Dynamics, stability and scaling of turbulent methane oxy-combustion

Abstract

Carbon capture and storage (CCS) is an important strategy for reducing CO₂ emissions, with oxy-fuel combustion being one of the most promising technologies because of it is high efficiency and low cost. In oxy-combustion, CH₄/O₂/CO₂ mixtures burn at low temperatures (~~1700 K), high pressures (~~40 bar), where laminar burning velocities are about 7 times lower than in traditional CH₄ /Air mixtures. Thus oxy-fuel combustors are more prone to blowoff and dynamic instabilities. In this thesis we examine turbulent oxy-combustion flame stabilization physics at the large and small scales using experimental studies and numerical simulations. Experimental measurements are used to establish the stability characteristics of flame macrostructures in a swirl stabilized combustor. We show that the transition in the flame macrostructure to a flame stabilized along both the inner and outer shear layers (Flame IV), scales according to the extinction strain rate, similar to air flames. To achieve accurate scaling, extinction strain rates must be computed at the thermal conditions of the outer shear layer, emphasizing the role of heat interactions with the wall boundary layer. Care must be exercised while modeling the chemical structure of oxy-flames. We show that the kinetics of CO₂ (used as a diluent in oxy-combustion) is important in determining the consumption speed and flame extinction strain rate. Specifically, the extinction strain rate was found to be heavily impacted by the reaction CO₂+ H -->/<-- CO + OH. Large Eddy Simulations (LES) models, first validated for various combustor geometries, fuels and oxidizers, are used to examine the stabilization mechanisms of these flames. First, we demonstrate the importance of choosing the correct global chemical kinetics mechanism in predicting the flow structures in multi-dimensional simulations and develop a priori criterion of selecting a reduced mechanism based on the extinction strain rate. Besides flame macrostructures, recirculation zone lengths are found to linearly scale with extinction strain rates. This scaling holds regardless of fuel or oxidizer type, Reynold's number, inlet temperature, or combustor geometry. It is thus very important that a chemical mechanism is able to correctly predict extinction strain rates if it is to be used in CFD simulations. We use the validated LES framework to model the transition to Flame IV in the swirl combustor for methane oxy-combustion mixtures. The 3D turbulent flame structure strongly resembles a ID strained adiabatic laminar flame structure in the combustor interior, and non-adiabatic flames near the combustor wall. The results support the earlier conclusions regarding the use of the extinction strain rate and the wall thermal boundary condition in scaling and modeling turbulent combustion dynamics.