  In this work we clarify the role of Markstein diffusivity on turbulent flame
speed and it's scaling, from analysis and experimental measurements on
constant-pressure expanding flames propagating in near isotropic turbulence.
For all C0-C4 hydrocarbon-air mixtures presented in this work and recently
published C8 data from Leeds, the normalized turbulent flame speed data of
individual mixtures approximately follows the recent theoretical and
experimental $Re_{T,f}^{0.5} $ scaling, where the average radius is the length
scale and thermal diffusivity is the transport property. We observe that for a
constant $Re_{T,f} $, the normalized turbulent flame speed decreases with
increasing Markstein Number. This could be explained by considering Markstein
diffusivity as the large wavenumber, flame surface fluctuation dissipation
mechanism. As originally suggested by the theory, replacing thermal diffusivity
with Markstein diffusivity in the turbulence Reynolds number definition above,
the present and Leeds dataset could be scaled by the new $Re_{T,M}^{0.5}
$irrespective of the fuel considered, equivalence ratio, pressure and
turbulence intensity for positive Mk flames over a large range of Damk\"ohler
numbers.
