Influencing factors of flow field characteristics of liquid-solid two-phase flow in converging-diverging tube heat exchangers

PENG Deqi; LIU Jian; FU Weng; YU Tianlan; TAN Zhuowei; WU Shuying; LI Fang

doi:10.13732/j.issn.1008-5548.2025.03.006

Abstract：Objective To optimize the design of the converging-diverging tube liquid-solid two-phase flow heat exchanger,a visual experi-mental platform was constructed.Particle image velocimetry(PIV)was employed to investigate the two-dimensional flow field characteristics within a vertical upward converging-diverging tube liquid-solid two-phase flow heat exchanger.The study ana-lyzed the influence of inlet fluid velocity,particle size,and particle volume fraction on the radial velocity and vorticity of the fluid,aiming to elucidate the mechanisms of enhanced mass and heat transfer in the liquid-solid two-phase flow within the converging-diverging tube. Methods A visual experimental platform was established,utilizing tap water as the circulating liquid medium,spherical acrylic particles as the solid phase material,and quartz glass as the converging-diverging tube structure.Based on the general prin-ciples of particle settling velocity in engineering applications and considering the constraints of experimental conditions,three key parameters were selected,i.e.,inlet fluid velocity,particle size,and particle volume fraction.Each parameter was set at five gradient levels.Two-dimensional PIV technology was employed to systematically investigate the flow field characteristics within a vertical upward converging-diverging tube liquid-solid two-phase flow heat exchanger.The influence of inlet fluid velocity,particle size,and particle volume fraction on the radial velocity and vorticity of the fluid was explored,and the optimal combination of operating conditions was determined. Results and Discussion The increase in the average radial velocity of the liquid-solid two-phase flow indicated enhanced radial fluid motion,which strengthened fluid exchange between different regions and promoted heat and mass transfer among these regions.Within a single pitch of the converging-diverging tube,the radial velocity of the liquid-solid two-phase flow at various radial positions exhibited positive and negative fluctuations as the axial height increased.The peak radial velocity occurred at axial heights of approximately 0,6,and 24 mm,aligning with the junctions between the expanding and contracting sections of the tube.Along the radial direction,the radial velocity gradually increased from the tube center to the tube wall.The average radial velocity increased with higher inlet fluid velocity and volume fraction but decreased with larger particle sizes.The vorticity of the liquid-solid two-phase flow showed a symmetric positive-negative distribution along the radial direction,with larger vor-ticity values near the wall and lower values in the central region.Both the overall average vorticity and the near-wall average vor-ticity increased with higher inlet fluid velocity and volume fraction but decreased with larger particle size.At an inlet fluid veloc-ity of 1.0 m/s,the overall average vorticity was 80.7 s-1,and the near-wall average vorticity was 126.4 s-1.When the volume fraction ranged from 1％to 5％,the near-wall average vorticity was approximately 56.6％to 69.4％higher than the overall aver-age vorticity.Under the conditions of an inlet fluid velocity of 1.0 m/s,a volume fraction of 5％,and a particle size of 1.5 mm,the fluid motion within the converging-diverging tube liquid-solid two-phase flow heat exchanger was the most intense,effec-tively enhancing mass and heat transfer within the tube. Conclusion The factors influencing the average radial velocity of the liquid-solid two-phase flow,in descending order of impact,are inlet fluid velocity,volume fraction,and particle size.Similarly,the factors affecting the average vorticity in the near-wall region,in descending order of influence,are inlet fluid velocity,volume fraction,and particle size.