An Experimental Study of the Effect of a Pilot Flame on Combustion Instabilities PDF Download

Author: Jihang Li
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Languages : en
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Book Description
Lean-premixed gas turbines, due to their superior emission performance, have been widely used in the industry. However, lean-premixed combustion is susceptible to combustion instability, which has become a major concern in the design and operation of lean-premixed gas turbines. Passive secondary flames, also known as pilot flames, are commonly used for control of combustion instability. However, the underlying mechanism whereby a pilot flame suppresses combustion instability is not fully understood. This limits the improvement of pilot systems.This dissertation presents an experimental study on the effect of a pilot flame on combustion instabilities in an atmospheric, laboratory-scale, single-nozzle, swirl-stabilized, lean-premixed combustor. The pilot flame is a central jet flame injected from the centerbody, which can operate in either the technically-premixed or the fully-premixed modes, depending on the types of pilot injectors. Piezoelectric sensors are utilized to measure the pressure fluctuation and the velocity fluctuation. High speed CH* chemiluminescence techniques are used to measure the dynamics of the flame. The instability characteristics of the technically-premixed unpiloted flame are measured at varying combustor length. Four distinct instability modes with different frequencies are observed. A one-dimensional simulation is conducted to calculate the natural frequencies and mode shapes of the instabilities. The effect of varying the percent pilot of a technically-premixed pilot flame on the technically-premixed combustion instabilities in different modes is studied. Instability maps to the percent pilot and the overall equivalence ratio are presented and discussed. The instability boundaries in each instability map, which separates the unstable regions from the stable regions, are discussed in detail by analyzing the high-speed images and Rayleigh index images. The results show that the pilot flame affects the main flame dynamics primarily through enhancing flame attachment and flame oscillation in the inner shear layer. The effect of independently varying the pilot air, pilot fuel and pilot mixture flow rates on the technically-premixed combustion instabilities are studied by utilizing a modified pilot injector. The results show that the effect of the pilot flame on the combustion instability is primarily determined by the equivalence ratio, but not the mixture flow rate of the pilot flame. The results support the statement that pilot flames influence the main flame dynamics by heat recirculation and demonstrate that the effect of the pilot flame is determined by its ability to change the time-averaged recirculation zone temperature. The structures of the pilot flame are presented and discussed.Fully-premixed flame transfer functions under the influence of a premixed pilot flame are investigated. The flame transfer functions show distinct behaviors at low frequencies and high frequencies. At low frequencies, the pilot flame has a weak effect on the FTF gain and phase, while at high frequencies, increasing the percent pilot reduces the FTF gain and shifts the FTF phase. High-speed chemiluminescence images show the pilot flame enhances the fluctuation near the base of the flame, which enhances the destructive interference within the inner shear layer, reduces the FTF gain and shifts the FTF phase at high frequencies. By separating the flame transfer function into different regions, it was found that a pilot flame only influences the inner shear layer, but not the near-wall region and the outer recirculation zone.