STRUCTURE OF A METHANOL AIR COAXIAL REACTING SPRAY NEAR THE STABILIZATION REGION

Planar laser Mie scattering, planar laser-induced fluorescence and two component phase-doppler interferometry have been used to study the re action zone structure near the stabilization region of a coaxial metha nol/air spray flame. The configuration of the experiment was chosen to approximate the atomizer geometry, surface tension and Weber number o f a single coaxial rocket injector. The measurements are made in a wat er-cooled, optically accessible confinement chamber at a pressure of 1 atm. Data are reported for two atomizing air velocity conditions. One yields a flame length of approximately 1 m, the other, half that valu e. Both the Weber number (characterizing the atomization process) and the Reynolds number (characterizing the gas-phase mixing process) vary between the cases, but the data suggest that it is the Weber number w hich has the dominant effect. In both cases OH imaging shows that the reaction zone is confined to a narrow region, with the OH field being similar in appearance to that of a single-phase turbulent mixing-contr olled (diffusion) flame. Size-classified mean velocity vectors derived from the phase-doppler data show striking differences in the flow pat tern for low and high Stokes number droplets. Droplets 5 mum in diamet er and below (Stokes number less than 3) appear to follow the recircul ating eddies that provide flame stabilization while droplets of large Stokes number travel ballistically through the flow. Increasing the We ber number by a factor of 2.5 decreased the Sauter mean diameter of th e spray by as much as one-third, and the arithmetic mean diameter by a s much as one-half. We believe that it is this decrease in the spray d roplet diameter that is primarily responsible for the very different f lame lengths in the two cases.