Immersed boundary method

In this paper, we report the extension of an earlier developed Direct Numerical Simulation (DNS) model to study coupled heat and mass transfer problems in particulate flows. The DNS model builds on an efficient ghost-cell based Immersed Boundary Method (IBM) which implicitly incorporates the boundary conditions into the discretized momentum, thermal and species conservation equations of the fluid phase. On the particles an exothermic surface reaction takes place. The heat and mass transport is coupled through the particle temperature, which offers a dynamic boundary condition for the fluid phase thermal energy equation. The present simulations are performed for four fluid-solid systems. Following the case of the unsteady mass and heat diffusion in a large pool of quiescent fluid, we consider a stationary sphere under forced convection. In both cases variable reaction rates are imposed at the particle surface, and the particle temperatures obtained from DNS show a good agreement with analytical/empirical solutions. After that, we apply our DNS model to the three-bead reactor and finally, a dense particle array consisting of hundreds of particles distributed in a random fashion is studied. The concentration and temperature profiles are compared with a ID heterogeneous reactor model and the heterogeneity inside the array is discussed.

Multiscale modelling of dense gas–particle flows

In large-scale industrial processes involving granulation, coating, and production of base chemicals and polymers dense particulate flows with coupled mass, momentum, and heat transfer are frequently encountered. Both (effective) fluid–particle and (dissipative) particle–particle interactions need to be accounted for because the mutual competition between these phenomena govern the key features of dense gas–particle flows such as regime transitions. These interactions prevail at different length scales and consequently a multiscale approach is adopted to arrive at a quantitative description of these complex flows. In this approach detailed models, taking into account the relevant details of fluid–particle interaction (DNS) and particle–particle interaction (DEM) are used to obtain closure laws to feed two-fluid models (TFMs) which can be used to simulate the flow on a much larger (industrial) scale. In this chapter, we will discuss recent advances in the multiscale simulation of dense gas-fluidized beds. The governing equations will be presented as well as the key features of the numerical solution method. For each model type, illustrative computational results will be presented. Finally, areas which need substantial further attention will be discussed

Weathering of a polyester-urethane clearcoat: lateral inhomogeneities

This paper is devoted to the surface analysis of a polyester-urethane coating during weathering under different conditions using artificial weathering machines. By means of atomic force microscopy (AFM), the evolution of the surface topology of the coatings is studied. Degradation is shown to be a laterally inhomogeneous process. The presence of water facilitates material removal and leads to an increase in the surface roughness and consequently a gloss loss. In addition, by comparing degradation under aerobic and anaerobic conditions, it is shown that oxidation reactions are the main cause of lateral inhomogeneous degradation of coatings

Depth-resolved infrared microscopy and UV-VIS spectroscopy analysis of an artificially degraded polyester-urethane clearcoat

Polyester-urethane resins are important candidates for high performance, outdoor coating applications. Despite their relevance, quantitative information regarding the photodegradation of these materials is scarcely available. In the present study, a model polyester-urethane clearcoat without additives is artificially degraded and the changes in optical properties and chemical composition are studied by UV-VIS spectroscopy and FTIR-ATR microscopy, respectively. The change of the optical properties throughout the coating thickness is quantified and interpreted using a newly developed optical model. Chemical changes are quantified in a depth-resolved manner by combining FTIR-ATR microscopy with optical profilometry in order to visualise the time evolution of compositional gradients during photodegradation by accurate assignment of the correct depth position to all recorded ATR spectra. The rate of change for the concentration of several chemical entities in the model polyester-urethane was found to become constant after the initial stage of weathering. The loss of urethane crosslinks in the coating occurs faster and to a much larger extent as compared to ester bond scission. Results from the optical and the chemical characterisation are combined to propose a kinetic model for ester bond photolysis that provides an estimate of the quantum efficiency of this process

Multi-scale simulation of degradation of polymer coatings: Thermo-mechanical simulations

In this work we simulate the full sequence of steps that are also typically performed in an experimental approach when studying photo-degradation of a polymer coating, namely, i) sample preparation, ii) photo-degradation and iii) thermo-mechanical analysis of the material during photo-degradation. In the current paper, we focus on performing several molecular dynamics simulations to study the thermo-mechanical properties of a virgin thermoset coating as well as degraded ones. Using an atomistic structure that is obtained by fine-graining the mesoscopic structure, we obtain consistent correlations between the simulated thermo-mechanical properties of the material and those measured experimentally. In addition, it is shown that by using oscillatory strain fields in MD - instead of the commonly applied linear tensile/compression strain fields - one can acquire greater knowledge on the structure-property relation of polymeric materials. Eventually, we show that our simulation approach gives rise to a remarkable insight into the mechanism of the photo-degradation process

Quantitative spectroscopic analysis of weathering of polyester-urethane coatings

Transmission FTIR analysis of polyester-urethane coatings (PUC), that were degraded under different accelerated laboratory weathering conditions, are compared. The aim of this comparison is to deepen our insight into the chemical pathways of weathering of polyester-urethane coatings. We monitored the chemical changes for different environments, such as aerobic or anaerobic conditions as well as wet or dry conditions, in order to increase our insight into the effect of each individual stress factor, i.e., photons, oxygen, temperature and water, on the chemical pathways of weathering. We showed that the degradation of urethane groups proceeds via photo-oxidative pathways and that the ester groups mainly degrade via photolytic reactions. The ester bond scission accelerates after an initially-slow-rate stage of weathering in the presence of urethane groups. This is due to an increase in the optical absorptivity of the coating as a result of degradation under anaerobic conditions, as shown before. By means of a kinetic analysis using a combination of FTIR and UV–Vis spectroscopy results (obtained before), we found that a first-order kinetic model can perfectly describe the rate of ester bond scission during the weathering and that the increase in the rate of reaction is due to the increase in the light absorptivity of the coating as a result of degradation. Finally, using the interference fringes of FTIR spectra, we showed that evaporation and water-caused removal of degraded material cause a particularly pronounced decay in the thickness of the coating. In the absence of water spray, the material loss takes place in the same period as urethane groups decompose and stops afterwards, even though the ester bond scission proceeds with higher rates. This supports the hypothesis of photo-oxidative pathways for the urethane group decomposition and photolytic mechanisms for ester bond scission. Dark experiments showed that PUC coatings are highly resistant to hydrolysis and thermal degradation