Development of a High-Pressure Variable Length Combustor Facility for Investigation of Thermoacoustic Mode Dynamics

A key challenge associated with studying both linear and nonlinear features of a thermoacoustic mode in realistic combustors is the presence of multiple, nonlinearly interacting modes. For example, two linearly unstable modes at a given operating condition may both grow and exhibit large amplitude limit cycle oscillations, but it is also possible that nonlinear interactions between them will cause one of the modes to be suppressed. As such, the absence of narrowband oscillations at a given natural frequency could mean that the mode is either linearly stable or that it is linearly unstable but suppressed through nonlinear interactions. This paper presents the development of a research facility to study these nonlinear interactions via a variable length exhaust. This allows for the tuning of the system's natural frequencies and thus tuning of the linear stability of various eigenmodes. The advantage of this approach is that it notionally fixes the fluid mechanic and combustion features of the problem and, thus, the critical time delays that influence the Rayleigh criterion. Additionally, these combustion time delays can be altered by operating on varied blends of natural gas and hydrogen fuel. This combination of fuel flexibility and length variation offers the opportunity to operate at conditions where two modes are simultaneously unstable. The periods of these modes are varied by changing the length of the combustor, inciting a switch in dominance and observing the resulting dynamics through high-speed acoustics and chemiluminescence. This paper presents details of the facility, as well as representative data that clearly show how the variable length capability allows new insights into nonlinear mode interactions.