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

The fuel composition effects of H-2/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 H-2 composition of the fuel altered FMS from a longitudinal fundamental mode (approximate to 250Hz) to a 7th harmonic mode (approximate to 1,750Hz). 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 (tau(conv)) 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 (phi) variation, which are vital parameters of the tau(conv) in terms of the flame length and mixing time. The phi variation caused obvious changes in the flame length and instability frequency/mode, while the V-air variation did not. The tau(conv) was analyzed by calculating the global convection time (tau(conv)_(global)) and the real convection time (tau(conv)_(real)) from the length of the OH-PLIF-based unburned mixture length divided by the averaged mixture nozzle exit velocity. The tau(conv_real) was calculated from the integral of the real velocity determined from PIV. Both calculations showed an inverse correlation between tau(conv) and CI frequency, which particularly signifies that the FMS is controllable and a specific mode of CI can be generated by adjusting the tau(conv). 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 tau(conv) variation. In conclusion, the tau(conv) is the main reason for the FMS. The importance of tau(conv) in understanding the CI characteristics was confirmed in a PP-MC using high H2 fuels. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.