Theoretical And Computational Analysis Of Thermoacoustic Oscillations For Non-premixed Flames

Non-premixed combustion widely exists in industrial equipments and propulsionsystems. Through theoretical analysis and numerical simulation, supported by relevantexperimental data, the thermoacoustic coupling relationships in non-premixed combus-tion are investigated. For the combustion in gas turbines, there are two preconditions forthermoacoustic oscillation occurrence. First, there should be initial disturbances in theflow field which could induce the thermoacoustic oscillations. Direct combustion noiseplays a significant role in it. And second, the corresponding acoustic boundary condi-tions should be satisfied to couple the acoustic wave propagation process with unsteadycombustion. This research includes two major branches of thermoacoustic oscillations ofnon-premixed combustion: the direct combustion noise and indirect combustion noise.The generation mechanisms, main characteristics and their internal relationships are in-vestigated in diferent chapters.First of all, based on theoretical analysis, the low order model of combustion dynam-ics and one-dimensional distributed flame transfer functions are obtained. These resultsare based on conservation equations and don’t depend on the combustion type and com-bustion modelling. They are established in premixed combustion, non-premixed com-bustion and partially premixed combustion. To demonstrate the implementation of aboveanalytical models, an idealized two-dimensional flat non-premixed flame with three slotsis employed. Based on Green’s function method, the one-dimensional distributed flametransfer function is calculated and the generation and evolution of hot spots are investi-gated.And then, based on the direct numerical simulation results, the two-point correlationfunctions of the rate of change of fluctuating heat release rate are investigated. Afternormalized by characteristic quantities, the correlation functions can be represented ingeneral form and independent with local turbulence and chemical reactions. These resultscan simplify the calculation of sound pressure level of direct combustion noise.In chapter5, by using a modelling gas turbine type burner for research, the ther-moacoustic combustion oscillations are investigated by numerical simulation. Based onlinear acoustic analysis, the acoustic impendence boundary conditions of the combustor’supstream and downstream are obtained and the CFD calculation can be limited only in the combustion zone.Finally, the frequency response of thermoacoustic combustion oscillations on broad-band disturbances is investigated. By numerical simulation and system identification, thebroadband distributed flame transfer functions are obtained. The non-uniform of flowfield is considered by shape factors in the one-dimensional analysis model and the acous-tic boundary conditions are described by local acoustic impedances. And then, a newmethod to predict the eigenfrequencies and corresponding initial growth rates of ther-moacoustic combustion oscillations is developed. And it depicts the transitions frombroadband direct combustion noise to thermoacoustic combustion oscillations which onlyhave one or a couple of eigenfrequencies.