Study On Flow Characteristics Of Single Phase And Gas-liquid Two Phase Flow In Packed Bed With Spheres

Water-cooled pebble bed reactor is a new concept reactor that the combination of the spherical fuel element and light water reactor technology.It has advantages of a simple design,modular construction,long life,high safety and inherent ability to prevent nuclear proliferation.Reactor core are closely packed with spherical fuels.Flow characteristics of single-phase and two-phase flow will provide reliable experimental basis and technical support to design and safety analysis of water-cooled pebble bed reactor.In this paper,Experimental research and numerical simulation were carried in the channels are packed with glass spheres of which the diameters are 2mm,3mm,5mm and 8mm for flow characteristics of single-phase gas-liquid two-phase.The main work is as follows:1.Build the pebble bed channel single-phase flow bench,and on this basis a multi-size,full-range study of resistance to single-phase flow experiments.The results show that:the pore velocity and hydraulic diameter of the characteristic velocity and Reynolds number correction feature sizes,better able to reflect the different structural parameters pebble bed channel uniform flow pattern.According to the overall flow resistance characteristics and local information channels ball bed Darcy single-phase flow area is divided into Darcy zone,Forchheimer zone and turbulent zone.Through present,the experimental data in the literature to calculate the single-phase relationship validation showed resistance:KTA correlation is not only in the form of simple expression,but also has high accuracy over the entire experimental range.In addition,the scope of KTA correlation was also extended correlation.2.Randomly packed channel was constructed by using discrete element method.In order to improve the degree of dense of the spheres,the method of reducing friction coefficient was used.In the meshing processing,the contact problem between the spheres was solved by the bridge method.Based on the verification and validation model,flow resistance was calculated under high Reynolds number,local pressure and velocity distribution were analyzed.Finally,grid applicability for the flow resistance and the heat transfer characteristics of packed channel under laminar and turbulent were analyzed,the reasonable meshing criteria was determined.3.By high speed camera,the gas-liquid flow patterns in packed channel were visualization.In the vertical upward flow conditions,typical flow patterns within the packed channel are:bubbly flow,pulse flow and annular flow.The flow maps were also presented,it was found that the pulse flow region was dominated.The comparisons of flow pattern map of between packed channel and non-packed channel show that the bubbly flow region in packed channel is narrower than that of non-packed channel,due to the packing.The comparisons of flow pattern map for three different packing size show that the cluster flow region expands with the increase of the packing diameter.4.Void fraction of packed channel was investigated experimentally.The results show that:under the same condition,void fraction was decreasing with the diameter of the sphere,void fraction is always less than the volume fraction,which is consistent with the traditional channel.But in different liquid and gas flow,there are two variations between void fraction and volume fraction,nonlinear and linear variation.On the basis of the drift flow model,a new void fraction calculation for packed channel was present,the calculated values in good agreement with the experimental values and also with literature values.5,The resistance characteristics of gas-liquid two-phase flow packed channel was studied under high Reynolds number.The Mass flow rates,flow pattern and parameters of channel influence on the flow resistance of two-phase were discussed.The results showed that:large pebble bed parameters on the two-phase resistance.The two-phase resistance correlations from literature were evaluated,using Lockhart-Martinelli method,a new correlation was derived,the calculated values is in agree well with the experimental data in the thesis.