Velocity fields, and extinction conditions for methane-air diffusion flames are measured for an opposed-flow nozzle-type burner system and calculated by a numerical-integration, routine for pressures from 0.25 to 2.5 atm and for dilutions having fixed stoichiometric mixture fractions with oxidizer-stream oxygen mass fractions from 0.233 to 0.190. Imposition of boundary conditions ranging from potential flow to plug flow reveals that changes on the order of a factor of two in the oxidizer-side strain rate at extinction can be produced by changes in opposed-flow burner design. It is shown that the maximum velocity gradient, however which occurs on the fuel side of the main reaction zone, achieves a value at extinction that is relatively insensitive to the boundary conditions of the flow. The results explain differences found by different investigators on influences of dilution on extinction strain rates and show that most counterflow burners are closer to the plug-flow limit than to the potential-flow limit. Strain rates at extinction without dilution are shown to increase with increasing pressure over the above-stated range, countrary to previously observed behaviors with dilution or at very high pressures. This behavior is explained as a consequence of decreasing peak radical concentration with increasing pressure.
ASJC Scopus subject areas
- Chemical Engineering(all)
- Fuel Technology
- Energy Engineering and Power Technology
- Mechanical Engineering
- Physical and Theoretical Chemistry
- Fluid Flow and Transfer Processes