With more and more stringent emission regulations, the need for gas turbine manufactures to develop dry low- NOx emission combustor is becoming changeling. Especially for mechanical drive application, the engines are asked to maintain low pollutant emissions for an extremely wide variety of fuel composition, from part-load to full load. On the other side, the extensive use of the Computational Fluid Dynamic (CFD) has become a useful tool also to reduce the cost associated to the new technology introduction.In this context, the present work concerns a novel CFD approach for the NOx emission assessment validated against the experimental data of a heavy-duty annular combustion chamber equipped with a swirl stabilized premixer. For a more physical modelling of the flame brush, in this research both the heat loss and the strain rate effects are introduced to correct the progress variable source term in the context of the premixed turbulent flame speed closure.The NOx are calculated in a Large-Eddy Simulation context considering two different contributions: the “in-flame” and the “post-flame” one. The cell-based switch between these two terms is a function of the characteristic NOx-Damköhler number. The calculation of the latter quantity implies the assessment of the characteristic formation time for NOx. Since the chemical mechanisms leading to the NOx formation involve many reactions with different time scales, in the present work the NOx formation time has been calculated starting from the global NOx production rate. At this purpose, a 1D freely propagating flame was used to pre-tabulate the NOx production rate and interrogated run time during the simulation through an additional user-defined function. &nbsp;When falling in the “in-flame” region, the NOx emission has a formation time that is proportional to the fast reaction rate of the combustion process (the so-called “prompt” contribution). But also the other mechanism like N2O-pathway and thermal NOx (due to the temperature rising in the flame front) have been included in the “in-flame” term. The extended Zeldovich mechanism has been implemented as per the “post-flame” term. Both the “in-flame” and “post flame” term concur to the definition of the source terms of the dedicated transport equation for the NOx.&nbsp;The code has been extensively validated at different operating conditions and fuel compositions, involving also different fuel pilot splits (i.e. different diffusive combustion regimes).Firstly, a comparison at mid-pressure with pure methane and a natural gas with moderate ethane content was conducted for 2 pilot split percentages, highlighting the impact of the different fuel composition on to NOx emission. Secondly, the model was tested at full-pressure conditions with pure methane to verify the rising of NOx with the operating pressure. Lastly, a generic fuel composition was simulated at full pressure.In any case, the model is able to capture the trend providing also a limited absolute error compared to the data.

A Novel Les-Based Process for Nox Emission Assessment in a Premixed Swirl Stabilized Combustion System

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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