Natural convection heat transfer analysis of self heating pebble bed nuclear reactor experiments with openfoam code

In this study, the finite volume method was used to numerically model the natural convection flow in the pebble bed geometry. Numerical analysis of natural convection experiments in pebble bed geometry made in the bilateral cooperation project of TÜBİTAK and South Korea (NRF) was performed using the OpenFOAM v2112 code. The mesh structure of the complex pebble bed geometry consisting of randomly placed spheres was successfully created. The diameters (d) of the spheres are 0,004; 0,006 and 0,01 m, and their associated Rayleigh numbers are 5,43e+06; 1,83e+07 and 8,48e+07, respectively. Pebble bed heights of H/d=5, 10 and 20 were used in the simulations. By comparing the OpenFOAM simulation results of the natural convection flow for the spheres in the pebble bed with the experimental results, it was shown that the velocity distributions and Nusselt values are in good agreement. In addition, it has been shown from the velocity and temperature distributions in the pebble bed geometry that there is a large temperature increase in the low velocity areas near the spheres and a small velocity increase in the middle and upper elevations. It has been shown that increasing the height of the pebble bed geometry and the consequent increase in the amount of spheres in it complicates the flow and also creates local hot spots. The Nusselt number correlation obtained as a result of the experimental study was confirmed by the OpenFOAM results. The significance of this study is that it uses the finite volume method to evaluate the natural convection flow in all pebble beds and simulates it using a significant number of spheres. These two factors contribute to the originality of this work.