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No description available.

No description available.

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.

This thesis describes an experimental investigation of the flame structure close to the extinction and the blow-off events of non-premixed and spray flames stabilized on an axisymmetric bluff body in a confined swirl configuration. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The first part of the thesis describes several velocity measurements in non-reacting and reacting flows. The main usefulness of this data is to provide the aerodynamic flow pattern and some discussion on the velocity field and the related recirculation zones. The velocity and turbulence information obtained are particularly useful for providing data, which is crucial for validation of computational models. The second part describes an experimental investigation of non-premixed stable flames very close to the blow-off condition. The measurements included visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, which allowed a quantification of the average duration of the blow-off transient. OH-PLIF images at 5 kHz for flames far from and close to extinction showed that the non-premixed flame intermittently lifts-off the bluff body, with increasing probability as the fuel velocity increases. The flame sheet shows evidence of localised extinctions, which are more pronounced as approaching blow-off. The measurements include blow-off limits and their attempted correlation. It was found that a correlation based on a Damkohler number does a reasonable job at collapsing the dataset. The final part examines the blow-off behaviour of swirling spray flames for two different fuels: n-heptane and n-decane. The measurements include blow-off limits and their att~mpted correlation, visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, and the quantification of the average duration of the blow-off transient. It was found that the average duration of the blow-off event is in order of the tens of ms for both spray flames (10-16 ms). The blow-off event is therefore a relatively slow process for the spray ~ames using n-heptane and decane fuels. This suggests that control measures, such as fast fuel injection, coupled with appropriate detection, such as with chemiluminescence monitoring, may have a reasonable chance of success in keeping the flame alight very close to the blow-off limit. These results, together with those obtained for the non-premixed gaseous case form a wide body of experimental data available for the validation of turbulent flame models. The quantification of some properties during the blow-off transient can assist studies of extinction based on large-eddy simulation that have a promise of capturing combustion transients.

No description available.

No description available.

No description available.

The successful design of a long-lived and efficient gas turbine engine requires a good knowledge of the thermal and aerodynamic performances of the components of the turbine. Of particular importance, is the heat transfer rate from the hot gases to the cooled turbine blades, since this limits the maximum turbine entry temperatures which can be obtained. Much gas turbine research is concentrated on experimental modelling and measurements to assist in the development of improved theoretical prediction techniques. The difficulties of instrumenting fully rotational rigs, which are necessary for a full understanding of the complex three dimensional flow in the turbine, have, however, to a large extent, limited most experimental research to stationary facilities. A technique is described which will allow heat transfer rate measurements to be made on fully rotating test facilities using mutlilayered model turbine blades comprising an electrical insulator on a metal base. An accurate and computationally efficient method for determining the surface heat flux to a multi-layered model turbine blade is developed theoretically, together with a method for calibrating the thermal properties of the multi-layered system. This method allows the existing successful heat flux measurement technique, which utilises electronic analogue circuitry in conjunction with thin film surface thermometers on a model made from a thermal insulator, to be extended for application to multi-layered models. The production of test models by the application of a vitreous enamel (as an electrical insulator), to a mild steel, is identified as the most suitable coating technique for experimental application. Radiant and wind tunnel testing of multi-layered cylindrical models are described, which confirm that the method is both practical and accurate.

Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (leaves 170-180).