Aibing Fang

Experimental Investigation of Thermoacoustic Oscillations in Syngas Premixed Multi-Swirler Model Combustors

This paper presents the experimental results of the thermoacoustic oscillations in several premixed syngas multi-swirler model combustors. Multi-swirler lean premixed combustion technology has been successfully applied to achieve an anticipative stability, lower noise and high efficiency in industrial gas turbines that burn natural gas or distillated oil. However, some critical operational issues, including combustion oscillations, flashback, blowout and autoignition, should be considered and balanced when burning the coal-derived syngas mainly composed of H2 and CO. In this paper several multi-swirler combustors are tested on an atmospheric-pressure downscaled test rig. Each multi-swirler combustor includes several elemental swirling nozzles with the equal Swirl Number and different array of port configurations. The dynamic pressure, dynamic heat release and critical flashback equivalence ratio are tested in these model combustors burning several kinds of simulated syngases with a similar low heat value. Firstly, the critical equivalence ratios of flashback are shown and compared with those in single-swirler combustors. Secondly, the paper presents the analysis of the temporal and spectral features of dynamic pressure oscillations using many data-processing methods. Thirdly, we describe the bifurcation and retardation phenomenon when the combustion transforms between stable and unstable operations. We also discuss how the equivalence ratio, the fuel composition and the combustor inlet velocity play important roles in determining the amplitudes, the frequencies, the bifurcation and retardation of the thermoacoustic oscillations. Finally, we use a wavelet transformation with a higher resolution in time domain than that with a PSD estimation by the AR model. The processes of amplitude “jump” and flashback are analyzed in details. The results in this paper could improve the current understanding of the nonlinear self-excited and combustion driven thermoacoustic oscillations in gas turbines and give us some references to the development of lean premixed syngas turbines for coal-based IGCC and co-generation systems.Copyright

Dynamic and Flashback Characteristics of the Syngas Premixed Swirling Combustors

Flashback and combustion thermoacoustic oscillation are two of the major problems in the lean premixed combustors of gas turbine, especially for hydrogen enriched syngas fuels. This paper introduced the experimental results regarding the characteristics research of thermoacoustic oscillation, dynamic heat release and flashback based on a premixed combustion experimental system. The critical flashback equivalence ratio and three dynamic parameters were tested in four types of combustor using eleven kinds of simulated syngas which have the same LHV. Partial flashback and full flashback are observed. With the increase of the hydrogen concentration, the critical equivalence ratio is decreasing and full flashback occurs more easily. The change of nozzle’s geometry structure has a consequent effect on the flashback characteristics. Flashback occurs more easily when the thermoacoustic oscillation is greater, and the corresponding critical equivalence ratio is smaller. Both flashback status and normal status are relative stable combustion within a certain range of equivalence ratio. As the generation and disappearance of the thermoacoustic oscillation and its amplitude-frequency characteristics somehow could be in some randomicity, the critical equivalence ratio is correspondingly not a constant. The dynamic chemiluminescence intensity which indicates the dynamic heat release has a good corresponding relationship in terms of amplitude and frequency characteristics with the dynamic pressure which indicates the thermoacoustic oscillation. Through power spectral density analysis, dynamic pressure and chemiluminescence intensity signals both reflect the dynamic characteristics of the combustor and can be applied in the research of the combustion instability, including the thermoacoustic oscillation characteristics, fluctuation of the chemical heat release rate and flashback characteristics and so on. The results and methods could give some reference to the development of hydrogen enriched gas turbines for future IGCC and poly-generation systems.Copyright

Experimental and Numerical Investigation of Non-Equilibrium Plasma Assisted Combustion of Low BTU Fuels

This paper mainly focuses on reliable ignition and flame stabilization of low-BTU gas fuels with non-equilibrium air plasma assist. The results of experimental and numerical analyses indicated that the flame propagation speed was increased while the ignition time was decreased by the effect of non-equilibrium air plasma. Active radicals observed in air plasma made great efforts to the generation of OH radical which can accelerate the fuel oxidation process greatly.Copyright

Non-Equilibrium Plasma Assisted Combustion of Low BTU Fuels

This paper presents preliminary experimental results of non-equilibrium plasma assisted combustion of low BTU fuels. The air flow and the fuel flow are separated by two co-axial tubes in which the air flow is excited by dielectric barrier discharge (DBD) plasma. A non-premixed simple jet flame is observed immediately when the DBD plasma-excited air flow mixed with the fuel flow. The measurements of the flame indicate that addition of a very small amount of energy in the form of DBD plasma can provide self-ignition in harsher conditions in contrast to usual spark ignition, and significantly improve the flame stability and combustion efficiency. The results also show that the effect of DBD plasma on the flame is most efficient in combustion at low equivalence ratios, and the chemical reactions can even occur at the equivalence ratios well below the lean flammability limit.Copyright

Experimental Test on a Syngas Model Combustor With Flameless Technology

The research of combustor for gas turbine using flameless combustion technology is in the stage of laboratory. This paper designs a flameless model combustor using syngas, which has similar flow structure with reverse-flow can-type combustors and therefore could be used in typical small gas turbine system. In this combustor, either the premixed or non-premixed mode can be selected, meanwhile the difference is discussed. CFD methods are used to design the flow field and emissions of a combustor with detailed mechanisms. How the distance between the nozzle and axis influences the recirculation zone is discussed and optimized in the Isight platform. In the experimental test, the emissions and dynamic pressure characteristics are the main focus. The results demonstrate that flameless combustion technology has potential to be applied in gas turbine with syngas for the aim of Ultra-low pollutant emission combustion, which can restrict emission in a low level meanwhile maintain combustion stability. Flameless combustion is a rising ultra-low-pollution combustion technology. However, in this combustor when pilot flame existed, in some conditions the thermoacoustic oscillation was observed. The CFD results illuminates that this combustor ensures the recirculation in the primary zone, meanwhile the position of the nozzle and injection’s velocity have significant effect to the flow field.Copyright