DRAG COEFFICIENTS OF SPHERICAL LIQUID DROPLETS .2. TURBULENT GASEOUS FIELDS

The drag of non-evaporating, spherical, liquid droplets was measured i n turbulent flow fields at parametric ranges relevant to spray combust ion, characterized by the droplet Reynolds number, and the intensity a nd spatial scales of turbulence. The experimental apparatus comprised a wind-tunnel and a piezo-electric droplet generator. The procedure wa s to inject water droplets of uniform size co-currently and continuous ly with vertical turbulent air flows while droplet velocity was measur ed at different elevations using laser-Doppler velocimetry. Turbulence was characterized using hot-wire anemometry prior to droplet injectio n. Drag coefficients were calculated using these main measurements and the law of conservation of mechanical energy. Reynolds numbers were i nvestigated in the range to-too, in terms of the equivalent spherical diameter of a droplet, and the mean relative speed between the ambient gaseous field and the droplets. Weber numbers were much less than uni ty so droplets were effectively spherical. Relative intensities of tur bulence were investigated in the range 20-65 percent, in terms of the mean relative speed. Spatial scales of turbulence were large in compar ison to the droplets; the ratio between the spatial integral scale and the droplet diameter was in the range 11-38, and the Kolmogorov scale was comparable in size or smaller than the droplet diameter. Experime ntal data showed that the drag in turbulent fields under these conditi ons is not significantly different than that of solid spheres in a qui escent field at the same Reynolds number.