The influence of hydrogen on the stability of a perfectly premixed combustor

Abstract

In this paper we investigate the effects of increasing the amount of hydrogen in the fuel mixture on the stability of a perfectly premixed combustor. Experiments are carried out on a single flame burner where the thermal power and equivalence ratio are kept constant while the amount of hydrogen is progressively increased from pure methane towards higher fractions of CH4−H2 blends. As hydrogen content is increased, the flame first experiences a series of topological changes until the system becomes unstable and a limit cycle is established. The changes experienced by the flame occur due to the increased burning speed of the mixture which shortens the flame. The flame changes from a lifted flame attached to the combustor wall, to a bluff body anchored flame also attached to the wall, to a bluff body anchored flame detached from the wall. For some CH4−H2 mixture fractions, there exists a bi-stable region where the last two flame shapes can be observed at the same operating conditions. As for the observed limit cycle, first a low amplitude limit cycle is established and a further increase in hydrogen content leads to a much higher amplitude one.The linear stability limit is predicted using a low-order network model where the flame response is modeled by a scaled distributed time lag model which provides good agreement with the experimental observations. The flame model only requires a measurement of the flame length and bulk velocities which significantly simplify the analysis when considering operating conditions where the flame transfer function has not been measured. In the region where the flame is bi-stable, the linear stability analysis shows that the flame attached to the combustor wall becomes unstable at lower hydrogen content when compared to the flame detached from the combustor wall. Therefore, from a linear stability perspective and in line with experimental observations, the flame changes shape in order to keep the system stable. Furthermore, a nonlinear model using the flame describing function approach is also implemented to analyze the transition between the two limit-cycles. The analysis shows that the system has a low amplitude stable limit cycle and a neighbouring unstable limit cycle with a slightly higher amplitude. This shows that the system is triggering until full saturation occurs at the much higher amplitude limit cycle, consistent with the experimental observations. The analysis presented in this paper demonstrates that the effect of premixed hydrogen on thermoacoustic instabilities can be predicted using well established methods.