Analysis Of Flow And Heat Transfer Characteristics In Radial Layered Packed Bed With Spheres

In the packed bed,the porosity rises sharply near the wall because the geometrical structure of the filler is interrupted by the container.In this radial porosity distribution,the velocity of fluid near the wall is relatively high when the fluid flows through the channel of the spherical bed,which is called "wall effect".Due to the influence of sidewall effect,the flow and temperature distributions near the tube wall and the center of the packed bed are quite different.Therefore,the pressure drop per unit resistance of the conventional structure(single spherical diameter)of the pebble bed channel is often higher.Because of the low velocity in the center of the channel and the existence of dead zone,the fluid distribution in the packed bed is not uniform,and there is obvious bypass flow on the wall,which reduces the convective heat transfer performance in the packed bed.Therefore,in order to solve the side wall effect of small tube diameter ratio spherical bed channel,the fluid distribution in the spherical bed filled with spherical bed channel is more uniform,thus improving the local heat transfer capacity.In this paper,the filling process is simulated by the discrete element method(DEM).The difference of radial pore distribution between radial layered composite pebble bed channel and conventional structure pebble bed channel was compared.Four colorless transparent glass spheres(diameter ratio range is 3-10)with 100 mm inner diameter and 8 mm,10 mm,12 mm and 14 mm inner diameter were selected.The spherical particles with 14 mm inner diameter were filled in the center of the radial layered composite structure,and the resistance pressure drop experiment and numerical simulation were carried out by changing the outer spherical particle size.By changing the spherical diameter ratio,different radial layered composite spherical beds are obtained,and the geometrical structure of these spherical beds is introduced into computational fluid dynamics(CFD)simulation.The fluid flow and heat transfer characteristics in a spherical bed were studied.The CFD model was validated by experiments and numerical simulation.Through model validation,porosity analysis,pressure drop resistance analysis and heat transfer characteristics analysis,it is found that radial layered composite spherical bed passage has advantages in reducing pressure drop and improving overall heat transfer efficiency.At the same flow rate,only about 30%-40%of the resistance pressure drop of conventional structure spherical bed passage is needed,and the heat transfer efficiency is significantly improved compared with the experimental and numerical results.The simulation results show that the average relative error of unit pressure drop is about 10%in the experimental process and numerical simulation process of conventional structure spherical bed channel,and that the experimental process and numerical simulation process of radial layered composite structure spherical bed channel are in good agreement with each other,with an average relative error of 12%,of which the minimum relative error is 4.5%,which meets the expected requirements.