Comparison of volume-average simulation and pore-scale simulation of thermal radiation and natural convection in high temperature packed beds

The phenomenon of natural convection and thermal radiation heat transfer in fluid-saturated high temperature packed beds has been widely studied due to its various applications ranging from solar collectors to high temperature gas cooled reactor. With the local thermal non-equilibrium model, the majority of the numerical simulation studies on natural convection and radiation heat transfer in fluid-saturated porous media have limits. In these studies the internal heat transfer coefficients have always been calculated as the existing formulas, which were obtained by the experiments of forced convection in porous media [1-2]. However, natural convection heat transfer in porous media is dominated by the temperature difference between solid particles and fluid, which is different with the forced convection in porous media. For thermal radiation in porous media, the Rosseland diffusion approximation model has always been used in simulations by researchers [3-4], in which the mean absorption coefficient are not calculated according to the experiments and need to be determined by ray-tracing Monte Carlo simulations. Based on high temperature packed pebble beds, this study is aimed to compare the volume-averaged simulations and pore-scale simulations of high temperature packed beds, and predict the effective thermal conductivities of packed beds with temperature up to 1600℃. The effective thermal conductivities of the pebble beds under different temperatures are essential parameters in simulation models to analyze the maximum fuel temperature and temperature distribution in the reactor core in the reactor safety analysis. The SANA test facility was installed at the Research Centre, Julich in Germany specifically to investigate the heat transport mechanisms inside the core of a high temperature gas cooled reactor (HTGR). The validation of the volume-averaged approach and pore-scale approach are based on the experimental data of SANA test [5]. In high temperature helium-saturated annular packed pebble bed, the inner wall has a heat source and the outer wall is isothermally cooled at a lower temperature. The top and bottom walls are kept adiabatic. In the volume-averaged simulations, local thermal non-equilibrium model with the revised internal heat transfer coefficients and radiative heat flux is applied as the energy equation, and no uniform porosity distribution is used. To describe the random packed structure, PFC 3D software is used to simulate the spheres packing, which is used for direct pore-scale numerical simulations. Natural convection and thermal radiation in a 2D circular cross section of the annular pebble bed have been carried out. The effective thermal conductivities and temperature distributions of volume-averaged and pore-scale simulations of the high temperature helium-saturated annular packed pebble bed are corresponded well with the existed experimental data with temperature below 1000℃, and predict the effective thermal conductivities of the pebble bed core with temperature up to 1600℃, which are vital references for thermal hydraulic designs of high temperature gas cooled reactor core

The influence of heterogeneous structure on salt precipitation during CO2 geological storage

The presence of rock heterogeneity and fractures may cause abrupt spatial changes in capillary action and flow characteristics, which eventually change the precipitation behavior during CO2 geological storage. Therefore, the salt precipitation mechanism of the heterogeneous structure needs to be studied. In this paper, the salt precipitation behavior in different heterogeneous structures was studied through pore-scale experiments at room temperature and atmospheric conditions. In the up-down heterogeneous structure, the salt precipitation has little effect on the injectivity regardless of the CO2 injection rate. When the CO2 injection rate is low, the salt tends to precipitate in situ in the small pore structure to form a crystal structure. When the CO2 injection rate is high, the salt tends to precipitate in the large pore structure to form a cluster structure. In the left-right heterogeneous structure, regardless of the CO2 injection rate, the precipitated salt is mainly in the cluster structure, and the salt is more dispersed in distribution, the impact on injectivity is small. The injection well can be selected in the formation with strong heterogeneity, to alleviate the blockage caused by salt precipitation. When CO2 leaks in the fractures, salt tends to grow until the fracture is plugged, which is of great significance for the self-healing of the fracture for the caprock.Cited as: He, D., Jiang, P., Xu, R. The influence of heterogeneous structure on salt precipitation during CO2 geological storage. Advances in Geo-Energy Research, 2023, 7(3): 189-198. https://doi.org/10.46690/ager.2023.03.0

An efficient and low-divergence method for generating inhomogeneous and anisotropic turbulence with arbitrary spectra

In this article, we propose a divergence-free method for the generation of inhomogeneous and anisotropic turbulence. Based on the idea of correlation reconstruction, the method uses the Cholesky decomposition matrix to re-establish the turbulence correlation functions, which avoids the time-consuming procedure that solves eigenvalues and eigenvectors in every location needed by the coordinate transformation in the conventional method and thus reduces the computational complexity and improves the efficiency of generating synthetic turbulence. Through adjusting the generation strategy of specific random vectors, the proposed method, which is based on the classical spectrum-based method widely used to generate uniform isotropic turbulence, can obtain inhomogeneous and anisotropic turbulence with a relatively low divergence level in practice with almost no additional computational burden. There are two versions of this new method: the shifter version and the inverter version. Both versions of the method are highly efficient, easy to implement, and compatible with high-performance computing. Suitable for providing high-quality initial or boundary conditions for scale-resolving turbulence simulations with large grid numbers (such as direct numerical simulation or large eddy simulation), this method can be quickly implemented either based on various open-source CFD codes or common commercial CFD software

Miscible density driven convective mass transfer process analysis based on Entransy dissipation theory

Density driven convective mass transfer process in porous media is one of the most universal phenomena in underground aquifer. In this study, an original model defining Nu (or Sh) number for miscible mass transfer system was derived, based on basic concept of integrated entransy dissipation rate. Numerical simulation results of density driven convective mass transfer process in a closed Hele-Shaw cell and porous media are analyzed. In the process of dilute brine-water mass transfer system in Hele-Shaw cell, three different stages were observed. Meanwhile, time dependent entransy variation and Nu number using our definition also show three different steps in accordance with the observing phenomenon which are perturbation growing stage, instable mass transfer stage and stabilized stage. Very different fingering patterns were observed in dilute brine-water system and PEG-Water system because the latter one has not only the Non-Monotonic Density-Concentration profile but also the strong dependence of viscosity on concentration which can cause viscous-instability accompanied with density driven instability

Performance Analysis and Optimization of a Vapor-filled Flat-plate Solar Collector

AbstractSolar thermal collectors are certain to play a primaryrole in reducingenergy consumption in buildings. At present, significant efforts are being made to findnew ways to enhance their performance. An emergingtechniqueis studied in this work based on thermodynamic analysis.The study shows how thermodynamic 2nd-law analysiscan be used to complement the 1st law analysisapproach for a FPSC (flat-plate solar collector) in which the confined air in the spacing between the absorber and glass cover is replaced by water-vapor. In the analysis, the influence of key parameters such solar irradiation, inclination angle, as well as the use of absorbers with different emissivity values, are investigated

NMR Relaxation of Gas Adsorbed in Microporous Material

NMR relaxometry has been widely applied to characterize fluid confined in porous media because of its versatility, chemical selectivity, and noninvasive nature. Here we extend its usage to gas adsorbed in microporous materials by establishing a new quantitative model based on the molecular level NMR relaxation mechanism revealed by the molecular simulation of a prototypical adsorption system, CH4 adsorbed in ZIF-8. The model enables new NMR relaxometry-based characterization methods for thermodynamic, dynamic, and structural properties of adsorption systems, as demonstrated and validated by the experiments where the adsorption capacity and self-diffusivity of H2, CH4, and small alcohols adsorbed in ZIF-8 are deduced from the NMR relaxation data. The findings can serve for a better understanding of the composition–structure–properties relationships of a wide range of adsorption systems which is essential for the development and application of new functional microporous materials