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 approximate to 1.43 at T = 423 K, hydrogen at the equivalence ratio phi = 0.6 with Le approximate to 0.58 at 298 K, and propane at phi = 0.7 with Le approximate to 1.62 at 298 K, 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 25 mm <= <R(t )> <= 45 mm. Using the density correction and Bradley's mean progress variable converting factor for schlieren spherical flames from (c) over bar 0.1 to 0.5, S-T,S-c=0.5 approximate to (rho(b)/rho(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,S-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,S-c=0.5 data with Le < 1 and Le>1 together with previous methane data at 300 K/423 K with Le approximate to 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. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.