Quantification of low Damköhler number turbulent premixed flames

Abstract

Combustion under highly strained conditions or at low Damköhler numbers requires external enthalpy sources to ensure stability. Such flames deviate from the conventional bimodal flame structure and chemically active fluid states become statistically relevant. The current work utilises a multi-fluid analysis in order to quantify the impact of such conditions on a turbulent (Ret ≃ 350) lean (Φ = 0.50) premixed DME / air flame with Da ≃ 0.29. The flames were aerodynamically stabilised in a back-to-burnt opposed jet configuration with the temperature of the external enthalpy support varied from 1200 ≤ THCP (K) ≤ 1600. Simultaneous Mie scattering, CH2O and OH–PLIF and PIV were used to quantify the transition from spatially distributed chemical reactions to reaction zones that appear flamelet-like. The analysis shows that in the current configuration such structures are only present at high THCP. By contrast, the low temperature chemistry is continuously active with CH2O increasingly more spatially distributed with reducing support temperature. The current analysis provides novel insights into low Damköhler number combustion and burning mode transitions by means of (i) multi-fluid probability statistics, (ii) the structure of formaldehyde and hydroxyl layers and (iii) their cross–correlation as well as (iv) the underlying strain rate statistics on material surfaces.