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 (S _T) of lean methane spherical flames at constant turbulent Reynolds numbers (Re _T≡u'L _I/ν), where u' and L _I 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'L _I in proportion to the decreasing ν at elevated pressure (p) up to 1.2MPa. Results show that when Re _T is fixed ranging from 6700 to 14,200, values of S _T decrease similarly as laminar burning velocities (S _L) with increasing p in minus exponential manners, revealing a global response of burning velocities to pressure. In general, the higher Re _T, the higher S _T/S _L at any fixed p. It is found that the curves of S _T/S _L as a function of u'/S _L all exhibit very strong bending under constant Re _T conditions. These results not only reveal that the important effect of Re _T on high-pressure S _T/S _L enhancement, but also suggest that recent findings related with the promotion effect of increasing pressure on S _T primarily due to the enhancement of flame instabilities via the thinner flame without any discussion on the influence of Re _T elevation at elevated pressure should be reconsidered. Moreover, we found that the modified values of S _T at mean progress variable c̄≈0.5 show good agreements between Bunsen-type and spherical flames, suggesting that S _T determined at flame surfaces with c̄=0.5 may be a better representative of itself regardless of the flame geometries. Finally, various general correlations of ST,c̄=0.5 are compared and discussed. It is found that the present scattering data under different p and Re _T conditions can be merged onto a single curve of (ST,c̄=0.5-S _L)/u'=0.14Da ^0.47, where Da is the turbulent Damköhler number.