MODELLING OF AIRFLOW IN WOOD KILNS UBC Mechanical Engineering CFD Modelling by E. Bibeau Process...
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MODELLING OF AIRFLOW MODELLING OF AIRFLOW IN WOOD KILNS IN WOOD KILNS UBC Mechanical Engineering CFD Modelling by E. Bibeau Process Simulations Ltd. Kiln Drying Course UBC June 1, 2000
MODELLING OF AIRFLOW IN WOOD KILNS UBC Mechanical Engineering CFD Modelling by E. Bibeau Process Simulations Ltd. Kiln Drying Course UBC June 1, 2000
MODELLING OF AIRFLOW IN WOOD KILNS UBC Mechanical Engineering
CFD Modelling by E. Bibeau Process Simulations Ltd. Kiln Drying
Course UBC June 1, 2000
Slide 2
CONTENTS lAirflow in kilns - Factors affecting airflow lAirflow
modelling lAirflow results - Plenum design, sticker thickness, and
roof design lWood drying model lConclusions
Slide 3
Research Group
Slide 4
PROCESS MODELS
Slide 5
UBC Other Institutions Government labs PSL Industry License
agreement Service agreements Consulting agreements Custom
agreements License agreements NUMERICAL MODEL lDeveloping wood kiln
model lPredict airflow, mass transfer, and heat transfer
Slide 6
DRYING KILN Automatic Vents Steam Spray Top Load Baffle Bottom
Load Baffle Fan Deck Reversible Fan Lumber Stack Booster Coil
Heating Coils
Slide 7
DRYING CYCLE Time Drying Stage I Convection Stage II
Convection- Diffusion Stage III Diffusion Free water Bound
water
Slide 8
KILN OPERATION STRESSES KILN OPERATOR CONTROL STRATEGY WOOD Wet
Bulb T o WATER Mass Transfer Heat Transfer HEAT Dry Bulb T o
Slide 9
IMPORTANCE OF AIRFLOW FLUID DYNAMIC CONTROL STRATEGY AIRFLOW
STRESSES Valid in Stage I & II WOOD Wet Bulb T o WATER Mass
Transfer Heat Transfer HEAT Dry Bulb T o
Slide 10
IMPORTANCE OF AIRFLOW AIRFLOW MASS TRANSFER (DRYING) HEAT
TRANSFER Relationship Valid in Stage I & II
Slide 11
KILN AIRFLOW CONTROL lFan speed (not always an option) lFan
reversal lFan positions and ducting lPackaging (sticker, aligning,
boxing) lAirflow devices (baffles, door strips) lKiln geometry
lMinimize leakage lLumber size control
Slide 12
SOME PARAMETERS AFFECTING AIRFLOW lDEVELOPING FLOW lGAPS
BETWEEN BOARDS lLUMBER IRREGULARITIES lTURBULENCE LEVELS
LITERATURE
Slide 13
DEVELOPING FLOW lAirflow between 2 plates creates a profile
lAir sticks to the wall thus slowing down the airflow at the wall
Wood Airflow Air Sticks Sticker Thickness
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DEVELOPING FLOW lThe profile changes as the air travels through
the wood stack lShear varies along wood stack lFlow is
turbulent
Slide 15
DEVELOPING FLOW lAir detaches from leading edge lFurther
increases shear and non- uniformity near leading edge Wood Airflow
Reticulation Bubble Airflow Detaches
Slide 16
DEVELOPING FLOW lCombined effect - Flow sticks to the wall -
Airflow detached from wood at the start lIncrease in drying rate
> 100% lRegion of influence: Sticker/L < 50 Wood Drying Rate
No Change High Shear Lower Shear
Slide 17
DEVELOPING FLOW lStrategy to avoid non- uniformity caused by
developing flow - Fan reversal Especially important in first stage
of drying
Slide 18
SMALL GAPS BETWEEN BOARDS lCause airflow exchange between the
air in the channel and the air trapped between the gaps lCause
increase in shear Wood Gap Airflow Increase Shear
Slide 19
SMALL GAPS BETWEEN BOARDS lUnsteady flow (period of 2 to 7 sec)
lLiterature reports overall mass transfer increase of 17% to 32%
for 1 to 5-mm gaps lInfluence felt 20 to 40 mm lLarge increases at
leading edge Wood D r y i n g
Slide 20
SMALL GAPS BETWEEN BOARDS lGaps are beneficial - Helps reduce
drying time - Offer more surface area to remove water lStrategy to
avoid non-uniformity caused by gaps between boards - Proper
stacking of wood - Fan reversal (Stage I and II) - Gaps should be
approximately equal and distributed evenly throughout charge
Slide 21
BOARD IRREGULARITIES lUnevenness in lumber height - Caused by
improper size control lLeads to additional shear upstream and
downstream of the variation Wood Airflow Board Irregularities
Increased Shear
Slide 22
BOARD IRREGULARITIES lThick to thin - Up to 100% increase
initially in mass transfer rate - Lower than normal afterwards
(15-30 mm) lThin to Thick - Larger influence - Lower than normal
afterwards (15-30 mm) lBoard height irregularities > gaps
lSuperposition of effects
Slide 23
BOARD IRREGULARITIES lIrregularities help reduce drying time in
Stage I and II lStrategy to avoid non-uniformity caused by board
irregularities - Fan reversal (Stage I and II) - Minimize
irregularities - Irregularities should be evenly distributed
throughout charge as much as possible
Slide 24
Gaps and Board Irregularities
Slide 25
TURBULENCE LEVELS lTurbulence Levels = small velocity
fluctuations in the mean flow lThe free stream turbulence of the
airflow can affects the mass transfer significantly lTurbulence
Level Turbulent Flow Mean flow Fluctuating component
Slide 26
TURBULENCE LEVEL lIncreasing the turbulence level increases the
mass transfer rate - 55% increase for 8% increase in turbulence for
flat plate lInfluences the velocity profile lTurbulence in wood
kilns are relatively high lTurbulence level may decrease inside the
wood stacks
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KILN GEOMETRY lPlenum width / roof height - Study show > 1
lPlenum width / (sticker x lumber pieces) - Experience claim
approximately 1 lSticker thickness - Between 1/2 to 1 1/4
Some Examples of CFD Applications ComputerJet engines Weather
Automotive Harrier jet
Slide 30
Mathematical Modelling IN OUT Principle of conservation Mass
Momentum Energy . IN = OUTOUT
Slide 31
KILN SIMULATED
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KILN SIMULATED SUMMARY lInlet Velocity 3 m/s (381 ft/s)
lSticker 3/4 l2 wood stacks (30 rows/stack) l4 gap between stacks
lOpening roof / stickers = 2.0 lOpening stickers / plenum = 1.2
lRough walls and fully turbulent lNo leakage, perfect packaging
lModel half of kiln
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KILN SIMULATED (GRID) BaseCase
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BASE CASE-FLOW VELOCITIES
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BASE CASE
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lUneven flow distribution lLower velocities at top lHigher
velocities at bottom lVelocity in gap between stack increases
because of lower resistance lFlow circulation at entrance of plenum
- Vertical flow reduces the flow entering the top flow
channels
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BASE CASE lVelocity distribution influenced by plenum entrance
geometry - Baffle and fan deck design - Elbow effect lBottom design
of baffle causes non- uniformity - Flow recirculates in lower
plenum cavity - Flow is reduced in first channel - Larger flow in
second channel
Slide 38
THREE PLENUM DESIGNS
Slide 39
PLENUM DESIGN WIDE PLENUM
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PLENUM DESIGN TAPERED
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PLENUM AVERAGE VELOCITY
Slide 42
PLENUM DESIGN (VELOCITY)
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PLENUM DESIGN (PRESSURE)
Slide 44
PLENUM DESIGN lInfluence of plenum is related to the flow
resistance through plenum and wood stack - K plenum smaller K
sticker - K plenum approximately equal to K sticker K plenum K
sticker
Slide 45
PLENUM DESIGN RESULTS lSlanted plenum does not offer the best
flow distribution lPressure buildup: Bernoulli lWider plenum causes
a better distribution - Better entrance effect with wider plenum -
Improvement is based on 90 0 roof angle - Better even downward flow
velocity lAll 3 designs have elbow effect
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DOUBLE PLENUM DESIGN Add Vertical Plates lMay want to add
vertical plates to obtain uniform flow
Slide 47
DOUBLE PLENUM DESIGN
Slide 48
Slide 49
STICKER THICKNESS (MESH) Base Case
Slide 50
STICKER THICKNESS (1)
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STICKER THICKNESS (1 1/4)
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STICKER THICKNESS
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STICKER THICKNESS-PRESSURE
Slide 54
STICKER THICKNESS lMain flow characteristics do not change
significantly with the sticker thickness lChoice of sticker
thickness is dependent on all the other parameters affecting
airflow lNeed better geometrical control for small sticker - Small
gaps - Height irregularities - Missing boards
Slide 55
STICKER THICKNESS lDecrease in sticker thickness - Increase in
flow resistance - Increase or decrease in flow velocity in the
channels - Reducing sticker thickness increases kiln capacity but
longer drying times lSmaller sticker is risky - Kiln more prone to
flow variations lSome mills found reduced drying using 1/2 rigid
stickers - Report an increase in moisture variation
Slide 56
STICKER THICKNESS lHow is the moisture variation in a channel
affected by change in sticker lAnswer: Depends - Did you preserve
same mass of air per channel air velocity - Related to shear stress
at the wall lIf shear and air mass are similar - No real effect on
moisture variation expected - Provided excellent geometry control
Wood Airflow
Slide 57
ROOF DESIGN (MESH)
Slide 58
ROOF DESIGN (45 o Baffle)
Slide 59
ROOF DESIGN (30 o Baffle)
Slide 60
ROOF DESIGN (VELOCITY)
Slide 61
Slide 62
ROOF DESIGN (PRESSURE)
Slide 63
ROOF DESIGN lRoof design affects how the flow enters the kiln
lThe baffle affects how the flow distributes in the top wood stack
lThe slanted roof causes the flow to accelerate before entering the
plenum - Velocity distribution in the top part of the plenum is
velocity dependent
Slide 64
DOUBLE TRACK KILN
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Wood Drying Model lThe lumber is assumed to be a porous,
homogeneous solid lThere are three kinds of water inside the
lumber: free water, bound water and water vapor lMoisture content
at the surface of the lumber is in equilibrium with the air
lShrinkage of the lumber during drying is neglected
Slide 66
Wood Drying Model Mass balance * Liquid phase * Vapor * Air n
mass flux density m phase change term M Moisture Content Energy
balance Three parameters are retained: M: Moisture Content T:
Temperature P: Total Pressure in gaseous Phase.
Slide 67
Wood Drying Airflow Two-way coupling Shear stress (Airflow)
Heat and mass transfer (wood surface) Temperature, Moisture (wood
surface) Temperature, Humidity (Airflow) Wood Shear stress (Result
of Airflow) M, T
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Moisture
Slide 69
Pressure
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Temperature
Slide 71
Develop Tools j i j jji i S x u x Process
ModelSimulatorCoreSimulatorCore PhysicalModel Measurements
OperatorexperienceProcessknowledge OperationalSimulators
TrainingSimulators VirtualCameras
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Neural network Training Data Super- heater Bank Generator Bank
Spray Numerical Model Process Simulators
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VIEWER EXAMPLE: Look at different states interactively
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CONCLUSIONS lImportance of airflow lFactors affecting airflow
lNumerical simulations of airflow - Plenum designs, roof shapes,
and sticker thickness lAirflow model can constitute a powerful tool
- Optimize functional and design kiln parameters - Help operators
better operate kilns without adding major costs
Slide 75
COPY OF PRESENTATION lGo to www.psl.bc.ca lPress on Public
Download button lGo to directory Woodkiln lDownload file
kiln_course.ppt