General correlations of high pressure turbulent burning velocities with the consideration of Lewis number effect

This study measures high-pressure turbulent burning velocities (S_T) of spherical expanding flames for various liquid and gaseous fuel/air mixtures with different effective Lewis number (Le), i.e. pre-vaporized stoichiometric iso-octane with Le»1.43 at T=423K, hydrogen at the equivalence ratio φ=0.6 with Le»0.58 at 298K, and propane at φ=0.7 with Le»1.62 at 298K, using the same dual-chamber, fan-stirred cruciform burner capable of generating near-isotropic turbulence. High-speed schlieren imaging is used to obtain the temporal growth of mean flame radii<R(t)>and the observed flame speeds, S_F and/or d<R>/dt, where S_F is the slope of<R(t)>which equals the average of d<R>/dt within 25mm≤<R(t)>≤45mm. Using the density correction and Bradley's mean progress variable converting factor for schlieren spherical flames from c=0.1 to 0.5, S_T,c=0.5»(ρ_b/ρ_u)S_F(<R>_c=0.1/<R>_c=0.5)², where the subscripts b and u indicate the burned and unburned gas. Results show that Le<1 flames have much higher S_T,c=0.5 than that of Le>1 flames at any given rms turbulent fluctuating velocities (u′) and pressure (p). We find that these very scattering S_T,c=0.5 data with Le<1 and Le>1 together with previous methane data at 300K/423K with Le»1 can be well represented by three modified general correlations originally proposed by Kobayashi et al. (2005), Chaudhuri et al. (2012), and Shy et al. (2012) when their scaling parameters are rescaling and grouping with Le^-1, each representing a single curve with small data scattering. This suggests a possible self-similar propagation for turbulent spherical flames, regardless of different fuels, T, p, u′ used. Discussion and comparison with the Bradley's correlation (1992) are offered and future studies identified.