A study of the global differences between axi-symmetric turbulent free jet flames from a smooth contraction and a pipe with well-defined boundary conditions

Abstract

A quantitative assessment is undertaken of the global differences between axisymmetric turbulent free jet flames from a smooth contraction and pipe nozzle by measurement of the radiant fraction, mean flame length, lift-off height and blow-off. The results show that the differences in these parameters are significant. The smooth contraction nozzle produces a flame less radiant, longer and more stable flame than the pipe. It resists lift-off at higher velocities and is self-sustaining at velocities 17% greater than that for the pipe. Similarly, lift-off is prevented till a velocity 40% higher compared to the pipe. The 25% higher radiant fraction of the smooth contraction is also significant. These results show that the trends found in previous studies [4,5] in non-reacting jets, where differences in the initial flow conditions are found to propagate throughout the flow, also apply for reacting flows. Thus the accepted approach of normalising global parameters based solely on bulk mean velocity and nozzle diameter is inadequate.