CFD Study On Single-phase Convective Heat Transfer In Structured Packed Beds

As a significant design concept for novel nuclear reactors,Pebble Bed Water-cooled Reactor(PBWR)has distinctive advantages in safety and economy,both as always ensuring that nuclear power plays a consequential role in the present global configuration of energy.Compared to the randomly packed bed,the structured packed bed possesses better overall heat transfer performance and a considerably vast prospect for the industrial application.Therefore,the researches into convective heat transfer within the structured packed bed are substantial for the optimal design of PBWR and perfection of the simulation methods in pebble beds.With CFD approach,the paper investigated the convective heat transfer characteristics and their effect factors in the ordered packings of Simple Cube(SC),Body-Centered Cube(BCC)and Face-Centered Cube(FCC)under turbulent condition,and devised rudimentarily the Particle Image Velocimetry(PIV)experiment in BCC structure in matching between particles and liquid working fluid and design of the test section to verify simulation results.The main content and results of the research are listed as follows:1.With contact points between particles being dealt with the bridge method,appropriate geometrical scale and bridge diameter of pebble bed models were determined to analyze local flow and heat transfer characteristics and effects of the bridge diameter on them.It was found that SC and FCC packed bed can be simplified to quar-unit channel,while it would be suitable for BCC to utilize unit channel in CFD simulation,resulting from a flow conversion from axisymmetry to centrosymmetry in turbulence,raising the pressure gradient and convective heat transfer coefficient notably.The location where flow symmetry changes is affected not by the number of layers in axial direction but by Reynold number.The increase of cylinder diameter reduces the local pressure drop in BCC and FCC packed bed,but it does not influence evidently the pressure drop and heat transfer characteristics of these packed bed when it is no higher than 10% of particle diameter,which was recommended here for the bridge diameter.2.The effects of particle diameter,heating condition and fluid variety were scrutinized on single-phase convective heat transfer characteristics.It indicated that the pressure gradient and convective heat transfer coefficient both decrease as the particle diameter rises,and itself respectively,and raising the particle diameter helps improve the overall heat transfer performance,but particle diameter does not affect the axial distribution characteristics of pressure drop and convective heat transfer coefficient.In addition,the coefficients of friction and Wakao-typed heat transfer correlation are not influenced by heating condition and fluid variety.The trend how the average friction coefficient varies with Reynold number in BCC packed bed is obviously different from that in SC and FCC one.3.The particle material and liquid working fluid were found to meet the requirements of the PIV experiment in packed bed,and the anticipated results and problems worthy of attention of test section designs were discussed.It is relatively proper that Pyrex glass as pebble material and NaSCN solution with mass fraction 57% as working fluid are matched.Then,as for the test section design,it is difficult in the whole-sphere packing project to weaken the interference of the wall effect on the symmetry change in the flow field,so the experiment objective cannot be achieved;BCC frame project could prove the flow change in BCC packed bed,but there would be a great deviation in the measured velocity and pressure drop;the project of half-spheres on the wall turned out to be best in the effect comparison,but the manufacture of half-spheres and quar-spheres made of Pyrex glass can not satisfy the demand of this experiment.