Investigation into the bag breakup and sub-droplet size distribution of tandem water droplet pair in an elevated-temperatures continuous airflow

Understanding droplet interaction mechanisms in dense sprays requires investigation of tandem droplet pair fragmentation. This investigation uses a high-speed camera to examine the fragmentation phenomena and sub-droplet distribution of tandem water droplet pair with separation distances (S) ranging from 1.9 to 13.3 under airflow temperatures between 293 K and 493 K. The separation distance is the ratio of the actual spacing to the droplet diameter. Within a critical separation distance, the bag breakup of the lead droplet is influenced by collision with the trailing droplet. The critical separation distance for the lead droplet bag breakup decreases with airflow temperature. Separation distance and airflow temperature influence the trailing droplet bag breakup through the collision with the lead droplet, the lead droplet shielding effect, and the airflow velocity dissipation. Further, the sub-droplet size distribution of the broken lead droplet is analyzed by dividing the broken droplet into three parts: bag, rim, and node. The correlation length governing the bag sub-droplet size primarily depends on the bag thickness, while the rim and node sub-droplet sizes depend on the liquid ring thickness. Separation distance and airflow temperature affect the sub-droplet size distributions by altering the volume fractions of the bag, rim, and node, as well as their correlation lengths. A semi-empirical model for predicting the Sauter mean diameter (SMD) of the sub-droplet of the broken lead droplet is proposed based on the volume fractions of different parts (bag, rim, and node) and their correlation lengths. The predictions align well with the experimental data among across a wide range in this investigation. This investigation can provide experimental data and theoretical references for understanding the droplet interaction mechanisms within dense sprays.