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MODELLING OF AIRFLOW IN WOOD KILNS UBC Mechanical Engineering CFD Modelling by E. Bibeau Process Simulations Ltd. Kiln Drying Course UBC June 1, 2000

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CONTENTS lAirflow in kilns - Factors affecting airflow lAirflow modelling lAirflow results - Plenum design, sticker thickness, and roof design lWood drying model lConclusions

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Research Group

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PROCESS MODELS

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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

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DRYING KILN Automatic Vents Steam Spray Top Load Baffle Bottom Load Baffle Fan Deck Reversible Fan Lumber Stack Booster Coil Heating Coils

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DRYING CYCLE Time Drying Stage I Convection Stage II Convection- Diffusion Stage III Diffusion Free water Bound water

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KILN OPERATION STRESSES KILN OPERATOR CONTROL STRATEGY WOOD Wet Bulb T o WATER Mass Transfer Heat Transfer HEAT Dry Bulb T o

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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

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IMPORTANCE OF AIRFLOW AIRFLOW MASS TRANSFER (DRYING) HEAT TRANSFER Relationship Valid in Stage I & II

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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

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SOME PARAMETERS AFFECTING AIRFLOW lDEVELOPING FLOW lGAPS BETWEEN BOARDS lLUMBER IRREGULARITIES lTURBULENCE LEVELS LITERATURE

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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

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DEVELOPING FLOW lAir detaches from leading edge lFurther increases shear and non- uniformity near leading edge Wood Airflow Reticulation Bubble Airflow Detaches

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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

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DEVELOPING FLOW lStrategy to avoid non- uniformity caused by developing flow - Fan reversal Especially important in first stage of drying

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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

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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

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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

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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

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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

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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

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Gaps and Board Irregularities

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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

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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

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AIRFLOW MODELING lPlenum Design lSticker Thickness lRoof Design Numerical Simulation (CFD)

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Some Examples of CFD Applications ComputerJet engines Weather Automotive Harrier jet

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Mathematical Modelling IN OUT Principle of conservation Mass Momentum Energy . IN = OUTOUT

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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

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THREE PLENUM DESIGNS

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PLENUM DESIGN WIDE PLENUM

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PLENUM DESIGN TAPERED

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PLENUM AVERAGE VELOCITY

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PLENUM DESIGN (VELOCITY)

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PLENUM DESIGN (PRESSURE)

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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

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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

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DOUBLE PLENUM DESIGN

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STICKER THICKNESS (MESH) Base Case

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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

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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

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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

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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

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ROOF DESIGN (MESH)

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ROOF DESIGN (45 o Baffle)

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ROOF DESIGN (30 o Baffle)

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ROOF DESIGN (VELOCITY)

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ROOF DESIGN (PRESSURE)

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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

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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

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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.

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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

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Pressure

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Temperature

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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

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COPY OF PRESENTATION lGo to www.psl.bc.ca lPress on Public Download button lGo to directory Woodkiln lDownload file kiln_course.ppt