Radiation Emissions From Turbulent Diffusion Flames Burning Large Hydrocarbon Fuels

The purpose of this study is to measure and compare radiation emissions from flames burning large hydrocarbon fuels to assist in understanding how the molecular composition changes radiation emissions. Radiative heat transfer is significant because it is a primary mode of heat transfer for many combustion devices. This study also provides quantitative data that can be available for validation of numerical efforts. Eleven liquid fuels were burned ranging from traditional and alternative jet fuels (e.g. Jet-A) to single and multi-component large hydrocarbons. The latter is used to investigate the role of hydrocarbon classes in radiation emissions and visible soot production. The flames were stabilized using a piloted turbulent diffusion burner. The Reynolds numbers ranged from 7,500 - 45,000 for the various flames. A premixed pilot flame burning ethylene and air was used to stabilize the central jet of vaporized fuel. The vaporized fuel exited the burner at 300° C. The radiative heat flux and the radiation intensity were measured using a radiometer and a mid-infrared camera (FLIR, SC6700), respectively. The radiant fraction ([chi]R), or the ratio of the global radiation emitted by the flames to the chemical energy released, is reported for the different fuels. For all fuels measured, [chi]R peaks near a Reynolds number equal to 20,000. The largest radiant fractions (e.g., 0.36) were observed for fuels with relatively large aromatic content (13-31%). A maximum 20% decrease in [chi]R was observed for fuels without aromatic content, with the exception of one single-component alkane based test fuel being 25% lower than aromatic containing fuels. This is significant because aromatic content increases soot production in flames. These findings suggest that radiative heat transfer in these flames is dominated by emissions from molecular species (i.e. carbon dioxide and water vapor) and not from soot. These findings were verified by the radiation intensity measurements. Measurements from a subset of fuels (with and without aromatic content) revealed peak emissions from CO2 and H2O varied less than 10% between flames while soot increased nearly 60% for aromatic containing fuels. Radiation intensity measurements further confirmed that CO2 and H2O emissions were evenly distributed throughout the flame while soot concentrations had peak emissions in the core of the flame. In this region, soot volume fraction and temperature are the greatest. Peak fluctuating emissions also followed this pattern, with CO2 and H2O emissions located close to the flame's boundary and soot along the central flame axis. Lack of entrained oxidizer within the core of the flame enhances soot production, which is a greater issue in fuels containing aromatics. This has implications on the use of these fuels in gas turbine engines (GTE) both due to considerations of pollution and the lifespan of engines.