Numerical Simulation of Premixed Flame Quenching in the Broken-Reaction-Zone Regime

Abstract

Lean perfectly premixed ∞ame propagation in a swirling ∞ow dump combustor is studied. The emphasis is on ∞ame propagation in the Broken Reaction-Zone (BRZ), where the reaction zone thickness (‐F) is larger than the Kolmogorov scale (·). The Linear-Eddy Mixing (LEM) model is used to simulate subgrid interactions between the ∞ame structure and the unresolved turbulent structures within the framework of a large-eddy simulation (LES) approach. A real case scenario is studied: the propagation of a lean premixed ∞ame in a dump combustion chamber where heat losses at the walls are taken into account via the use of a simple heat loss model. Results show ∞ame lift-ofi, caused by ∞ame quenching, for low equivalence ratio ('=0.45). The dynamics of ∞ame quenching, and the subsequent ∞ame lift-ofi, can be linked to the natural combustion chamber dynamics. The frequency of ∞ame lift ofi is equal to the half-flrst longitudinal combustion chamber oscillation mode. Since the early 90’s, gas turbine based ground power generation is considered an environmentally-friendly alternative to the more polluting coal-burning power plants. As large investments are made to transport and distribute natural gas in a liquid form (Liquid Natural Gas, a.k.a. LNG), the number of gas turbine power plants that will enter into service will soar in the next few decades. Independently to the development of LNG technologies, gasifled coal (syngas) is also considered as a possible fuel for gas turbine power plants, even though syngas is less energy-dense than natural gas. These state-of-the-art gas turbines use the Lean-Premix-Prevaporized (LPP) technology and are operated at an equivalence ratio that is only slightly higher than the lean extinction limit. Although it is often assumed that the turbulent combustion process taking place in these gas turbines is in the ∞amelet regime, where ‐F is larger than ·, and the chemical time scale (?C) is smaller than the characteristic turbulent ∞ow time scale (?F), other turbulent combustion regimes are also locally present. In regions of high turbulence, the smallest eddies can be smaller than the ∞ame thickness. In this case, eddies penetrate the preheat zone, increase heat and species transport, and increase the ∞ame thickness. Furthermore, experiments 1{3 have shown that very high level of turbulence can result in ∞ame quenching, and that the stabilized ∞ame can exhibit local extinction (quenching of a very small portion of the ∞ame) without global extinction (quenching of a large portion in the ∞ame creating a hole in the ∞ame where unburnt reactants penetrate into the product region). Both local and global ∞ame quenching are not fully understood. In theory, ∞ame quenching is a direct result of the action of turbulent structures small and/or powerful enough to break-up the structure of the reaction zone, but this has never been formally demonstrated, neither by experiments nor by numerical simulations. Other factors such as reactants equivalence ratio distribution, combustion chamber geometry, heat losses, etc., all have to be considered when ∞ame quenching is studied. This work focuses on the numerical investigation of lean, swirl stabilized turbulent premixed ∞ames with the objective to study ∞ame quenching phenomena and its impact on the overall combustion process. In