FUNDAMENTAL SCALING EFFECTS OF ACETYLENE AND NATURAL GAS FREE-JET FLAMES

Abstract

The aerodynamic and turbulent structures of combusting, partially premixed free-jet flames, with varying size and for different fuels at constant equivalence ratio, are compared. The first fuel, natural gas, produced predominantly blue flames and was investigated using laser Doppler anemometry measuring techniques for three round-jet exit nozzle diameters to a distance X = 40 diameters downstream. Each jet had the same exit velocity, which was limited only by the blowoff limit. A similar set of experiments was repeated with acetylene, a fuel of higher calorific value producing highly radiative yellow flames. Constant exit velocities were maintained for all cases; in this case the higher flame speed of acetylene allowed higher values of exit velocity to be used. The natural gas flames were then revisited and the flow was seeded with fine coal particles of constant mean diameter and their effect on the flow dynamics was explored using particle image velocimetry for acetylene jet flames it was found that mean velocities scale to X/D = 10, whereas fluctuating velocities are seen to increase across the jet from just beyond the nozzle exit. Normalized flame length and potential core decrease with increasing scale. Self-similarity was seen for mean velocity and different nozzle diameters, less so for fluctuating velocities. Relaminarization of the flames occurred between X/D = 5 and 10. Compared to acetylene, natural gas flames produce self-similarity to greater distances downstream due to longer flame lengths. They also have lower fluctuating velocity levels and exhibit only a slight increase in large-scale turbulence. Comparison of results with other studies shows that induced turbulence into the jet at small scale may be needed to simulate larger scale systems. Finally, the addition of fine coal particles to natural gas flames is shown to delay vorticity formation and mixing, the latter increasing with nozzle diameter.

Keywords:

• free jets
• acetylene
• natural gas
• scaling