Transition from turbulence-dominated to instability-dominated combustion regime in lean hydrogen-air flames
Journal article, 2024

To explore the importance of thermodiffusive and hydrodynamic instabilities of laminar flames in turbulent flows, previously generated direct numerical simulations of statistically one-dimensional complex-chemistry lean hydrogen-air flames in forced turbulence were continued by switching-off turbulence forcing. Three sets of flames characterized by different ratios of initial root-mean-square velocity to laminar flame speed, i.e., (A) u′/SL=2.2, (B) u′/SL=4.0, and (C) u′/SL=8.3, were addressed. Moreover, new complementary simulations of unstable laminar flames were performed. Analyses from the obtained numerical results indicate (but do not prove) that laminar flame instabilities play a minor role at sufficiently high Karlovitz numbers Ka. This supposition is supported, first, in cases B and C, where the initial value of turbulent burning velocity UT is significantly higher than burning velocity evaluated during non-linear stage of laminar flame instability development in the same computational domain. Second, regular large-scale perturbations of instantaneous flame surface are prominent in the unstable laminar flame but are not observed at high Ka. Third, Karlovitz numbers associated with appearance of such perturbations as the turbulence decays are scattered between 4 and 10 and are consistent within an order of magnitude to a recently proposed criterion (Chomiak and Lipatnikov, Phys. Rev. E 107: 015102, 2023) of importance for laminar flame instabilities in turbulent flows. Fourth, at such transition instants, a ratio of potential and solenoidal turbulent kinetic energies, averaged over the flame-brush leading edge, is close to 2.0 in 11 studied cases and a ratio of UT/SL varies between 3.0 and 3.5. Fifth, the maximum fuel consumption rate (over the computational domain) decreases as the turbulence decays. Thus, the initial maximum rates are significantly higher than the counterpart rate in the unstable laminar flame. Together these results show that the hypothesis that laminar flame instabilities play only a minor role at high Ka deserves further study.

Turbulent burning velocity

Premixed flame

Hydrogen

Thermodiffusive instabilty

DNS

Author

Andrei Lipatnikov

Chalmers, Mechanics and Maritime Sciences (M2), Energy Conversion and Propulsion Systems

Hsu Chew Lee

Southern University of Science and Technology

Peng Dai

Southern University of Science and Technology

Minping Wan

Southern University of Science and Technology

V. A. Sabelnikov

ONERA - The French Aerospace Lab

Central Aerohydrodynamic Institute (TsAGI)

Combustion and Flame

0010-2180 (ISSN) 15562921 (eISSN)

Vol. 259 113170

Subject Categories

Energy Engineering

Fluid Mechanics and Acoustics

DOI

10.1016/j.combustflame.2023.113170

More information

Latest update

7/9/2024 3