Characteristics and Mechanisms of Particle Adhesion

Patterns in an Aerodynamic Cyclone

Yuanye Zhou1,2*, Shan Zhong1,2, Lin Li1,2

1 School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Oxford

Road, Manchester, M13 9PL, United Kingdom

2 Laser Processing Research Centre, The University of Manchester, Oxford Road, M13 9PL, United

Kingdom

*Corresponding author at: School of Mechanical, Aerospace and Civil Engineering, The University of

Manchester, Oxford Road, M13 9PL, United Kingdom

E-mail address: yuanye.zhou@manchester.ac.uk

Abstract

Characteristics of particle adhesion (deposition) patterns in an aerodynamic cyclone were

studied by both experimental methods and computational fluid dynamic (CFD) simulation

methods. The cyclone used in the experiment was made of Acrylonitrile Butadiene Styrene

(ABS). The particles were a plaster material, with an average size of 1.13 μm and a density of

2300 kg/m3. Four levels of particle load rates were examined, ranging from 0.28 g/m3 to 0.96

g/m3 at a fixed mass flow rate of 2.1 g/s. Experimental results showed three key features of

particle adhesion patterns. They are large-scale spiral patterns (SPs), small-scale wave

patterns (WPs) and thick adhesion layer (TAL) at the cyclone tip region. It was observed that

the SPs had 5 turns and the WPs were periodic discrete patterns that crept slowly against the

flow direction. The formation of WPs was explained based on the Barchan sand dune

mechanism. Under zero particle load rate, six different mass flow rates ranging from 1.24 g/s

to 3.16 g/s were simulated using CFD. It was found that the precessional bent vortex end

(PBVE), precessing along the circumference of the cyclone tip, occurred close to the cyclone

tip. The PBVE was believed to be the cause of the TAL, because there was a weak wall shear

stress region below the PBVE. In addition, particle trajectories were simulated at a mass flow

rate of 2.26 g/s. Simulation results showed that particles had spiral trajectories that were

supposed to be linked with the SPs.

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Key words: cyclone, particle, adhesion, deposition, CFD

Nomenclature

a Width of cyclone inlet [m]

b Height of cyclone inlet [m]

d Diameter of cyclone tip [m]

D Diameter of cyclone cylindrical body [m]

De Diameter of cyclone vortex finder [m]

Hc Length of cyclone cylindrical body [m]

L Length of cyclone conical body [m]

pw Wall static pressure of cyclone [Pa]

∆ p0 Pressure drop of cyclone without particles [Pa]

S Length of cyclone vortex finder [m]

th Thickness of vortex finder [m]

Abbreviations

CoR(er) Coefficient of Restitution

PVC Precessing Vortex Core

PBVE Precessional Bent Vortex End

SPs Spiral Patterns

TAL Thick Adhesion Layer

TCG Triboelectric Charge Generator

URSM Unsteady Reynolds Stress Model

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WPs Wave Patterns

1. Introduction

Cyclones are widely used devices in the particle processing technology. A typical cyclone

consists of a tangential inlet, a cone shape body, and a vortex finder at the top centre, as

shown in Figure 1. Air and mixed particles enter the cyclone through the rectangular

tangential inlet. Particles are separated due to the centrifugal force and are collected in the

dust collector. Clean air leaves the cyclone through the vortex finder.

It is important to avoid particle adhesion (deposition) in the cyclone, as particle adhesion can

cause the blockage of the cyclone, which deteriorates the performance of the cyclone

(O’Callaghan and Cunningham, 2005). In the industry, there are some measures for

preventing the blockage of the cyclone, such as placing a jet flow tube at the tip of a cyclone

(Huang et al., 2013; He et al., 2014), the use of a portable central rod (Mozley, 1979) and the

use of vibrating rubber nozzle in a commercial vacuum cleaner.

However, it remains unclear about characteristics and mechanisms of particle adhesion

patterns in the cyclone. Previously, particle adhesion patterns were known as spiral patterns

on the wall (Yuu et al., 1978; Ranz, 1985). The said spiral patterns were caused by the

swirling flow in the cyclone. But they did not explain why spiral patterns had several bands.

Recently, some small-scale discrete droplet shapes and chevron shapes of particle adhesion

patterns were also observed on the wall (Bogodage and Leung, 2016; Houben, 2011). But

they did not give the explanation for these patterns. Therefore, there is need to understand

characteristics and mechanisms of these particle adhesion patterns, so as to provide the

fundamental knowledge for the reduction of particle adhesion in the cyclone.

Theoretically, particle adhesion in the cyclone is affected by the capillary force, the van der

Waals force, the electrostatic force and the aerodynamic force. These forces together induce

the friction and the removal force on the particle over a surface. According to the particle

sliding detachment model (Wang, 1990), if the friction is larger than the removal force,

particle adhesion would occur.

In detail, these forces are dependent on a number of parameters including the environmental

condition, material properties and the air flow distribution in the cyclone. For example, the

capillary force is related to the relative humidity (RH) of the environment; the van del Waals

force is associated with the Hamaker constant between the particle and the surface; the

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electrostatic force is determined by the particle charge; the aerodynamic force is determined

by the air flow velocity (Mittal and Jaiswal, 2015).

In the cyclone, it is possible to control the environmental condition and material properties,

so that the impact of the capillary force, the van der Waals force and the electrostatic force

can be uniform for the whole cyclone. However, the aerodynamic force in the cyclone is

usually complex, due to different flow characteristics in the different region of the cyclone.

The basic flow structure in the cyclone is the ‘Rankine’ vortex, with a solid vortex core and a

free outer vortex. But there are some secondary flows as well, such as the roof secondary

flow and the axial secondary flow. There is also a large-scale coherent structure existing in

the cyclone, known as the precessing vortex core (PVC) phenomenon (Yazdabadi et al.,

1994). At the cyclone tip region, the PVC phenomenon is the precessional bent vortex end

(PBVE) attached to the wall surface (Hoffmann and Stein, 2002). In addition, it is found that

the particle concentration also affected the local air flow velocity and the aerodynamic force

(Liang et al., 1996). Therefore, the prediction of particle adhesion patterns in the cyclone

needs to consider all of flow characteristics mentioned above on a case-by-case basis,

because detailed flow characteristics are unique for different region of the cyclone.

In this work, a small aerodynamic cyclone was used to study characteristics and mechanisms

of particle adhesion patterns. Particle adhesion patterns were visualised during the experiment

and after the experiment. In order to explain experimental results, the CFD simulation

method was adopted to obtain detailed flow characteristics of the cyclone. The PBVE and the

particle trajectory were revealed in the simulation. Based on simulation results and the

Barchan sand dune mechanism, experimental observations of particle adhesion patterns were

explained.

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Figure 1 Illustration of a typical cyclone and its key dimensions

2. Material and method

2.1.Particles

As plaster particles can easily form particle adhesion, Thistle Dura-Finish plaster,

manufactured by British Gypsum Ltd. was used in the experiment. The density of this plaster

particle is about 2300 kg/m3. Before the experiment, particles were stored in a sealed tank at

room temperature, so that particles were not wetted by the humidity.

Figure S1 shows the 2D image of particles under an optical microscope (GXML 3230) and

the particle size distribution. Since particles were irregular, the particle size distribution was

given in terms of dynamically equivalent diameter, which was measured by a particle sizer

(TSI 3321) in a sedimentation tank with diluted particle samples. The average dynamically

equivalent diameter of plaster particles was 1.13 μm.

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2.2.Experimental method

2.2.1. Particle adhesion test rig

An experimental test rig was built to investigate particle adhesion patterns in the cyclone. The

test rig consisted of a triboelectric charge generator (TCG), a cyclone, a fibre filter, an air

pump with a pump voltage adjustor and sensors for pressure and mass flow rate

measurement, as shown in Figure 2.

In this study, the mass flow rate of the cyclone was fixed at 2.1 g/s. Four particle load rates

were tested. In terms of per gram particle in per volume flow, the particle load rates were

0.28 g/m3, 0.60 g/m3, 0.75 g/m3, and 0.96 g/m3, respectively. The duration of each test was 20

minutes.

During the experiment, the mass flow rate and pressure drop of the cyclone were measured.

The measurement range of mass flow sensor (TSI 40241) was 0 - 6 g/s and the accuracy was

± 3% of the reading. The pressure sensor (Sensortechnics HDIM series) read the differential

wall pressure between the inlet and outlet. The measurement range of pressure sensor was

±20 kPa and the accuracy was ± 100 Pa. The measurement was taken every 15 seconds. The

measured data were transmitted to a data acquisition card (model NI PCI-6221) and were

stored in a PC computer.

Dimensions of cyclones used in the experiment are listed in Table 1. Two types of cyclones

were used. They had the same inlet, vortex finder and dust collector. The only difference was

the conical part of the cyclone. One had a grey conical part, the other one had a transparent

conical part, as shown in Figure S2. Both conical parts were made from Acrylonitrile

Butadiene Styrene (ABS) material with a similar surface roughness (7.63 μm and 7.85 μm,

respectively, measured by Keyence VK-X200K 3D Laser Scanning Microscope). In the

experiment, the grey conical part was used for observing particle adhesion patterns after the

experiment, while the transparent conical part was used for observing particle adhesion

patterns during the experiment. Before each experiment, cyclones were cleaned by the water

and were dried by the compressed air.

The environmental temperature and humidity during the experiment was 20±2℃ and 50±5%

RH, respectively. The variation of room pressure was less than 5% of 101 kPa over one year,

according to the data from the centre of atmospheric science in the university. Thus, the test

environment condition is regarded as constant.

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Table 1 Dimensions of cyclones used in the experiment

Feature Dimension, mm Feature Dimension, mma 5 S 13.55b 11.6 Hc 12.1D 35 L 87.11De 8.54 d 6.6th 1

* Measurement errors are within 0.01 mm

Figure 2 Particle adhesion test rig

2.2.2. Flow characteristics test rig

A flow characteristics test rig was built to validate results of the CFD simulation, as

illustrated in Figure 3. The cyclone used in the flow characteristics test rig had the same

geometry as the cyclone used in particle adhesion test. During the test, the clean air moved

into the cyclone directly. The mass flow rate was 1 g/s to 3 g/s.

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The pressure drop and mass flow rate measurements were the same as that described in the

previous section. In addition, the wall pressure distribution of the cyclone was also measured.

Five wall pressure sensors (HDIM series) were placed along the wall of the conical part. The

axial location of the wall pressure tapping from the cyclone tip was 5 mm, 15 mm, 25 mm, 35

mm and 45 mm, respectively. The sampling frequency of the pressure measurement was 200

Hz and the duration was 15 seconds.

In order to measure the frequency of the PBVE, a microphone (RS Pro Microphone) was

placed next to the cyclone tip. This measurement technique was reported as an approximate

frequency measurement method for the PBVE frequency in the cyclone (Grimble and

Agarwal, 2015). In this test, the measurement range of the microphone was 50 Hz to 16 kHz,

which was enough to cover two times the PBVE frequency. An 8 kHz low-pass signal filter

was applied to the measured acoustic signal before it was recorded by the data acquisition

card. The sampling frequency and duration of the acoustic measurement were 16 kHz and120

seconds.

Figure 3 Schematic of flow characteristics test rig

2.3.CFD simulation method

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Commercial CFD software (Star CCM+ 9.02) was adopted to investigate the flow in the

cyclone. The computational domain was a section of the flow characteristics test rig, which

was the section between two pressure tappings. Structured meshes were generated for the

computational domain, as shown in Figure S3. The finest mesh was 0.08 mm at the tip of the

cyclone (1.2% of the cyclone tip diameter). The total number of meshes was ~3 million.

The mesh sensitivity study was conducted by changing the mesh density of the computational

domain. The coarse meshed domain had ~1 million meshes, the medium meshed domain had

~3 million meshes, and the fine meshed domain had ~5 million meshes. After running the

simulation under the same condition, the difference of the Euler number between the coarse

meshed domain and the medium meshed domain was 16%. The difference between the

medium meshed domain and the fine meshed domain was 5%. Therefore, the medium

meshed domain (~3 million meshes) was chosen, considering the fact that the computational

time of one case for the coarse meshed domain, the medium meshed domain and the fine

meshed domain was typically 1 week, 2 weeks and 4 weeks, respectively.

The turbulence model for the simulation was chosen to be the Unsteady Reynolds Stress

Model (URSM), so that the unsteady PBVE can be predicted. In addition, the two-layer all y+

wall treatment was chosen, because the flow velocity varied in different region of the

cyclone. Boundary conditions of the CFD simulation (Star CCM+ 9.02) were given based on

the experiment condition, as shown in Table 2. The wall of cyclone is set to be wall

boundary condition. In total, six cases were designed for the simulation. The mass flow rate

for these cases ranged from 1.24 g/s to 3.16 g/s.

After obtaining the flow field of the cyclone, the unsteady particle trajectory was simulated

by the Lagrangian method. Particles were injected from an injector grid consisting of a 11 x

11 array at the inlet as shown in Figure S4. Each time step, a hundred and twenty one

particles were injected into the cyclone. Based on the experiment condition, the particle size

was set to be 1.13 μm and the particle density was set to be 2300 kg/m3. The two-way

coupled particle-particle interaction and particle-flow interaction were not simulated.

Furthermore, only the drag force was considered in the simulation. The lift force, turbulence

dispersion effect and gravity were not accounted, as their magnitude was one to two orders

smaller than that of the drag force. Particle-wall interaction was simplified to be the elastic

collision. For simplicity, the tangential coefficient of restitution (CoR) was assumed to be the

same as the radial CoR.

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Table 2 Boundary conditions and time steps

Case 1 2 3 4 5 6Inlet total pressure, Pa 0 0 0 0 0 0

Outlet static pressure, kPa -2 -5 -8 -10 -12.6 -17Time step, µs 49 36 28 26 24 2

3. Results and discussion

3.1.Experimental results and discussion

3.1.1. Particle adhesion patterns in the cyclone

Pictures of particle adhesion patterns were taken after the experiment. As shown in Figure 4,

particle adhesion patterns (white part on the wall) were formed in the cyclone. The area of the

tip blockage to the tip area was measured with a Matlab image procession tool, showing that

the blockage became serious when the particle load rate increased. At the highest load rate

condition (0.96 g/m3), the cyclone was completely blocked. After that, particles were not able

to move into the dust collector.

What’s more, particle adhesion patterns under the highest load rate condition (0.96 g/m3)

appeared to be thinner than other load rate conditions. It was probably because particle

adhesion patterns were worn by particles that were not able to move out of the cyclone. At

other load rate conditions, particle adhesion patterns looked similar, except for the thickness

of the adhesion.

Based on the pictures, three key features of particle adhesion patterns were identified. They

were large-scale spiral patterns (SPs), small-scale wave patterns (WPs) and the thick

adhesion layer (TAL) close to the cyclone tip (within 10 mm away from the tip), as shown in

Figure 5. Large-scale SPs started from the upper part of the conical part and continued to the

cyclone tip. These SPs were similar to previous findings (Yuu et al., 1978; Ranz, 1985). In

this study, there were 5 turns in the SPs. Visually, the thickness of SPs slowly increased as

patterns moved down to the cyclone tip. It could be explained by the decrease of the wall

surface area. However, such mechanism did not hold for the TAL close to the cyclone tip,

because the thickness of the TAL increased rapidly. The cause for the TAL was explained

with the aid of the CFD simulation later. In addition, small-scale WPs were found to be

embedded in the large-scale SPs. The WPs were similar to droplet patterns and chevron

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patterns (Bogodage and Leung, 2016; Houben, 2011). They were small-scale approximate

periodic discrete patterns in the cyclone.

Figure 4 Pictures of particle adhesion patterns after experiment at different particle

load rates (mass flow rate 2.1 g/s)

Figure 5 Key features of particle adhesion patterns in cyclone

3.1.2. Development of particle adhesion patterns

By using a transparent conical part of the cyclone, the development of particle adhesion

patterns were visualised during the experiment at the highest particle load rate condition (0.96

g/m3). As shown in Figure 6, after loading particles, the WPs appeared firstly around the

cyclone tip. Then the WPs became larger and thicker, as particles were continually loaded.

With the continually growing of the WPs, the SPs were formed. After 24 seconds, particle

adhesion at the cyclone tip was severe. It can be regarded as the initial state of the TAL.

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Interestingly, it was also found that the WPs crept backward (against the air flow direction),

as shown in Figure 7. The white arrow indicated a fixed location and the white rectangular

window tracked the same individual WP. It can be seen that the relative location of the white

window and arrow changed, suggesting that the individual WP crept from the left to right (the

local flow velocity direction was from the right to left). From 30 second to 40 second, this

individual WP travelled about 1 mm. Thus, the migration velocity of the WP was relatively

small compared with the flow velocity. In addition, although the resolution of the picture was

not very high, it can still be seen that the shape of the WP deformed during the experiment.

Similar creeping motion could be found for other individual WPs.

Figure 6 Development of particle adhesion patterns in the cyclone at particle load rate

0.96 g/m3 (mass flow rate 2.1 g/s)

Figure 7 Backward creeping motion of the WPs in the cyclone at particle load rate 0.96

g/m3 (mass flow rate 2.1 g/s)

3.1.3. Discussion on experimental results

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A hypothesis was proposed to explain the motion of the WPs. It was found that the motion of

the WPs was similar to the motion of the Barchan sand dune. The Barchan sand dune is an

arc-shaped dune, widely appearing in the desert region. This sand dune can individually exist

or form a chain of sand dunes, as shown in Figure S5. The WPs and Barchan sand dune had

similarities of their appearance and the slowly moving speed.

Based on the mechanism of slowly moving Barchan sand dune, the mechanism of the WPs

moving against the air flow was illustrated, as shown in Figure 8. The movement of the WPs

was determined by the progress of the particle deposition and removal. For the Barchan sand

dune, the flow blew sand particles on the windward side and transported them to the leeward

side, where these sand particles settled. Therefore, it led to the slow migration of the sand

dune in the flow direction. However, in the cyclone, particles were separated to the windward

side due to the centrifugal force. At the leeward side of the WPs, particles were removed by

the flow, may be due to the flow separation. As the flow was faster in the cyclone than the

flow over the sand dune, the flow separation at the leeward side of the WPs was strong

enough to remove particles. Therefore, the WPs crept against the flow direction. The reason

for the slow moving was that the progress of the particle deposition and removal involved

large amount of particles. It required time to accumulate and remove large amount of

particles.

The reasons why the SPs and TAL occurred in the cyclone were explained with the aid of

CFD simulation in the next section.

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Figure 8 Sketch showing the mechanism of the backward creeping motion of the WPs

3.2.CFD simulation results and discussion

3.2.1. Validation of CFD simulation

The CFD simulation was validated by comparing the pressure drop and wall pressure

distribution in the cyclone with that of the experimental results obtained from the flow

characteristics test rig.

A comparison of the pressure drop between simulation results and experiment results is

shown in Figure 9(a). It can be seen that the experiment results agreed well with the

simulation results. Furthermore, the comparison of the wall pressure distribution was

conducted. The wall pressure was plotted in a non-dimensional way, as the pressure drop was

not the same for different mass flow rates. The non-dimensional wall pressure was defined as

pw /∆ p0, where pw is the wall pressure and ∆ p0 is the pressure drop without particles loaded.

It was noted that since the wall pressure was lower than the atmospheric pressure, the sign of

the non-dimensional wall pressure was negative. A comparison of the wall pressure

distribution between simulation results and experiment results are given, as shown in Figure

9(b). Again, it can be seen that experiment results agreed well with simulation results.

As both pressure drop and wall pressure distribution predicted by the simulation were close to

experiment results, simulation results were regarded as reliable.

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In addition, it was found that the non-dimensional wall pressure followed a declining trend

from the upper part of the cyclone to the cyclone tip, in both experiment and CFD simulation.

It meant that the wall pressure decreased along the wall. A faster declining ratio of the non-

dimensional wall pressure was also observed between z=15 mm and z=5 mm. It was

supposed that the PBVE occurred around this region, as describe by Peng et al. (2005) that

the PBVE would lead to a faster decrease of the wall pressure in the region above the vortex

end and a rapid increase of the wall pressure in the region below the vortex end. According to

the measured wall pressure in this study, only faster decrease of wall pressure was observed.

Therefore, the location of the vortex end should be below z=5 mm in this study.

a b

Figure 9 Comparison of CFD simulation and experiment (mass flow rate 1 g/s to 3 g/s )

(a) on the pressure drop (b) wall pressure distribution

3.2.2. Precessional bent vortex end in the cyclone

The precessional bent vortex end (PBVE) was visualised by using the pressure iso-surfaces in

the CFD simulation. Instantaneous results of the static pressure in the vertical cross section of

the cyclone at the mass flow rate of 1.24 g/s are shown in Figure 10(a). It can be seen that

the vortex was bent near the cyclone tip. In other locations, the vortex was slightly twisted

but was almost straight. In Figure 10(b), the vortex core was eccentric, which was associated

with the bent vortex end. As the time step advanced, the vortex core changed its location

periodically (the precession motion of vortex core represents the precession motion of vortex

end). Similar results were found at other mass flow rates, as shown in Figure S6, S7, S8, S9

and S10. All these results showed that the location of the PBVE in the CFD simulation was

less than 2 mm away from the cyclone tip, which was the same as the prediction of

experimental results.

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In order to support the CFD simulation, the PBVE frequency was experimentally measured

by placing a microphone next to the cyclone tip. The measured acoustic signal was processed

by the Fourier transform method in Matlab2012a. Results are shown in Figure 11. It can be

seen that the frequency of the acoustic signal had several peaks. Some of peaks did not

change as mass flow rate increased, such as 0 Hz, 3000 Hz, 4000 Hz and 6000 - 8000 Hz.

They were regarded as the hum frequency of cyclone or the environment noise. However,

some of the peak frequencies moved as the mass flow rate changed, as indicated by the

arrows. At the lowest mass flow rate, the peak was approximately 1700 Hz. As the mass flow

rate increased from 1.3 g/s to 3.0 g/s, the peak moved to around 2500 Hz, 3100 Hz, 3600 Hz,

4000 Hz, 4500 Hz and 5100 Hz, respectively.

In the CFD simulation, the frequency was counted for 100 precessional cycles. A comparison

between the CFD simulation and the experiment on the PBVE frequency is shown in Figure

12. Both experimental results and simulation results showed that the frequency changed

nearly linearly with the mass flow rate. However, experimental results were always higher

than simulation results. The difference between the experiment and the CFD simulation was

about 25%. It was probably because the URSM used in the CFD simulation had difficulty to

exactly capture the PBVE. However, considering the fact that the linear trend of the

frequency and the location of the vortex end were properly predicted by the CFD simulation,

the CFD simulation was able to reveal the PBVE in the cyclone.

Figure 10 CFD simulation results of PBVE at mass flow rate 1.24 g/s (time step: 49 µs)

(a) vertical cross section; (b) horizontal cross section (Z =2 mm)

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Figure 11 Spectrum of the measured acoustic signal of the cyclone

Figure 12 Comparison of the CFD simulation and the experiment on the PBVE

frequency

3.2.3. Particle trajectory in the cyclone

Results of particle trajectory in the cyclone at different coefficient of restitution and a fixed

mass flow rate (2.26 g/s) are shown in Figure 13. Particles were enlarged to make them

visible. It can be seen that all particles moved down in a spiral trajectory into the dust

collector. By counting the band of the trajectory, the number of the spiral trajectory turns was

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6, which was different from experimental results (5 turns). This difference was supposed due

to the particle-particle interaction in the experiment, as the particle-particle interaction was

found to reduce the number of turns (Chu et al., 2011; Chan et al., 2008).

Comparing results of different coefficient of restitution (CoR), it can be seen that the CoR did

not significantly affect the spiral particle trajectory but did affect the particle velocity. For a

small CoR (0.25), the particle velocity was low in some region, which appeared like vertical

bands (the dark colour bands in Figure 13). As the CoR increased, the area of low particle

velocity bands decreased, which meant that the particle velocity increases in the bands.

Figure 13 Particle trajectory in the cyclone at different coefficient of restitution (mass

flow rate 2.26 g/s)

3.2.4. Discussion on simulation results

Based on the results of CFD simulation, the formation of TAL and SPs can be explained.

The TAL near the cyclone tip, observed in our experiment, was supposed to be explained by

the PBVE. In the experiment, the location of the TAL was close to the location of the PBVE

in the CFD simulation. However, the location of the TAL in the experiment was 0 mm to 10

mm away from the cyclone tip. Some parts of the TAL were slightly above the location of the

PBVE in the CFD simulation (0 mm to 2 mm). In previous study, the location of PBVE was

found to be lifted up by the loading of particles (Hoffmann et al., 1995; Peng et al., 2005). A

possible explanation was that downward moving particles damped the flow and pushed the

flow and PBVE moving upward. Therefore, the location of the PBVE in the experiment was

higher than that in the CFD simulation.

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Since the PBVE means the end of vortex, the flow wall shear stress was small below the

PBVE (wall shear stress distribution can be found in S3 section). Thus, the weak flow below

the PBVE in the experiment was believed to be located at the same position as the TAL,

which was supposed to be the major cause of the TAL. Similar opinion was proposed by

Hoffmann and Stein (2002), but they did not show experimental results of particle adhesion

that was associated with the PBVE. In this study, experimental results of particle adhesion

were supported by relevant CFD simulation results, so that this opinion was confirmed.

In addition, the SPs can be explained by the spiral particle trajectories. In the simulation, the

spiral particle trajectories had 6 turns in the cyclone. In the experiment, there were 5 turns of

the SPs. Because the particle-particle interaction decreased the number of turns (Chu et al.,

2011; Chan et al., 2008), it was believed that if the CFD simulation considered the particle-

particle interaction, the number of turns would be the same as that in the experiment.

Moreover, as there was no significant change in the wall shear stress (see S2 section) on most

parts of the cyclone, the aerodynamic force on the particle near the wall did not change

significantly. Therefore, the spiral SPs were the footprint of the spiral particle trajectories.

4. Conclusion

Particle adhesion patterns in aerodynamic cyclones were studied by using plaster particles.

The size and density of the plaster particle was 1.13 μm and 2300 kg/m3. In the experiment,

the mass flow rate of the cyclone was 2.1 g/s, and particle load rates were 0.28 g/m3, 0.60

g/m3, 0.75 g/m3, and 0.96 g/m3, respectively. Experimental visualisation results showed that

particle adhesion patterns had three key features:

Large-scale spiral patterns (SPs)

Small-scale wave patterns (WPs)

Thick adhesion layer (TAL) near the cyclone tip

The CFD simulation was conducted to help explain these features. In the CFD simulation,

structured meshes were generated for the cyclone, and the unsteady RSM was chosen for the

turbulence model. Combined with results of the CFD simulation, the features observed in the

experiment were explained as follows:

The spiral particle trajectory explained the SPs

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The PBVE was believed to be the cause of the TAL near the cyclone tip

In addition, with the aid of the Barchan sand dune mechanism, the WPs was explained as

follow:

Barchan sand dune mechanism helped to illustrate the backward creeping motion of

the WPs

Acknowledgments

The author would like to acknowledge the Dyson Ltd. for funding this research. Also thanks

for James Allan in National Centre for Atmospheric Science (NCAS) for the assistance with

particle size measurements.

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