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Spatial distributions of droplet size and velocity in air heated spray Measured by interferometric laser imaging technique

The behaviour of a water spray centrally injected into a swirling annular jet (initial outer diameter 56.5mm) was studied experimentally. Two flow conditions were compared, namely a non-heated and a heated airflow issuing into quiescent air (Figure 1). The water spray was produced using a full cone nozzle mounted centrally into the injection pipe. The instantaneous spatial distribution of droplet size and velocity was measured applying the interferometric laser imaging technique (ILIDS). The circular out-of-focus images were first optically squeezed for allowing an easier and more reliable analysis of the images. The fringe spacing, which is proportional to the droplet diameters, was determined by discrete Fourier transform and a Gaussian interpolation technique. The droplet velocity was obtained from the displacement of the squeezed images recorded or two subsequent images. The results demonstrate that the interaction of the spray with the swirling free jet flow and the established recirculation zone results in bimodal droplet size distributions for both the non-heated and heated case (Figure 2). As expected, droplet evaporation in the heated case yields a faster decrease of the mean droplet size along the spray than for the non-heated case. Fig. 1. Schematics of experimental set-up and interferometric laser imaging technique Fig. 2. Size distributions and correlation maps between diameter and axial velocity at (z,r)=(75,0)