Optimizing Prediction Model For Effective Thermal Conductivity Of High Temperature Gas-cooled Pebble Bed Reactor

The high temperature gas-cooled pebble bed reactor is recognized internationally as one of the safest fourth generation advanced reactors.Its safety features are mainly based on the passive decay heat removal mechanism of the core.That is,under accident conditions,the pebble bed core can removal the core decay heat purely through the heat conduction,radiation and natural convection of the structure.The effective thermal conductivity,as an important parameter to characterize the comprehensive heat transfer capability of the pebble bed reactor,is of great significance to the thermal design and safe operation of the pebble bed reactor.The ZBS(Zehner-Bauer-Schlunder)model has a good predictability for the effective thermal conductivity of the pebble bed.However,the empirical parameter φ that characterizes the contact thermal conductivity in the model still needs to be determined,and the applicability of ZBS model in the near-wall region also needs to be improved.Therefore,this paper carries out a numerical study on the effective thermal conductivity of the pebble bed under the passive heat transfer mechanism,and improves the ZBS model based on theoretical analysis.In this paper,a pebble bed model with a simple cubic packed structure is first established.And the effective thermal conductivity of the pebble beds are numerical simulated under heat conduction、radiation and convection.The research results show that under low temperature conditions,the heat transfer of the pebble bed is mainly based on heat conduction.As the temperature rises,the main heat transfer mechanism of the pebble bed gradually transitions to radiation,while natural convection has a small contribution to the effective thermal conductivity of the pebble bed.It is almost negligible under high-temperature conditions.At the same time,it is found that by adjusting the value of the contact area coefficient φ,the prediction results of the ZBS model are in good agreement with the numerical results.Then the contact area coefficient φ in the ZBS model is analyzed.Through the numerical analysis of the effective thermal conductivity of the pebble bed under different packing structures,the effective thermal conductivity of the pebble bed and the corresponding φ value under 12 sets of contact diameter ratios and coordination numbers is obtained.And then obtain the calculation expression of φ through multivariate linear analysis.By comparing with the German SANA experimental results,it is found that the improved ZBS model has better predictive ability than other models.Meanwhile,the prediction results of the improved ZBS model are in good agreement with the effective thermal conductivity of the bulk region of the pebble bed measured by the South African HTTU experiment and previous experiments.However,the improved ZBS model still needs to improve the prediction accuracy of the near-wall region of the pebble bed under high temperature conditions.Finally,the discrete element method is used to construct a random packed pebble bed model to study the effective thermal conductivity in the near-wall region.The results show that the local effective thermal conductivity in the near-wall region of the pebble bed exhibits an oscillating attenuation and is inversely proportional to the local porosity.The average effective thermal conductivity of the wall region of the pebble bed is significantly reduced by about 21.46% compared to that in the bulk and the near-wall region.By comparing the improved ZBS model,a correction coefficient is introduced to correct the predicted value of the ZBS model in the wall region.And the revised ZBS model can be applied to predict the effective thermal conductivity in wall region of the pebble bed.