Correlations of high-pressure lean methane and syngas turbulent burning velocities: Effects of turbulent Reynolds, Damköhler, and Karlovitz numbers

This paper investigates correlations of high-pressure turbulent burning velocities (S_T) using our recent S_T measurements of lean methane and syngas spherical flames at constant elevated pressures (p) and constant turbulent Reynolds numbers (Re_T = u′ L_I/_ν), where u′, L_I, and ν are the r.m.s. turbulent fluctuation velocity, the integral length scale of turbulence, and the kinematic viscosity of reactants, respectively. Such constant constraints are achieved by applying a very large high-pressure, dual-chamber explosion facility that is capable of controlling the product of u′ L_I in proportion to the decreasing m due to the increase ofp. We have found that, contrary to popular scenario for S_T enhancement with increasingp at any fixed u′, S_T actually decreases similarly as laminar burning velocities (S_L) with increasing p in minus exponential manners when values of Re_T are kept constant. Moreover, S_T increases noticeably with increasing Re_T varying from 6700 to 14,200 at any constantp ranging from 1 atm to 10 atm. It is found that a better correlation for the normalization of S_T is a power-law relation of S_T/u′ = aDa^b where Da = (L_I/u′)(S_L/dF) is the turbulent Damköhler number, δ_F ≈ α/S_L is the laminar flame thickness, and α is the thermal diffusivity of unburned mixture. Thus, the very scattering S_T data for each of lean methane and syngas mixtures can be merged on their S_T/u′ vs. Da curves with very small data fluctuations. For lean methane flames with the Lewis number (Le) ≈ 1, S_T/u′ ≈ 0.12Da^0.5. supporting a distributed reaction zone model anticipated by Ronney (1995), while for lean syngas flames with Le ≈ 0.76 < 1, S_T/u′ ≈ 0.52Da^0.25. supportinga theory predicted by Zimont (1979). A simple physical mechanism is proposed in attempt to explain what causes the aforesaid discrepancy on the power-law constants.