Correlations of high-pressure lean methane and syngas turbulent burning velocities: Effects of turbulent Reynolds, Damköhler, and Karlovitz numbers
Journal article, 2015
This paper investigates correlations of high-pressure turbulent burning velocities (ST) using our recent ST measurements of lean methane and syngas spherical flames at constant elevated pressures (p) and constant turbulent Reynolds numbers (ReT ≡ u′LI/ν), where u′, LI, 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′LI in proportion to the decreasing ν due to the increase of p. We have found that, contrary to popular scenario for ST enhancement with increasing p at any fixed u′, ST actually decreases similarly as laminar burning velocities (SL) with increasing p in minus exponential manners when values of ReT are kept constant. Moreover, ST increases noticeably with increasing ReT varying from 6700 to 14,200 at any constant p ranging from 1 atm to 10 atm. It is found that a better correlation for the normalization of ST is a power-law relation of ST/u′ = aDab, where Da = (LI/u′)(SL/δF) is the turbulent Damköhler number, δF ≈ α/SL is the laminar flame thickness, and α is the thermal diffusivity of unburned mixture. Thus, the very scattering ST data for each of lean methane and syngas mixtures can be merged on their ST/u′ vs. Da curves with very small data fluctuations. For lean methane flames with the Lewis number (Le) ≈ 1, ST/u′ ≈ 0.12Da0.5 supporting a distributed reaction zone model anticipated by Ronney (1995), while for lean syngas flames with Le ≈ 0.76 ≪ 1, ST/u′ ≈ 0.52Da0.25 supporting a 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.