Analysis of external and internal mass transfer resistance at steady state diffusion experiments on small clear wood specimens

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1 COST FP0802 Workshop Experimental and Computational Micro-Characterization Techniques in Wood Mechanics, Vila Real, April 2011 Analysis of external and internal mass transfer resistance at steady state diffusion experiments on small clear wood specimens A. Straže, Ž. Gorišek University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Ljubljana, Slovenia;

2 Agenda Background Diffusion cup method Fick s first law Basic principles Preferences and drawbacks Real wood structure and surface properties Experimental Material & Methods Diffusion experiments - climatic conditions, determination of water mass flow and moisture content Structural and surface properties Results and discussion Concluding remarks COST FP0802, Vila Real,

3 Background Measuring of diffusion coefficients by steady state principle, i.e. Diffusion cup method RH 1 MC 1 RH 2 MC 2 ΔL Fick s first law: m A c D L Substitution of concentration gradient (Δc) with moisture content gradient: SOLUTION G w MC c 100 Various potentials: moisture content, relative humidity, partial vapour pressure, chemical or water potential, free energy, spreading pressure COST FP0802, Vila Real,

4 Background Drawbacks of diffusion cup method: surface equilibrium moisture content has to be available Solution: 2 specimens the water vapour flux causes a differential of relative humidity Solution: correction of potential RH 1 MC 1 RH 2 MC 2 ΔL RH 1 MC 1 RH 2 MC 2 Δ(RH) ΔL SOLUTION RH s SOLUTION COST FP0802, Vila Real,

5 Background Drawbacks of diffusion cup method: the effect of convective surface resistance is neglected Solution: correction factors? Salin, J.G Mass transfer from wooden surfaces. Drying Tech., 14(10): COST FP0802, Vila Real,

6 Aim and research objectives to test the hypothesis of addition of internal and external mass transfer resistance during steady state diffusion experiments. Fick s first law: c m Da A D a apparent diffusion coefficient L Flux in the material: c R s R 1 1 m D A s L S A Resistances: R s - convective surface R D internal diffusion R t total ΔL R D R D L D A R t L Rt RD 2R D A a s Flux at the surfaces: m S A c 2 R s R s 1 S A COST FP0802, Vila Real,

7 Aim and research objectives COST FP0802, Vila Real, Adding of resistances: L 1 L 2 L 3 R t A L S 1 2 intercept D slope 1 D L S A R R A R D s t 1 2 2

Material and methods: Sampling Material:

Diameter: 45 mm Thickness: 2 mm, 4 mm, 6 mm (R,

8 Material and methods: Sampling Material: European spruce (Picea abies Karst.) Initial MC: 8%, industrial dried Sampling: radial (R), tangential (T) and longitudinal (L) Diameter: 45 mm Thickness: 2 mm, 4 mm, 6 mm (R, T) 4 mm, 10 mm, 18 mm (L) R T L COST FP0802, Vila Real,

9 Material and methods: Conditioning Equipment: Diffusion cups and thermostatic climatic chambers with saturated salt solutions. lid washer specimen cup Temperature: 20 ± 0.1 C COST FP0802, Vila Real,

10 Material and methods: Diffusion experimentation Procedure: pre-drying of specimens at state 0 successive exposing of specimens to different RHs (1 st to 4 th run) at constant temperature (20 ± 0.1 C) State Medium Relative humidity [%] 0 T = 40 C 10 1 LiCl 18 2 K 2 CO NaNO ZnSO Distilled water 97 6 T = 103 ± 2 C 0 1 st run 2 nd run 3 rd run 4 th run COST FP0802, Vila Real,

11 Material and methods: Surface properties Procedure: Light emission microscopy Visual assessment Surface roughness Tactile needle method COST FP0802, Vila Real,

12 (2Rs / Rt)R [ ] (Rt)R [s/m] Results: Radial direction 1.0E E+07 R t = 7,762,740.49L + 17,391, R² = st run; MC avg = 8.4% 4 th run; MC avg = 22.8% 1.0E E R t = 77953L R² = L [mm] y = 0.909e -0.12x R² = Total resistance increases with decrease of MC. External mass transfer resistance has strong influence (> 25%) y = 0.794x R² = L [mm] COST FP0802, Vila Real,

13 D R [m 2 /s] S R [m/s] Results: Radial direction 1.0E E E E E E E E E-11 MC [%] 1.0E-08 MC [%] Decreasing of diffusion coefficient and surface emission coefficient with reduction of average moisture content. COST FP0802, Vila Real,

14 (2Rs / Rt)T [ ] (Rt)T [s/m] Results: Tangential direction 1.0E E+07 R t = 9,400,198.29L + 21,061, R² = st run; MC avg = 8.2% 4 th run; MC avg = 23.0% 1.0E E R t = 70538L R² = L [mm] y = e x R² = Total resistance increases with decrease of MC. External mass transfer resistance has strong influence (> 20%) y = x R² = L [mm] COST FP0802, Vila Real,

15 D T [m 2 /s] S T [m/s] Results: Tangential direction 1.0E E E E E E E E E-11 MC [%] 1.0E-08 MC [%] Decreasing of diffusion coefficient and surface emission coefficient with reduction of average moisture content. COST FP0802, Vila Real,

16 (2Rs / Rt)L [ ] (Rt)L [s/m] Results: Longitudinal direction 1.0E E+07 R t = 178,139.70L + 2,047, R² = st run; MC avg = 8.4% 4 th run; MC avg = 23.9% 1.0E E R t = L R² = L [mm] y = e x R² = 0.99 Total resistance increases with decrease of MC. External mass transfer resistance has strong influence ( 40%) y = e x R² = L [mm] COST FP0802, Vila Real,

17 D L [m 2 /s] S L [m/s] Results: Longitudinal direction 1.0E E E E E E E E E-11 MC [%] 1.0E-08 MC [%] Increasing of diffusion coefficient and surface emission coefficient at average MC bellow 20% Significantly higher values at average MC above 20% - possibility of capillary condensation. COST FP0802, Vila Real,

18 Results: Surface characteristics cross section (RT plane) Raised wood tissue with low connection to the underlying material have influence on: convective surface resistance, rate of equilibration, equilibrium moisture content (?) COST FP0802, Vila Real,

19 Results: Surface characteristics longitudinal direction x Roughness depending on processing characteristics and on wood structure influencing: air movement at surface layer. COST FP0802, Vila Real,

20 Results: Surface characteristics RL plane Raised wood tissue with low connection to the underlying material have influence on: convective surface resistance, rate of equilibration, equilibrium moisture content (?) COST FP0802, Vila Real,

21 Results: Surface characteristics RL plane x Roughness depending on processing characteristics and on wood structure influencing: air movement at surface layer. COST FP0802, Vila Real,

22 Concluding remarks There is a need to precise experimentation using diffusion cup method problems with thin specimens, having low internal resistance. Internal and external mass transfer resistance at steady state experimentation can be analysed by varying the material thickness. Material homogeneity is needed (successive, parallel samples) to achieve reliable results problem at wood material! COST FP0802, Vila Real,

23 COST FP0802 Workshop Experimental and Computational Micro-Characterization Techniques in Wood Mechanics, Vila Real, April 2011 Analysis of external and internal mass transfer resistance at steady state diffusion experiments on small clear wood specimens A. Straže, Ž. Gorišek Thank you for the attention! University of Ljubljana, Biotechnical Faculty, Department of Wood Science and Technology, Ljubljana, Slovenia;

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