Critical Comparison of Electrostatic Effects on Hydrodynamics, Heat Transfer and Chaotic Analysis in a Bubbling Fluidized Bed with a Central Jet

Critical comparison of electrostatic effects on hydrodynamics, heat transfer and chaotic analysis in a bubbling fluidized bed with a central jet

Haotong Wang, Lungu Musango, Jingdai Wang, Yongrong Yang

Abstract: Gas-solid fluidized bed has a very high heat transfer and mass transfer efficiency and can handle a large number of particles, and thus has a wide range of applications in many industrial processes. In a gas-solid fluidized bed, the flow behavior exhibits randomness and structural instability due to the existence of bubble agitation, particle motion and gas-solid interaction which results in a typical nonlinear transient system. The study of gas-solid fluidized bed is mainly focused on the time-averaged parameters. Compared with the time-averaged parameters, the fluctuating parameters (pressure and porosity fluctuation, etc.) of the fluidized bed also contain the information of flow structure and transport behaviors. Therefore, the combination of time domain analysis and frequency domain analysis can be a more in-depth study of fluidized bed, especially for gas and solid flow, heat and mass transfer characteristics. In particular, electrostatic phenomenon is unavoidable in many industrial processes, such as ethylene polymerization, due to particle-particle and particle-wall collision and friction ^{1, 2}. The electrostatic and hydrodynamic effects are interdependent. After the particles are charged, parameters such as bubble size and particle velocity will also be affected, which further influences the heat and mass transfer ability in the fluidized bed reactor. Whereas changes in hydrodynamic properties, bubble size, particle velocity and phase fraction distribution, etc., also affect the electrostatic potential distribution of the fluidized bed.

In this paper, a comprehensive study of the electrostatic effect on bubble behavior, particle fluctuation velocity and heat transfer coefficient in the fluidized bed with a central jet has been evaluated by Eulerian-Eulerian two fluid model coupled with electrostatic model and energy model. The simulated voidage profiles at different positions, bubble detachment time and initial bubble diameter are compared with experiment results in the literature. The predicted bubble behaviors including bubble frequency and bubble numbers, and particle fluctuation parameters, including particle fluctuation velocity, granular temperature and Reynolds stress are analyzed in both charged and uncharged system. Electrostatic effect on two kinds of heat transfer coefficients, namely bubble to emulsion phase heat transfers based on the gas throughflow velocity and gas solid local heat transfer are quantitatively compared. Simulation results show that electrostatic charges decrease bubble numbers and granular temperature, whereas the averaged heat transfer coefficients are enhanced. A more chaotic behavior is observed by time series analysis and phase space reconstruction. Overall, the electrostatic effect on the hydrodynamic and heat transfer characteristics can be revealed.

Keywords: Fluidization, Bubble dynamics, Electrostatics, heat transfer

References

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