Ie061304i - Artigo Pneumatic Transport
Ie061304i - Artigo Pneumatic Transport
Ie061304i - Artigo Pneumatic Transport
This paper presents three flow patternssdispersed flow, reverse flow, and half-ring flowsin the post-bend
region of a 45° inclined pneumatic conveying pipe. Solid concentration and velocity distribution were measured
using electrical capacitance tomography (ECT), particle image velocimetry (PIV), and a high-speed camera.
The axial velocity of particles and solid transverse motion in the pipe cross section were obtained using a
modified cross-correlation method from the ECT data for the three flow patterns. The axial velocity profile
over the pipe cross section was compared with data obtained from the PIV system. In particular, emphasis
has been placed on the characterization of the reverse-flow pattern, where negative values of solid velocities
were observed. By analyzing the images captured with the high-speed camera, three distinct regions in the
solid phase of the reverse-flow regime may be discerned. A dense region with high solid concentration formed
next to the bottom wall of the inclined conveying pipe, while a dilute region with a relatively low solid
concentration formed near the center of the pipe. Reverse flow was observed to occur predominantly in the
dense region and a transition region between the two. Finally, the dynamic forces acting on single particles
were analyzed for the three flow patterns. This analysis showed that the phenomena of reverse-flow and
half-ring flow structure formation may be attributed to the effects of electrostatics. This finding has also been
validated with results of numerical simulations performed in the present study.
Figure 1. Schematic of the pneumatic conveying experiment facility. Legend: 1, air control valve; 2, dryers; 3, rotameter; 4, hopper; 5, solids feed valve;
6, rotary valve feeder; 7, feed control valve; 8, computer; 9, DAM; 10, ECT plane 1; 11, ECT plane 2; 12, plane of PIV measurements; 13, plane of
measurements for the high-speed camera; 14, measurements for induced current; 15, measurements for particle charge; 16, pressure transducer sensor 1; and
17, pressure transducer sensor 2.
Table 1. Experimental Conditions pipes: one is positioned at a distance of 3.41 m, and the lower
parameter value one is positioned at a distance of 4.12 m. A 45° inclined pipe
air flow rate 1600 L/min, 1100 L/min, 1000 L/min
(4-cm ID) was connected to the two horizontal pipes. The entire
solids feed valve 75% opening conveying loop was made of poly(vinyl chloride) (PVC) pipes.
roll speed of rotary valve 25 rpm 2.2. Electrical Capacitance Tomography (ECT). ECT is a
pipe material poly(vinyl chloride), PVC noninvasive technique for measuring and displaying the con-
pipe diameter (inner) 40.0 mm centration distribution of a mixture of two dielectric fluids. In
pipe thickness 5.0 mm
particles conveying style cycle this experiment, two sets of 12-electrode (10 cm long) ECT
particle material polypropylene, PP sensors (labeled as “10” and “11” in Figure 1) were arranged
particle size 2.80 mm on the inclined pipe at distances of 1.45 and 2.18 m from a
particle density 1123 kg/m3 bend connecting this inclined riser to a horizontal duct at the
bottom. A data acquisition module (DAM) (Process Tomogra-
insights into the relationship between solids flow behavior and phy, Wilmslow, Cheshire, U.K.) was connected to the ECT
electrostatic effects. sensors to gather capacitance data. The ECT was sampled at
40 Hz, corresponding to a time period of 0.025 s for each frame.
The resolution of each frame was 1024 (32 × 32) pixels. The
2. Experimental Section
ECT system was calibrated for the lower and upper permittivity
2.1. Experimental Setup. The experimental facility, which bounds: air was used for the lower bound (with 0 as the
was modified from a previous study4,7 and used to perform the reference value) and solids were used as the upper bound (with
measurements, is shown schematically in Figure 1; the experi- 1 as the reference value).
mental conditions are listed in Table 1. Air from the compressor By post-processing the ECT data at each instant of time t,
was metered and sent to the rotary air lock feeder (General using the simultaneous iterative reconstruction technique (SIRT)
Resources Corp., Hopkins, MN), where it entrained the particles. described by Su et al.,8 the particle concentration in each pixel
The rotary feeder equipped with eight pockets and a vent at the of one plane, R(x,y,z,t), was obtained. Here, as shown in Figure
body of the rotary valve, providing the passage of the air 1 by the viewing section A-A, x and y denote Cartesian
leakage, was operated at a fixed speed of 25 rpm. The test coordinates in the cross-sectional plane, made dimensionless
section consisted of two 4-cm-inner-diameter (ID) horizontal using the pipe diameter as the characteristic length, and z is the
6068 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
axial coordinate denoting the location of the ECT electrodes. direction perpendicular to the lightsheet and transmitted to the
The solid velocity can be calculated based on cross-correlation computer for processing. The cross-correlation yields the
of twin plane data, which was described by Hua et al.,9 Rao et distance traveled by the granules in a small time interval (dT )
al.,7 and Zhu et al.4 The time-averaged particle concentration 50-58000 µs) from the first snapshot to the second and the
R
j t was found by averaging R(x,y,z,t) over a time period T (30 velocity distribution of granules in the plane can be determined
s in this case): by dividing by the time interval. In this study, the variation of
velocity in the axial direction of the 45° inclined pipe was
R
j t(x,y,z) )
1
T
∫0T R(x,y,z,t) dt (1) measured.
2.4. Electrostatic Measurements. During the pneumatic
conveying process, frictional contacts between the solid particles
The instantaneous value of the cross-sectional average particle
and pipe wall generate electrostatic charges. With proper pipe
concentration, R
j s(z,t), is defined as
connections, current induced on the surface of the pipe wall
can be measured as a function of time using a digital
j s(z,t) )
R
1
A
∫∫ R(x,y,z,t) dx dy (2) electrometer (ADVANTEST R8252, Tokyo, Japan). The mea-
surement section is labeled “14” in Figure 1 while details of
j s(z,t) is denoted by 〈R〉(z):
and the time-averaged value of R the measurement methods have been described previously by
Yao et al.6 In addition, the particle charge density was measured
〈R〉(z) )
1
T
∫0T Rjs(z,t) dt ≡ A1 ∫∫ Rj t(x,y,z) dx dy (3) using a Faraday cage at the horizontal segment6 (labeled “15”
in Figure 1).
The correlation coefficient, C(d), was then computed as
3. Numerical Method
C(d) )
1
T
∫0 T
j s(z1,t) - 〈R〉(z1)) (R
(R j s(z2,t + d) - 〈R〉(z2)) dt The method of combining the discrete element method (DEM)
(4) with CFD is well-established in the literature for numerical
studies of various types of solid-fluid systems. Recently, it has
Here, z1 and z2 refer to upstream and downstream planes, also been applied successfully to reproduce the various flow
respectively; d denotes the delay time. The dominant pattern regimes observed in pneumatic conveying of granular materials
propagation velocity (V*) was estimated from V* ) L/D, through vertical and horizontal pipes.11 Lim and co-workers12
where L ) z2 - z1 (L ) 2.18-1.45 ) 0.73 m in the current have also described, in detail, a method of incorporating an
work), is the axial distance between the two ECT sensors and electrostatic force model into the CFD-DEM model for
D is the value of d at which C(d) assumes the largest value. V* numerical studies of pneumatic conveying processes with
estimated in this manner is clearly based on cross-sectional electrostatic effects. It was shown that the eroding dunes regime
averages. One can also define pixel-pixel cross-correlation observed in previous experimental studies using inclined
function as pneumatic conveying pipes could be reproduced computation-
ally. Reversed flow of particles was observed in a dense region
c(x,y,d) )
1
T
∫0T (R(x,y,z1,t) - close to the bottom wall of the conveying pipe and forward
flow in a more dilute region in the space above. The same
R
j t(x,y,z1)) (R(x,y,z2,t + d) - R
j t(x,y,z2)) dt (5) methodology has been applied in the present study to comple-
ment the experimental investigations conducted. A brief outline
The dominant pattern propagation velocity (V(x,y)) was esti- of the main component of the CFD-DEM model is provided
mated from eq 6: in the following section.
L 3.1. Discrete Element Method. The translational and rota-
V(x,y) ) (6) tional motions of individual solid particles are governed by
D(x,y) Newton’s laws of motion:
where D(x,y) is the value of d at which c(x,y,d) assumes the
maximum value (for that x and y). dvi N
these collision forces. The normal (fcn,ij, fdn,ij) and tangential (fct,ij, where u is the velocity vector, the local average porosity, P
fdt,ij) components of the contact and damping forces are the fluid pressure, and F the source term due to fluid-particle
calculated according to the following equations: interaction.
The computational domain was divided into uniform grid
fcn,ij ) -κn,iδn,ij (9) cells, and all quantities such as velocities and pressure were
assumed constant over each cell. The source term F for a
fct,ij ) -κt,iδt,ij (10) particular computational cell was calculated by summing the
fdn,ij ) -ηn,i (Vr‚ni)ni (11) fluid drag forces on all particles present within that cell:
∑
i)1
ff,i
where fcn,ij, fdn,ij and fct,ij, fdt,ij are the normal and tangential F) (20)
components of the contact and viscous contact damping forces, ∆V
respectively; κn,i, δn,ij, ni, ηn,i and κt,i, δt,ij, ti, ηt,i are the spring
constants, displacements between particles, unit vectors, and where ∆V is the volume of a computational cell and n is the
viscous contact damping coefficients in the normal and tan- number of particles present in the cell.
gential directions, respectively; Vr is the relative velocity between 3.4. Electrostatic Effects. During pneumatic conveying, solid
particles; and Ri and Rj are the radius vector (from particle center particles gain electrostatic charges as a result of repeated
to a contact point) for particles i and j, respectively. If |fct,ij| > collisions and impacts against other particles and with the walls
|fcn,ij| tan φ, then “slippage” between the two contacting surfaces of the conveying pipe. The total electrostatic force acting on
is simulated by a Coulomb-type friction law, each particle may then be written as the sum of electrostatic
forces due to charges carried by other particles and the pipe
|fct,ij| ) |fcn,ij| tan φ walls:
where tan φ is analogous to the coefficient of friction. fE,i ) fEp,i + fEw,i (21)
3.2. Fluid Drag Force. In a multiphase system such as the
gas-solid pneumatic conveying system considered in this study, where fEp,i and fEw,i are the electrostatic forces due to other
interactions between the two phases take the form of fluid drag charged particles and the pipe walls acting on particle i,
forces on the solid particles exerted by the interstitial fluid and respectively.
arise from velocity differences between the two phases. In this The electrostatic force arising from charges carried by other
study, the model due to Di Felice,13 which is applicable over a particles may be calculated by assuming each particle to be a
wide range of particle Reynolds numbers, was used to evaluate constant point charge:
the fluid drag force:
N
Q2
ff,i ) ff0,i-(χ+1)
i (13) fEp,i ) ∑ ni (22)
j)1 4πorij2
j*i
ff0,i ) 0.5cd0,iFf πRi2i2|ui - Vi|(ui - Vi) (14)
[ ]
where Q is the constant charge assumed to be carried by all
(1.5 - log10 Rep,i)2 particles, o the permittivity of free space, rij the distance
χ ) 3.7 - 0.65 exp - (15)
2 between particles i and j, and ni the unit normal vector in the
( )
direction of the line joining the two particle centers.
4.8 2
cd0,i ) 0.63 + (16) In the present work, a dimensionless quantity Λ depicting
Rep,i0.5 the ratio of the electrostatic force arising from the charged pipe
wall to the gravitational force exerted on each particle, fEw,i/
2Ff Rii|ui - Vi|
Rep,i ) (17) mig, is defined. In other words,
µf
fEw,i ) Λmi g (23)
where ff0,i is the fluid drag force on particle i in the absence of
other particles, χ is an empirical parameter, i is the local average 4. Results and Discussion
porosity in the vicinity of particle i, cd0,i is the drag coefficient,
Rep,i is the Reynolds number based on particle diameter, Ff is 4.1. Flow Patterns and Velocities for Particle Transport
the fluid density, µf is the fluid viscosity, and ui is the fluid in a 45° Inclined Conveying Pipe. Depending on the different
velocity of the computational cell in which particle i is located. air flow rates, three distinct phenomenasnamely, dispersed flow,
3.3. Computational Fluid Dynamics. The motion of the reverse flow, and half-ring flowsfor particle transport in a 45°
continuum gas phase is governed by the Navier-Stokes inclined conveying pipe was observed and schematically il-
equations with an additional source term in the momentum lustrated in Figures 2a, 2b, and 2c, respectively. When the air
equation to represent the reaction force acting on the fluid by flow rate is high, particle flow is dilute and particles move in
the particles: the direction of gas flow as shown in Figure 2a, the dispersed
flow pattern. When air flow rate is decreased, some particles
∂ near the pipe center moved forward, while most of the particles
+ ∇‚(u) ) 0 (18) formed layers at the bottom of the pipe and slid downward,
∂t
which is called reverse flow (shown in Figure 2b). When the
∂(Ff u) air flow rate was further decreased, particles formed a layer on
+ ∇‚(Ff uu) ) -∇P + ∇‚(µf ∇u) + Ff g - F the pipe wall, which resulted in an annular structure. This flow
∂t
(19) pattern is called half-ring flow (shown in Figure 2c). An
6070 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Figure 2. Schematic of particle flow in three different flow patterns in pneumatic conveying: (a) dispersed flow; (b) reverse flow; and (c) half-ring flow,
Panel (d) shows unsteady reverse flow with pulsating wave.
interesting unsteady flow patternsreverse flow with pulsating pixels from the second plane are chosen from the corresponding
wave on the surfacesis also discussed in section 4.2.4. pixel and those in its neighbors (not just the nearest neighbors).
4.1.1. Dispersed Flow Pattern. Figure 3 shows the images Equation 24 describes the function in detail:
of particles concentration captured by ECT on plane 1 and plane
2, respectively. When the air flow rate is 1600 L/min, the particle
concentration is generally low and particles do not concentrate
c(x,y,i,j,d) )
1
T
∫0T (R(x,y,z1,t) - Rt(x,y,z1))(R(i,j,z2,t + d) -
on the pipe wall. As such, it forms the dispersed flow pattern. Rt(i,j,z2)) dt (24)
Figure 4 shows the result of velocity calculated by the best-
correlated pixel method proposed by Mosorov and colleagues.10 where (i,j) are dimensionless coordinates of the pixel in plane
The Mosorov method is modified from the classical cross- 2, R(x,y,z1,t) and R(i,j,z2,t + d) are the values associated with
correlation method (section 2.2) but is not limited by the basic pixel (x,y) from the image at time t obtained from plane 1 and
assumption of the latter that the solid’s motion between the pixel (i,j) from the image at time (t + d) obtained separately
sensors is parallel to the pipe axis and perpendicular to the sensor from plane 2. Therefore, similar to the classical cross-correlation
plane. This method is to calculate cross-correlation between a method, when c(x,y,i,j,d) reaches its maximum, d is D(x,y) and,
pixel from the first plane and a number of pixels from the second according to eq 6, the axial solid velocity of each pixel, Vz-
plane. Therefore, the essential difference between the classical (x,y), can be obtained. It is observed that velocities are high
cross-correlation method and the best-correlated pixel method over the entire pipe (see Figure 4).
is the choice of pixels in the second plane. In the former, only Furthermore, the coordinates (i, j) on plane 2 (nondimen-
corresponding pixels are correlated; however, in the latter, the sionalized by the pipe diameter as the characteristic length scale)
Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6071
Figure 3. Images of particle concentration (R) at twin planes captured by ECT for the dispersed-flow pattern (air flow rate ) 1600 L/min).
()
overall space among particles is fairly large, thus resulting in a
Vx dilute solid phase. For illustration purpose, one particle was
B
V ) Vy (25) chosen as an example and traced over several images. The
Vz positions of this particle in two images are marked in circles,
as shown in Figure 5a. Generally, particle motion is observed
Here, Vx, Vy, and Vz are the magnitudes of the velocity to follow the gas flow direction. Subsequently, 10 particles were
components in the three perpendicular directions, respectively. chosen randomly from 1000 pictures over a period of 4 s and
6072 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Figure 5. Images of particle transport captured by a high-speed video camera: (a) dispersed flow (time interval between two successive pictures ) 0.008
s, air flow rate ) 1600 L/min); (b) reverse flow (time interval between two successive pictures ) 0.072 s, air flow rate ) 1100 L/min); (c) half-ring flow
(time interval between two successive pictures ) 0.096 s, air flow rate ) 1000 L/min); (d) reverse flow with pulsating wave (time interval between two
successive pictures ) 0.176 s, air flow rate ) 1100 L/min, PP with an antistatic agent).
the average velocity of particles was computed as the distance 4.1.2. Reverse Flow Pattern. When the air flow rate is
between the two positions in selected images divided by the decreased to 1100 L/min, it is observed in the experiment that
duration between these two images. The average particle velocity some particles near or above the pipe center moved forward
value obtained was 1.59 ( 0.41 m/s. with fairly high velocities, whereas most particles were deposited
Figure 6 displays the velocity profile of particles at different and formed layers at the bottom of the pipe. These slid backward
positions along the pipe diameter. The data were obtained in a direction opposite to that of the general flow direction at
from ECT and PIV experiments, respectively. The figure shows relatively slow velocities. These are represented by the red areas
that velocity profiles obtained from these two different meth- in Figure 8.
odologies exhibited similar trends. However, there are some Figure 9 presents the resulting velocity obtained with the best-
quantitative differences, probably because of the different correlated pixel method for the reverse-flow pattern. In this
extents of experimental errors associated with the two instru- figure, the entire solid phase is divided into three parts. It is
ments. Here, velocities are normalized with respect to the observed that velocities are relatively high and positive near
maximum value of velocity observed within the pipe cross and above the center of the pipe, where the corresponding
section. The normalized velocities are high near the pipe center coordinate along the y-axis are 0.35-1, whereas those at the
but decrease toward the pipe wall. Such profile was observed bottom (y ) 0-0.18) are very small and negative. These two
in both side view (Figure 6a) and top view (Figure 6b). This areas are defined as dilute and dense regions, respectively.
was also predicted by CFD simulation of pneumatic conveying Particles at the interface between these regions (referenced as
of granular solids in an inclined pipe.3 It may also be observed the transition region with y ) 0.18-0.35) flowed backward with
from the velocity profiles in both side and top views in Figure relatively higher negative velocities. The lateral velocities
6 that lateral velocities are much lower than those of axial calculated from eq 26 also show three distinct velocity scales,
velocities, implying that axial movement is primary for such increasing from 10-6 m/s, to 10-5 m/s, to 10-4 m/s, from the
flow pattern, thereby validating the results obtained by eq 26 dense region to the dilute region, respectively.
from the ECT data. The limitation of the cross-correlation method is that it is
Figure 7 shows the side view image from the top to the difficult to get exact values when data are the same on two
bottom of the pipe of the dispersed flow pattern captured by correlation planes, because the principle of correlation is based
PIV, where the vector arrows verify the observation that the on variation of the data on two planes. For the dense region,
particle flow directions are consistent with the air flow. the concentrations of particles measured by two ECT sensors
Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6073
Figure 8. Images of particle concentration (R) at twin planes captured by ECT for the reverse flow (air flow rate ) 1100 L/min).
Figure 10. Diametrical distribution of particle axial velocity for the reverse flow (air flow rate ) 1100 L/min).
q) ∫0T I dt (27)
hI )
1
T
∫0T I dt (28)
Figure 12. Images of particle concentration (R) at twin planes captured by ECT for the half-ring flow (air flow rate ) 1000 L/min).
E) ∫ 4π
1 dq
0 r
2
)
λ
∫π/2 cos θ dθ ) 2πλ 0l
2π0l 0
(29)
Figure 14. Diametrical distribution of particle axial velocity for the half-ring-flow pattern (air flow rate ) 1000 L/min).
DEM work12 also predicted that the average electrostatic force where µg is the gas viscosity, and b
ug and b
up are the translational
acting on each particle in similar systems was only 10-9 N and, velocity vectors of the gas and particle, respectively.
thus, confirmed that electrostatic effects due to charged particles (ii) Electrostatic force is assumed to be mainly contributed
are negligible, as compared with other types of forces. Evidence by the lower pipe wall in this simplified model, FE:
for this claim is that particle agglomeration is seldom observed
in the present operation. On the contrary, clustering is prevalent FE ) EQ (34)
for fine powders. This might be because of the fact that the
electrostatic interactions among fine particles are significant, where Q is the electrical charge on each particle and can be
because of the small size and weight. obtained from multiplying the charge-mass ratio (charge den-
Summarizing the aforementioned discussion, the major forces sity)6 by the mass of a single particle.
that act on each particle therefore include the following (iii) Friction between particle and pipe wall: the product of
components (besides gravity): normal force and coefficient of friction, which is estimated to
(i) Aerodynamic drag force, B FD:15 be ∼0.56 from an internal angle of friction experiment.16
Table 3 displays the comparison of forces on a single particle
πFgDp2 for three flow patterns, where the particle velocity shown is
B
FD ) bg - b
cD(u bg - b
u p)|u u p| (30) the averaged value obtained from a high-speed video camera.
8 For the dispersed-flow pattern, the magnitude of the electrostatic
where Fg is the gas density, Dp the particle diameter, and cD force, on average, is only 10-6 N and, thus, can be neglected;
the drag coefficient: whereas the calculated aerodynamic drag force is more than 4
times and 300 times that of gravity and electrostatic force,
cD )
24
Rep
(1 + 0.15Rep0.687) )
24
f
Rep D( ) (for Rep e 1000)
(31)
respectively, showing that the primary driving force is indeed
the drag force, which is also responsible for transporting the
particles upward.
For the reverse-flow pattern, the aerodynamic drag force in
cD ) 0.44 (for Rep > 1000) (32) the dilute region was more than 10-4 N. This is still the
FgDp|u
bg - b
u p| dominant force, as compared to gravity and electrostatic forces.
Rep ) (33) This implies that particles would also move upward in this
µg region. In the dense region, aerodynamic drag force decreased
6078 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Figure 15. Averaged current on pipe wall for three flow patterns (AF ) air flow rate; Tc ) charging time).
is the charging time Tc. c Charge per particle (Q) is obtained from multiplying the charge density by the mass of a single particle. d Drag force (F BD) is
calculated from eq 30, where cD ) 0.44. e Electrostatic force (FE) is calculated from eq 34.
to the same range as gravity and could not provide enough seem to confirm that the reverse-flow pattern is most significant
driving force to transport particles upward, so that particles are in the transition region.
likely to slide downward, because of the pull of gravity. In addition, the formation of a transition zone may be
However, the extents of electrostatic force and associated friction interpreted to indicate the presence of shearing instabilities at
were high (∼10-3 N). It is probable that the electrostatic force the interface between the dilute and dense regions. Particles in
causes particles to stick to the pipe wall and friction limits the the dilute region move at higher velocities than those in the
reverse speed to almost zero. However, in the transition region, dense region. This relative velocity between the two regions
the magnitudes of the three types of forces were almost in the causes significant shearing effects in a transition region between
same ranges, indicating that particles should move either upward the two. Particles in this transition region are more unstable
or downward. Some particles may be attracted and approach than those in the dense region. Furthermore, in the transition
the dense region by electrostatic force. These particles are close region, aerodynamic drag forces are smaller than those in the
to, but do not touch, the pipe wall and flow reversely; however, dilute region and electrostatic forces are smaller than those in
the reverse velocities in this region were greater than that of the dense region. This resulted in the particularly negative
the bottom particles, because of the absence of friction for such velocities of solids within the transition region observed in the
suspended particles. Therefore, in this region, the track of present study. Similarly, according to the report by Goldfarb et
particle motion was more wavelike, as shown in Figure 2b. On al.,17 a transition region formed at the interface between a steady
the other hand, some particles may still move ahead but also state and a wavy state formed by two shearing granular flows
may collide with particles moving in the opposite direction, thus in their experiment involving two streams of identical grains
resulting in such particles turning around. Generally, the results flowing on an inclined chute downstream of a splitter plate.
Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6079
Figure 16. Comparison of electrostatic characters on the bottom of the pipe and the top of the pipe for the half-ring-flow pattern (air flow rate ) 1000
L/min): (a) induced current measurement section, (b) averaged current on the pipe wall, and (c) wall charge from integration of the induced current.
Table 4. Electrostatic Forces on Single Particle in the Entire Reverse Area of Inclined Pneumatic Conveying (Air Flow Rate ) 1100 L/min)
Value
parameter PP without Larostat-519 PP with Larostat-519
particle velocity (m/s) (-5.4 × 10-2)-(-2.0 × 10-3) (-3.3 × 10-1)-(-1.7 × 10-1)
equilibrium charge on pipe (C) -1.0 × 10-6 -5.2 × 10-8
charge per particle (C) 2.6 × 10-11 3.0 × 10-12
electrostatic force (N) 2.3 × 10-4-1.2 × 10-3 1.3 × 10-6-6.7 × 10-6
friction on the bottom of pipe (N) 6.9 × 10-4 5.35 × 10-5
measured Λ valuea 1.8-9 0.01-0.05
a Λ ) FE/G.
As for the half-ring-flow pattern, the ratio of the three types measurement is modified from the induced current test and is
of forces and the observations in the two regions were both shown in Figure 16a, where the test section is divided into two
similar to those of the reverse-flow pattern. As such, the details parts and the experimental data can be obtained from the top
of the analysis will not be repeated here. Furthermore, particle and bottom of the pipe, respectively. The results of average
velocity was much lower, because of a smaller air flow rate in currents calculated from eq 28 were shown in Figure 16b, in
the dilute region and particles had a tendency to remain which equilibrium times, i.e., charging times (Tc) can be read
stationary and form a ring structure on the pipe wall in the dense as fairly similar values for two cases. Consequently, the induced
region and part of the dilute region, probably because of the currents were integrated with time to obtain the charge on the
force balance described previously; thus, the ring structure here pipe wall (Figure 16c), according to eq 27. It is observed that
was relatively stable during the entire transport process. the wall charge at the top of the pipe is greater than that at the
In addition, a further experiment was performed to clarify bottom of the pipe during the entire experiment, as well as at
the reason why a ring or half-ring structure formed. This the moment of equilibrium time. Therefore, the magnitude of
6080 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Figure 17. Results of reverse-flow pattern with pulsating wave (air flow rate ) 1100 L/min): (a) normalized signal from two pressure transducers against
time (4 s); (b) particle concentration (R) captured from two ECT planes against time (4 s); (c) power spectral density of ECT data in plane 1; and (d) power
spectral density of ECT data in plane 2.
electrostatic force at the top of the pipe is high (up to 10-3 N); the one under observation, were performed. A commercially
thus, such a value would be large enough to overcome the available antistatic agent, Larostat-519 powder,18 has been
gravity and attract particles to stick on the upper part of the demonstrated to be a suitable means to reduce electrostatic
pipe wall. effects effectively in a pneumatic conveying system.6 Larostat-
4.2.4. Validation of Control Experiments. The unique 519 is a non-inflammable white powder with a bulk density of
feature of the reverse-flow phenomena, especially the velocity 520 kg/m3; it is composed of ∼60% soyadimethylethyl am-
difference between the transition and dense region, is attributed monium and ∼40% ethasulfate amorphous silica. In this
to the electrostatic effect, as analyzed previously. Thus, it is experiment, 0.5% (by weight) of Larostat-519 powder was
anticipated that particles at the bottom of the pipe would slide mixed with PP granules and other conditions were fixed to be
downward with higher speed and, thus, the transition region the same as those for reverse flow. In comparison with previous
would disappear, when the electric force was reduced. To verify cases, where no such agent was used, the equilibrium charge
this postulation, control experiments, which isolate the electro- on the pipe wall and on each particle obtained in the presence
static effect on a system by holding constant all variables but of the Larostat-519 powder was smaller, by ∼2 orders of
Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6081
Figure 18. ECT images of particle concentration (R) at test plane 2 when the air flow rate is 1000 L/min: (a) PP without an antistatic agent and (b) PP with
an antistatic agent (Larostat-519 powder).
magnitude and 1 order of magnitude, respectively, as shown in by adding the antistatic agent, it was observed that particles
Table 4. Correspondingly, a drastic drop in electric force acting did not slide downward in a smooth and steady manner. Instead,
on each particle at the bottom of the pipe, as well as frictional from the observation made in the present study, the speed of
forces, were observed. Under the same air flow rate (1100 the reversing particles showed periodic fluctuations and solids
L/min), such small aerodynamic drag force cannot afford enough moved downward in alternating pulses of high and low
power to move particles upward, as mentioned in the last section. concentration, as shown in Figure 2d. The slow motion captured
Consequently, particles on the bottom of the pipe slid downward by the high-speed video camera presented this phenomenon in
with higher velocities, compared to the situation without an detail: initially, a stream of solids flowed down at a relatively
antistatic agent, because the small electric force and associated high speed from downstream to upstream along the conveying
friction would not restrict particles in this region. Furthermore, pipe; then, the reverse speed of some particles decreased; these
bottom particles would drive other particles in the dense region particles blocked those flowing back from downstream and
with almost the same velocities. Thus, particles in the entire accumulated on the upstream side of the pipe wall, making the
dense region flowed in the reverse direction. solids stream intermittent. (This continued until friction between
This process was captured by a high-speed video camera, as the particles and the wall or among the particles was unable to
shown in Figure 5d. The range of the dilute region in these support the gravity of the group of particles.) The process was
three images is basically the same as that in Figure 5b; however, then repeated.
no obvious transition region is observed. This is because In conclusion, the behavior of solids motion in the dense
particles A on the bottom of the pipe and particle B at the region exhibits a pulsating type of movement. Such flow pattern
interface of the dense and dilute regions are moving with almost can be identified by the pressure fluctuations in the conveying
the same speed (-0.25 ( 0.078 m/s). of solids.19 Therefore, an additional measurement was performed
Furthermore, during the experimental process of reverse-flow to obtain the pressure data using two pressure transducers (Gems
pattern without the influence of electrostatic forces achieved 2200BGA1002A3UA, Basingstoke, England) at distances of
6082 Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007
Table 5. Material Properties and System Parameters for Figures 17c and 17d show that both power spectral density
CFD-DEM Simulations profiles exhibit a dominant peak at ∼1 Hz. This is also consistent
parameter value/comment with the fluctuation period of 1 s, as observed from Figures
shape of particles spherical 17a and 17b.
type of particles polypropylene, PP Another experiment was performed for the purposes of
number of particles 1000 comparison and confirmation, by mixing an antistatic agent into
particle diameter 2.80 mm
particle density 1123 kg/m3 particles at a fixed air flow rate of 1000 L/min. The ring-flow
spring constant, κ 5.0 × 103 N/m pattern then disappeared, as shown in Figure 18.
coefficient of restitution 0.9 4.2.5. Validation of Numerical Results. To verify the
coefficient of friction 0.3
gas density 1.205 kg/m3
aforementioned analysis, the experimental results for reverse
gas viscosity 1.8 × 10-5 N s/m2 flow were compared with a numerical simulation of pneumatic
pipe diameter 40.0 mm conveying of granular solids through a 45° inclined pipe, which
pipe length 1.0 m was achieved using a simple electrostatic field model and DEM
pipe inclination 45° coupled with CFD. The geometry of the pneumatic conveying
computational cell size 4 mm × 4 mm
simulation time step, ∆t 10-7 s system and the type of particles used in the simulations are based
on the experimental work. Material properties and system
parameters are listed in Table 5. The number of particles used
0.85 and 1.85 m from the bend, respectively as labeled “16” was 1000, corresponding to an overall solid concentration of β
and “17” in Figure 1. The pressure data were acquired ) 0.16, where β is defined as the overall volume fraction of
simultaneously for 100 s with the sampling rate set at 100 Hz. particles divided by the volume fraction of particles at maximum
Figure 17 shows the results of the normalized signal from packing, which is generally taken to be 0.64. A dimensionless
the two pressure transducers against time (4 s). It is observed quantity (Λ) that indicates the ratio of electrostatic force arising
that two pressure waveforms from the two sensors are out of from the charged pipe wall to the gravitational force acting on
phase with each other, with each signal having a regular each particle is defined in section 3.4. The specific values used
period of ∼1 s. The time lag (∆t) between the two waves for the various parameters, such as the spring constant and
indicated the time taken for one cluster of particles to slide from coefficients of restitution and friction, have been shown to have
test point 1 to test point 2. The average ∆t value was obtained minimal effects on the flow patterns produced from the CFD-
statistically to be 0.5 ( 0.3 s. The corresponding time lag DEM simulations in a sensitivity analysis performed by Lim et
between two particle concentration waves measured by two sets al.11 In all simulations performed, particles were first allowed
of ECT sensors was also observed to be similar in mag- to settle freely under gravity for 0.5 s and form a packing at
nitude (see Figure 17b). The power spectral densities for both the “bottom” of the pipe before gas flow was initiated. Periodic
waves were obtained by a fast Fourier transform (FFT) of the boundary conditions were applied to the solid phase to simulate
pressure and concentration data, to have a more quantitative an open flow system while a uniform gas velocity profile was
comparison of the dominant frequencies for both waveforms. maintained at the inlet. Particles that were carried out of the
Figure 19. Numerical result of reverse flow [β ) 0.16 (1000 particles)]: (a) enlarged image of one section in pneumatic conveying through a pipe (inlet
gas velocity ) 10 m/s, Λ ) 5.0) and(b) diametrical distribution of particle axial velocity.
Ind. Eng. Chem. Res., Vol. 46, No. 19, 2007 6083
conveying pipe by the flowing gas were simulated to re-enter downward. Thus, the electrostatic force was essential for the
from the inlet of the pipe with the same velocities and radial occurrence of three regions in reverse flow, and this was also
positions. validated by the control experiments and the results of CFD-
Figure 19a successfully shows that reverse-flow behavior can DEM simulation.
be observed in particles moving along the bottom wall, when
the inlet gas velocity is 10 m/s and Λ ) 5.0. Figure 19b Acknowledgment
describes the solid velocity distributions along the pipe diameter
from the bottom of the pipe (y ) 0) to the top of the pipe (y ) This study has been supported by the SERC (A*STAR), under
1). Here, particle velocities are actual values and not normalized, Grant No. R-279-000-208-305. The authors are grateful to Dr.
to have a good comparison between the two groups of data. In Jun Yao, Dr. Kewu Zhu, Fong Yew Leong, and Lai Yeng Lee
for many helpful discussions on this project.
this figure, the symbols at the bottom right-hand corner represent
the inlet gas velocities and the dimensionless quantity Λ.
Consequently, when electrostatic force was 5 times the mag- Literature Cited
nitude of the gravitational force (Λ ) 5), negative velocities (1) Levy, A.; Mooney, T.; Marjanovic, P.; Mason, D. J. A comparison
appeared. This shows that some solids at the bottom of the pipe of analytical and numerical models with experimental data for gas-solid
may move in a direction opposite to the air flow. In particular, flow through a straight pipe at different inclinations. Powder Technol. 1997,
93, 253-260.
a peak negative value (-0.48 m/s) in the velocity between the (2) Hirota, M.; Sogo, Y.; Marutani, T.; Suzuki, M. Effect of mechanical
positions y ) 0.2 and y ) 0.3 corresponds to the transition properties of powder on pneumatic conveying in inclined pipe. Powder
region and illustrates a quantitative agreement with the experi- Technol. 2002, 122, 150-155.
mental data (-0.077 m/s to -0.031 m/s). However, when the (3) Zhu, K;, Wong, C. K.; Rao, S. M.; Wang, C. H. Pneumatic conveying
electrostatic force was absent (Λ ) 0), the velocity profile of granular solids in horizontal and inclined pipes. AIChE J. 2004, 50, 1729-
1745.
showed more significant backflow (-0.55 m/s to -0.38 m/s) (4) Zhu, K.; Rao, S. M.; Wang, C. H.; Sundaresan, S. Electrical
in the dense region. Furthermore, the change from negative capacitance tomography measurement on vertical and inclined pneumatic
values to positive values from the bottom to the top of the pipe conveying of granular solids. Chem. Eng. Sci. 2003, 58, 4225-4245.
indicates the gradual disappearance of the transition region. (5) Maré, T.; Voicu, I.; Miriel, J. Numerical and experimental visualiza-
Therefore, these results demonstrate that the electrostatic force tion of reverse flow in an inclined isothermal tube. Exp. Therm. Fluid Sci.
2005, 30, 9-15.
is an essential determinant of negative solid velocities in the (6) Yao, J.; Zhang, Y.; Wang, C. H.; Matsusaka, S.; Masuda, H.
transition zone of the reverse-flow regime. Generally, the Electrostatics of the granular flow in a pneumatic conveying system. Ind.
simulation results show good agreement with the experimental Eng. Chem. Res. 2004, 43, 7181-7199.
results and physical observations. (7) Rao, S. M.; Zhu, K. W.; Wang, C. H.; Sundaresan, S. Electrical
capacitance tomography measurements on the pneumatic conveying of
solids. Ind. Eng. Chem. Res. 2001, 40, 4216-4226.
5. Conclusions (8) Su, B. L.; Zhang, Y. H.; Peng, L. H.; Yao, D. Y.; Zhang, B. F. The
use of simultaneous iterative reconstruction technique for electrical
The concentration distribution of polypropylene (PP) particles capacitance tomography. Chem. Eng. J. 2000, 77, 37-41.
transported in a 45° inclined pneumatic conveying pipe was (9) Hua, J. S.; Wang, C. H. Electrical capacitance tomography measure-
measured using electrical capacitance tomography (ECT). By ments of gravity driven granular flows. Ind. Eng. Chem. Res. 1999, 38,
applying the best-correlated pixel method, the characteristics 621-630.
of velocities and directions of particle motions in three different (10) Msosorov, V.; Sankowski, D.; Manzurkiewicz, L.; Dyakowski, T.
The ‘best-correlated pixels’ method for solid mass flow measurements using
flow patterns were described and compared with results of electrical capacitance tomography. Meas. Sci. Technol. 2002, 13, 1810-
particle image velocimetry (PIV) and high-speed video camera 1814.
measurements. It is concluded that when the air flow rate is (11) Lim, E. W. C.; Wang, C. H.; Yu, A. B. Discrete element simulation
high, the entire solid phase is dilute and particles are dispersed for pneumatic conveying of granular material. AIChE J. 2006, 52, 496-
509.
over the entire cross section of the pipe with high velocity,
(12) Lim, E. W. C.; Zhang, Y.; Wang, C. H. Effects of an electrostatic
especially in the pipe center. In contrast, at low air flow rates, field in pneumatic conveying of granular materials through inclined and
most of the solid particles are deposited at the bottom of the vertical pipes. Chem. Eng. Sci. 2006, 61, 7889-7908.
pipe and some of them are transported in the upper part of the (13) Di, Felice, R. The voidage function for fluid-particle interaction
pipeline, forming the dense and dilute regions, respectively; the systems. Int J. Multiphase Flow 1994, 20, 153-159.
corresponding velocity distribution shows the presence of (14) Yao, J.; Zhang, Y.; Wang, H, C.; Liang, Y. C. On the Electrostatic
Equilibrium of Granular Flow in Pneumatic Conveying Systems. AIChE J.
reverse flow in a transition region between these two regions. 2006, 52, 3775-3793.
The reverse-flow pattern is also observed in the computational (15) Sommerfeld, M. Analysis of collision effects for turbulent gas-
fluid dynamics-discrete element method (CFD-DEM) simula- particle flow in a horizontal channel: Part I. particle transport. Int. J.
tion results. When the air flow rate is decreased further, a half- Multiphase Flow 2003, 29, 675-699.
(16) Zhang, Y.; Wang, C. H. Particle Attrition due to Rotary Valve
ring flow structure is formed and the majority of particles
Feeder in a Pneumatic Conveying System: Electrostatics and Mechanical
adhered to the pipe wall. The major forces acting on single Characteristics. Can. J. Chem. Eng. 2006, 84, 663-679.
particles were analyzed for the three flow patterns, including (17) Goldfarb, D. J.; Glasser, B. J.; Shinbrot, T. Shear instabilities in
the drag force, electrostatic force, gravity, and friction. For the granular flow. Nature 2002, 415, 302-305.
dispersed-flow pattern, the dilute region of the reverse-flow (18) Zhang, Y. F.; Yang, Y.; Arastoopour, H. Electrostatic effect on
the flow behavior of a dilute gas/cohesive particle flow system. AIChE J.
pattern, and the half-ring-flow pattern, aerodynamic drag force 1996, 42, 1590-1599.
was the major driving force transporting particles upward. For (19) Dhodapkar, S. V.; Klinzing, G. E. Pressure fluctuations in pneumatic
the dense region of the reverse-flow pattern and half-ring- conveying systems. Powder Technol. 1993, 74, 179-195.
flow pattern, the electrostatic force predominated and at-
tracted particles onto the pipe wall. For the transition region of ReceiVed for reView October 12, 2006
reverse flow, three major forces were in the same ranges and ReVised manuscript receiVed March 23, 2007
Accepted March 30, 2007
caused particles to be in a suspended state. In this region,
particles would be drawn by the gravitational force to flow IE061304I