Numerical investigation of the adequacy of chemiluminescence as a heat release rate marker in turbulent, stratified propane/air and n-heptane/air bluff body flames

• Chemiluminescence of OH*, CH*, CO 2 * evaluated as heat release rate marker in stratified flames. • Propane/air and n-heptane/air mixtures are considered. • LES simulations using Thickened Flame Model and an algebraic chemiluminescence model. • Flame Index method assessed local flame regimes’ impact on heat release rate marker performance. • Chemiluminescence signals adequate for heat release rate topology under low-turbulence stratified conditions. This study numerically investigates the adequacy of chemiluminescent species, specifically OH*, CH* and CO 2 *, as topological markers of Heat Release Rate (HRR) in turbulent stratified propane/air and n-heptane/air flames. Chemiluminescence, a widely utilized non-intrusive diagnostic tool in combustion research, provides insight into flame topology and dynamics. However, its reliability as an HRR marker, particularly in flames propagating through inhomogeneously mixed reactants remains a subject of debate. Therefore, the spatial correlation between chemiluminescence signals and HRR is examined, using Large Eddy Simulations (LES) and the Dynamically Thickened Flame combustion model (DTF) coupled to short skeletal chemical kinetic schemes for propane/air and n-heptane/air oxidation. The concentrations of OH*, CH*, and CO 2 * were post-processed under the quasi-steady state assumption (QSSA) using an algebraic model. Correlation analysis, employing the Pearson Correlation Coefficient (PCC), quantified the spatial overlap between chemiluminescent species and HRR, while their predictive performance was compared to well-established HRR markers, such as Planar Laser-Induced Fluorescence (PLIF) signals of HCO and the product of the PLIF signals of OH and CH 2 O. Additionally, the Flame Index method was used to classify local flame regimes and evaluate their influence on the observed trends. Results suggest that fundamentally the excited species remain adequate markers of HRR even under the presence of mixture gradients, experiencing a weak dependence on the fuel type in the presently studied low-turbulence, u ′ / S L 0 O ( 1 ) flames with Re t = 150 . However, the non-trivial effect of the line-of-sight (LoS) integration of the experimentally acquired signals should be taken into consideration when assessing the feasibility to obtain both spatially and temporally resolved information of the HRR topology based on those chemiluminescent species.