Examination of laminar-flamelet concept using vortex/flame interactions

The laminar-flamelet concept for turbulent non-premixed combustion is examined through a study of vortex/flame interactions in a hydrogen/air, opposing-jet non-premixed flame. Vortices with sizes ranging from centimeter to sub-millimeter are injected toward the flame surface. The vortex-injection velocity is selected such that every interaction results in (1) flame extinction and (2) the ratio between the vortex-turnaround and the chemical time scales falling in the laminar-flamelet regime of turbulent combustion. The dynamic changes that occur to the flame structure during its interaction with the vortex are mapped onto a scalar-dissipation-rate scale. It is found that not only the scalar-dissipation rate but also the size of the vortex (eddy) is required for describing the flame-stretching process. The large centimeter-size vortex, irrespective of the propagation velocity, wrinkles and strains the flame before causing local extinction, which represents typical laminar-flamelet behavior. On the other hand, the small sub-millimeter-size vortex replaces the local fluid in the flame zone with fresh air and destroys the flame structure without causing any wrinkling or stretching of the reaction zone, which represents non-flamelet behavior. Interactions with millimeter-size vortices are found to deviate gradually from the flamelet behavior as the vortex size decreases, Vortex/flame interactions that do not follow laminar-flamelet behavior produce very high heat-release rates that are not observed in stretched planar flames. Similar deviations from flamelet behavior are observed in methane and ethylene flames. Since turbulent eddies are two to three orders of magnitude smaller than a millimeter-size vortex, caution must be exercised when applying laminar-flamelet theory to turbulent-combustion modeling. (c) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.