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Numerical and Experimental Investigation on the Performance of Various Drag Models in Predicting the Hydrodynamics of Tapered Beds

Ghayour Najafabadi, Mohammad Amin | 2025

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 58402 (06)
  4. University: Sharif University of Technology
  5. Department: Chemical and Petroleum Engineering
  6. Advisor(s): Molaei Dehkordi, Asghar
  7. Abstract:
  8. Gas–solid fluidized beds are widely utilized in petrochemical, energy, and metallurgical industries due to their efficient mass and heat transfer as well as excellent mixing capabilities. However, in the presence of particles with broad size distributions and exothermic reactions, conventional cylindrical beds often exhibit poor performance, and tapered fluidized beds have therefore been introduced as more efficient alternatives. In the design and simulation of such beds, the drag coefficient model plays a decisive role in predicting hydrodynamic behavior, and an inappropriate choice may lead to significant errors in analysis and design. In this study, using a combined approach of laboratory experiments and numerical simulations (two-fluid model coupled with the kinetic theory of granular flow, KTGF), the effects of key parameters including superficial gas velocity, bed apex angle, initial bed height, and particle diameter were investigated on bubble diameter, bubble rise velocity, bed expansion, and pressure drop, and experimental data were compared with numerical simulations employing different drag models (Wen–Yu, Gidaspow, Syamlal–O’Brien, Gibilaro, a developed model, and EMMS). The results showed that increasing gas velocity promoted bubble growth and acceleration, leading to enhanced bed expansion while maintaining nearly constant pressure drop, whereas increasing apex angle and initial bed height tended to reduce bubble size and create semi-stagnant wall regions; larger particle diameters decreased pressure drop and limited bubble size, rise velocity, and bed expansion. Comparison of drag models revealed that for gas velocity, the Syamlal–O’Brien model predicted bubble diameter, rise velocity, pressure drop, and bed expansion with mean errors of 10.24%, , 6.19%, and , respectively, while the developed model achieved , 10.05%, , and 6.05%; for apex angle, Syamlal–O’Brien yielded errors of 10.41%, 10.39%, 10.08%, and 4.59% compared to 9.18%, 9.17%, 6.86%, and 4.21% for the developed model; for initial bed height, the developed model predicted with mean errors of 14.86%, 13.43%, 5.44%, and 2.17%; and for particle size, the developed model gave 11.68%, 12.33%, 2.44%, and 3.03% compared to , 13.96%, 8.64%, and 6.00% for Syamlal–O’Brien. Overall, the Syamlal–O’Brien model can be considered a reliable choice for simulating tapered fluidized beds, while the developed drag model also provided promising predictions but requires further experimental validation. These findings emphasize that accurate selection of drag models and consideration of tapered geometry are crucial for reliable design and scale-up of fluidized beds, and the outcomes of this research can contribute to improved reactor design in industrial applications as well as the development of more advanced drag models in future studies
  9. Keywords:
  10. Tapered Fluidized Bed ; Computational Fluid Dynamics (CFD) ; Drag Coefficient ; Gas Bubbles ; Bed Expansion Ratio ; Pressure Drop ; Gas Bubbles Rising Velocity ; Gas Bubble Size Distribution

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