On use of OH∗ and CO₂∗ chemiluminescence for heat release rate measurement in diluted flames

Chemiluminescence emission of the OH∗ radical is commonly used as an indicator of heat release rate in combustion measurements. In the presence of dilution, an environment of interest for exhaust gas recirculation applications, these diagnostics may be affected due to impacts on the chemical processes leading to these radicals. The present work was conducted as an initial test for the validity of OH∗chemiluminescence as a heat release rate indicator in the presence of dilution by nitrogen and carbon dioxide. Measurements of OH∗ and CO₂∗ chemiluminescence were made in a turbulent premixed combustor, with varying levels of diluent. Both species reflect the commonly reported linear relationship between fuel flow rate and emitted intensity. CO₂∗ chemiluminescence exhibited a correlation with temperature for nitrogen dilution, but exhibited significant increases in intensity with carbon dioxide dilution at high flame temperatures. This is possibly due to the higher CO concentrations found with increased CO₂ dilution, which may increase the rate of the overall chemiluminescent reaction. When corrected for the CO₂∗ baseline, OH∗ chemiluminescence intensity exhibited a correspondence in intensity to the flame temperature for both diluents. When appropriate care is taken to account for these effects, OH∗ and CO₂∗ chemiluminescence appear to be suitable indicators of flame heat release rate.