CFD Droplet Boundary Conditions from
Black Liquor Spray Experiments
Markus Engblom*, Anders Brink*, Mikko Hupa*,
Ari Kankkunen**, Mika Järvinen**
*) Åbo Akademi University, Turku, Finland
**) Aalto University, Espoo, Finland
Black liquor
• Kraft chemical pulping
Fibers
Fibers
Lignin etc.
+
Wood
(Honghi Tran)
Pulping
chemicals
(Na2S,
NaOH)
155°C
900 kPa
Black liquor
Black liquor recovery boiler
• Recovery of chemicals and heat
• Wood
• Chemicals
• Water
Recovered
chemicals
Heat
Chemical
pulping
Fibers
Black liquor
• Organic matter
• Spent chemicals
• Water
Recovery processes
• Recovery Boiler
• Black liquor is burned
Black liquor combustion – recovery boiler
• Black liquor spray
• In-flight combustion
Kankkunen et al
~60 m
• Char bed
• Material input by droplets
• Char bed burning
~12 m
Black liquor spraying
(Rick Wessel)
Black Liquor
Black liquor spraying
CFD simulation input data
Initial droplet density
- Droplet boundary condition
- Currently unknown
BL density
e.g. 1466 kg/m3
Droplet density
? kg/m3
• Flashing conditions
• Experimental data/model
not available
Initial droplet density important CFD input
Initial droplet density (kg/m3)
(°C)
430
140
20
Aalto university spray characterization
• To obtain initial data for CFD
• Special arrangement for video recording liquor spray
Droplet tranparency
suggest low density
For images at > 2m,
black liquor spayed along the wall
Droplets 2.3 m from
the spray nozzle
A. Kankkunen
78 mm
10 mm
Objectives
• Better understanding of liquor spraying experiments
• Initial droplet density for CFD
Modeling - Spraying geometry
Gas
velocity
(m/s)
View from above
Spraying along the wall
Wall
Selected and
terminated
droplet trajectories
Gun
opening
Wall
Wall
Gun
opening
Cross flow – furnace gas
Gas BC: - velocity (varied)
- temperature 1200°C
- non-reactive
Wall
Gun
orientation
Gun
opening
and liquor gun
Liquor spray model
Gas:
- Turbulent flow (k- )
Droplets:
- Discrete Phase Model (DPM)
- Järvinen/Mueller droplet model
- Turbulent dispersion
Gas-Droplet interaction:
- Heat, mass, and momentum exchange
Droplet-Droplet interaction:
- Not considered
Liquor spray model parameters (1/3)
Based on data from spray experiments (Kankkunen et al.)
• Opening angles
- horizontal 120°
- vertical 14°
• Size distribution
- Rosin-Rammler
- Dave = 17.6 mm (after in-flight swelling)
- spread 1.93
• Drop sphericity 0.96
(aspect ratio 1.71)
• Liquor feed
- 5.8 liters/s
- 1466 kg/m3
- T = 142 °C
- Tboilling = 122 °C
• Initial drop velocity:
17.4 m/s
Liquor spray model parameters (2/3)
Mass flow distribution over opening angles (Miikkulainen et al.)
• No mass flow distribution data available for these experiments
• Linear distribution assumed – spray chamber data
Model assumption
Spray chamber data
Liquor spray model parameters (3/3)
4.1 m/s
1466 kg/m3
Liquor in-gun
swelling
17.4 m/s
345 kg/m3
Droplet in-flight swelling (=simulation cases)
- 1.5 m ( 0.086 s) (based on Järvinen)
- to density (kg/m3)
345 (all remaining steam to gas phase)
120
58
32
20 (all remaining steam retained in drop)
Järvinen:
Predicted bubble growth within droplet
assuming all steam retained in droplet
-0.5
0
0.5 1.0 1.5
Axial distance (m)
2.0
Results - Steam Vs. Droplets
Gas velocity
(m/s)
Steam only
With droplets
View from above spray
Gas flow and nearby wall
Gas velocity
(m/s)
Wall
”Coanda effect”
Spray center line
The Coanda effect - Impact of cross flow velocity
Gas velocity
(m/s)
Cross flow velocity 1 m/s
Cross flow velocity 7 m/s
Small Vs. Large droplets
17.6 mm drops
2 mm drops
• At low cross flow
Segregation of large
and small drops
Spray spreading
Cross flow 1 m/s
Cross flow 7 m/s
Droplet segregation – Predicted Vs. Experimental
Experimental:
Droplets smaller on wall side
Far from wall
17.6 mm drops Cross flow:1 m/s
2 mm drops
Center
(Kankkunen et al.)
Wall side
Spray spreading – Predicted Vs. Experimental
24°
Experimental: (Kankkunen et al.)
- near nozzle 14°
- 2.3 m from nozzle 26°
14°
17.6 mm drops Cross flow:1 m/s
2 mm drops
Droplet density
Experimental drop velocities (Kankkunen et al.)
• Which modeled initial droplet density gives
measured droplet velocity behavior at 2.3 from nozzle?
Scatter
Droplet density
• Large-droplet velocities by tracking individual droplets
• Within exp. range when 20-80 kg/m3
18
Drop velocity (m/s)
at 2.3 m from nozzle
17
Legend:
Swell time_drop size _cross flow
16
90ms_10mm_1m/s
90ms_10mm_4m/s
15
90ms_10mm_7m/s
Exp. large drop
velocity range
14
90ms_18mm_1m/s
90ms_18mm_4m/s
90ms_18mm_7m/s
180ms_10mm_4m/s
180ms_18mm_4m/s
13
density_low
density_high
12
11
0
100
200
300
Drop density (kg/m3)
400
Conclusions
• Modeling
New insight into BL spray experiments
• The Coanda effect – proposed contributor to
experimentally observed
segregation of small and large droplets
and
spray spreading
• Initial droplet density for CFD: 20-80 kg/m3
Acknowledgements
This work has been carried out within FUSEC (2011-2014)
as part of the activities of the Åbo Akademi Process Chemistry Centre.
Other research partners are VTT, Lappeenranta University of Technology,
Aalto University and Tampere University of Technology.
Support from the National Technology Agency of Finland (Tekes),
Andritz Oy, Metso Power Oy, Foster Wheeler Energia Oy,
UPM-Kymmene Oyj, Clyde Bergemann GmbH, International Paper Inc.
and Top Analytica Oy Ab is gratefully acknowledged.