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- Type of Document: M.Sc. Thesis
- Language: Farsi
- Document No: 40565 (45)
- University: Sharif University of Technology
- Department: Aerospace Engineering
- Advisor(s): Farshchi, Mohammad
- Abstract:
- Lean premixed flames bring both benefits and challenges to combustion systems. While they have low emission levels and high efficiencies, they are also very susceptible to combustion instabilities. Low swirl burners have been shown to be very capable in stabilizing lean premixed flames.
In this thesis, behavior of low swirl flames under conditions of variable equivalence ratio (0.6 – 0.8), mean flow velocity (3.5 – 5.5 m/s), and excitation frequency is investigated. In static stability diagram of the flame, a region is observed in which stability mode of the flame is dependent on the path that takes the flame to its operating point (hysteresis region). Increasing the recess distance of the burner increases the equivalence ratio at which the lean lifted flame flashbacks. To characterize the flame shape changes, three parameters of flame length, flame lift-off distance and flame angle are used. Increasing flame equivalence ratio and decreasing mean flow velocity causes steady (unexcited) flame to become flatter. Under acoustic excitation with low frequency (less than 400 – 500 Hz) flame shows considerable shape change. With decreasing equivalence ratio, flame shape change becomes more pronounced. The frequency of maximum flame shape change is observed to increase almost linearly with mean flow velocity such that maximum flame shape change in all cases is observed at St≈2.2. Maps of flame response magnitude show considerable fluctuations in flame shear layer. These fluctuations are attributed to vortex roll-up phenomenon.
- Keywords:
- Combustion Instability ; Low Swirl Burner ; Swirling Flames ; Lean Permixed Combustion ; Acoustic Excitation ; Flame Imaging
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