| The radial dispersion of water droplets injected into a turbulent pipe flow of air has been investigated. The eddy diffusivity, (epsilon)(,p), mean-square fluctuating velocity, v(,p)('2), and Lagrangian integral time scale, (tau)(,p), of droplets with diameters of 50, 90, and 150 (mu)m are estimated. The centerline mean air flow velocity varied from 400 to 2900 cm/sec.;Dispersion data have been collected in a region where X(,p)('2) varies linearly with time permitting the direct measurement of (epsilon)(,p) = 1/2(dX(,p)('2)/dt). The value of v(,p)('2) is estimated with a dispersion model based on an exponential autocorrelation function for the fluid velocity fluctuations that are experienced by a droplet. The value of (tau)(,p) is calculated from Taylor's theory, (tau)(,p) = (epsilon)(,p)/v(,p)('2).;For the range of conditions studied, the effect of droplet inertia dominates the dispersion process. As inertia increases, the ratio of the droplet to fluid eddy diffusivity, (epsilon)(,p)/(epsilon)(,f), remains nearly constant (greater than one) for the smaller droplets, but decreases sharply for the larger (150-(mu)m diameter) droplets. Non-turbulent effects are believed to have a significant influence on the dispersion of these large diameter droplets.;Experiments were conducted in a fully developed turbulent air - flow directed downwards in a 2-inch diameter vertical pipeline. A liquid jet formed by forcing water through a small orifice fastened to the end of an injector tube was aimed down the centerline of the pipe. A continuous stream of droplets of uniform size was created by disturbing the liquid jet with a vibrating piezoelectric transducer. An optical detection technique was developed to measure local droplet fluxes. A He-Ne laser beam, directed along a diameter of a transparent test section, illuminated passing droplets. Receiving optics, aimed at a particular radial position along the laser beam, collected light scattered by passing droplets and focussed it onto a photomultiplier tube. Radial droplet flux distributions were measured by traversing the receiving optics horizontally. The droplet mean-square displacement from the pipe centerline, or dispersion, X(,p)('2), was determined from the flux profiles. The entire optical apparatus was mounted on a vertical traversing bed so as to allow dispersion measurements to be made as a function of the time that the droplets had spent in the flow. |
