Pore scale simulation of transient spectral radiative properties during evaporation of moisture from a porous substrate with adsorbed droplets

Radiative heating has been widely used to enhance drying and evaporation in porous media, where spectral radiative properties directly affect radiative energy deposition and temperature field evolution. For microscale droplets attached to porous skeletons, the droplet size can be comparable to the infrared wavelength, and the apparent radiative response is strongly affected by wave optical effects, substrate reflection, and morphology evolution. Therefore, fixed radiative parameters are insufficient to describe the time dependent radiative energy absorption during evaporation. In this work, a pore scale coupled computational framework for evaporation, heat transfer, and radiation was developed to obtain the transient apparent absorptance, reflectance, and transmittance of a droplet substrate system. Nickel and alumina substrates were selected as representative opaque and semitransparent substrates, respectively. The transient spectral responses were investigated at 3.1, 4.5, and 8.3 μm, and the effects of radiative intensity, initial droplet size, and Jakob number were evaluated. The results show that the apparent absorptance is closely related to the incident wavelength, substrate material, and evolving droplet radius. A higher absorption level and stronger nonlinear variation are obtained under short wave irradiation, whereas a smoother response is observed at longer wavelength. On the metallic substrate, secondary absorption and oscillations are enhanced by strong substrate reflection. In contrast, lower absorption and weaker oscillations are obtained on the semitransparent substrate because part of the radiative energy is transmitted through the base. The radiative intensity and Jakob number mainly change the evaporation time scale, while the initial droplet size affects the absorption level during evaporation. The obtained transient spectral radiative parameters can serve as pore scale inputs for macroscale heat transfer, flow, and phase change models in porous media.