Experiments on Turbulent Nonpremixed Flames at Elevated Pressures

Abstract

Understanding reacting flows in conditions relevant to practical combustion devices is a challenging but critically important task. In such devices, combustion nearly always occurs in a turbulent flow field and at high pressure. The formation of soot is highly sensitive to these parameters. However, little research has been conducted in conditions that replicate the complex physics of such devices in simplified configurations. This body of work focuses on the development of a rig suitable for investigating turbulence-chemistry interactions in simple jet flames at high pressure and high Reynolds numbers and discusses results from the initial experiments in that rig.

First, the flame structure of syngas flames at pressures up to 12 bar and at Reynolds numbers up to 83,500 is investigated using OH-PLIF. A corrugation factor is used to characterize the wrinkling of the flame fronts and PDFs of this factor show that the corrugation of the flame front is a very strong function of the Reynolds number, but in most cases, the pressure has no effect. Separations in the OH layers become less probable as the pressure increases if the Reynolds number remains constant.

Next, the flame structure of nitrogen-diluted ethylene flames at pressures up to 5 bar and Reynolds numbers up to 50,000 are examined using OH-PLIF. Again, the corrugation factor is used to show that the flame fronts become more wrinkled as the Reynolds number increases. Further analysis shows that the extent of wrinkling is limited and further increases in turbulence result in more frequent breaks in the OH layer.

Lastly, two soot studies on the ethylene flames are presented. The soot particle size distribution is characterized in two flames at atmospheric pressure. The time-averaged, mean particle diameter on the centerline increases as the distance from the nozzle increases. Soot volume fraction measurements are made with LII in three flames at different pressures and Reynolds numbers. Soot production is found to be much more sensitive to changes in pressure than changes in Reynolds number. Increases in the mean soot volume fraction as the pressure increases are due to higher instantaneous soot concentrations and lower soot intermittency.