Blowout Limits of Flames in High-Speed Airflows: Critical Damkohler Number

Abstract

೘ For best correlation of the data, n and m were found to be 1.0. H is the cavity height, Uoo is the free stream velocity ant T is the free stream static temperature. The lean blowout limits for the case of aft wall fuel injection was found to be correlated by the relation: φo = 0.0043 Da -0.26 , where φo is the overall equivalence ratio that is based on the properly defined characteristic air mass flow rate. For the floor fuel injection cases, the lean blowout limit was correlated by: φo = 0.0028 Da -0.80 . The correlations are valid over a range of conditions, including Mach numbers from 2-3, four fuel types (hydrogen, ethylene, methane, acetylene having different laminar flame speeds SL0), and static pressures (p) from 0.5 to 1.0 atm. Results showed that the location of the fuel injection into the cavity is important. To explain the importance of the location of fuel injection, some images are presented that were obtained by recording the chemiluminescence. The results can be explained by three simple ideas: the injected fuel jet creates a locally fuel-rich region, the entrained air creates an effective air-jet (that starts at the upper rear corner of the cavity where the shear layer impacts on the aft wall), and the products in the recirculation zone supply heat that elevates the flame speed. The flame is observed to be nearly attached to the injector port if there is sufficient air near the fuel injector. For other conditions it can be slightly and the fuel does not burn at the injector but burns a few millimeters downstream of the injector. For large fuel flow rates, the flame is highly lifted such that the cavity is flooded with fuel, and the fuel elements are convected around the cavity by the recirculation zone without burning. Then the fuel enters the shear layer at the top of the cavity and burns entirely within the shear layer. Previous theories have ignored the location of the fuel injector and have assumed that fuel-air mixing takes place immediately, creating a 'well-stirred reactor; the present results indicate that this concept is not realistic.