Investigation On Pore-scale Spectral Radiative Transfer In High-porous Foam Materials

As a new material for heat exchange,high-porous open-cell foams are promising in many thermal engineerings,such as volumetric solar receivers,electronics cooling,enhanced heat exchangers,and energy storage.In these high-temperature applications,precise analysis of radiative transfer is of fundamental importance for better performance of system.So far,however,there is much less investigation on the complex pore level radiative transfer mechanism in open-cell foams.Meanwhile,the pore-scale interaction between the radiation and porous structure is not clearly known yet.The data of radiative properties at high temperature are inadequate,and aviailable predictive method is also relatively scarce.These defects limit the development and application of advanced techniques involving open-cell foams.The present study aims to investigating the radiative transfer process in high-porous open-cell foams at pore level.Five parts are included:(1)parametric characterization and reconstruction of porous structure;(2)pore-scale radiative transfer modeling and analysis;(3)prediction of radiative properties data;(4)experimental measurements and validation of radiative behaviors;(5)a new scale-coupled approach.The structure morphologies and commonly used parameters of nickel and alumina foams are obtained by 2D scanning electron microscope(SEM)and 3D X-ray micro-computed tomography(?-CT)techniques.Based on the morphologies information,the parametric foam geometry is reconstructed by Voronoi cell models.Furthermore,typical solid struts are precisely redefined.Considering open-cell foams made of opaque(metal)and semitransparent(ceramic)components,the Monte Carlo ray-tracing(MCRT)method is used to simulate the pore-scale radiative transfer in the limit of geometric optics.Besides,a predicted mo del is established to obtain volumetric radiative properties directly from the pore-scale porous structures.Relative algorithms are compiled and validated.Specially,an acceleration algorithm is added into the MCRT one to cope with the computational challenge.Nickel foams are investigated based on the ?-CT structures in the aspects of emitting and reflection characteristics at pore level.Meanwhile,generalized analytical relations of extinction coefficients,scattering albedos,and scattering phase functions are established on the reconstructed artificial foams.Furthermore,the volumetric properties of nickel and copper foams at high temperat ure are predicted.Experiments are conducted on several nickel foams at two set-ups for validation:(1)normal-normal transmittance is measured by Fourier Transform Infrared(FTIR)Spectrometer;(2)normal-hemispherical transmittance and reflectance are measured by an in-house system established on an integrating sphere.The numerical results are confronted with the measured ones,and they are in good agreements.The models and methods are applied in analyzing the blackbody temperature of nickel foams,and the validation of contious-scale apprach when simulating the blackbody temperature is considered.For the open-cell foams made of semitransparent components,alumina foams,as an interesting case,are investigated in the aspects of solid struts and foam sh eets.Based on the artificial foams,generalized analytical relations of extinction coefficients,scattering albedos,and scattering phase functions are established.The volumetric properties of alumina and zirconia foams at high temperature are predicted.Directional and hemispherical measurements are conducted on alumina foams to validate the numerical simulation and predictive relations.A novel modeling method called scale-coupled approach(SCA)is proposed to overcome the defects of classical continuous-scale approach(CSA)and discrete-scale approach(DSA).Compared with the CSA and DSA,the SCA can offer information on the geometry-dependent radiative quantities and at the same time on the equivalent continuous quantities through a single simulation.Besides,this new approach leads to a reduction in computational time by approximately one order of magnitude compared to the DSA which relies on a full-size foam geometry.Through present study,parametric geometry is developed according to real foams.The pore-scale modeling method for radiative transfer analysis in foams made of semitransparent struts are established.A novel scale-coupled apprach is proposed to overcome the defects of classical contiunou-scale apprach and discrete-scale apprach.The high-temperature radiative properties of nickel,copper,alumina foams are predicted.The relationships between radiative properties and structural parameters as well as component properties are established.These findings can provide a deeper understanding of radiative transfer mechanism in porous structures and support the application and design of such materials.