Air-traffic growth measured in terms of revenue passenger kilometers has averaged 6.7% over the last five years and is expected to keep growing at an average annual rate of 4.6% over the next twenty years. Low air fares, higher living standards, increased tourism and leisure travel, and new airline business models, are largely responsible for this growth in the commercial aviation industry. The introduction of technological innovations in the development of aircraft and engines has allowed some decoupling of this air-traffic growth from carbon dioxide (CO~2~) emissions in recent years, but even so there is plenty of room for further reductions in the levels of emissions produced on a given journey. In particular, the formation of pollutants such as nitrogen oxides (NOx) that have large negative effects on human health and the environment need to be properly addressed.  
Gas turbine thermal efficiency is directly reflected in engine fuel burn and is very dependent on turbine entry temperature and overall pressure ratio (OPR). Unfortunately increases in these two parameters compromise other key aspects of engine operation and tend to increase NOx emissions. Therefore the European Horizon 2020 ULTIMATE project researched advanced-cycle aero engines having synergistic combinations of novel technologies to increase thermal efficiency without increasing emissions. One such technology is the addition of secondary combustion to increase the mean temperature of heat addition for higher thermal efficiency, while limiting the primary combustor flame temperatures and NOx formation. However, an overall reduction in NOx will only be achieved if the secondary combustor is also a low-NOx design.  
Accordingly, this paper describes numerical studies carried out on novel aero engine secondary combustor concepts developed in two MSc-thesis research projects. More-specifically, the studies explored the potential of the oxy-poor-flame combustion concept. These annular combustor designs featured two distinct regions: (i) the vortex zone, which promotes recirculation of combustion products, a prerequisite for low-oxygen combustion, and (ii) the through-flow region, which mixes the incoming vitiated air with the new combustion products.  
Tridimensional computational fluid dynamics (CFD) simulations were conducted with both inert and reactive flows. These numerical studies were performed on 15º segments of the annular combustors for take-off, climb and cruise conditions. The following approaches were adopted:  
• Reynolds-averaged Navier-Stokes (RANS) and large eddy simulations (LES) were used for turbulence modelling.  
• Cold-flow simulations were run first to provide insight into the internal-flow aerodynamics.  
• Combustion was then modelled using a Flamelet Generated Manifold (FGM).  
• A kerosene-surrogate fuel was used in conjunction with a CHEMKIN-based mechanism to model turbulence-chemistry interactions.  
• NOx emissions were computed as a post-processing task.  
The main conclusions were as follows:  
• Pressure drops across the combustor were shown to be less than 5% in all cases.  
• RANS-based simulations were found to under-predict turbulence throughout the whole combustor.  
• Overall combustor flow temperatures were kept below 2000 K and the final design achieved an acceptable outlet temperature traverse for the downstream turbine.  
• The NOx emissions at take-off computed by LES were about half the amount predicted by RANS.  
• Big differences in NOx emissions appeared depending on the NOx formation pathways accounted for.  
• The nitrous oxide (N~2~O) contribution to overall NOx formation was found to significantly increase at lower fuel-to-air ratios.  
These studies have demonstrated the advantages and limitations of the proposed designs in comparison with results obtained from previous secondary combustor studies.

Keywords: Aero engine, Secondary combustor, Computational fluid dynamics, Turbulence-chemistry interactions, NOx emissions.


Numerical Studies of Novel Aero Engine Secondary Combustors for Low-NOX Emissions

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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