Effects of convection time on the high harmonic combustion instability in a partially premixed combustor

Abstract

The fuel composition effects of H2/CH4 syngas in a partially premixed model combustor (PP-MC) were examined for the unique phenomenon of combustion instability (CI) frequency/mode shifting (FMS), which is a transition of mode as well as frequency. The increase in the H2 composition of the fuel altered FMS from a longitudinal fundamental mode (≈250Hz) to a 7th harmonic mode (≈1750Hz). The cause and characteristics of this FMS were investigated using OH planar laser-induced fluorescence (OH-PLIF) measured at 10Hz, particle-image velocimetry (PIV), and the flame transfer function (FTF).The convection time (τconv) was assumed to be the key parameter of the FMS. Thus, tests were conducted to determine the air flow rate (V˙air) and equivalence ratio (φ) variation, which are vital parameters of the τconv in terms of the flame length and mixing time. The φ variation caused obvious changes in the flame length and instability frequency/mode, while the V˙air variation did not. The τconv was analyzed by calculating the global convection time (τconv_global) and the real convection time (τconv_real) from the length of the OH-PLIF-based unburned mixture length divided by the averaged mixture nozzle exit velocity. The τconv_real was calculated from the integral of the real velocity determined from PIV. Both calculations showed an inverse correlation between τconv and CI frequency, which particularly signifies that the FMS is controllable and a specific mode of CI can be generated by adjusting the τconv. The FTF was measured to determine the intrinsic characteristics of the flame. The FTF phase was normalized by the Strouhal number (St) and identified a direct relationship between FTF gain and τconv variation.In conclusion, the τconv is the main reason for the FMS. The importance of τconv in understanding the CI characteristics was confirmed in a PP-MC using high H2 fuels.