Experimental and theoretical study of the spectral radiation characteristics of lean premixed flames

Turbulent premixed flames were originally used only in spark ignition engines. Their use is increasing in other types of engines, most notably power generation gas turbines for low NOx emissions and no particulate emissions. Motivated by these applications, turbulent premixed flames have been studied for different configurations. Past work has focused on the flame structure, and on chemical mechanisms and analyses of pollutant emissions. Studies are lacking on the radiation characteristics of turbulent premixed flames. In the present study, the spectral radiation intensities from laminar lean premixed flat flames were measured to evaluate an existing narrow band radiation model. The spectral radiation intensities leaving diametric paths at different axial locations in the turbulent premixed jet flames were measured over the wavelength range 2.5 to 5.0 μm, using a monochromator and a liquid nitrogen cooled InSb detector. Measurements of the probability density functions (PDF) of the spectral radiation intensities were compared with Gaussian distributions. Predictions of the spectral radiation intensities from turbulent premixed jet flames can be made if the temperature and radiating species concentration distributions are known. A novel narrow band thin filament pyrometry (TFP) technique was established in the present work to make precise temperature measurements in turbulent lean premixed jet flames. A laminar lean premixed flat flame, stabilized on a McKenna burner, combined with the narrow band radiation model based emission spectroscopy was used to calibrate the TFP system. Two other laminar lean premixed flat flames were used to evaluate the calibration procedure, and they yielded accuracy estimates within 20 K or 1% approximately. The narrow band TFP was then used to measure the radial temperature time series of two turbulent lean premixed jet flames. A stochastic spatial-temporal series analysis technique was developed for the treatment of turbulence-radiation interactions. The predictions of the spectral radiation intensities from the turbulent lean premixed jet flames using this technique show excellent comparisons with the measurements provided the integral length scales at different axial locations are prescribed correctly. Independent evaluation of the integral length scales is recommended in future work