methodology for the simulation of stratified premixed flames is developed. Firstly, several sub-models for the dissipation rate of a reacting scalar are evaluated by the simulation of two laboratory scale flames, a turbulent stratified V-flame and a dump combustor fed by two streams of different mixture strength. This work highlights the importance of this quantity and its influence on the simulation results. Any model for stratified combustion requires at least two variables to describe the thermochemical state of the gas: one to represent the mixing field and another to capture the progress of reaction. In turbulent stratified flames the joint probability density function (pdf) of these variables can be used to recover the mean reaction rates. A new formulation for this pdf based on copula methods is presented and evaluated alongside two alternative forms. The new method gives improved results in the simulation of the two test cases above. As it is likely that practical stratified combustion devices will have some unsteadiness to the flow the final part of this work applies the modelling methodology to an unsteady test case. The influence of the unsteady velocity forcing on the pollutant emissions is investigated. Finally the methodology is used to simulate a developmental, liquid fuelled, lean burn aero-engine combustor. Here the model gives reasonable predictions of the measured pollutant emissions for a relatively small computational cost. As such it is hoped that the modelling methodology presented can be useful in the iterative industrial design process of stratified combustion systems.

The main topic of this thesis concerns the development and application of laser diagnostic techniques for accurate temperature measurements and for the determination of flamefront properties in premixed flames that can serve as input data for computational fluid dynamical (CFD) models in technical combustion. The work comprises of a number of related studies, to address problems of relevance in the field of combustion research. The first part of this work involves the development and testing of an improved method for the computation of flamefront curvature in lean premixed turbulent flames. Measurements of spatially resolved heat release rate along the flamefront were then compared with the curvature data and it could be shown that a significant correlation exists between local rate of heat release and flamefront curvature. The results here agree with predictions from CFD models and improve on previous experimental attempts to find a correlation between curvature and heat release. In the second part of this work, the focus was shifted towards the development and application of improved thermometry techniques. One study was on the improvement and application of a coherent anti-Stokes Raman spectroscopy (CARS) setup to an acoustically-forced turbulent lean premixed flame stabilised on a burner, whose design was modelled to mimic phenomena of relevance in industrial combustors. In a related previous study reported in the literature two-line OH planar laser induced fluorescence had been applied to this flame and it was suspected that the results were inaccurate. Using CARS, these inaccuracies could be verified, amounting to discrepancies in temperature of up to 47% compared to the true temperatures. A major effort towards the end of this project was focused on the improvement of traditional thermometry techniques, in order to make them more accurate, faster, and spatially resolved. A technique based on indium two-line atomic fluorescence (TLAF) thermometry was developed and applied, which employed a novel extended cavity diode laser design, and it was shown for the first time that temperature measurements with high accuracy and precision could be performed in low pressure sooting flames without recourse to calibration. Both the high precision and accuracy of the technique allowed for the deduction that the temperature in the flames studied here is relatively insensitive to changes in pressure in stark contrast to the soot volume fraction. Finally, it is shown for the first time that low power diode lasers can be used in combination with indium TLAF to measure spatially and temporally highly resolved temperatures in a quasi-continuous fashion. We demonstrated such measurements at effective rates of 3.5 kHz in a steady laminar test flame yielding an unprecedented precision of 1.5 % at ~2000 K at this measurement rate.