An analysis of image evaluation procedures for measurements of diesel spray characteristics

In order to investigate the influence of the methods applied for measuring spray parameters such as liquid phase spray penetration and dispersion cone angle, Diesel spray experiments in a high pressure / high temperature chamber were performed under non-evaporating (30 ºC, 21.1 bar), evaporating (350 ºC, 43.4 bar) and combusting (600 ºC, 60 bar) conditions with a constant gas density 24.3 kg/m3. A high-speed video camera was used to record shadowgraph images. The images were analyzed both as individual images and as averages of 100 images at each time step. The influence of the threshold level to define the spray boundary on the measured spray penetration and dispersion cone angle was analyzed. Under non-evaporating and evaporating conditions, it was found that a change in threshold level has a larger influence on the values for the liquid spray penetration determined from the averaged images, compared to the values from the instantaneous images. The penetration lengths determined from the instantaneous and averaged images are generally quite similar. Under combusting conditions, during the early phase of the injection there are only small differences in the penetration length between the instantaneous and averaged images or when the threshold level is changed, which is in strong contrast to the later part where there is a high sensitivity to changes in the threshold level and there can be significant differences in lengths determined from instantaneous and averaged images. The spray dispersion angle was calculated along the whole spray based on the width at each distance from the nozzle tip. It was found that there is a region around 12 mm from the nozzle tip where the measured spray cone angle is constant during most of the injections. Further downstream the dispersion angle increases till about 75 % of the penetration length under non-evaporating and evaporating conditions, whereas it decreases under combusting conditions