AUTOMATED QUANTITATIVE INTERROGATION OF VOLUMES TO SIZE HIGH SPEED SPRAYS

A recently developed digital image analysis technique capable of sizing particles of arbitrary shape is examined. A series of experiments were conducted in order to assess the robustness and accuracy of the technique in its application to the sizing of relatively small droplets whose diameters are predominantly in the range 5 to 30µm. Following a description of the fundamental principles of the technique, a thorough description of the calibration procedure is provided in which the image processing routine has been calibrated down to 18µm. The depth of field characteristics as a function of object diameter were determined experimentally for the optical set-up and confirmed an approximately linear relationship. In order to verify whether this linearity was still valid for object diameters in the range 10<D<18µm, simulations using a single lens approximation were performed and is of particular interest when measuring fuel and other finely atomised sprays. The measurement performance of the PDIA system has been assessed in terms of individual object diameters, number and volume probability density functions of object diameters and this data has been compared to phase Doppler anemometry (PDA) results of the same spray in space and time. It is shown that the larger objects are measured correctly and PDIA is arguably more accurate since object diameter is based upon the measured area/perimeter rather than local object curvature as is the case of PDA. Smaller objects may also be measured correctly by both methods although sensitivity to signal-to-noise ratio, for both methods can generate spurious and contradictory errors. 1