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Counterflow flames of air and methane, propane and ethylene, with and without periodic forcing

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Abstract

The extinction of forced and unforced turbulent premixed counterflow flames has been quantified with lean mixtures of air and each of methane, propane and ethylene. Symmetric flames were produced with two streams of equal equivalence ratios between 0.6 and 1.0, and nozzle separations from 0.2 to 2.5 D, while acoustic drivers were used to force the flow at discrete frequencies. Photographs confirmed visual observation of unforced twin flames and their merging with increasing strain rate into one reaction zone at the stagnation plane before extinction. Propane flames merged at velocities closer to the extinction limit. At separations less than 0.4 D local quenching and extinction and relight occurred at equivalence ratios less than 0.7, independent of fuel type. Unforced extinction times were determined by igniting mixtures with equivalence ratios of 0.6 to 0.9 and bulk velocities above the extinction limit, and observing the extinction process with high-speed video: they were found to increase quasi-exponentially with reduction in strain rate, and were strongly dependent on equivalence ratio and fuel type. Forced extinction times also increased with decrease in strain rate and with reduction in forcing amplitude and instantaneous strain rates greater than the unforced limit were observed. Ethylene flames were more sensitive to the cyclic weakening with more rapid temperature decay rates and shorter extinction times.

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Abbreviations

f :

Forcing frequency (Hz)

H :

Nozzle separation (m)

D :

Nozzle diameter (m)

\( S_{{\text{b}}} \) :

Bulk strain rate, 2U b/H, (s-1)

\(S^{{{\text{ext}}}}_{{\text{b}}} \) :

Bulk strain rate at extinction (s-1)

\(S^{{\max }}_{{\text{f}}} \) :

Maximum instantaneous forced strain rate (s-1)

\(S^{{\max }}_{{{\text{unf}}}} \) :

Maximum instantaneous unforced strain rate (s-1)

\( t_{{{\text{ext}}}} \) :

Forcing time to extinction (s)

\( T_{{\text{p}}} \) :

Time of one period of forcing oscillation (s)

\( U_{{\text{b}}} \) :

Bulk velocity, flow rate/nozzle exit area (ms-1)

\(U^{{{\text{ext}}}}_{{\text{b}}} \) :

Bulk velocity at extinction (ms-1)

u′:

Fluctuating component of turbulent velocity (ms-1)

\({u}'_{{{\text{osc}}}} \) :

Fluctuating component of forced velocity (ms-1)

Φ :

Equivalence ratio (dimensionless)

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Acknowledgements

This work was carried out with financial support from the US Office of Naval Research under Contract N000014-02-1-0664.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Thermofluids Section, Imperial College, SW7 2BX, London , England

    D. Luff, E. Korusoy, P. Lindstedt & J. H. Whitelaw

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  1. D. Luff
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  2. E. Korusoy
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  3. P. Lindstedt
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  4. J. H. Whitelaw
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Correspondence to J. H. Whitelaw.

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Luff, D., Korusoy, E., Lindstedt, P. et al. Counterflow flames of air and methane, propane and ethylene, with and without periodic forcing. Exp Fluids 35, 618–626 (2003). https://doi.org/10.1007/s00348-003-0710-1

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  • DOI: https://doi.org/10.1007/s00348-003-0710-1

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