An Experimental and Computational Study of Turbulent Lean Premixed Flames

摘要

Applications of lean premixed combustion lead to significant reductions in emissions of oxides of nitrogen (NO_x). Our past studies have established that these reductions are due to lower temperatures and CH radical concentrations in the combustion zone and lower temperatures in the post combustion zones. These studies utilized laser Doppler velocimetry (LDV) for measurements of the mean and RMS velocities, thin filament pyrometry (TFP) for measurements of mean and RMS temperature and laser induced fluorescence (LIF) for measurements of the CH radical concentrations. One and two dimensional computational studies of the corresponding opposed flow and McKenna burner flames were completed to further understand the details of the thermal and prompt NO reductions using quantitative reaction path diagrams. Line of sight measurements of infrared radiation intensity emitted by H_2O and CO_2 molecules were completed to understand the flame structure. The present work utilizes transient filtered fast infrared planar imaging. The details of the turbulence chemistry interactions are treated using a partially premixed flame model with major properties represented by the joint statistics of the reaction progress variable and the mixture fraction, with the latter representing mixing with the products of the pilot flame and the co-flow air. Turbulence was treated using a standard K - e model with the Zimont model allowing an estimation of the local flame speed and flame structure. The results show that the computed mean axial velocity and mean temperature match relatively well with the experimental data except near the flame tip. A narrow band radiation model (RADCAL) and a line by line radiation model using HITRAN and HITEMP databases were utilized for rendering the computational results in the form of infrared images for comparison with the measurements. The results show that the quantitative computational and experimental infrared imaging techniques provide useful insights into turbulence-NO_x chemistry interactions. Both techniques are now ready for studying practical lean premixed flames with high pressure, preheat, exhaust gas recirculation and steam addition.