59351 - Response of Autoignition-Stabilized Flames to One-Dimensional Disturbances: Intrinsic Response 

Growing concern about global warming gives strong incentive for the usage of carbon-free fuels such as hydrogen for power production in gas turbines. Conventional single-stage combustor architecture is, however, presently not capable of burning undiluted hydrogen because of its high reactivity. A more suitable design is a two-stage combustor with an autoignition-stabilized flame in the reheat stage. On the other hand, unsteady combustion processes due to the interaction between the flame and acoustic modes of the combustor are a major deterrent to the stable operation of combustion systems. While this undesirable phenomenon has been thoroughly studied for conventional single-stage combustors with propagation-stabilized flames, the mechanisms leading to unsteady combustion in a reheat system are not well understood. One particular mechanism that may lead to self-sustained flame oscillations involves modulation of the inlet reactant mixture by upstream traveling isentropic perturbations that are generated by the unsteady heat release rate in the ignition front. Previous simulations have shown that this may result in an unstable feedback loop that manifests in strong oscillations of the ignition front location and the heat release rate. In the present work, we analyze this intrinsic feedback mechanism in an elementary 1D combustor configuration and develop an efficient modeling framework for studying this phenomenon.

We consider the Lagrangian evolution of a fluid particle that convects with the mean flow and, at the same time, is harmonically forced by upstream-traveling isentropic temperature and pressure fluctuations. For a given initial perturbation phase, the energy and species mass balance equations are integrated in time to obtain the ignition location. The calculations are then repeated for different initial perturbation phases to obtain the temporal response of the ignition length and the heat release rate over one complete forcing cycle. Performing these calculations for a range of forcing frequencies allows one to construct transfer functions of the heat release rate and the ignition front response to upstream traveling acoustic perturbations.

We first validate the framework with fully compressible DNS computations, where a lean vitiated hydrogen-air mixture is forced by upstream entropic and acoustic disturbances. The temporal response of the ignition front location and the heat release rate show good agreement with the DNS data. Next, we calculate the flame transfer functions by imposing low-amplitude acoustic temperature and pressure perturbations individually and in combined form. The ignition front location and the heat release rate are found to be highly sensitive to upstream traveling temperature fluctuations. The addition of acoustic pressure fluctuations either increases or decreases the transfer function gain depending on the mean pressure. Furthermore, the flame transfer functions show a distinct frequency dependence with substantial variations in gain and a pronounced time-delay character in phase. The frequency dependence of the transfer functions is strongly affected by the mean gas temperature. This effect is found to scale well when using a Doppler-shifted frequency, non-dimensionalized with the unforced ignition time. The results of the present study are essential for understanding and modeling the intrinsic feedback mechanism in reheat combustors.