Thermal Radiation Properties of Turbulent Lean Premixed Methane Air Flames

Thermal radiation properties of turbulent premixed flames have received little attention in the past perhaps because of the lower radiative heat loss compared with that for non-premixed flames. However, the high-temperature sensitivity of NO kinetics and the importance of radiation in near-limit laminar premixed flames provide fundamental reasons for studies of radiation properties of turbulent premixed flames. Reduced cooling airflows in lean premixed combustors, miniaturization of combustors, and the possible use of radiation sensors in combustion control schemes are some of the practical reasons for studying radiation heat transfer in these flames. Motivated by this, we report the first (to our knowledge) study of spectral radiation properties of turbulent premixed flames. Measurements of mean, root mean square (rms) and probability density functions (PDFs) of spectral radiation intensities leaving diametric paths at five heights in two turbulent lean premixed methane/air jet flames stabilized using small H2/air pilot flames in a coflow of air were completed. Measurements of spectral radiation intensities leaving three laminar flames were also completed. These data were used to evaluate narrowband radiation calculations independent of the treatment of turbulent fluctuations. Stochastic spatial series analysis was used to estimate instantaneous distributions of temperature. The analysis requires the specification of mean and rms temperature distributions, integral length scale distributions, and an assumption of exponential spatial correlation function. We specified the mean and rms temperature distributions measured by calibrated narrowband thin filament pyrometry. A simple flame and mixing model was used to relate the concentrations of CO2 and H2O to the temperature. We used scalar spatial series in conjunction with a radiation model to calculate the mean, rms, and PDFs of spectral radiation intensities. Overall, the model predictions are in reasonable agreement with the data. The only improvement needed is in the area of capturing correlated occurrences of high temperatures along the radiation path.