Experimental Investigation And Numerical Simulation Of The Effective Thermal Conductivity Of Pebble Bed Under Passive Heat Transfer Mechanism

The high temperature gas cooled pebble bed reactor(HTGR)is one of the advanced fourth generation nuclear reactors.During loss of forced cooling conditions,the decay residual heat of the core must be derived by passive heat transfer mechanisms,including conduction and radiation.The effective thermal conductivity of HTGR represents the macroscopic heat transfer ability and is of great significance for thermal hydraulic design and safety of HTGR.In this paper,the effective thermal conductivity of the pebble bed under different working conditions is measured by developing an experimental facility,such as temperature,the porosity,the diameter of the pebbles,the material of the pebbles and packing structure.Based on the pore-scale numerical method,the effective thermal conductivity of a simple cubic packed pebble bed is simulated.The simulation results are compared with the experimental data.The contribution of three main heat transfer mechanisms:(1)conduction through the pebbles and stagnant fluid,(2)conduction through contact points of the pebbles,(3)thermal radiation in the core to effective thermal conductivity are analyzed.The main contents are as follows:An experimental facility has been developed to measure the temperature distribution in the pebble bed under passive heat transfer mechanism.The effective thermal conductivity is obtained by solving the Fourier law of heat conduction.It is found that the porous structure of the packed pebble bed changes significantly near the wall,which led to the downward trend of the effective thermal conductivity in the near-wall region.At the same time,the effects of temperature,pebble diameter,porosity and the pebbles thermal conductivity are analyzed.The graphite thermal conductivity is much higher than nitrogen thermal conductivity and the heat mainly transfers through solid phase conduction.Meanwhile,graphite thermal conductivity decrease with the rising temperature.Therefore,as the temperature increases,the effective thermal conductivity of graphite pebble bed decreases.On the other hand,as the temperature increases,the radiation heat transfer gets stronger,which leads to downward trend of the effective thermal conductivity with the temperature becomes slower.The effect of particle diameter on the effective thermal conductivity is not obvious in the experiment,but under the same conduction,the effective thermal conductivity of the large pebbles is slightly larger than the small pebbles.The porosity of the pebble bed has great influence on the effective thermal conductivity.When the porosity gets larger,the proportion of the pebbles gets smaller and the effective thermal conductivity is smaller.The effective thermal conductivity of the stainless pebble bed increases as temperature rises because the thermal conductivity of the stainless increases slowly with the increase of the temperature.The experimental results are compared with the correlations of the effective thermal conductivity.It is found that,experimental results fit reasonably well with the correlations.Based on the pore-scale numerical method,effective thermal conductivity of a simple cubic pebble bed is numerically studied.The heat transfer characteristics are analyzed.Meanwhile,the numerical results are compared with experimental results.At last,the contributions of three heat transfer mechanism including conduction through the pebbles and stagnant fluid,conduction through contact regions and thermal radiation to the effective thermal conductivity are analyzed in this paper.It is found that the numerical results are in agreement with the experiment data.The contact conduction is the dominant heat transfer mechanism in the temperature range of present simulations(less than 500°C),which contribution could reach 95.96% to 59.11%.The contribution of radiation to effective thermal conductivity gets greater as the temperature increases ranging from 1.8%-36% when temperature rising from 30-500°C.The solid-fluid conduction plays an insignificant role to effective thermal conductivity,which accounts for only 2.26%-5.33%.