Evaluation of PDA measuring results obtained from sprays of optically inhomogeneous liquids

Abstract

For control and optimization ofatomization processes measurement techniques are employed beeing able to determine the diameter and the velocity of the generated droplets. The so-called "Phase-Doppler-Anemometry" (FDA) measuring simultaneously both distributions has been successftilly applied to sprays of homogeneous liquids like water, molten metals etc. In a lot of other technical processes liquids contain optical inhomogeneities consisting of suspended superfme particles. In general the measured droplet size distributions of such process fluids appear to be broadened compared to the "real-one". In the following an approximate method for fmding the "real-size" distributions is introduced. A blurring ftinction is obtained by performing FDA measurements on single-sized droplets. An iterative constrained inversion algorithm is applied evaluating the real-size distribution. The precision of the algorithm is checked by companng the calculated distributions with undistorted measuring results of an atomized optically homogeneous liquid.Keywords: p article-sizing, Phase-Doppler-Anemometiy, inhomogeneous process fluids, ill-posed problems, inversion problems, constrained iterative inversion algorithm INFRODUCTIONWith the advance of process automatization, the demand for fast and high precision measuring and automatic control techniques increases. Avoiding or minimizing low-grade product charges, in line measuring techniques which allow in-situ process control are favoured. In a lot ofdifferent areas ofindustrial processes the atomization ofliquids has become an important unit operation. The quality ofthe atomization process defines product's qualities. For control and optimization ofthese processes the diameter and the velocity of the atomized droplets have to be measured continuously. Optical measurement techniques are prefered because they do not affect the formation of droplets and they need short measuring times.A well established method of theses techniques is the so-called "Phase-Doppler-Anemometry" (PDA)"2 allowing simultaneous measurements of droplet and velocity distributions. In this method an interference fringe field is produced by two crossing laser beams. The light-intensity scattered by a passing single particle is detected at two different locations. The frequency of each signal is proportional to the velocity ofthe particle. The particle diameter is related to the phase difference between both signals. The application of Mie-theory enables the evaluation of this relation3. Whereas the measurement of the velocity distribution is normally uncomplicated problems are encountered by determining the correct size distribution.Mie theory is applicable to homogeneous spherical particles with constant refraction index. Therefore FDA has been successfully applied to sprays ofhomogeneous media like water, mineral oil, molten glass, metals and gas bubbles. Difficulties arise when measuring optical inhomogeneous liquids containing compounds -inmost cases suspended superfine particles -in the order of the laser wavelen...