Combustion modeling with conditional source-term estimation and laminar flamelet decomposition

• Main Author: Grout, Ray W. S.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/15580

Conditional source-term estimation with laminar flamelet decomposition has been utilized to model the mean chemical source term in a predictive RANS simulation of two different problems. With this model, integral equations relating the unconditional mean temperature and species fields to the conditional means are used to determine the flame structure as a combination of basis functions formed from solutions to the unsteady laminar flamelet equations. In the simulation of the Sandia Flame 'D', a well studied co-flowing piloted jet flame with a steady average solution, a converged solution was obtained which captured the trends in the temperature and major species, although the nitric oxide prediction overestimated the peak concentration by a substantial margin. Simulation of the autoignition process of non-premixed methane produced simulation predictions in excellent agreement with the experimental data. Using a new, more stringent, criteria to define ignition than earlier studies, the effect of ambient temperature on the ignition delay was captured, as was the expected physical behaviour prior to ignition. The errors in the simulation of the co-flowing piloted jet can be attributed to a large degree to the lack of ability of the basis functions used to account for the effect of the pilot flame. That the autoignition simulation was much more successful highlights the importance of including all relevant physics in the library of basis functions. Earlier formulations of the model were extended by expanding the sample space for the basis functions to include a wide variety of solutions to the laminar flamelet equations; in order to distinguish between the larger set, a new method of stabilizing the numerics was found to be necessary and the number of scalars used to determine the flame structure was increased. The formulation is easily extensible to libraries of basis functions which are capable of including effects such as pilot and edge flames not captured by the laminar flamelet solutions.