Study On The Motion Characteristics And Heat Transfer Characteristics Of Liquid-solid Two-phase Flow In Converging-diverging Tube

With the increasing demand for efficient utilization of energy,the composite technology based on the combination of multiple heat transfer enhancement methods can effectively improve the heat transfer efficiency of the heat exchanger.Therefore,the composite heat transfer technology of liquid-solid two-phase flow in the scaling tube is proposed.By exploring the interaction process between liquid-solid two-phase flow in the scaling tube,it is of great significance to analyze the work efficiency of the liquid-solid two-phase flow heat exchanger.In this paper,the development status of the liquid-solid two-phase flow technology in the zoom tube is introduced.The flow field characteristics of the liquid-solid two-phase flow in the zoom tube are observed by PIV experiment,and the motion and heat transfer characteristics are further numerically simulated.The corresponding conclusions are obtained.The dissertation mainly includes the following contents.1.The research status of the converging-diverging tube and the liquid-solid two-phase flow is briefly analyzed.Through the summary and comparative analysis of the research methods of the liquid-solid two-phase flow,it is shown that the CFD-DEM coupling method has advantages in the numerical simulation of the liquid-solid two-phase flow in the converging-diverging tube.2.The flow field characteristics of the central longitudinal section of the vertical tube were experimentally studied by PIV technology,and the results were compared with the CFD-DEM coupling simulation results.The experimental results were consistent with the simulation results.The results show that the total average vorticity of the longitudinal section increases with the increase of the inlet velocity of the fluid and the inlet concentration of the particles.The fluctuation of the fluid velocity near the converging-diverging tube wall is mainly affected by the structure of the converging-diverging tube,and the fluctuation of the fluid velocity in the central region is affected by the particles in the tube.3.The CFD-DEM coupling method is used to simulate and study the effects of scaling ratio?,rib height e,fluid inlet velocity?and particle inlet concentration?on the motion characteristics of the particles in the tube.The results show that the force fluctuation of the particle group is the largest in the scaled tube with e=2.0 mm and?=2.0.When??0.6m/s,??3%,the bottom-up distribution of particles in the zoom tube presents the characteristics of gradual concentration from the initial uniform to the near wall.When e?1.5mm,??3%,??0.8m/s,??1.0,the concentration of particles in the center of the tube is higher,which is prone to particle agglomeration.The axial average velocity of the particles increases first and then fluctuates around the fixed value,and the fluctuation is small.In the fully developed phase of liquid-solid two-phase,the velocity distribution of particle group in the radial region is roughly inverted U-shaped,and the particle velocity difference at the same radial position increases with the increase of the scaling ratio?and rib height e.4.On the basis of the research on the motion behavior of particle group in the converging-diverging tube,the influence of scaling ratio?,rib height e,fluid inlet velocity?and particle inlet concentration?on the heat transfer characteristics of liquid-solid two-phase flow in the tube was analyzed,and the heat transfer performance was evaluated by comprehensive evaluation index.The results show that the influence of scaling ratio?and smaller particle inlet concentration?on heat transfer is greater than that on resistance,and the influence of fluid inlet velocity?,rib height e and larger particle inlet concentration?on resistance is greater than that on heat transfer.When particle inlet concentration?=3%,fluid inlet velocity v=0.4m/s,rib height e=1.0mm,scaling ratio?=2.0,PEC_max=1.246>1,the heat transfer enhancement effect is the best.