Benedetta Franzelli

On the generalisation of the mixture fraction to a monotonic mixing-describing variable for the flamelet formulation of spray flames

Spray flames are complex combustion configurations that require the consideration of competing processes between evaporation, mixing and chemical reactions. The classical mixture-fraction formulation, commonly employed for the representation of gaseous diffusion flames, cannot be used for spray flames owing to its non-monotonicity. This is a consequence of the presence of an evaporation source term in the corresponding conservation equation. By addressing this issue, a new mixing-describing variable, called the effective composition variable η, is introduced to enable the general analysis of spray-flame structures in composition space. This quantity combines the gaseous mixture fraction Zg and the liquid-to-gas mass ratio Zl, and is defined as . This new expression reduces to the classical mixture-fraction definition for gaseous systems, thereby ensuring consistency. The versatility of this new expression is demonstrated in application to the analysis of counterflow spray flames. Following this analysis, this effective composition variable is employed for the derivation of a spray-flamelet formulation. The consistent representation in both effective composition space and physical space is guaranteed by construction and the feasibility of solving the resulting spray-flamelet equations in this newly defined composition space is demonstrated numerically. A model for the scalar dissipation rate is proposed to close the derived spray-flamelet equations. The laminar one-dimensional counterflow spray-flamelet equations are numerically solved in η-space and compared to the physical-space solutions. It is shown that the hysteresis and bifurcation characterising the flame structure response to variations of droplet diameter and strain rate are correctly reproduced by the proposed composition-space formulation.

Numerical Modeling of Soot Production in Aero-Engine Combustors Using Large Eddy Simulations

Numerical simulations are regarded as an essential tool for improving the design of combustion systems since they can provide information that is complementary to experiments. However, although numerical simulations have already been successfully applied to the prediction of temperature and species concentration in turbulent flames, the production of soot is far from being conclusive due to the complexity of the processes involved in soot production. In this context, first Large Eddy Simulations (LES) of soot production in turbulent flames are reported in the literature in laboratory-scale configurations, thereby confirming the feasibility of the approach. However numerous modeling and numerical issues have not been completely solved. Moreover, validation of the models through comparisons with measurements in realistic complex flows typical of aero-engines is still rare.This work therefore proposes to evaluate the LES approach for the prediction of soot production in an experimental swirl-stabilized non-premixed ethylene/air aero-engine combustor, for which soot and flame data are available. Two simulations are carried out using a two-equation soot model to compare the performance of a hybrid chemical description (reduced chemistry for the flame structure/tabulated chemistry for soot precursor chemistry) to a classical full tabulation method. Discrepancies of soot concentration between the two LES calculations will be analyzed and the sensitivity to the chemical models will be investigated.© 2015 ASME