A DNS Analysis of the Correlation of Heat Release Rate with Chemiluminescence Emissions in Turbulent Combustion

The essential correlation of heat release rate and chemiluminescence emission from turbulent combustion is quantitatively analyzed by means of direct numerical simulation (DNS) of premixed methane/air flames, employing a detailed reaction mechanism with 18 species and 69 elementary reactions, and the mixture-averaged transport method. One-dimensional freely propagating laminar flames have first been studied for different stoichiometries varying from fuel-lean to fuel-rich conditions. There, the local generation of the chemiluminescent OH* species correlates strongly with the heat released by the combustion reaction, especially in the fuel-lean range. Three-dimensional DNS have then been applied to calculate a synthetically propagating flame front subjected to different turbulent inflow conditions. Joint probability density functions of OH* concentration and heat release rate have been generated from the DNS results, showing a stronger scattering of the correlation curve compared to the corresponding laminar flame. As the chemiluminescence measurement gathers light only along one viewing direction, the line-of-sight integrated values of heat release and OH* concentration have been evaluated from the DNS, where the domain has been decomposed into a number of rays defined by a fixed viewing direction and a specific area. A quasi-linear relationship has been identified for these integral values, where the correlation becomes stronger for flames subjected to lower turbulence intensities or larger cross-section areas of the rays. A computational grid with 16 million finite volumes has been used for the DNS of the turbulent flames and the simulations have been performed in parallel with 3,600 processor cores from the Hazel Hen cluster of HLRS. Scale-up performance of the DNS code, which is based on the open-source program OpenFOAM, has been evaluated.