The turbulent propagation speed of a premixed flame can be significantly enhanced by the onset of
Darrieus–Landau (DL) instability within the wrinkled and corrugated flamelet regimes of turbulent com-
bustion. Previous studies have revealed the existence of clearly distinct regimes of turbulent propagation,
depending on the presence of DL instabilities or lack thereof, named here as super- and subcritical respec-
tively, characterized by different scaling laws for the turbulent flame speed.
In this study we present experimental turbulent flame speed measurements for propane/air mixtures at
atmospheric pressure, variable equivalence ratio at Lewis numbers greater than one obtained within a Bun-
sen geometry with particle image velocimetry diagnostics. By varying the equivalence ratio we act on the
cut-off wavelength and can thus control DL instability. A classification of observed flames into sub/super-
critical regimes is achieved through the characterization of their morphology in terms of flame curvature
statistics. Numerical low-Mach number simulations of weakly turbulent two-dimensional methane/air slot
burner flames are also performed both in the presence or absence of DL instability and are observed to
exhibit similar morphological properties.
We show that experimental normalized turbulent propane flame speeds S T =S L are subject to two distinct
scaling laws, as a function of the normalized turbulence intensity U rms =S L , depending on the sub/supercrit-
ical nature of the propagation regime. We also conjecture, based on the experimental results, that at higher
values of turbulence intensity a transition occurs whereby the effects of DL instability become shadowed by
the dominant effect of turbulence
