Flujo de Aire en Secadores Spray

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Drying 2004 Proceedings of the 14th International Drying Symposium (IDS 2004)

So Paulo, Brazil, 22-25 August 2004, vol. B, pp. 775-781

775

AIRFLOW PATTERNS IN A COUNTER-CURRENT SPRAY DRYING TOWER

SIMULATION AND MEASUREMENT

Andrew E. Bayly, Paul Jukes, Michael Groombridge and Clare McNally

Procter & Gamble, Newcastle Technical Centre, Whitley Rd., Longbenton,

Newcastle upon Tyne, NE12 9TS, United Kingdom.

E-mail: [email protected]

Keywords: spray drying, airflow patterns, swirl, counter-current, CFD

ABSTRACT

Airflow profiles in a counter-current spray drying tower with a swirling airflow were

measured using an LDV (Laser Doppler Velocimeter) system at a series of axial

locations. The experiments were done in cold conditions without any spray present. Acommercial computational fluid dynamic code (FLUENT) was used to develop a model

of the experimental conditions. Experimental and simulated airflow profiles are

compared and a good agreement is observed.

INTRODUCTION

During spray drying the air and particles can be contacted in a counter-current or co-current fashion.

Co-current towers are typically chosen for heat sensitive products as the particle temperature remains

lower than in the counter-current case. However for drying more robust materials the counter-current

process offers higher thermal efficiency and can lead to different, in some cases desirable, product

characteristics. The counter-current dryer was chosen by the synthetic detergent industry as its process of

choice 50 or more years ago and it remains by far the most economically important process for themanufacture of granular detergent products around the globe.

Although basic research into spray drying has been undeservingly limited in the past (Masters, 2002),

several groups have studied airflow patterns in spray dryers. A review of this work can be found in

Southwell & Langrish (2000). The vast majority of this work has been focused on co-current towers,

where the importance of the airflow pattern on dryer operation and product properties has been

highlighted. This is no less the case for counter-current dryers yet to date quantitative measurements of

airflow profiles have not been reported. This has meant that recent work modelling the airflow patterns in


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counter-current dryers (Harvie et al., 2001), whilst providing useful insights into the flow, has been

restricted by the lack of experimental data for validation.

The work reported here seeks to address this issue by making accurate experimental measurements of

air flow profiles within a small scale, counter-current tower in cold, no spray, conditions. A single base

case is presented alongside a computational fluid dynamic (CFD) model of the same situation.

GEOMETRY

The scaled down counter-current tower studied by Sharma (1990) was used for this investigation. The

geometry is shown in Figure 1. The cylindrical section of the tower is 1.22 m wide and around 5 m high.

The tower is constructed from a transparent PVC material. In the set-up studied the air enters through

eight equally spaced cylindrical air inlets set around the tower hip. These directable air inlet nozzles

were fixed in a position for the experiments. The axis of the cylinder was set 25 below the horizontal

and 25 to the tower radius in the horizontal plane, thus imparting a significant swirl to the flow in the

tower. For these experiments the nozzle exits were covered with a perforated mesh. This mesh both

ensured an equal distribution of air through each of the inlets and helped straighten the flow into the

tower. The inlet nozzle diameter was 0.102 m i.d. and the circular mesh holes were 2 mm diameter, the

overall open area was 2.41x10

-3

m

3

per inlet. Air can also enter the tower through an inlet at the basewhich is 0.05 m diameter. The outlet, at the top of the tower, is 0.68 m diameter.

Figure 1 Tower Geometry

1.71m0.36m

0.07m

0.05m

1.67m

0.17m

1.21m

2.73m

0.68m

5.28m

a) tower dimensions

Perforated plate

b) Schematic of one of the eight inlets


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EXPERIMENTAL METHODS

Fans supplied air to the tower at a total flowrate of 3814 m3/hr through the 8 main inlets and a

flowrate of 239 m3/hr through the inlet at the base of the tower (known as the leakage airflow). The air

temperature was approximately 20 C. Tangential and axial velocity profiles were measured at a series of

heights using a Laser Doppler Anemometer (Flowlite, Dantec Ltd.). Care was taken at each position tocheck alignment and measurement position and errors in this are estimated to be less than +/- 3 mm.

Each measurement was taken over a period between 30 s and 120 s. The measurement was stopped if

10,000 velocity samples were recorded. The flow was seeded using smoke injected into the inlet at the

base of the tower. The heights where the measurements were taken are shown in Table 1.

Table 1. Measurement Position Locations

Position 1 2 3 4 5 6 7 8

Distance from base (m) 0.71 1.45 1.73 2.00 2.39 2.90 3.58 4.77

Diameter of tower (m) 0.76 1.51 1.71 1.49 1.21 1.21 1.21 1.21

The repeatability of the velocity experiments was checked and found to be excellent for the tangentialprofiles. For the axial profiles some small differences were seen between repeats on different days,

particularly in the region close to the tower base, position 1. This is due to some variability in the leakage

airflow rate and direction, further up the tower these differences were reduced, and apart form the central

region of the tower, where leakage flow has an effect, the velocities matched to within 0.2 m/s. The

symmetry of the profiles was also checked by measuring profiles along horizontal axes 90 apart. At

position 6 the profiles were not significantly different; however at position 1, some significant asymmetry

was noted in the tangential and axial profiles. Possible reasons for this are discussed below.

CFD MODEL

The airflows were simulated using the commercially available Fluent 6.0 code available from Fluent

Inc.. The grid used to represent the geometry was composed mainly of hex cells and had of the order of

500,000 cells with higher densities in the areas with the highest velocity gradients, for example at the

centre of the tower and at the walls. A Reynolds Stress Model was chosen to model the turbulence due to

the highly swirling nature of the flow. A SIMPLEC solution method was used for the pressure-velocity

coupling and 2nd order differencing for momentum terms. Initially a steady-state simulation was

performed. In this case the mass flow residuals did not reduce to as low a level as typically aimed for. If

this flow solution was then used to initialize an unsteady simulation the residuals could be reduced

further. However, the difference in the velocity profiles between the unsteady and steady simulations

was not large. The magnitude of the profiles remained almost identical; though a movement of the central

vortex core could be seen in the unsteady case.

In order to model the inlet geometry the perforated plate was modelled as an orifice plate with an openarea of identical size to the total open area on the perforated plate. This therefore gives the same inlet

velocity and consequently angular momentum as the experimental case which is important in order to

generate the same tangential velocity profiles. The airflow profile at the inlets and the base of the tower

were modelled as uniform with constant velocity inlets.


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AIRFLOWS

Experimentally measured and simulated airflow profiles are shown in Figure 2. The simulated profiles

are instantaneous profiles taken from the unsteady simulation and demonstrate some of the non-

uniformity that this leads to. Overall one sees good agreement between the magnitude and shapes of the

tangential and axial velocity profiles. The flow patterns that these profiles reveal are the same as those

reported by Sharma (1990) and Harvie et al. (2001), however they provide also provide a quantitative

comparison. The profiles in the different areas of the tower are considered below:

Inlet area

At around the inlet level, positions 3 and 4, there is good agreement, see Figure 2, between the overall

shape and magnitude of the axial and tangential velocity profiles, and the expected forced vortex flow

pattern is seen. The exact details, maximum and minima, are not matched for two reasons. Firstly, the

profile is dependent on the circumferential position of the profiles which are not identical, and secondly

due to the approximate modelling of the inlet profile.

Base of tower

The airflow in the base of the tower is asymmetric due to non-uniformity in the jet of leakage air

entering the tower. This jet is also unsteady and when visualized using ribbons is seen to flap with no

clear periodicity, however it remains mainly on one side of the tower as indicated by the quantitative

measurements. The precession of the vortex core in the tower could be responsible for this as well as

natural unsteadiness and asymmetry in the leakage airflow which was not straightened or made uniform.

Interestingly the tangential profile remains quite symmetric in both the simulated and the measured cases

where the axial profile is significantly asymmetric in both cases. The asymmetry in the simulated profile

emphasizes the unsteady nature of this region of the tower.

Tower cylindrical section and exit

In the tower cylindrical section, the tangential profile changes from a forced vortex shape to Rankinetype vortex shape, the peak tangential velocity moving into the centre of the tower slightly as this

happens. The model predicts the overall shape of the tangential profile well though it flattens the profile

at the centre of the tower, where the measured velocity gradient of the profile is steep. This is perhaps

due to either the leakage air inlet boundary condition being uniform with no swirl or perhaps the exit

boundary condition.

This Rankine vortex shape differs from the simulations of Harvie et al. (2001) which predict a forced

vortex within this section of the tower (with the 25, 25 inlet set-up as used here). Whilst the overall

flow rates in Harvie et al.s simulation are not identical to those used here, it is thought that the profile

shapes should not change significantly with airflow rate. The most likely reason therefore for the forced

vortex profiles is the use of the model, as these profile shapes have been noted previously by theauthors when using this turbulence model to simulates these flows.

The axial profile is again well predicted by the model and the same features, which are governed by the

swirl profile, are seen. The axial profile starts with maxima at the walls and at the centre, with a slight

down flow at the minima between these. The wall maxima weaken further up the tower and a minimum

forms in the central peak towards the exit of the tower as the flow is constrained by the exit geometry.

This is more clearly defined by the simulation, again probably due to the exit boundary condition.


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Position 8 (4.77m)

-2

-1

0

1

2

3

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 8 (4.77m)

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

LDA

Position 7 (3.54m)

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 7 (3.58m)

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

Position 6 (2.9m)

-2

-1

0

1

2

3

-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 6 (2.9m)

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

LDA

Position 5 (2.39m)

-4

-2

0

2

4

6

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 5 (2.39m)

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

a) Axial Velocity Profiles b) Tangential Velocity Profiles

Figure 2 Measured and simulated tangential and axial velocity profiles


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Position 4 (2m)

-25

-20

-15

-10

-5

0

5

10

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 4 (2m)

-30

-20

-10

0

10

20

30

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

LDA

Position 3 (1.73m)

-8

-6

-4

-2

0

2

4

6

8

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

Position 3 (1.73m)

-15

-10

-5

0

5

10

15

20

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

Position 2 (1.45m)

-10

-8

-6

-4

-2

0

2

4

-1 -0.5 0 0.5 1

Distance (m)

Yvelocity(m/s)

CFD

LDA

Position 2 (1.45m)

-15

-10

-5

0

5

10

15

-1 -0.5 0 0.5 1

Distance (m)

Zvelocity(m/s)

CFD

LDA

Position 1 (0.71m)

-2

-1

0

1

2

3

4

-0.4 -0.2 0 0.2 0.4

Distance (m)

Yvelocity

(m/s)

CFDLDA

Position 1 (0.71m)

-15

-10

-5

0

5

10

15

-0.4 -0.2 0 0.2 0.4

Distance (m)

Zvelocity

(m/s)

CFDLDA

a) Axial Velocity Profiles b) Tangential Velocity Profiles

Figure 2 Measured and simulated tangential and axial velocity profiles - continued

CONCLUSIONS

The airflow profiles within a scaled down, swirling, counter-current spray drying tower have been

accurately measured and provide a quantitative resource for comparison with simulations. A CFD

model, using a Reynolds Stress turbulence model, has been developed for the base case and shows good

agreement with the experimental measurements. This model can therefore be reapplied to full-scale

situations and acts as a basis for simulating the two-phase flows in spray drying towers.


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LITERATURE

Harvie, D.J.E., Langrish, T.A.G. and Fletcher, D.F. (2001), Numerical simulations of gas-flow patterns

within a tall-form spray dryer, Trans IChemE, Vol. 79, Part A, pp. 235-248

Masters, K. (2002), Spray Drying in Practice, SprayDryConsult International ApS, Charlottenlund,

Denmark

Sharma, S. (1990), Spray dryer simulation and air flow pattern studies, Ph.D. Thesis, The University of

Aston, Birmingham, United Kingdom

Southwell, D.B. and Langrish, T.A.G. (2000), Observations of flow patterns in a spray dryer, Drying

Technology, Vol. 18., no. 3, pp. 661-685

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Flujo de Aire en Secadores Spray