An experimental investigation of non-premixed turbulent spray flames and their self-similar behavior

An experimental investigation was conducted in well-defined, turbulent non-premixed spray flames of methanol. An ultrasonic atomizer was used to disperse the liquid into droplets with a small velocity slip with respect to the gas. As a result, the influence of the liquid injector on the flowfield was deliberately minimized. Turbulence was enhanced by the use of a combination of perforated plate and contoured geometry. Close to the burner exit, where the liquid loading was high, the spray flames had a dense droplet core providing the vapor source, and an outer envelope flame. Two components of the velocity and temperature of the gaseous phase, as well as size, velocity, and concentration of the droplet phase, along with the relevant statistical quantities, were measured by phase Doppler anemometry and spontaneous Raman thermometry. The characterization of the temperature field, and, indirectly, of the gaseous density field, via equation of state, allowed for the mapping of the experimental data to a constant density space, by using a Howarth transformation. In this transformed space, the spray flames could be described as a combination of point source of vaporization and point source of momentum. In particular, detailed characterization of two spray flames showed that the vapor source in the gas-phase mass governing equation and the axial momentum behave in a self-similar fashion, the former throughout the spray flame. Close to the burner exit, where the droplet concentration was large, the scaling of point source of momentum did not apply because of the two-way coupling between the phases.