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Description
Evaporation in confined and porous-like systems is commonly described using diffusion-limited models that assume local thermodynamic equilibrium and steady thermal boundary conditions. However, many practical processes involve continuously varying temperatures, for which the validity of equilibrium-based evaporation laws remains uncertain. This work investigates the transition from equilibrium to nonequilibrium evaporation under controlled linear temperature ramping, using both sessile droplet and pool (filled crucible) configurations as model systems.
Thermogravimetric analysis (TGA/DSC) experiments were conducted on deionized water subjected to rates of temperature increase ranging from 5 to 100 °C min⁻¹ under controlled nitrogen purge conditions. The experimental mass-loss dynamics were compared with predictions from a simplified analytical diffusion-based model and detailed multiphysics finite-element simulations coupling heat transfer, vapor diffusion, fluid flow, and interface motion. At low rates of temperature increase, both models accurately reproduce the experimental evaporation behavior, consistent with well-established diffusion-controlled evaporation studies under near-equilibrium conditions. As the rate of temperature increase rises, systematic deviations emerge between experiments and model predictions.
Above a critical rate of temperature increase of approximately 40°C min⁻¹, evaporation accelerates abruptly, drying times are increasingly overpredicted by equilibrium-based models, and pronounced evaporation-rate instabilities appear, particularly in the filled crucible configuration. These instabilities are associated with localized superheating and intermittent boiling events, which are not captured by diffusion-limited formulations. A timescale-based equilibration analysis reveals that these deviations coincide with the breakdown of both thermal and vapor-phase equilibration, indicating that rapid temperature increases reduce the separation between evaporation, heat-transfer, and vapor-adjustment timescales.
The influence of vapor removal was further examined by varying the nitrogen purge rate. Enhanced vapor removal stabilizes the evaporation process by promoting evaporative cooling and suppressing vapor-phase accumulation, thereby delaying or mitigating nonequilibrium effects. Overall, the results demonstrate that the rate of temperature increase is a key control parameter governing the transition from diffusion-limited evaporation to nonequilibrium, boiling-influenced regimes.
These findings highlight the limitations of classical evaporation models under transient thermal conditions and provide quantitative guidance on their domain of validity. The study offers insight into phase-change dynamics relevant to drying, thermal processing, and evaporation in confined and porous media subjected to non-steady thermal forcing.
| Country | France |
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