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Citation for published version
Zhang, Wenbiao and Yan, Yong and Qian, Xiangchen and Guan, Yanjun and Zhang, Kai (2017)
Measurement of Charge Distributions in a Bubbling Fluidized Bed Using Wire-Mesh Electrostatic
Sensors. IEEE Transactions on Instrumentation and Measurement, 66 (3). pp. 522-534. ISSN
0018-9456.
DOI
https://doi.org/10.1109/TIM.2016.2639238
Link to record in KAR
http://kar.kent.ac.uk/59573/
Document Version
Author's Accepted Manuscript
1
Revised version of IM-16-13557
Title: Measurement of Charge Distributions in a Bubbling Fluidized Bed Using
Wire-Mesh Electrostatic Sensors
Author: Wenbiao Zhang
a
Yong Yan
b
, (Corresponding author)
Xiangchen Qian
a
Yanjun Guan
c
Kai Zhang
c
Address: a) School of Control and Computer Engineering
North China Electric Power University
Beijing 102206, P.R. China
wbzhang@ncepu.edu.cn, xcqian@ncepu.edu.cn
b) School of Engineering and Digital Arts
University of Kent
Canterbury, Kent CT2 7NT, U.K.
y.yan@kent.ac.uk
c) Beijing Key Laboratory of Emission Surveillance and Control for
Thermal Power Generation
North China Electric Power University
Beijing 102206, P.R. China
gyj0627@gmail.com, kzhang@ncepu.edu.cn
2
Abstract—In order to maintain safe and efficient operation of a fluidized bed, electrostatic
charges in the bed should be monitored continuously. Electrostatic sensors with wire-mesh
electrodes are introduced in this paper to measure the charge distribution in the cross section of the
fluidized bed. A Finite Element Model is built to investigate the sensing characteristics of the
wire-mesh sensors. In comparison with conventional electrostatic sensors, wire-mesh sensors have
higher and more uniform sensitivity distribution. Based on the induced charges on the electrodes
and the sensitivity distributions of the sensors, the charge distribution in the cross section of the
fluidized bed is reconstructed. However, it is difficult to directly measure the induced charges on
the electrodes. A charge calibration process is conducted to establish the relationship between the
induced charge on the electrode and the electrostatic signal. Experimental studies of charge
distribution measurement were conducted on a lab-scale bubbling fluidized bed. The electrostatic
signals from the wire-mesh sensors in the dense phase and splash regions of the bed for different
fluidization air flow rates were obtained. Based on the results obtained from the charge calibration
process, the estimated induced charges on the electrodes are calculated from the Root Mean
Square values of the electrostatic signals. The characteristics of the induced charges on the
electrodes and the charge distribution in the cross section under different flow conditions are
investigated, which proves that wire-mesh electrostatic sensors are able to measure the charge
distribution in the bubbling fluidized bed.
Index Terms–wire-mesh electrostatic sensors; bubbling fluidized bed; charge distribution
measurement; induced charge; Finite Element Modeling
I. INTRODUCTION
Bubbling fluidized beds, which have excellent heat and mass transfer efficiencies, are widely
applied in chemical engineering, biomolecular engineering and food processing industries. Due to
the contact and frictions between the particles and between the particles and wall, electrification is
inevitable in a fluidized bed. The presence of electrostatic charges in the bed affects the operation
of the bed. The hydrodynamics in the bed, such as bubble size and shape and solids mixing rate,
changes with the level of electrostatic charges in the bed. If the charges on the particles exceed a
critical value, the particles in the bed may adhere to the wall and even cause discharges and
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explosion [1]. In order to maintain safe and efficient operation of the fluidized bed, the
electrostatic charges and the flow dynamics of solid particles in the fluidized beds should be
continuously monitored.
As an off-line measurement tool, Faraday cups were used to directly measure the charge density in
the fluidized beds [2, 3]. However, charge generation and dispassion during the sampling process
would influence the measurement result. Apart from Faraday cups, electrostatic probes were
developed to measure the electrostatic charges in fluidized beds. A theoretical model was
developed by Chen et al. to explain the electrical current signals due to the passage of isolated gas
bubbles in a fluidized bed [5, 6]. Based on this model, a collision probe was built to measure the
particle charge-to-mass ratios in a 2D bubbling fluidized bed. An induction probe, which was
mounted flush with the outside wall of the fluidized bed, was also developed by Chen et al.. They
applied a number of induction probes to measure the induced charge signals due to the passage of
bubbles and the charge distribution around the bubbles was reconstructed with different algorithms
[7-9]. He et al. [10, 11] developed a dual-tip electrostatic probe for the measurements of particle
charge density and bubble properties in a bubbling fluidized bed. The estimated particle charge
density and bubble rise velocity were in reasonable agreement with those obtained using a Faraday
cup and video imaging. However, electrostatic probes can only provide localized charge
distribution information near the electrode. In order to maintain an effective operation of the
fluidized bed, the electrostatic charge distribution in the whole cross section of the bed should be
monitored.
As a noninvasive tomography method, electrostatic tomography (EST) was applied to visualize
the flow pattern and reconstruct the charge distribution in the pneumatic conveying pipeline
[12-15]. A 16-electrode system was applied by Green et al. to reconstruct the concentration profile
in a gravity conveyer [12]. Machida et al. combined a back projection algorithm with the least
squares method to reconstruct the electrostatic charges carried by particles [13]. Zhou et al. used
the permittivity distribution acquired from an electrical capacitance tomography (ECT) system to
improve the charge sensitivity field of an EST system and to reduce the uncertainty relating to the
charge distribution reconstruction [14]. However, the sensitivity distribution of the sensor used for
the EST system is not uniform, which may result in reconstruction errors, especially in the central
area of the pipe. In addition, as charges stay on particle surface, the EST method requires the
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information of the size and the shape of the particles to reconstruct the solids concentration
profile.
In order to overcome the above drawbacks in the charge distribution measurement, wire-mesh
electrostatic sensors are introduced in this paper. In comparison with ring-shaped and arc-shaped
electrodes, the wire-mesh electrodes have higher and more uniform spatial sensitivity especially in
a large diameter fluidized bed. The drawback of the electrodes is that the wire-mesh can obstruct
the flow of particles and hence suffer from wear problems. However, the degree of obstruction
depends on the diameter of the wire and the spacing between them and a wear resistant material
can be used to prevent the abrasion of the wire. A detailed comparison of the existing methods for
particle charge measurement is summarized in Table 1.
Table 1: Comparison of the existing methods for particle charge measurement
Methods Sampling requirement Intrusiveness Sensitivity
distribution
Measurement results
Faraday cups Sampling of particles is
required, charge generation
and dispassion during the
process may influence the
result.
Intrusive Sensitivity to the
particles inside the
cup
Electrostatic charge on the
samples
Intrusive
probes
Not required Intrusive and the
blockage depends on
the size and the
number of the
electrodes
Only sensitive to the
particles near the
probes
Charge on the particles near
the probe
Electrostatic
tomography
Not required Non-intrusive Sensitivity
distribution is not
uniform, especially
in the central area of
the pipe
Reconstructed charge
distribution may not
represent the true
distribution due to
non-uniform sensitivity.
Wire-mesh
electrostatic
sensors
Not required Intrusive and the
blockage depends on
the size and the
number of the
electrodes
Higher and more
uniform sensitivity
distribution
Cross-sectional charge
distribution
Wire-mesh sensors have already been widely applied in the measurement of gas-liquid and
liquid-liquid two-phase flows based on the capacitive and conductive methods. Pena and
Rodriguez presented a review of the applications of wire-mesh sensors to multiphase flow
measurement [16]. The advantages and disadvantages of wire-mesh sensors were analyzed.
