Planer measurement of velocity vector, size and spatial distri bution of transient spray by novel interferometric imaging technique

摘要

Laser techniques investigating spray characteristics are demonstrated aiming at the simultaneous measurement of size, velocity vector and concentration. The spray has almost always a certain broad size distribution, so that the droplet sizing has been carried out by PDA system which provides a size and velocity data for individual droplets one after one in the manner of Eulerian observation. The conventional PIV system came to be able to provide flow vector field of fluids in a certain area so that we can observe a flow regime by both Eulerian and Lagrangian manners, while the displacement is estimated by comparing the distributed pattern of tracer particles in a small interrogation window and searching the similar pattern in the next frame under the assumption that the pattern is not deformed in the time interval. The PIV system has been hardly applied for the spray with a distributed size of droplets, which have different velocities and the particle sizing ability lacked basically in the system. Not only the size and velocity distribution of droplets, but the planer information of those instantaneous properties of individual droplets is required especially to understand the unsteady behavior of flow fields. The authors developed an improved interferometric imaging technique, which can measure the planar or volumetric distribution of diameter and velocity vectors of droplets instantaneously and in the time series. As the size evaluation, using the interferometric imaging and optical image compression techniques for the acquisition of the dense image and the simultaneous sub-pixel frequency estimation by a Gaussian interpolation technique, enables the highly accurate determination of the droplet size, the images in the successive images give a high correlation. Consequently the velocity vectors of individual droplet can be determined even from the images of denser spray. The direct digital acquisition of images and fully automated image processing software provide the high quality data, up to six hundred droplets in 10mm times 10mm observation area from a single frozen image. Further, the time-series properties of flow at various periods after the start of injection are obtained by the phase-averaged flow resume. The present system intends to have both of abilities, the sizing and planer velocity vector information for individual droplets from a frozen image and to obtain the time series information by processing frames sequentially. The results showed the significant difference of droplet behavior depending on the group of diameters in the transient spray. A series of the works has been examined a combined measurement techniques with a LIF to investigate vaporization of liquid droplet of acetone. Make the instantaneous image of LIF by UV coincide with the image by the interferometric method and we obtain the droplet size and velocity vector and the vapor concentration of the acetone surrounding the droplet.