MODELLING OF AIRFLOW IN WOOD KILNSUBC Mechanical Engineering CFD Modellingby

E. BibeauProcess Simulations Ltd.Kiln Drying CourseUBCJune 1, 2000

CONTENTSAirflow in kilnsFactors affecting airflowAirflow modellingAirflow resultsPlenum design, sticker thickness, and roof designWood drying modelConclusions

Research Group

Dr. Martha Salcudean, Principal Investigator

Dr. Ian Gartshore, Co-Investigator

Bian Zhengbing

Feng Xioasi

Mohammad Shariati

Dr. Eric Bibeau

Dr. Pingfan He

Dr. David Stropky

Chris Chiu

Lu Hua

Zhu Zhi Xiao

Suqin Dong

Dr. Emil Statie

Dr. Jerry Yuan

Michael Georgallis

Dr. Paul Nowak

Kegang and Xun Zhang

Note: UBC and PSL Personnel (Pulp & Paper Section)

PROCESS MODELS

PROCESS

IN PROGRESS

INDUSTRIAL APPLICATION

SIMULATOR & TRAINER

WOOD KILNS

(

LIME KILNS

(

DIGESTERS

(

HEADBOXES

(

HYDROCYCLONES

(

BARK BOILERS

(

RECOVERY BOILERS

(

NUMERICAL MODELDeveloping wood kiln modelPredict airflow, mass transfer, and heat transferUBCOther InstitutionsGovernment labsPSLIndustry License agreementServiceagreementsConsultingagreementsCustomagreementsLicenseagreements

DRYING KILNAutomatic VentsSteam SprayTop Load BaffleBottom Load BaffleFan DeckReversible FanLumber StackBooster CoilHeating Coils

DRYING CYCLETimeDryingStage IConvectionStage IIConvection-DiffusionStage IIIDiffusionFree waterBoundwater

KILN OPERATIONSTRESSESKILN OPERATOR CONTROL STRATEGYWOODWet Bulb ToWATERMass TransferHeat TransferHEATDry Bulb To

IMPORTANCE OF AIRFLOWFLUID DYNAMIC CONTROL STRATEGYAIRFLOWSTRESSESValid inStage I & IIWOODWet Bulb ToWATERMass TransferHeat TransferHEATDry Bulb To

IMPORTANCE OF AIRFLOWAIRFLOWMASS TRANSFER(DRYING)HEAT TRANSFERRelationshipRelationshipValid inStage I & II

KILN AIRFLOW CONTROLFan speed (not always an option)Fan reversalFan positions and ductingPackaging (sticker, aligning, boxing)Airflow devices (baffles, door strips)Kiln geometryMinimize leakageLumber size control

SOME PARAMETERS AFFECTING AIRFLOWDEVELOPING FLOWGAPS BETWEEN BOARDSLUMBER IRREGULARITIESTURBULENCE LEVELSLITERATURE

DEVELOPING FLOWAirflow between 2 plates creates a profileAir sticks to the wall thus slowing down the airflow at the wall

DEVELOPING FLOW The profile changes as the air travels through the wood stackShear varies along wood stackFlow is turbulent

DEVELOPING FLOWAir detaches from leading edgeFurther increases shear and non-uniformity near leading edge

DEVELOPING FLOWCombined effectFlow sticks to the wallAirflow detached from wood at the startIncrease in drying rate > 100%Region of influence: Sticker/L < 50

DEVELOPING FLOWStrategy to avoid non-uniformity caused by developing flowFan reversalEspecially important in first stage of drying

SMALL GAPS BETWEEN BOARDSCause airflow exchange between the air in the channel and the air trapped between the gapsCause increase in shearWoodWoodGapAirflowIncreaseShear

SMALL GAPS BETWEEN BOARDSUnsteady flow (period of 2 to 7 sec)Literature reports overall mass transfer increase of 17% to 32% for 1 to 5-mm gapsInfluence felt 20 to 40 mmLarge increases at leading edge

SMALL GAPS BETWEEN BOARDSGaps are beneficial Helps reduce drying timeOffer more surface area to remove waterStrategy to avoid non-uniformity caused by gaps between boardsProper stacking of woodFan reversal (Stage I and II)Gaps should be approximately equal and distributed evenly throughout charge

BOARD IRREGULARITIESUnevenness in lumber height Caused by improper size controlLeads to additional shear upstream and downstream of the variation

BOARD IRREGULARITIESThick to thinUp to 100% increase initially in mass transfer rate Lower than normal afterwards (15-30 mm)Thin to ThickLarger influenceLower than normal afterwards (15-30 mm)Board height irregularities > gapsSuperposition of effects

BOARD IRREGULARITIESIrregularities help reduce drying time in Stage I and IIStrategy to avoid non-uniformity caused by board irregularitiesFan reversal (Stage I and II)Minimize irregularitiesIrregularities should be evenly distributed throughout charge as much as possible

Gaps and Board Irregularities

TURBULENCE LEVELSTurbulence Levels = small velocity fluctuations in the mean flow

The free stream turbulence of the airflow can affects the mass transfer significantlyTurbulence Level Turbulent FlowMean flowFluctuating component

TURBULENCE LEVELIncreasing the turbulence level increases the mass transfer rate 55% increase for 8% increase in turbulence for flat plateInfluences the velocity profileTurbulence in wood kilns are relatively highTurbulence level may decrease inside the wood stacks

KILN GEOMETRYPlenum width / roof height Study show > 1Plenum width / (sticker x lumber pieces)Experience claim approximately 1Sticker thicknessBetween 1/2 to 1 1/4

AIRFLOW MODELINGPlenum DesignSticker ThicknessRoof DesignNumerical Simulation (CFD)

Some Examples of CFD ApplicationsComputerJet enginesWeatherAutomotiveHarrier jet

Mathematical ModellingINOUTOUT

KILN SIMULATED

KILN SIMULATED SUMMARYInlet Velocity 3 m/s (381 ft/s)Sticker 3/42 wood stacks (30 rows/stack)4 gap between stacksOpening roof / stickers = 2.0Opening stickers / plenum = 1.2Rough walls and fully turbulentNo leakage, perfect packagingModel half of kiln

KILN SIMULATED (GRID)BaseCase

BASE CASE-FLOW VELOCITIES

BASE CASE

BASE CASEUneven flow distributionLower velocities at topHigher velocities at bottomVelocity in gap between stack increases because of lower resistanceFlow circulation at entrance of plenumVertical flow reduces the flow entering the top flow channels

BASE CASEVelocity distribution influenced by plenum entrance geometryBaffle and fan deck designElbow effectBottom design of baffle causes non-uniformityFlow recirculates in lower plenum cavityFlow is reduced in first channelLarger flow in second channel

THREE PLENUM DESIGNS

PLENUM DESIGN WIDE PLENUM

PLENUM DESIGN TAPERED

PLENUM AVERAGE VELOCITY

PLENUM DESIGN (VELOCITY)

PLENUM DESIGN (PRESSURE)

PLENUM DESIGNInfluence of plenum is related to the flow resistance through plenum and wood stackKplenum smaller KstickerKplenum approximately equal to KstickerKplenumKsticker

PLENUM DESIGN RESULTSSlanted plenum does not offer the best flow distribution Pressure buildup: BernoulliWider plenum causes a better distribution Better entrance effect with wider plenumImprovement is based on 900 roof angleBetter even downward flow velocityAll 3 designs have elbow effect

DOUBLE PLENUM DESIGNMay want to add vertical plates to obtain uniform flowAdd Vertical Plates

DOUBLE PLENUM DESIGN

DOUBLE PLENUM DESIGN

STICKER THICKNESS (MESH)Base Case

STICKER THICKNESS (1)

STICKER THICKNESS (1 1/4)

STICKER THICKNESS

STICKER THICKNESS-PRESSURE

STICKER THICKNESSMain flow characteristics do not change significantly with the sticker thicknessChoice of sticker thickness is dependent on all the other parameters affecting airflowNeed better geometrical control for small stickerSmall gapsHeight irregularitiesMissing boards

STICKER THICKNESSDecrease in sticker thickness Increase in flow resistanceIncrease or decrease in flow velocity in the channels Reducing sticker thickness increases kiln capacity but longer drying timesSmaller sticker is riskyKiln more prone to flow variationsSome mills found reduced drying using 1/2 rigid stickers Report an increase in moisture variation

STICKER THICKNESSHow is the moisture variation in a channel affected by change in stickerAnswer: DependsDid you preserve same mass of air per channelair velocityRelated to shear stress at the wallIf shear and air mass are similarNo real effect on moisture variation expectedProvided excellent geometry controlWoodAirflow

ROOF DESIGN (MESH)

ROOF DESIGN (45o Baffle)

ROOF DESIGN (30o Baffle)

ROOF DESIGN (VELOCITY)

ROOF DESIGN (VELOCITY)

ROOF DESIGN (PRESSURE)

ROOF DESIGNRoof design affects how the flow enters the kilnThe baffle affects how the flow distributes in the top wood stackThe slanted roof causes the flow to accelerate before entering the plenumVelocity distribution in the top part of the plenum is velocity dependent

DOUBLE TRACK KILN

Wood Drying ModelThe lumber is assumed to be a porous, homogeneous solidThere are three kinds of water inside the lumber: free water, bound water and water vaporMoisture content at the surface of the lumber is in equilibrium with the airShrinkage of the lumber during drying is neglected

Wood Drying ModelMass balance* Liquid phase

* Vapor

* Airn mass flux densitym phase change termM Moisture ContentEnergy balanceThree parameters are retained:

M: Moisture ContentT: TemperatureP: Total Pressure in gaseous Phase.

Wood DryingAirflowTwo-way couplingShear stress (Airflow)Heat and mass transfer (wood surface) Temperature, Moisture (wood surface)Temperature, Humidity (Airflow)WoodWoodShear stress (Result of Airflow)M, TM, TM, T

Moisture

Pressure

Temperature

Develop ToolsSimulatorCorePhysical ModelMeasurementsOperatorexperienceProcessknowledgeOperationalSimulatorsTrainingSimulatorsVirtual Cameras

Neural networkTraining DataSuper-heaterBankGeneratorBankSprayNumerical ModelProcess Simulators

VIEWER EXAMPLE:Look at different states interactively

CONCLUSIONSImportance of airflowFactors affecting airflow Numerical simulations of airflow Plenum designs, roof shapes, and sticker thicknessAirflow model can constitute a powerful tool Optimize functional and design kiln parameters Help operators better operate kilns without adding major costs

COPY OF PRESENTATIONGo to www.psl.bc.caPress on Public Download buttonGo to directory WoodkilnDownload file kiln_course.ppt