Polydisperse Droplet Size Growth in a Turbulent Air Flow - Effects of Droplet Number Density and Size

Interaction of polydisperse droplets in a turbulent air flow features prominently in a wide range of phenomena. Here, we present an experimental study on the collective effects of droplet number density ($ND$) and mean droplet size ($D_{10}$) on the droplet size growth. For each ($ND, D_{10}$), we observe an optimum turbulent intensity and a corresponding maximum droplet size growth rate $R^*$. Interestingly, an increase (decrease) in $ND (D_{10})$ doesn't influence $R^*$ up to some threshold values of $ND$ and $D_{10}$, beyond which a sudden increase in $R^*$ is observed. The observed trend is understood in terms of droplet pair dispersion, as observed from Long Distance Microscopy images. The dispersion of droplet pairs is negligible till the threshold conditions are reached. Surprisingly, a substantial increase in dispersion is observed beyond the threshold values, which potentially increases the collision probability, and hence $R^*$. The opposing effects of $ND$ and $D_{10}$ on collision rates cause a negligible variation of $R^*$ up to the threshold values; however, the effect of $ND$ overrides the effect of $D_{10}$ beyond the threshold values, and hence causes a sudden increase in $R ^*$.