Drop size and velocity instrumentation

Abstract

A general review is made of a wide range of commercially available instruments for investigating the structure of liquid sprays. The techniques include photographic recording, visualization, drop size and velocity measurement. A brief mention is made of non-probing methods including phase discriminating suction probes and a submicron particle size analyzer using a radioactive source. Following the discussion of the major requirements for detailed spatial and temporal analysis of continuous and pulsed liquid sprays, the major portion of the paper is devoted to discussion of optical techniques. Imaging techniques including still and motion photography, holography and video recording are reviewed. The basic principles and practice in the use of optical interferometry for simultaneous particle sizing and velocity measurement of individual particles are examined and described in detail. The concepts of visibility and peak amplitude of doppler signals are related to problems associated with light intensity distributions within the measurement control volume. Calibration methods and spatial and temporal averaging procedures are presented. In the section on anemometry, simultaneous measurement of drop size and velocity as well as differentiation between drop and gas velocity is examined. The laser Fraunhofer diffraction method for particle size analysis of droplet clouds is also discussed. This instrument is being widely used for rapid and convenient testing of overall spray characteristics. Additional spatial resolution is achieved by using tomography in which slices of a spray are examined at various locations and at different angles. The laser diffraction meter has also been specially adapted for making more rapid measurements in high frequency pulsed Diesel sprays. The final section of the paper is devoted to recent and future developments. These include uniform laser light intensity distributions within the measurement control volume to reduce spastial ambiguity; use of 3-D motion picture photography for determination of individual droplet trajectories; automatic image analysis and numerical holographic reconstruction.