Conference Dates

June 22-27, 2014

Abstract

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.

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