EXTINCTION KARLOVITZ NUMBERS OF PREMIXED COUNTERFLOW FLAMES UNDER VARIOUS PRESSURES

摘要

The dependence of extinction limit of counterflow premixed flames on pressure was experimentally and numerically investigated. Stretch rates at extinction of various mixtures with different Lewis numbers were measured in the pressure range between 0.1 MPa and 0.5 MPa. An increase in the pressure resulted in a decrease in the stretch rate at extinction of a mixture near the flammability limit. On the other hand, it increased with pressure for a mixture with a relatively large equivalence ratio, and the maximum point appeared at a certain pressure in some cases. To explain those experimental results, numerical simulations were conducted. In the analysis, it was elucidated that the Karlovitz number defined as the ratio of a characteristic chemical time to a characteristic flow time was a dominant parameter for extinction behavior. The characteristic chemical time was evaluated by the ratio of flame thickness to burning velocity (δ/S_L), and the characteristic flow time was evaluated by the reverse of stretch rate (1/K). The dependence of stretch rates at extinction on pressure was well explained by the Karlovitz number. However, the Ka number evaluated from a non-stretched flame failed to predict the order of stretch rates at extinction between three mixtures with different Lewis numbers (CH_4/O_2/N_2 flame (Le = 0.97), He-20% diluted flame (Le = 1.40), and CO_2-20% diluted flame (Le = 0.90)). The local Ka number (Ka~L) evaluated from the local burning velocity (S_L~L) and the local flame thickness (δ~L) of a stretched flame was needed for precise prediction of the order of stretch rates at extinction for mixtures with different Le numbers.