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An experimental determination of the critical diffusion coefficient and critical relative humidity (RH) of drying air when optimizing the drying of three hardwood species (birch, aspen, and black alder) Cover

An experimental determination of the critical diffusion coefficient and critical relative humidity (RH) of drying air when optimizing the drying of three hardwood species (birch, aspen, and black alder)

Forestry Studies
Volume 79 (2023): Issue 1 (December 2023)
By: Hannes Tamme,  Regino Kask,  Peeter Muiste and  Valdek Tamme  
Open Access
|Apr 2024

Figures & Tables

Figure 1.

(a) interior view of the climate chamber, and (b) sensor placement schematic.
(a) interior view of the climate chamber, and (b) sensor placement schematic.

Figure 2.

A comparison of the average moisture content levels, and the moisture content levels which were obtained using the slicing method at a depth of 12 mm on the example of alder wood.
A comparison of the average moisture content levels, and the moisture content levels which were obtained using the slicing method at a depth of 12 mm on the example of alder wood.

Figure 3.

Alder wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase (Tamme et al., 2021a). Drying curves for the layers of alder wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.
Alder wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase (Tamme et al., 2021a). Drying curves for the layers of alder wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.

Figure 4.

Alder wood drying experiment. Thermocouple data, with the temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.
Alder wood drying experiment. Thermocouple data, with the temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.

Figure 5.

Aspen wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase. Drying curves for layers of aspen wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.
Aspen wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase. Drying curves for layers of aspen wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.

Figure 6.

Aspen wood drying experiment. Thermocouple data, with temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.
Aspen wood drying experiment. Thermocouple data, with temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.

Figure 7.

Birch wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase. Drying curves for layers of birch wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.
Birch wood drying experiment. Identification of the critical RH of the drying air according to the separating line of the first and second drying phase. Drying curves for layers of birch wood at different depths, and the RH graph for the drying air on an axis with a different scale, while the time axis is unchanged.

Figure 8.

Birch wood drying experiment. Thermocouple data, with temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.
Birch wood drying experiment. Thermocouple data, with temperature input based on the drying plan (see Table 1), and displacement sensor data depending on the drying time.

Figure 9.

A comparison of drying curves in different hardwood species under the same drying plan (see Table 1).
A comparison of drying curves in different hardwood species under the same drying plan (see Table 1).

Figure 10.

Moisture profiles for different tree species at different times of drying: (a) alder; (b) aspen; (c) birch; and (d) pine, with moisture profiles-based data which is used in the article (Tamme et al., 2021a).
Moisture profiles for different tree species at different times of drying: (a) alder; (b) aspen; (c) birch; and (d) pine, with moisture profiles-based data which is used in the article (Tamme et al., 2021a).

Figure 11.

A comparison of differences in surrounding air temperature and wood surface temperature for different tree species: (a) alder; (b) aspen; (c) birch; and (d) pine (according to the paper by Tamme et al. (2021a)).
A comparison of differences in surrounding air temperature and wood surface temperature for different tree species: (a) alder; (b) aspen; (c) birch; and (d) pine (according to the paper by Tamme et al. (2021a)).

Figure 12.

A comparison of the behaviour of the surface layer displacement sensor as a function of time, for different tree species.
A comparison of the behaviour of the surface layer displacement sensor as a function of time, for different tree species.

Figure 13.

A comparison of the total deformation in the surface layer depending on the drying time, for different tree species.
A comparison of the total deformation in the surface layer depending on the drying time, for different tree species.

Hypothesis testing results for hardwood species_ Independent x-value is moisture content (MC) in the depth level of 1 mm from the surface_ Dependent y-value is the MC in the depth level of 12 mm from the surface_

Type of woodEquation: 12mmMC (y) ~ 1mmMC (x)RR2p-value
Black aldery = 0.951x + 17.3030.9890.977< 0.001
Aspeny =1.275x + 11.80.9790.958< 0.001
Birchy =1.302x + 12.0280.9650.931< 0.001
*Pine sapwoody =0.9377x + 57.8980.9920.984< 0.001

The same drying schedule for drying all hardwoods_

Time (h)Surrounding air temp. (°C)Wet bulb temp. (°C)Surrounding air RH (%)
02019.4460
14746.1195
124847.1095
365048.1390
605248.0280
845245.5469
1085243.0559
1325240.2749

A comparison of the effective diffusion coefficient (EDC) (Deff) for different tree species under quasi-stationary drying conditions_

Type of woodDeff (*10−4 mm2/s)
Black alder11.92
Aspen11.44
Birch4.18
*Pine sapwood9.58

Equations of experimentally determined parabolic moisture profiles for different tree species, and an R-squared approximation_ Independent x-value is the depth level (mm) from the board surface_ Dependent y-value is the moisture content (MC%) at the depth level_

Type of woodTime (h)Equation of parabolic MC profileR2
Black alder (Figure 10a)0---------------
61y = − 0.0976x2 +3.0933x +30.20.9512
96y = − 0.0655x2 +2.0568x+ 12.8840.9964
119y = − 0.047x2 +1.5694x + 6.46650.9974
Aspen (Figure 10b)0-------------------
50y = − 0.1197x2 +3.4894x+32.5920.8463
99.5y = − 0.0597x2 +1.9168x + 11.80.9916
125y = − 0.0315x2 +1.18x +9.18380.993
Birch (Figure 10c)0-------------------
46y = − 0.1154x2 +3.9316x +23.50.9698
94y = − 0.0599x2 +2.0472x +13.2740.9981
122y = − 0.0547x2 +1.9479x +10.2740.9984
*Pine sapwood (Figure 10d)22y = − 0.054x2 +1.5471x +91.4840.87
92y = − 0.0781x2 +2.3962x +24.7910.8978
116y = − 0.0867x2 +2.2641x +18.5330.8615
140y = − 0.0625x2 + 1.8709x +11.1910.9769

The experimentally determined moment of time, for the separating line (SL) and the corresponding average moisture content in the wood at the point at which the wood's surface layer transitions from the first drying phase into the second drying phase, and figures related to that transition_

Type of woodSeparating line (SL) (h) of drying phasesAvg MC (%)Related figures
Black alder6155Figure. 9, line 1; Figure. 11a
Aspen5062Figure. 9, line 2; Figure. 11b
Birch4664Figure. 9, line 3; Figure. 11c
*Pine sapwood9460Figure. 11d

Section-linear calibration functions determined on the basis of Formula (7) at depths of 1 mm and 4 mm from the surface, using the example of alder wood_ Independent x-value is electrical resistance of wood (unit 10LogR)_ Dependent y-value is the moisture content (MC) of wood_

Depth (mm)ABBCCD
1 mmy = −8.0269x+475.3y = −0.676x+61.97y = −0.41x+43.335
4 mmy = −12.813x+714.47y = −1.474x+119.696y = −0.721x+69.18

The critical diffusion coefficient (Dcr) for the surface layer of three hardwood species and pine sapwood, along with the corresponding critical air humidity (RHcr) and the ratio of the diffusion coefficients for the first drying phase and the second drying phase in the surface layer (Dcr/D2ph), for different tree species_

Type of woodDcr (*10−4 mm2/s)RHcr (%)D2ph (*10−4 mm2/sDcr/D2ph
Black alder36.5775.613.872.64
Aspen30.7185.411.722.62
Birch16.3585.46.922.36
*Pine sapwood2781181.5
DOI: https://doi.org/10.2478/fsmu-2023-0009 | Journal eISSN: 1736-8723 | Journal ISSN: 1406-9954
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Language: English
Page range: 3 - 20
Published on: Apr 13, 2024
Published by: Estonian University of Life Sciences
In partnership with: Paradigm Publishing Services
Publication frequency: 2 issues per year
Keywords:
wood drying,
critical diffusion coefficient,
effective diffusion coefficient
Related subjects:
Life sciences,
Plant science,
Ecology,
Life sciences, other

© 2024 Hannes Tamme, Regino Kask, Peeter Muiste, Valdek Tamme, published by Estonian University of Life Sciences
This work is licensed under the Creative Commons Attribution 4.0 License.

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