Fundamental studies in hydrogen-rich combustion : instability mechanisms and dynamic mode selection

Abstract

Hydrogen-rich alternative fuels are likely to play a significant role in future power generation systems. The emergence of the integrated gasification combined cycle (IGCC) as one of the favored technologies for incorporating carbon capture into coal-based power plants increases the need for gas turbine combustors which can operate on a range of fuels, particularly syngas, a hydrogen-rich fuel produced by coal gasification. Lean premixed combustion, the preferred high-efficiency, low-emissions operating mode in these combustors, is susceptible to strong instabilities even in ordinary fuels. Because hydrogen-rich fuels have combustion properties which depend strongly on composition, avoiding the dynamics that energize combustion instability across all operating conditions is a significant challenge. In order to explore the effect of fuel composition on combustion dynamics, a series of experiments were carried out in two optically-accessible laboratory-scale combustors: a planar backward-facing step combustor and an axisymmetric swirlstabilized combustor. Fuels consisting of carbon monoxide and hydrogen, or propane and hydrogen were tested over a range of equivalence ratios and at various inlet temperatures. Dynamic pressure and chemiluminescence measurements were taken for each case. High-speed video and stereographic particle imaging velocimetry were used to explore the dynamic interactions between the flame and the flow field of the combustor. Stable, quasi-stable, and unstable operating modes were identified in each combustor, with each mode characterized by a distinct dynamic flame shape and acoustic response which is dependent on the composition of the reactants and the inlet temperature. In both combustors, the quasi-stable and unstable modes are associated with acoustically driven flame-vortex interactions in the combustion-anchoring region. In the planar combustor, the flame is convoluted around a large wake vortex, which is periodically shed from the step. In the swirl-stabilized combustor, the flame shape is controlled by the dynamics of the inner recirculation zone formed as a result of vortex breakdown. In both cases, the unstable mode is associated with velocity oscillation amplitudes that exceed the mean flow velocity. The apparent similarity between the response curves and flame dynamics in the two combustors indicate that the intrinsic local dynamics--instead of global acoustics--govern the flame response. Analysis shows that for each combustor, the pressure response curves across a range of operating conditions can be collapsed onto a single curve by introducing an appropriate similarity parameter that captures the flame response to the vortex. Computations are performed for stretched flames in hydrogen-rich fuels and the results are used to explain the observed similarity and to define the form of the similarity parameter. This similarity parameter works equally well for both experiments across fuel compositions and different inlet conditions, demonstrating that it fundamentally embodies the reciprocity between the flow and the combustion process that drives the instability. A linear model of the combustor's acoustics shows that the onset of combustion instability at a particular frequency can be related to a time delay between the velocity and the exothermic response of the flame that is inversely proportional to the local burning velocity. This analysis captures the impact of the fuel composition and operating temperature on the mode selection through an appropriately-weighted strained flame consumption speed, further emphasizing the influence of local transport-chemistry interactions on the system response. This new result confirms the role of turbulent combustion dynamics in driving thermoacoustic instabilities.