Stringent emissions regulations have led engine manufacturers to focus on fuel-efficient low emission technologies. Basic understanding and modeling of fundamental mechanisms governing formation and destruction of NOx, CO and UHC is essential to reduce pollutant emissions. Recent advances in turbulent combustion modeling have enabled designers to use CFD as a design tool for evaluating low emission concepts at the conceptual design phase.
Prediction of pollutant NOx for gas turbine combustors has proven successful for design validation applications. The challenge is to provide quick and accurate estimates of NOx for application to gas turbine combustor preliminary design phase, which can be characterized by multiple design changes, varying operating conditions and a variety of fuel staging concepts. NOx formation processes are typically slow compared to the fast hydrocarbon oxidation reactions. As a result, NOx predictions are typically performed as a post-processing step on thermal field obtained from reacting flow simulations. This work builds on prior work on flamelet approach [1,3] by suitably blending it with FLUENT®’s species transport. NOx production within gas turbine combustors has contributions from two major sources: flame front & post-flame thermal NO. The flame front contributions are obtained from flamelet based computations involving detailed chemistry whereas the slow evolution of post-flame NOx is tracked by explicitly solving for NO species transport. The closure of turbulence-chemistry-interactions is derived from Girimaji’s [2] assumed PDF closure using temperature-composition correlations. A Gaussian PDF shape is used with mean and variance of temperatures accounting for the first and second moments, required for PDF weighting computations. The formulation has been validated against SANDIA D flame, and then extended to GE Aviation’s fielded combustors over a wide range of operating conditions, with errors within 11% at Take-Off condition. The model has also been used for pre-test predictions on a number of combustors under development.
