Standard PDA for measuring the size of inhomogeneous droplets

A method to obtain the diameter of inhomogeneous droplets by using phase-Doppler anemometry (PDA) is described. Inhomogeneities included in a fluid droplet disturb light scattered by the droplet. Consequently the phase difference measured between two Doppler bursts deviates from the value calculated by geometrical optics theory for a droplet of diameter d. Because evaluation of the measured phase differences is based on a linear relationship between phase difference and particle diameter the accuracy of PDA is deteriorated. The difference between the real and the measured diameters of such droplets can be characterized by convolution with a Gaussian probability function. For monodisperse droplets and for a given type of inhomogeneity the standard deviation of the Gaussian function increases linearly with increasing mass concentration of these inhomogeneity. Applying infrared laser-PDA (IR-PDA) with a wavelength of 1312 nm, the measuring range to investigate droplets of, for instance, water-latex suspensions containing 450 nm polystyrene spheres can be extended to suspensions with six times the measurable concentration standard PDA using a He-Ne laser at lambda = 632.8 nm. The relative improvement obtained by applying IR-PDA (in comparison with a standard PDA system equipped with a 632.8 nm laser) can be estimated by the decrease of the extinction efficiency Q(ext) of the inclusions with laser wavelength. The linear relation between extinction E and standard deviation sigma can be used by a blind deconvolution algorithm. Starting from the measured diameter distribution of the inhomogeneous droplets the real diameter distribution and the standard deviation of each size class of the particles can be calculated by the algorithm. From this standard deviation it is possible to determine the extinction E and the mass concentration of the inhomogeneities.