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MODELLING OF AIRFLOW IN WOOD KILNS

MODELLING OF AIRFLOW IN WOOD KILNS. UBC Mechanical Engineering CFD Modelling. Process Simulations Ltd. by E. Bibeau. Kiln Drying Course UBC June 1, 2000. CONTENTS. Airflow in kilns Factors affecting airflow Airflow modelling Airflow results

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MODELLING OF AIRFLOW IN WOOD KILNS

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

  2. CONTENTS • Airflow in kilns • Factors affecting airflow • Airflow modelling • Airflow results • Plenum design, sticker thickness, and roof design • Wood drying model • Conclusions

  3. Research Group

  4. PROCESS MODELS

  5. NUMERICAL MODEL • Developing wood kiln model • Predict airflow, mass transfer, and heat transfer License agreement UBC PSL Custom agreements Service agreements Consulting agreements License agreements Government labs Industry Other Institutions

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

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

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

  9. IMPORTANCE OF AIRFLOW FLUID DYNAMIC CONTROL STRATEGY AIRFLOW Mass Transfer Heat Transfer o Wet Bulb T o Dry Bulb T WATER HEAT WOOD Valid in Stage I & II STRESSES

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

  11. KILN AIRFLOW CONTROL • Fan speed (not always an option) • Fan reversal • Fan positions and ducting • Packaging (sticker, aligning, boxing) • Airflow devices (baffles, door strips) • Kiln geometry • Minimize leakage • Lumber size control

  12. SOME PARAMETERS AFFECTING AIRFLOW • DEVELOPING FLOW • GAPS BETWEEN BOARDS • LUMBER IRREGULARITIES • TURBULENCE LEVELS LITERATURE

  13. Wood Sticker Thickness Air “Sticks” Airflow Wood DEVELOPING FLOW • Airflow between 2 plates creates a profile • Air “sticks” to the wall thus slowing down the airflow at the wall

  14. Wood Lower No Change High Shear Shear Airflow Wood DEVELOPING FLOW • The profile changes as the air travels through the wood stack • Shear varies along wood stack • Flow is turbulent

  15. Wood Reticulation Airflow Detaches Bubble Airflow Wood DEVELOPING FLOW • Air detaches from leading edge • Further increases shear and non-uniformity near leading edge

  16. Wood High Lower Shear Drying No Change Shear Rate Wood DEVELOPING FLOW • Combined effect • Flow sticks to the wall • Airflow detached from wood at the start • Increase in drying rate > 100% • Region of influence: Sticker/L < 50

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

  18. SMALL GAPS BETWEEN BOARDS • Cause airflow exchange between the air in the channel and the air trapped between the gaps • Cause increase in shear Airflow Increase Shear Wood Wood Gap

  19. Wood g n i y r D Wood SMALL GAPS BETWEEN BOARDS • Unsteady flow (period of 2 to 7 sec) • Literature reports overall mass transfer increase of 17% to 32% for 1 to 5-mm gaps • Influence felt 20 to 40 mm • Large increases at leading edge

  20. SMALL GAPS BETWEEN BOARDS • Gaps are beneficial • Helps reduce drying time • Offer more surface area to remove water • Strategy 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

  21. Increased Shear Airflow Wood Board Irregularities BOARD IRREGULARITIES • Unevenness in lumber height • Caused by improper size control • Leads to additional shear upstream and downstream of the variation

  22. BOARD IRREGULARITIES • Thick to thin • Up to 100% increase initially in mass transfer rate • Lower than normal afterwards (15-30 mm) • Thin to Thick • Larger influence • Lower than normal afterwards (15-30 mm) • Board height irregularities > gaps • Superposition of effects

  23. BOARD IRREGULARITIES • Irregularities help reduce drying time in Stage I and II • Strategy 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

  24. Gaps and Board Irregularities

  25. TURBULENCE LEVELS • Turbulence Levels = small velocity fluctuations in the mean flow • The free stream turbulence of the airflow can affects the mass transfer significantly • Turbulence Level Turbulent Flow Mean flow Fluctuating component

  26. TURBULENCE LEVEL • Increasing the turbulence level increases the mass transfer rate • 55% increase for 8% increase in turbulence for flat plate • Influences the velocity profile • Turbulence in wood kilns are relatively high • Turbulence level may decrease inside the wood stacks

  27. KILN GEOMETRY • Plenum width / roof height • Study show > 1 • Plenum width / (sticker x lumber pieces) • Experience claim approximately 1 • Sticker thickness • Between 1/2 to 1 1/4”

  28. AIRFLOW MODELING • Plenum Design • Sticker Thickness • Roof Design Numerical Simulation (CFD)

  29. Some Examples of CFD Applications Weather Computer Jet engines Harrier jet Automotive

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

  31. KILN SIMULATED

  32. KILN SIMULATED SUMMARY • Inlet Velocity 3 m/s (381 ft/s) • Sticker 3/4” • 2 wood stacks (30 rows/stack) • 4” gap between stacks • Opening roof / stickers = 2.0 • Opening stickers / plenum = 1.2 • Rough walls and fully turbulent • No leakage, perfect packaging • Model half of kiln

  33. KILN SIMULATED (GRID) Base Case

  34. BASE CASE-FLOW VELOCITIES

  35. BASE CASE

  36. BASE CASE • Uneven flow distribution • Lower velocities at top • Higher velocities at bottom • Velocity in gap between stack increases because of lower resistance • Flow circulation at entrance of plenum • Vertical flow reduces the flow entering the top flow channels

  37. BASE CASE • Velocity distribution influenced by plenum entrance geometry • Baffle and fan deck design • “Elbow effect” • Bottom design of baffle causes non-uniformity • Flow recirculates in lower plenum cavity • Flow is reduced in first channel • Larger flow in second channel

  38. THREE PLENUM DESIGNS

  39. PLENUM DESIGN WIDE PLENUM

  40. PLENUM DESIGN TAPERED

  41. PLENUM AVERAGE VELOCITY

  42. PLENUM DESIGN (VELOCITY)

  43. PLENUM DESIGN (PRESSURE)

  44. PLENUM DESIGN • Influence of plenum is related to the flow resistance through plenum and wood stack • Kplenum smaller Ksticker • Kplenum approximately equal to Ksticker K s t i c k e r K p l e n u m

  45. PLENUM DESIGN RESULTS • Slanted plenum does not offer the best flow distribution • Pressure buildup: Bernoulli • Wider plenum causes a better distribution • Better entrance effect with wider plenum • Improvement is based on 900 roof angle • Better even downward flow velocity • All 3 designs have elbow effect

  46. DOUBLE PLENUM DESIGN • May want to add vertical plates to obtain uniform flow Add Vertical Plates

  47. DOUBLE PLENUM DESIGN

  48. DOUBLE PLENUM DESIGN

  49. STICKER THICKNESS (MESH) Base Case

  50. STICKER THICKNESS (1”)

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