59215 - A Novel Les-Based Process for Nox Emission Assessment in a Premixed Swirl Stabilized Combustion System 

With more and more stringent emission regulations, the need for gas turbine manufactures to develop dry low- NO_x 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 NO_x 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 NO_x 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 NO_x-Damköhler number. The calculation of the latter quantity implies the assessment of the characteristic formation time for NO_x. Since the chemical mechanisms leading to the NO_x formation involve many reactions with different time scales, in the present work the NO_x formation time has been calculated starting from the global NO_x production rate. At this purpose, a 1D freely propagating flame was used to pre-tabulate the NO_x production rate and interrogated run time during the simulation through an additional user-defined function.  

When falling in the “in-flame” region, the NO_x 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 NO_x (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 NO_x. 

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 NO_x 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.