Heat-release actuation for control of mixture-inhomogeneity-driven combustion instability

A low-order heat-release model, which accounts for the impact of flame surface area and the equivalence ratio oscillations. was used in conjunction with system acoustics to design a hierarchy of control strategies designed to mitigate combustion instability. The model assumes premixed combustion which burns a mixture with a temporally dependent equivalence ratio at high Damkohler number and low turbulence intensity (i.e,, a wrinkled laminar flame). When the acoustic field is of the Helmholtz-resonator type, the instability is dominated by the ratio of the convective time lag between the primary fuel injection and die burning zone and the time constant of the resonant acoustics. The model was applied to a practical combustor, and the instability condition predicted agreed with the measurements. The model structure was then utilized to design several active control approaches which incorporated periodic fuel injection at an arbitrary location between the primary injector and the burning zone. We showed that by exploiting the instability mechanism, as captured by the model, one can add flexibility and robustness to die control design (e.g., where the injector is located, how much fuel is introduced, the settling time). Several control algorithms, including an internal controller located at the fuel supply, an integral-plus-proportional controller located at the burning zone, and a Posi-Cast controller located at an arbitrary location in between, are proposed. While we find that the secondary injector can be successful in suppressing the instability, irrespective of its location with respect to the flame zone, results show that the control effort and the robustness against uncertainty and changes in operating conditions depend on the delay associated with the combustion of the extra control fuel (i.e., the location of the injector and the control algorithm).