A modal analysis of the size distribution for an hollow-cone spray in cross-flow

Studying sprays in cross-flow is a challenge as well in the design of combustion engines as for spray cooling of heat exchanger. In the latest applications, small particles with high cooling efficiency are injected in the air flow. Some studies have documented particle velocities and diameters in spray plume using Phase Doppler Analysis or photographic techniques. The present contribution focusses on the particle spatial distribution during their far field dispersion in airflow. Size distribution measurements inside sprays were made with a high spatial resolution using a Phase Doppler Analyzer. Afterwards, the particle size in measurement planes are analysed using a statistical procedure of data analysis, which breaks down the particle cloud into groups. It uses the same concept than "Modal Models", whose decompose the aerosol cloud into different modes or bins. This method is applied to a spray generated with an hollow-cone nozzle, in both with and without airflow. The clustering approach allows to identify a cloud of fine particles, that is spatially distinct of the coarse and intermediate groups. In a resting atmosphere, coarse particles are concentrated in the periphery of the spray whereas small droplets are confined in the centre, revealing the typical shape of hollow-cone nozzle spray. Cloud trajectories of coarse, intermediate and fine modes are dissociated in cross flow conditions. The classification facilitates the trajectory monitoring for each group and localisation of falling regions: the fall of coarse particles may be roughly approximated with the equation of motion for a single coarse particle. The fine and intermediate particles falls more rapidly than the equation's prognostic. Noticeable lateral deviations of clouds are also observed. The kidney shape observed for the coarse cloud fits the thermal footprint of spray on heat exchanger. Other shapes, like horse-shoe and vortexbag, are founded for intermediate and fine particles. Large particles acts as curtain and induces airflow perturbations that are associated with the particular shape of fine and intermediate clouds. The modal analysis provides an efficient tool for studying particles and should be further used with 2 or 3-dimensional measurements in order to study drift vortices.