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- Type of Document: Ph.D. Dissertation
- Language: Farsi
- Document No: 41725 (45)
- University: Sharif University of Technology
- Department: Aerospace Engineering
- Advisor(s): Farshchi, Mohammad
- Abstract:
- This thesis is a complementary experimental and theoretical investigation on stability of premixed flames and a study of combustion instability and combustion dynamics in a swirl-stabilized combustor, aiming to understand the fundamental mechanisms responsible for combustion oscillations in gas turbine combustors. Theoretical investigations on acoustic modeling of a simple combustor and a study on kinematic response of premixed flames to flow perturbations have been discussed in the first part of this work. In another part of thesis, experimental studies on the response of premixed flames to acoustic perturbations have been performed. In addition, experimental investigations on combustion dynamics and NOx emission of a swirl stabilized combustor with secondary fuel injection, for control and suppression of combustion instabilities, have been fulfilled in the last part of thesis. A simple model was used to study the acoustic characteristics of a combustor in presence of a heat release source. It is worth to note that the study on the response of premixed flames to flow perturbation is a major part of investigations on stability of premixed flames which is the main objective of the present thesis. To study the transfer functions of flame response to flow perturbations the response of premixed flames to flow oscillations and acoustic excitation was investigated considering the direct effects of the flame speed and heat of reaction perturbations on the flame’s heat release as well as the indirect effect of flame speed on the flame area disturbances which can also lead to fluctuations of heat release. Due to the importance of studies on swirl-stabilized premixed flames and their related combustion instabilities in industrial combustors the effect of secondary fuel injection on behavior of confined premixed swirl-stabilized flames was experimentally studied. This investigation was performed by considering the influence of flow rate, diameter and offset of secondary fuel injection on combustor noise level, pressure fluctuation and NOx emission of the combustor for overall equivalence ratio () of 0.7 ~ 0.9 and flow rate of secondary fuel (Q sec) from 0.6 to 4.2 L/min. The experimental results on the response of laminar premixed flames (such as M-shaped flames and V-shaped flames) to external acoustic perturbations indicated that M-shaped flames have a narrower stability range compared to that of V-flames. This would limit the range of equivalence ratio for acoustic excitation experiments. It is important to note that V-flames attach to an anchoring rod at the centre of the burner, whereas M-flames attach to the anchoring rod as well as the burner rim. The experimental and theoretical results suggested that the phase response of V-flame increases quasi-linearly with excitation frequency indicating that the fluctuations require a certain time to reach the flame surface. Regarding the experimental investigations on combustion dynamics and NOx emission of swirl stabilized combustor it should be noted that secondary fuel injection with offset leads to reduction of NOx emission for the whole range of equivalence ratios compared with those of the injection without offset. Low emission index by flame lifting off from the swirl injector which was also reported in another related study, is more pronounced with injection offset by increase of the secondary fuel rate to 4.2 L/min in near stoichiometric operating condition (). The undesirable increase in combustion noise and fluctuating behavior of the flame for the case of injection with offset indicates the flame structure sensitivity to trajectory of the secondary fuel. Spectral analysis of pressure fluctuations revealed that larger amount of secondary fuel by the injectors with injection offset induce a lot of new modes in the wide range of frequency. The results obtained in this part of study suggested that the emission index and the dominant mode of instability are deeply related to flame shapes and local flame structure.
- Keywords:
- Flame Structure ; V-Flame ; Equivalence Ratio Perturbation ; Secondary Fuel Injection ; Pressure Fluctuation
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محتواي پايان نامه
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