Effect of velocity and fuel concentration fluctuations on non-premixed jet flames.

摘要

Fundamental research is needed to monitor and control hydrocarbon combustion processes because of the economic and environmental impact of efficient combustion. The focus of this study is to experimentally examine the effects of velocity and fuel concentration fluctuations on the combustion characteristics of buoyant jet flames and to develop optimum conditions for burner efficiency and pollutant abatement. Full characterization of the flame consisted of flame shape, height, temperature, velocity and species radiation emission measurements (soot, unburned hydrocarbons, CO2, CO).; Frequency-dependent experimental measurements demonstrate potential conditions of potential enhancement in combustion efficiency by altering the exit velocity using acoustic forcing. The fuel stream was modulated at moderate amplitude with frequencies associated with the natural flame flickering (7.5Hz), the burner resonance (30Hz) and non-resonance (60Hz) conditions. All experiments are compared with the unpulsed flame (0Hz) to determine the most efficient combustion condition. It was determined that the low frequency burner resonance was the most efficient and produced the least amount of pollutants because of the enhanced mixing. Increasing the amplitude at the burner's resonance resulted in velocities with reverse flow and potentially partially premixing the fuel stream. The mixing associated with the natural flickering produced more soot and CO than the unpulsed flame, while the non-resonant frequency was comparable to the unpulsed flame.; Mixing within the fuel-rich centerline of the flame can be enhanced through fuel concentration or velocity fluctuations. However, the mechanism by which mixing is achieved differs. Fuel concentration fluctuations seek to alter the spatial distribution of the fuel rather than inducing higher near-nozzle velocities. For short fuel injections, it was determined that low momentum-moderate frequency fluctuations produced the most significant increase in radiation associated with products of complete combustion. This condition was further explored by comparing the same mass of fuel from an unpulsed flame, for a particular period, to the following composition variations: CH4-No Gas, CH4-N2 and CH4-Air. It was determined that the flame height, temperature, and radiative emission for CH4-Air fuel concentration fluctuations has the potential to produce ideal conditions. Both of these control strategies produced conditions that are important for industrial burner design.