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Stable lifted flames and blow-out phenomenon

Observations of high-momentum jet fires reveal that the base of the flame oscillates backwards and forwards in space. The flame will be blown out should the turbulent timescale be much smaller than the timescale associated with the chemical reactions. A re-ignition of the jet further downstream can-not be ruled out, though this is dependent on the availability of an ignition source.

Broadwell et al. (1984) put forward an expression to predict blowout velocity of hydrocarbon-air mix-tures:

(U_b)_{mix} = (U_b)_{fuel} \left[ \left( \frac{(S_{b,max})_{mix}}{(S_{b,max})_{fuel}} \right)^2 \left( \frac{(Y_{max})_{mix}}{(Y_{max})_{fuel}} \right)^2 \left( \frac{(\rho)_{mix}}{(\rho)_{fuel}} \right)^{1/2}\right],

where (U_b)_{fuel} is the blowout velocity for pure fuel, (S_{b,max})_{mix} is the maximum laminar flame speed for the mixture, (S_{b,max})_{fuel} is the maximum laminar flame speed for pure fuel, Ymax,mix is the ratio of fuel air mass fraction corresponding to the maximum flame speed for the mixture, Ymax,fuel is the ratio of fuel air mass fraction corresponding to the maximum flame speed for the fuel, and and are the densities of the mixture and pure fuel, respectively. Broadwell et al (1984) proposed that flame blow-out occurs when the ratio between the local mixing time and characteristic chemical timescale is less that a certain critical threshold. The expression yielded poor agreement with observed blowout ve-locities for hydrogen-air mixtures.

It is possible to predict the blowout behaviour of a jet from calculations of the local velocities of a cold isothermal jet mixing, Choudhuri and Gollahalli (2000).

Broadwell, J. E., Dahm, W. J. A., and Mungal, M. G., Blowout of turbulent flames, 20th Symposium (International) on Combustion, Combustion Institute, Pittsburgh, PA, USA, pp. 303-310, 1984
Choudhuri, A. R., and Gollahalli, S. R., Effects of isothermal mixing processes on the stability of hy-drogen-hydrocarbon blend turbulent diffusion flames, ASME paper no. IJPGC2000-15022, Proceed-ings of 2000 International Joint Power Generation Conference Miami Beach, Florida, July 23-26, 2000, 2000.

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Page last modified on December 10, 2008, at 03:17 PM