High-energy droplet collisions in multi-interacting hollow cone sprays

TL;DR

This study systematically investigates droplet collisions in multi-interacting hollow cone sprays, revealing how high-energy collisions produce smaller satellite droplets and alter size distributions, with implications for spray dynamics.

Contribution

It provides a detailed analysis of collision mechanisms and droplet size evolution in combined hollow cone sprays using advanced imaging and measurement techniques.

Findings

Collision outcomes include reflexive, stretching, splashing, fingering, and multi-droplet interactions.

Higher Weber numbers increase collision frequency and size reduction, especially via stretching separation.

Droplet size distributions are broader and well modeled by the gamma distribution, indicating ligament-mediated breakup.

Abstract

Droplets collide in several complex spray environments ranging from sea sprays to combustion chambers, altering their size and velocity characteristics. The present work offers a systematic investigation of such collisions within the interacting region formed by three hollow-cone sprays, termed the combined spray, at two elevated liquid sheet Weber numbers (Wel). The integrated analysis employs Phase Doppler Interferometry (PDI) and microscopic high-speed backlight imaging to characterize the collision dynamics. PDI indicates a notable reduction (11-15%) in Sauter mean diameter (SMD) at the onset of the interaction region. Images reveal frequent and high-energy droplet collisions, capturing structures associated with binary collision outcomes, namely reflexive and stretching separations, splashing, fingering, and stretching with digitations, along with complex multi-droplet collisions. These collisions produce numerous smaller satellite droplets at the expense of larger parent droplets, leading to a decrease in local SMD. Increasing Wel elevates the frequency of these outcomes, particularly highlighting stretching separation as the dominant mechanism. Furthermore, joint probability density functions from PDI and image-based analysis confirm that most satellite droplets predominantly exhibit axial motion, in contrast to the initial trajectories of parent droplets. The satellite droplets continue to move downstream, colliding with others and resulting in a cascade effect that produces finer droplets. Rescaled droplet size distributions, normalized by mean droplet diameter, are broader in the combined spray due to enhanced size reduction from collisions. These distributions are well captured by the compound gamma distribution, reflecting ligament-mediated breakup dynamics.