Flash Freeze--Thaw Phenomenon in Sprayed Evaporating Micrometer Droplets

Abstract

Two-fluid spray nozzles are widely used in combustion, chemical processing, pharmaceutical coating, environmental control, and spray drying to atomize liquids with pressurized gas. However, the adiabatic cooling and resulting flash freeze--thaw exposure of atomized droplets remain underexplored. Using high-fidelity computational fluid dynamics coupled with droplet-scale nucleation modeling, we show that the atomizing gas temperature at the nozzle exit can fall from $22\,^{\circ}\mathrm{C}$ to below $-130\,^{\circ}\mathrm{C}$, initiating rapid ice nucleation and freezing in micro-scale droplets. For atomizing gas at $5\,\mathrm{bar}$ (gauge) and $22\,^{\circ}\mathrm{C}$, all droplets smaller than $1.5\,\mu\mathrm{m}$ freeze, whereas droplets larger than $3\,\mu\mathrm{m}$ remain liquid. These frozen droplets thaw within $O(10)\,\mu\mathrm{s}$ upon leaving the cold zone, subjecting sensitive actives to intense freeze--thaw thermomechanical stresses near the nozzle even when the bulk drying gas is warm. Parametric studies show that ice formation is eliminated at atomizing gas temperatures above $110\,^{\circ}\mathrm{C}$ for all gas-to-liquid mass ratios (GLRs) between 8 and 25, or at $\mathrm{GLR}<12$ for all atomizing gas temperatures; the chamber drying gas does not influence near-nozzle freezing. Additionally, we demonstrate that swirling flow intensifies flash freeze--thaw by deepening gas cooling, whereas non-swirling flow extends cold-zone residence time, yet both designs produce similar iced-droplet fractions. We construct an operating map delineating conditions that avoid flash freeze--thaw and show that the no-ice boundary provides a conservative criterion for both swirl and non-swirl nozzles. These findings identify a previously unrecognized freeze--thaw stress mechanism that can compromise spray-dried pharmaceutical product stability.