Planar Laser Induced Fluorescence Studies of Radicals in Precessing Jet Flames

Researcher: Dr. Zeyad Alwahabi

The major source of energy for the world is, and will continue to be, the combustion of hydrocarbon fuels. Despite their value as an energy source, hydrocarbon fuels produce serious atmospheric, and other, pollutants. During the last two decades the oxides of nitrogen, NO_x, have come under scrutiny because of their adverse impact locally on photochemical smog, regionally on acid rain and globally on the greenhouse effect and damage of the ozone layer. More recently the problems associated with CO₂ emissions have been recognised, resulting in the signing of the Rio de Janeiro Agreement by many nations.

Early industrial gas burner designs utilised simple turbulent jet diffusion flames. Such jet diffusion flames are stable over a relatively small operating range, and with natural gas as the fuel, produce long heat flux profiles with only a modest radiative component. Hence their application has required system designs which rely heavily on convective heat transfer or has resulted in poorer efficiency relative to other fuel that might be employed. The fine scale mixing of the turbulent diffusion jet results in flames with high local combustion temperatures, low radiant heat transfer and accompanying high levels of thermally generated NO_x.[1]

There is now growing evidence that the distribution of scales in turbulent mixing can have a significant influence on the combustion in jet flames. Modifying the character of the turbulent mixing field from that produced by a simple jet can be beneficial in increasing flame emissivity and reducing NO_x emissions, at least from natural gas flames. The need to match burners to processes and thus to produce high process thermal efficiency, good heat transfer characteristics, low emissions, high turn-down and good operating stability has led researchers to investigate the use of burner nozzles with fundamentally different mixing characteristics. A recent means of changing the mixing characteristics is the fluidic precessing jet (FPJ) nozzle. [2,3] The flow field generated by this nozzle greatly augments the dominant scale of mixing, relative to that generated by shear stress in a mixing layer, by the natural precession of a jet about the geometric axis of the nozzle.

The precessing jet (PJ) nozzle is the ongoing subject of investigation by the combustion and fluid dynamics research group at the University of Adelaide with fundamental research conducted in parallel with commercial developments. Whilst the efficacy of the FPJ nozzle has been proven in practice, it produces a flow field which is extraordinarily difficult to study at the fundamental level. Primarily this is because it is continuously unstable, and it is this very instability which appears to be the source of the efficacy. The continuous instability manifests itself in several forms, the most apparent of which is the precession of the emergent jet. As a step toward dissecting the naturally occurring precessing jet flow into its component features a fundamental investigation of the effects of precession on a jet has been undertaken by mechanically rotating a nozzle, from which an inclined simple jet emerges with its origin on the nozzle axis, about that nozzle axis. This causes the inclined jet to precess about that axis. Unlike the FPJ nozzle, the jet which emerges from the mechanical precessing jet (MPJ) nozzle (Figure 2) has well defined conditions at its origin that can be varied independently: viz., the exit diameter, velocity, angle and precessional frequency. This flexibility allows the controlled study of the separate and combined effects of these parameters on the external flow field both in cold flow and with chemical reaction. By changing these parameters a wide range of "non self-similar" turbulent mixing characteristics can be generated in the region corresponding to the base of the flame, so allowing a definitive investigation of the effect of the scale and intensity of turbulence on the mixing and combustion characteristics of a jet.

A major part of the fundamental research on the PJ nozzle is a detailed study of the influence of the spectrum of turbulent mixing scales on the radiation and NO_x emission. One contribution to this is to obtain simultaneous planar measurements of mixture fraction in the unreacted fluid and of the position of the flame front. Simultaneous planar measurements of OH radicals, to mark the position of the reaction zone, and acetone, to determine the mixture fraction of the unreacted fluid upstream from the flame front, can be conducted using the planar laser-induced fluorescence (PLIF) technique, a very powerful laser-diagnostic tool for use in combustion research. Using PLIF, images of key intermediate combustion radicals, such as OH, CH, C₂ or NO, can be recorded within the flame. The acetone will only survive in the non-reacting fluid but will provide planar data that are complementary to the single point data and will also allow comparison with existing planar measurements of concentration in cold flow.

Preliminary PLIF measurements of OH images provide some insight into the fundamental behaviour of precessing turbulent jet flames. Further studies are in progress.

References

1. Turns, S. R. and Myhr, F. H., 1991, Combustion and Flame, 87: 319.
2. Nathan, G. J., 1988, PhD Thesis, (University of Adelaide).
3. Nathan, G.J ., Hill, S. J and Luxton, R. E., 1996, J. Fluid Mechanics, to appear.