| Autor: |
Attili, Antonio, Lamioni, Rachele, Berger, Lukas, Kleinheinz, Konstantin, Lapenna, Pasquale E., Pitsch, Heinz, Creta, Francesco |
| Zdroj: |
Proceedings of the Combustion Institute; 2020, Vol. 38 Issue 2, p1973-1981, 9p |
| Abstrakt: |
The effect of pressure on the hydrodynamic stability limits of lean methane–air premixed flames is investigated with Direct Numerical Simulation based on multi-step chemistry and using a simplified one-step chemistry formulation. The dependency on pressure p of the cut-off length scale λ c , that separates stable from unstable wavelengths of the initial perturbation, is computed for a number of different conditions. An increase of pressure causes a significant decrease of the cut-off length, as observed already in previous simulations and experiments. However, this decrease cannot be ascribed only to the decreased flame thickness due to elevated pressures, but the cut-off is reduced significantly even if normalized by either the thermal flame thickness ℓ T or the diffusive flame thickness ℓ D. For the conditions analyzed, the cut-off can be well approximated by the power-law λ c ∝ p − 0.8 , while the thermal and diffusive flame thicknesses, in accordance with previous experiments, are proportional and scale as ℓ T ∝ ℓ D ∝ p − 0.3 . Therefore, the non-dimensional cut-off scales as λ c / ℓ T ∝ λ c / ℓ D ∝ p − 0.5 . This behavior is linked to the increase of the Zeldovich number with pressure, caused by higher inner layer temperatures at higher pressures, which is a result of increased importance of chain termination reactions. The same behavior is observed also in a one-step chemistry approach if the Zeldovich number, appearing explicitly in the one-step model equations, is varied with pressure according to the results from multi-step chemistry. The analysis is extended to the non-linear phase of the instability, when typical strong cusps are observed on the flame surface, simulating a two-dimensional slot burner for different pressures; it is confirmed that the same pressure effects are observed also in more complex settings and in the non-linear regime. [ABSTRACT FROM AUTHOR] |
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