High-pressure burning velocities measurements for centrally-ignited premixed methane/air flames interacting with intense near-isotropic turbulence at constant Reynolds numbers

This paper measures high-pressure turbulent burning velocities (ST) of lean methane spherical flames at constant turbulent Reynolds numbers (ReT ≡ u′LI/ν), where u′ and LI are the r.m.s. turbulent fluctuation velocity and the integral length scale of turbulence and ν is the kinematic viscosity of reactants. This is achieved by adopting a recently-built double-chamber, fan-stirred cruciform burner with perforated plates that can be used to generate intense near-isotropic turbulence with negligible mean velocities while controlling the product of u′LI in proportion to the decreasing ν at elevated pressure (p) up to 1.2 MPa. Results show that when ReT is fixed ranging from 6700 to 14,200, values of ST decrease similarly as laminar burning velocities (SL) with increasing p in minus exponential manners, revealing a global response of burning velocities to pressure. In general, the higher ReT, the higher ST/SL at any fixed p. It is found that the curves of ST/SL as a function of u′/SL all exhibit very strong bending under constant ReT conditions. These results not only reveal that the important effect of ReT on high-pressure ST/SL enhancement, but also suggest that recent findings related with the promotion effect of increasing pressure on ST primarily due to the enhancement of flame instabilities via the thinner flame without any discussion on the influence of ReT elevation at elevated pressure should be reconsidered. Moreover, we found that the modified values of ST at mean progress variable c ¯ ≈ 0.5 show good agreements between Bunsen-type and spherical flames, suggesting that ST determined at flame surfaces with c ¯ = 0.5 may be a better representative of itself regardless of the flame geometries. Finally, various general correlations of S T, c ¯ = 0.5 are compared and discussed. It is found that the present scattering data under different p and ReT conditions can be merged onto a single curve of ( S T, c ¯ = 0.5 − SL)/u′ = 0.14Da0.47, where Da is the turbulent Damköhler number.