Speaker
Description
Thermal imaging offers a unique opportunity to study gas/vapour–liquid mass transfer processes and liquid-phase hydrodynamics. In this work, we develop quantitative infrared thermography (QIRT)-based methods for the study of flowing liquid films under absorption and distillation conditions.
Depending on the system under study, the thermographic outcomes are two-fold. For semi-transparent liquids in the long-wave infrared range, such as selected organic solvents and their mixtures, the recorded signal contains information related to local film thickness distribution and hydrodynamic patterns.
If infrared-opaque liquids are studied (typically aqueous systems and alcohol mixtures), information about the liquid–vapour interface temperature of the film is acquired, which opens possibilities for the evaluation of local interfacial mass transfer intensity distributions, active interfacial area assessment, or monitoring of thermal effects of chemisorption. These techniques are not sufficiently developed, and the corresponding quantities are experimentally inaccessible under distillation conditions, leading to significant uncertainties and simplifying assumptions in current distillation models.
This specific application of quantitative infrared thermography is associated with substantial radiometric challenges. Observed gas–liquid systems must be closed with IR-transparent windows. Moreover, if there is a layer of vapour between the film and the window, the observed system is complex, and the obtained thermograms must be carefully evaluated using a specifically developed radiometric model. This model takes into account absorption and emission of the vapour, apparent absorption of the window caused by multiple internal reflections at the optical interfaces, and reflected background radiation from the surroundings. Additionally, both the liquid and vapour phases often behave as spectrally selective emitters, which must be accounted for in quantitative analysis.
This contribution focuses on the influence of the vapour layer and the window in the optical path between the target and the camera on the apparent temperature measured by the camera during observations of infrared-opaque liquid films. A radiometric evaluation model and methodology are developed to reconstruct the true liquid–vapour interface temperature of the radiating liquid film under distillation-relevant conditions. The experimental apparatus, consisting of multiple parallel KBr windows and a layer of semi-transparent vapour with controlled temperature between them, is employed for model validation. The results demonstrate the necessity of detailed radiometric modelling for reliable quantitative thermographic measurements in distillation-relevant gas–liquid systems, as well as the applicability limits of this methodology.
