The stretched laminar flamelet model provides a convenient mechanism for incorporating realistic chemical kinetics into calculations of turbulent nonpremixed flames. In the standard flamelet model, two scalars, a mixture fraction ξ and scalar dissipation η (a measure of flamelet stretch) suffice to specify the local instantaneous thermochemical state in the turbulent flow. One shortcoming of the flamelet approach is the implicit assumption that the reaction zone structure of laminar flamelets can respond on a time scale that is fast compared to the time scale of changes in mean scalar dissipation in the turbulent flow. analysis of the response of a one-dimensional unsteady strained diffusion layer to arbitrary time-dependent-strains is presented. This analysis suggests that laminar flames cannot approach the equilibrium (zero strain) structure as rapidly as the flamelet model implies far downstream in the jet; the flamelet model breaks down in the far jet. An ad hoc modification to the flamelet model is made to account for the limited response of the diffusive layer to time-varying strain rates. In a CO/H2/N2-air turbulent jet diffusion flame, the modified flamelet model yields a substantially slower approach to chemical equilibrium and improved agreement with experimental data compared to the standard flamelet model. This suggests that the final approach to equilibrium may be limited by the nonzero response time of flamelet chemical structure to the rapidly decaying strain rates encountered downstream in the turbulent jet. In the context of flamelet models, transient effects may be more important than previously assumed.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology
- Mechanical Engineering
- Physical and Theoretical Chemistry
- Fluid Flow and Transfer Processes