Modeling and measurement of sodium chloride and ammonium sulfate droplet growth in laminar flow
Zhao, Zhiyu
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https://hdl.handle.net/2142/132627
Description
Title
Modeling and measurement of sodium chloride and ammonium sulfate droplet growth in laminar flow
Author(s)
Zhao, Zhiyu
Issue Date
2025-10-06
Director of Research (if dissertation) or Advisor (if thesis)
Brewster, M. Quinn
Department of Study
Mechanical Sci & Engineering
Discipline
Mechanical Engineering
Degree Granting Institution
University of Illinois Urbana-Champaign
Degree Name
M.S.
Degree Level
Thesis
Keyword(s)
droplet growth
Abstract
Cloud droplet growth is governed by a balance between vapor diffusion, Kelvin effect, and Raoult’s law, as expressed in K¨ohler theory. In this study, a thermodynamic droplet growth framework initially developed by Brewster and Li (2021) for radiation-induced cooling of pure water droplets is extended to include the effects of dissolved salts through an ion-number-based weighting in the Kelvin–Raoult balance. Two atmospherically relevant salts—sodium chloride (NaCl) and ammonium sulfate ( (NH4)2SO4)—are investigated at multiple concentrations in a controlled laminar-flow environment.
Droplets are generated via ultrasonic nebulization, transported through a polyethylene tube under well-characterized temperature and humidity, and measured using optical particle sizing at multiple downstream positions. The model predicts the temporal evolution of droplet radius by solving the coupled mass and heat transfer equations with supersaturation determined from the modified K¨ohler equilibrium.
Results show that the extended model captures the droplet size distribution’s primary mode diameter and general shape for both salts across the tested concentrations. Higher ion yield and molar mass effects for ( NH4)2SO4 lead to greater growth enhancement compared to NaCl, consistent with the Raoult term’s suppression of equilibrium vapor pressure. Discrepancies at high solute concentrations and in the large-droplet tail (> 20 μm) are attributed to neglected hydrodynamic coalescence and concentration-dependent surface tension effects.
This work demonstrates that pure water thermodynamic growth models can be extended to salt droplets with modifications, providing a validated basis for parameterizing cloud and climate models.
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