Predicting NOx Emissions In Gas Turbines Using Finite Rate Approach

Emission standard agencies are coming up with more stringent regulations on Nitrogen Oxides (NOx), given their adverse effect on the environment. The aircraft engines operate at varying operating conditions and temperature-dependent emissions like NOx are significantly affected by varying conditions. Computational Fluid Dynamics (CFD) simulations are playing a key role in the design of gas turbine combustors and an accurate NOx model will be an important tool for the designers. The new stringent regulations will require new computational approaches over the traditional methods so that the NOx can be predicted accurately under a wide range of operating conditions. Traditionally, the high temperature NOx is predicted using a three-step Zeldovich mechanism. However, it has been observed that the NO (Nitrogen oxide) mass fraction predicted by the Zeldovich mechanism is not accurate for low power conditions due to its predominantly high-temperature kinetics. A significant amount of NO2 (Nitrogen dioxide) is observed in the experimental data at lower temperatures. This requires the inclusion of NO2 chemistry in the NOx mechanism. With the increase in the available computational power, a detailed chemistry simulation is gaining attention, especially for pollutant prediction. In this work, we explore the finite rate (FR) chemistry approach for the prediction of total NOx (NO + NO2) in a gas turbine combustor designed for Aerospace applications. Two reduced mechanisms are investigated namely, the PERK mechanism with 31 species and the Hychem mechanism with 71 species. Simulations with both mechanisms show good comparison with the experimental data and predict the individual contribution of NO and NO2 reasonably well. Further, it is observed that the spray breakup model has a significant impact on the NOx prediction, and it is important to capture the fuel spray correctly to predict the right amount of NOx.