Model Of Discrete Particle Heat Transfer With Constitutive Model

High-temperature gas-cooled pebble bed reactor,the fourth-generation reactor,is one of the most promising for its outstanding efficiency and safety.Its main heat transfer methods are conduction,natural convection,and radiation.Effective thermal conductivity is an important parameter to measure the heat transfer capability of the pebble bed.At low temperatures,the heat transfer mode of the particles in the pebble bed pile is mainly contact heat conduction.Measuring or modeling effective thermal conductivity and thermal contact resistance is one of the critical issues in the study of pebble bed particle systems.A large number of researches have been carried out in the world,and most of the research results focus on the effect of porosity and thermal conductivity ratio on the effective thermal conductivity without considering the influence of coordination number.Based on this background,this paper will systematically study the effective thermal conductivity of packed beds from point-contact heat conduction,surface-contact heat conduction,and particle-fluid heat conduction.In this paper,based on the thermal discrete element method(TDEM),the relationship model between effective thermal conductivity and contact thermal resistance is established,and the total effective thermal conductivity of the pebble bed at low temperature is deduced from this model.This model can embed the existing effective thermal conductivity model into the open-source discrete element particle simulation software LIGGGHTS to simulate the heat transfer of particles in a randomly packed bed.The effective thermal conductivity of the pebble bed under vacuum and interstitial fluid is solved analytically and compared with the experimental data to verify its accuracy.The temperature simulation of the steel ball experiment was carried out using the relationship model between the effective thermal conductivity and the contact thermal resistance,and the reliability of the model was analyzed by comparing the simulated temperature with the experimental temperature.The temperature simulation of the steel ball experiment is accelerated,and the accuracy of the acceleration method is verified by comparing the simulation results before and after.At the same time,the heat transfer simulation of each empirical correlation was carried out,and the simulation results were compared with the experimental results.The total effective thermal conductivity is simulated to the prediction results of the HTTU experiment,and the simulation results are compared with the experimental results.The results show that the analytical solution for the effective thermal conductivity as a function of porosity under vacuum agrees well with the experimental data.Under the condition of interstitial fluid,it is obtained that when the thermal conductivity ratio of particles to liquid is greater than 10000,surface contact conduction dominates.In contrast,point contact conduction dominates when the thermal conductivity ratio is less than 2000.The experiment of measuring the effective thermal conductivity of the steel ball is simulated,and the simulated temperature results are consistent with the experimental results.By comparing the simulated data before and after the acceleration,it is found that the simulated temperature data of the two are almost the same,and the error is not more than 1%.The method of accelerating the simulation is feasible.The heat transfer simulation of the steel ball experiment is carried out for each empirical correlation of the effective thermal conductivity,and it is found that the simulation results are consistent with their prediction results.Finally,the bulk heat transfer simulations performed for the HTTU experiment in South Africa show that the simulated results agree with the experimental results in the radial distribution.The effective thermal conductivity model can effectively predict the effective thermal conductivity of low-temperature packed beds.