Characteristics And Control Study Of Combustion Induced Thermoacoustic Instability

Gas turbine will be used more and more widely in the power engineering field when the Integer Gasification Combined Cycle (IGCC) technology becomes mature enough, since gas turbine offers various advantages, such as compact structures, reliability, wide ranges of capacity, and so on. As the environmental demand becomes more stringent, the lean premixed combustion technology is adopted widely in gas turbines in order to meet the NOx emission requirements. However, this technology is proved to be notoriously susceptible to combustion induced thermoacoustic instability during the running of the devices, which leads to large-amplitude and low-frequency pressure oscillations. When the thermoacoustic instability becomes strong enough, it results in fatigue damages of the combustor, lower combustion efficiency, increased NOx emissions, and even accident flameout.This work focused on the characteristics as well as the controlling of the combustion induced thermoacoustic instability. Experimental studies and numerical modeling were employed to explore the influences of various system parameters on the features of the thermoacoustic instabilities in a Rijke-type combustor and a swirl combustor. These parameters included equivalence ratio, inlet velocity, swirl intensity, et al. Based on the above studies, turbulent coherence structures created by introducing a transverse jet in the swirl combustor were used to interrupt and control the combustion induced thermoacoustic instability. Besides, the exhaust gas components as well as the combustion efficiency before and after the controlling were measured and calculated, comparisons were made and discussed.First, features of thermoacoustic instability in a Rijke-type combustor were investigated. The pressure oscillations inside the Rijke-type combustor are time-constant signals when they reach the limit-cycle state, like the standard sinusoid waves. The start-oscillation process of thermoacoustic instability included three stages, named the initial excitation stage, the fast grow stage and the fine tuning stage. The main peak amplitude of the thermoacoustic instability inside the Rijke-type combustor increased with the equivalence ratio, and reached the maximum value when the equivalence ratio equals to 1.1, then decreased with the equivalence ratio. On the other hand, the main peak amplitude of the thermoacoustic instability increased sharply with the inlet velocity, showing a linear change law when uRijke < 0.1 m/s, yet the linear change laws were affected seriously by the equivalence ratio.Second, Computational Fluid dynamics (CFD) were employed to model the self-excited thermoacoustic instability in a Rijke tube. The modeled sound pressure level and resonant frequency is 156 dB and 294 Hz separately, which were consistent with the experimental data, i.e. 147 dB and 269 Hz. Moreover, when the cooling effect of the wall was taken into consideration, the simulated sound pressure level and the resonant frequency is 157 dB and 269 Hz separately. The critical temperature difference needed for the start oscillation between the heat source and the inlet gas was determined by the inlet boundary conditions. Numerical results showed that the critical temperature difference lied in the range of 240 K-290 K, and that the resonant frequency increased with the inlet gas temperature. In this work, the working medium had no impact on the velocity of the start oscillation, yet affected significantly the pressure amplitude and the resonant frequency, i.e. CH4 as the working medium excited oscillations with lowest pressure amplitude (159 dB) and highest resonant frequency (391 Hz), however, SO2 as the working medium excited oscillations with highest pressure amplitude (164 dB) and lowest resonant frequency (195 Hz). The inlet velocity had an important influence on the velocity of the start oscillation, showing that the velocity of the start oscillation increased with the inlet velocity.Third, detail features of the thermoacoustic instability inside the swirl combustor were explored. The largest pressure amplitude of the thermoacoustic instability occurred at a certain equivalence ratio in the lean combustion zone, indicating that the pressure amplitude decreased when the equivalence ratio is larger or smaller than the critical equivalence ratio. However, the combustion efficiency of the swirl combustor increased with the equivalence ratio in the range studied. On the other hand, the effective pressure amplitude and the resonant frequency increased with the inlet velocity. The swirl combustor excited a sound pressure level of 166.4 dB when the inlet velocity equals to 39.8 m/s and the equivalence ratio equals to 0.9. Besides, as the swirl intensity decreased, both of the main peak pressure amplitude and the effective pressure amplitude decreased sharply, and the combustion efficiency also decreased to a smaller value.Fourth, the controlling characteristics of the thermoacoustic instability inside the swirl combustor using Jet In Cross-Flow (JICF) were explored. The mass flow rate of air QAir is 1333.3 ml/s, the equivalence ratioφis 0.9, and the swirl intensity Sw is 0.77. Experimental results showed that the intensity of the thermoacoustic instability inside the swirl combustor was greatly suppressed by JICF with a small ratio the air flow rate XJICF.The effective pressure amplitude decreased from 1712 Pa to 185 Pa when XJICF= 6%, which resulted in a controlling effect of 89.2%. The resonant frequency before and after controlling is 233 Hz and 254 Hz separately, showing a slight increase. On the other hand, the combustion efficiency before the introduction of JICF is 81.68%, yet it increased to be 96.69%, showing an increase of 15.01% was achieved. Besides, JICF was found to be much more effective in controlling thermoacoustic instabilities than Jet Flame in Cross-Flow (JFCF). In order to obtain a controlling effect of 32.5%, the ratio of the fuel flow rate XJFCF should be larger than 50% when the JFCF method was adopted.Fifth, CFD method was employed to model the combustion process and the thermoacoustic features in a swirl combustor. The modeled gas components and the pollutant emissions were consistent with the experimental data. The obtained NOx concentration is 23.4 ppm, which is 3.92 ppm higher than that measured in the experiment. The numerical resonant frequency of thermoacoustic instability and the main peak amplitude are 273 Hz and 1191 Pa, separately, while the corresponding values measured during the experiment are 222 Hz and 1108 Pa, separately, indicating that the numerical results agreed well with the experimental data. On the other hand, the oscillating velocity was found to fluctuate with an amplitude of 4.25 m/s, which is 73% large than the average velocity 2.45 m/s. The phenomena revealed that the fluid inside the combustion chamber could be drived to flow upstream temporary.According to the above study, characteristics of combustion induced thermoacoustic instability in a Rijke-type combustor and a swirl combustor were investigated in detail, and an effective and feasible controlling method for the swirl combustor was proposed. All of these experimental and numerical data served as important reference in the design and running a gas turbine.