Influence of flame-generated vorticity on reaction-zone-surface area in weakly turbulent flows
Paper in proceedings, 2019
Direct Numerical Simulation (DNS) data obtained from two statistically stationary, 1D, planar, weakly turbulent premixed flames are analyzed in order to examine the influence of flame-generated vorticity on the area of the reaction surface. The two flames are associated with the flamelet combustion regime and are characterized by significantly different density ratios, i.e. 𝜎=7.53 and 2.5, with all other things being roughly equal. Results indicate that generation of vorticity due to baroclinic torque within flamelets can impede wrinkling the reaction surface, reduce its area, and, hence, decrease burning rate. Thus, these results call for revisiting the widely-accepted concept of combustion acceleration due to flame-generated turbulence. In particular, in the case of 𝜎=7.53, the local stretch rate, which quantifies the local rate of an increase or decrease in the surface area, is predominantly negative in regions characterized by a large magnitude of enstrophy or a large magnitude of baroclinic torque term in the transport equation for the enstrophy, with the effect being more pronounced at larger values of the mean combustion progress variable. If 𝜎=2.5, baroclinic torque weakly effects vorticity field within the mean flame brush and the aforementioned effect is not pronounced.