Soot Emission Optimization of a Helicopter Engine: From Injector Design to Engine Tests Validation

Even though no regulation currently exists on helicopter gas turbines, soot production in aeronautic engines is of paramount importance to comply with future rules, as well as to offer environmental-friendly products on the market. Thus, design modifications of the combustion liner and fuel injectors are one way to explore in order to reduce soot emission levels of existing combustors. These design changes are driven both by fundamental knowledge of soot production mechanisms and by advanced combustion and pollutants modelling. The major difficulty is to reduce soot emissions while not deteriorating other combustion performances: NOx and CO emissions, lean blow-off limits and service lifetime.

The objective of the present study is to optimize fuel injectors of a recent Safran Helicopter Engines research combustor. The injector design modifications are driven by one main guideline: reducing soot emissions can be achieved in lowering the equivalence ratio downstream of the injector. Detailed designs are achieved thanks to advanced RANS injector and LES combustion computations.

Then, in order to mitigate main identified risks – management of soot emissions and lean blow-off limits – engine tests were performed very soon in the demonstration process. A combustor is successively equipped with one standard and two modified geometries of fuel injectors on an engine test bench at Safran Helicopter Engines facility. Experimental results show that the two modified injector geometries reduce smoke numbers by a factor of respectively 2 and 5 and slightly deteriorates lean blow-off limits. These measurements are also compared to CFD computations. Leung et al. model (Combust Flame 1991), relying on phenomenological descriptions of soot formation combined with a LES computation of the combustor, well predicts a significant decrease in smoke level, even if it does not perfectly match engine data. Concerning lean blow-off limits, LES modelling predict a slight deterioration in lean blow-off limits, which do not agree qualitatively with engine test results.

As a conclusion, this study identifies a design driving factor for soot reduction, with possibly acceptable impacts on other combustion performances like lean blow-off limits.