Coordinator: Dr. Klaus-Peter Geigle
Within the European project FIRST (Fuel Injector Research for Sustainable Transport, FP7/n265848) we generated a comprehensive data set for validation of complex soot models. Several laser-based diagnostics were applied to a dual swirl burner at increased pressure, operated with ethylene as fuel. The data set contains flow field information, temperature statistics, quantitative soot distributions and OH and PAH (polycyclic aromatic hydrocarbons) distributions for different operating conditions.
For the simulation of soot emissions from aero-engine combustors, we developed a quite complex PAH and soot model. Soot precursors (PAH) and soot particles are lumped in size classes and treated as quasi-species in separate transport equations, identical to those of gas phase species. The model includes acetylene addition, agglomeration and oxidation. It is optimized to cover different combustion conditions equally well (rich and lean conditions, premixed, non-premixed, increased and ambient pressure) and without the need of flame-condition dependent parameterization. Particularly challenging is the correct treatment of the recirculation of hot reaction products (contains unburnt hydrocarbons, UHC) and inclusion of oxidation or quenching air which can be injected downstream of the primary combustion zone.
A valuable tool to support the understanding of soot formation is the simultaneous application of different diagnostics to deduce correlations between quantities. This revealed that OH and soot do not co-exist at a given instant and location. Soot filaments fill the gaps present in the OH distribution (see Fig. 1). Simulations, presented in Fig. 1, right, correctly predict this behavior.
Additional understanding is accessible by correlation of soot precursors (PAH) and soot (see Fig. 2). The soot distribution changes significantly when adding oxidation air downstream of the primary combustion. Without oxidation air (left) soot filaments, yellow-red in the plot, are widely distributed, while the flame is characterized by compact soot structures close to PAH clouds (blue-green) when adding oxidation air (right).