UC Irvine

Turbulent combustion modeling within Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulations (LES) is a critical aspect of aerospace design, particularly for jet and rocket combustors and ground-based gas turbines. One common approach for modeling turbulent combustion is the flamelet model. While there has been substantial improvement to flamelet theory over its near 50-year history, there are still areas where models fail to include and predict critical physics. This work aims to address two of these areas. First, among other assumptions, existing flamelet models do not incorporate the three-dimensional effects of vortex stretching on the sub-grid fluid-chemical structure through a solution to the momentum equations. Including three-dimensional vortex stretching in the flamelet model produces a centrifugal effect which substantially increases the flammability limit predicted for the reacting mixture. This effect is explored in detail and incorporated into a new flamelet model, called the Rotational Flamelet Model (RFM). Second, a new coupling procedure is developed between the flamelet model and the larger CFD program. This coupling procedure is two-fold as it addresses mechanical coupling; i.e., the influence of turbulent straining and rotation on the flamelet model, and scalar coupling; i.e., how the thermodynamic state in a given CFD cell informs the scalar boundary conditions of the counterflow flamelet. Most flamelet models operate under the modeling assumption that the turbulent flame samples the internal structure of a single canonical counterflow diffusion flame under various strain rates. Under this assumption, flame-local quantities, i.e. parameters defined in the center of the flamelet reaction zone, are used to couple the flamelet to the CFD program. These include scalar dissipation rate (SDR) or flamelet progress variable (FPV) for the mechanical constraint, and mixture fraction for the scalar constraint. Unfortunately, turbulence cascade theory does not present a clear manner of relating or scaling a quantity from RANS or LES to a quantity in the center of a flame that occurs below the mean in RANS or on a much smaller scale in LES, due to the complicated physics that determine the qualities of the reaction zone. Instead, this work adopts the view that turbulent cascade theory better relates the mean or resolved-scale quantities to velocity gradients far removed from the center of the flamelet. The flame-local qualities are then determined through a solution to the sub-grid governing equations, including vortex stretching and density variation in the momentum equations. It is shown that the inclusion of three-dimensional momentum equations with varying levels of vorticity and asymmetric strain rates yields substantially different heat release rates and species production rates compared to an irrotational, symmetric flamelet. Furthermore, by exploring various scalar boundary conditions for the flamelet model, many flamelet families are produced, i.e., flamelets with different scalar boundary conditions, which show further differences in heat release rates and species production rates. For situations in which the flamelet inflows mirror the global fuel and oxidizer inflows (both in composition and enthalpy, which is the practice in the classical flamelet approach), the RFM reproduces classical flamelet behavior such as S-curves, extinction/flammability limits, stable and unstable branches, etc., albeit with the modifications of three-dimensional vortex stretching. The classical behavior is reproduced because the inflows in these situations do not contain high active radical concentrations nor high enthalpy; thus, the flame must provide enough heat to self-sustain or else it extinguishes. However, in situations where the flamelet inflows have high enthalpy, the behavior is substantially different from that of classical flamelets because the inflows themselves may provide enough heat to sustain reactions. It is shown that varying inflow enthalpies substantially alters flammability limits, an effect that is not included in most flamelet libraries. In summary, by including new sub-scale physics and boundary conditions that follow established turbulent scaling laws, a flamelet model is developed to better-predict non-premixed turbulent combustion.