(74f) 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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Hendrickson, Electrostatics and gas phase fluidized bed polymerization reactor
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