(568f) Soret Effects on Gaseous Diffusion Flame Temperatures and Positions

ABSTRACT Adiabatic gaseous diffusion flame temperatures and positions are usually estimated by assuming that the effective fuel and oxidizer Fick diffusivities are not only close to each other but also nearly equal to the thermal diffusivity of the prevailing gas mixtures. While it is well-known that unequal Fick diffusivities can systematically alter these important flame characteristics (Rosner(2000)), little attention has been paid to an interesting consequence of gas-kinetic theory--viz., when the fuel and/or oxidizer diffusivity is very different from the thermal or momentum diffusivity of a gas mixture, Ludwig-Soret (LS-) transport of that species (ie., species transport driven by temperature gradients) also becomes important (Rosner, et al.(2000). In this paper we therefore examine the consequences of these two effects operating simultaneously, adopting a simple yet rational physicochemical model amenable to mathematical analysis. Since our primary interest is in 'air-breathing' combustion, it will be the fuel Lewis number ((Le)F) which departs significantly from unity, corresponding to appreciable fuel vapor Soret factors--especially for 'heavy' fuels. For simplicity, we consider the limiting case of rapid homogeneous chemical kinetics in planar gas flows--ie. both unsteady/unstrained, and steady strained (counterflow) situations. Since flame temperature has such an important effect on IR radiation emission and the production of oxides of nitrogen in practical combustors, special attention is directed here to systematic flame temperature 'shifts' associated with non-zero Soret transport.

The results show that for high molecular weight (MW) fuels, fuel vapor dilution not only reduces adiabatic diffusion flame temperatures (Tf) for thermodynamic reasons, it also magnifies the 'retarding' effect of Soret diffusion, augmenting the transport effects on Tf already associated with reduced Fick diffusivity (cf., gas mixture heat diffusivity). As a consequence of the combined effects of reduced (D)F and simultaneous LS-transport, we find that diffusion flame temperatures for high MW fuels can be reduced by hundreds of Kelvins (from their thermodynamic 'expected values'), with significant consequences (eg., for NOx production and IR emission).

The simplicity of our results lends itself to making rational estimates in new situations, clarifying when these additional 'cross-diffusion' effects must be considered. Our results are also expected to have important implications for chemical synthesis applications using unconventional oxidizers/fuels, in which cases flame temperature may influence additional characteristics (eg., the sintering of particle aggregates)(see, eg., Rosner(2005) and Rosner and Pyykonen(2002)). ________________________________________________ Supported by ACS/PRF Grant 40062-AC9 and NSF CTS 0522944; For: AIChE '06 11/06 Rosner, D.E.(2000), Transport Processes in Chemically Reacting Flow Systems, DOVER Publications, New York Rosner, D.E., Israel, R.A. and La Mantia, B.(2000), Comb. & Flame, vol. 123, 547-560' Rosner, D.E.,(2005) Ind. & Eng. Chem.-Res(ACS), vol 44(8); August (2005) Rosner , D.E. and Pyykonen, J.J.(2002) AIChE J , vol 48(3) 476-491