Soot formation in laminar diffusion flames
Example: Soot formation in a pulsed laminar diffusion flame
Soot formation and oxidation in diffusion flames is affected by mixture fraction, scalar dissipation rate and temperature. The question is how do these non-linear effects combine in unsteady combustion (diffusion flames)? Here, the experimental investigation of pulsating flames give access to a broader range of scalar dissipation rates. In experiments it was found, that the soot volume fraction in the steady flame is smaller than the phase averaged soot volume fraction in the oscillating flame. Soot formation in unsteady flames is enhanced compared to the corresponding steady flames.
The figure shows the qualitative comparison between experiment and calculation for a pulsed laminar diffusion flame (fuel methane) which was pulsed with a frequency of 10 Hz. The upper images are taken at different phases of the oscillation and show the OH distribution. The numerical simulation is shown in the lower pictures (cut and isoplane of temperature field) and indicates a behaviour similar to the flames in the experiment. Chunks separate from the flame while it is collapsing.
For the calculations adaptive tables with data the from 2-domain-1-step kinetics scheme have been used. The generated data included also the source term for the transport equation of the soot volume fraction. The second figure shows the calculated soot volume fraction and the line of stoichiometric mixture (white). The left picture shows the result for a steady-state flame, the right pictures for different times during the pulsation period.
The last two pictures show the comparison of normalized measured (exp) and calculated soot volume fraction for two heights above burner. It can be seen, that the use of the tabulated source terms of soot volume fraction (advanced approach) leads to much better results than using the tabulated soot volume fractions itself (classic approach).
M. Charwath and J. Hentschel and R. Suntz and H. Bockhorn. Characterisation of the flame properties of moderately oscillating sooting methane-air diffusion flames. In Combustion generated fine carbonaceous particles, H. Bockhorn, A. D’Anna, A. F. Sarofim, H. Wang (ed.), KIT Scientific Publishing, Karlsruhe, p. 589-604, 2010