Difference in Thermo-Acoustic Instability Frequency Between Partially and Fully Premixed Flames

Abstract

The difference in thermo-acoustic instability frequency was analyzed between partially premixed (PP) and fully premixed (FP) flames in a methane-fueled gas turbine combustor. The instability frequencies of both PP and FP flames shifted to high frequencies as the equivalence ratio increased. However, the instability frequencies of two flames were different at some equivalence ratio conditions, despite the same air and fuel flow rates. The convection time was introduced to determine the reason for the difference, and values of two flames were calculated quantitatively using OH-planar laser-induced fluorescence imaging and particle image velocimetry. The PP flames had a longer than the FP flames, and incomplete premixing of fuel and air in PP flames was determined to be the cause of this result. A low-order network model was adopted to confirm that whether the difference in between PP and FP flames can cause different instability frequencies. The network model well-predicted that two flames featured different instability frequencies owing to the difference in despite the same equivalence ratio. In conclusion, the PP flames had longer than FP flames owing to incomplete premixing of fuel and air, and it consequently contributed to the difference in instability frequency.