Visualization experimental study on NO2 condensation process: Insights into gas-fog-liquid-ice mode evolution.

• A visualization experimental system for NO 2 gas condensation is designed. • Fog, droplet-streamlet-film (DSF), and freezing condensation modes are revealed. • Formations of DSF mode divided into 4 periods at varying conditions are quantified. • Relationship between mode evolution and heat transfer performance is demonstrated. • Insight into evolution of each mode regarding NO 2 -N 2 O 4 dimerization effect is gained. Pure nitrogen dioxide (NO 2) is a profitable feedstock for production of various chemicals. The condensation technology plays an essential role in NO 2 enrichment during production or resource recovery from industrial exhausts, which, however, remains challenging in process optimization due to the lack of thorough comprehension on NO 2 condensation behavior. In this work, the visualization experiments of NO 2 condensation on the vertical SS316 surface at varying temperatures, NO 2 mole fractions and volume flow rates in the absence and presence of non-condensable gas (NCG) were conducted. Three major NO 2 condensation modes, the fog mode, the droplet-streamlet-film (DSF) coexistence mode, and the freezing mode, have been identified. The formation process of NO 2 gas-fog-droplet-DSF mode as well as the occurrence frequency of streamlets (OFS) was quantified under different conditions. The higher OFS was found to contribute to greater overall heat transfer coefficient and condensate amount, showing the optimal parameters at each condition. Deeper insights into gas-fog, gas-liquid, and gas-ice phase transformations for NO 2 were gained, revealing the unique effects of dimerization of NO 2 to N 2 O 4 on condensation. The dimerization triggers the facile formation (< 10 s) of N 2 O 4 fog with increased intermolecular forces, the heterogeneity of heat and mass transfer in DSF mode with complex heat resistance network and condensate states, and the melting-thickening cycle of ice layer in the freezing process along with the release of dimerization reaction heat. The findings render a microscopic view of favorable heat and mass transfer towards optimal NO 2 condensation strategies in practical uses. [Display omitted]. [ABSTRACT FROM AUTHOR]