Determination of Thermal Properties of Wood and Wood Based Products by Using Transient Plane Source

Transcription

1 Determination of Thermal Properties of Wood and Wood Based Products by Using Transient Plane Source Bijan ADL-ZARRABI Ph. D. SP Fire Technology Sweden Lars BOSTRÖM Ph.D. SP Fire Technology Sweden Summary Thermal properties of Norway spruce, particleboard and low-density fibreboard have been determined by using the Transient Plane Source method in different temperatures and at different moisture content. The measured results are compared with results from other studies. The comparisons shows that Transient Plane Source can be used for determination of thermal properties of wood and wood based material. 1. Introduction Analysing behaviour of a wood products or structures at high temperatures is multi principal. Thermal calculations can be needed in different applications such as calculations of drying processes or fire resistance. In order to make valid predictions, reliable data of the physical and the mechanical properties of the material are required at elevated temperatures. There are several different methods available for the determination of the thermal properties at ambient conditions but at elevated temperatures only few methods can be applied. Furthermore, wood is an orthotropic material and thus the method must be capable to handle the anisotropy. The purpose of this paper is to introduce a new measuring technique, Transient Plane Source (TPS), for measuring of thermal properties of wood and wood based materials. The TPS methodology enables measurements of thermal conductivity and thermal diffusivity simultaneously at ambient as well as elevated temperatures. Gustafson et. al have described the principal of using a thermal plane source in different papers [1, 2]. The TPS method is a transient technique for determination of thermal conductivity and thermal diffusivity of materials. A flat source (sensor) is located between two specimen halves and acts as heater as well as detector of the temperature increase. The flat plane source is insulated between two layers of Kapton or Mica. The thickness of the insulation is about 25 µm. The sensor insulated by Kapton can be used in measurement in temperatures up to 200 C and the Mica insulated sensor can be used for temperatures up to about 700 C. The technique is in itself transient since it measures the temperature response when a predefined effect is applied to the heater. Although, the sample must be in thermal balance with the surrounding media (the air in most cases) before the measurement can be made. By using the measured temperature increase in the sample and the effect applied to the heater it is possible to solve the partial differential equation of heat transfer by using iterative methods. Thermal conductivity and thermal diffusivity can be measured simultaneously. Furthermore, anisotropy can be handled by this method. Wood is an anisotropic cellular material with a large variability in its material properties. The

2 properties vary between different species and also within a specific species. Due to the orthotropic nature of wood the determination of the thermal properties is more complex. The thermal conductivity of wood in the radial direction is about 5 to 10 % greater than tangential direction and the conductivity in longitude direction is about times the conductivity across the grain [3, 4]. The specific heat of wood should not vary dramatically by different wood species since the chemical composition only varies slightly between different species. Although measured specific heat given in literature of different wood species varies in the range of kj/kgk and the specific heat of cellulose is about 1.55 kj/kgk [3, 4, 5]. Furthermore, since wood is a hygroscopic material it will always contain more or less water. The amount of the water has a profound influence on almost all properties of wood which also is the case for the thermal properties. Thus it is of great importance that the determined properties are given together with the actual moisture content. Also the temperature affects the thermal properties. Kollmann and Cote suggested equations (1) to (4) for calculation of the influence of temperature, moisture and density on the thermal properties of wood [3]. c d T (1) c ( 4.19u c ) /(1 u) (2) w d 2 1 ( ( u1 u2 )) (4) c d is the specific heat of dry wood in kj/kgk and c w is the specific heat of wet wood. is the thermal conductivity in W/mK and is the density in the range kg/m 3. T is the temperature and finally u is moisture content by mass. Eurocode 5, part 1-2, give informative values for the thermal properties at elevated temperatures [6]. These values do not consider different species, density, moisture content or the orthotropic behaviour. Hence, the values given must be used with care. 2. Experiments 2.1 Specimens Thermal properties of Norway spruce (Picea abies), particleboard and low density fibre board (LDF) have been measured at different temperatures up to 150 C. Particleboard and LDF have been assume as an isotropic material and spruce as an anisotropic material. Two pieces of the material is needed for the measurement since the sensor is placed between these two pieces. Three different specimens (six specimens halves) were used in measurements concerning particleboard and LDF. In order to investigate anisotropic behaviour of spruce five specimens (ten specimens halves) were prepared. The size of the specimens was 60x60x30 mm (length, width and thickness). The density of the materials is presented in table 1. All specimens of spruce were taken of same piece of timber. The density of spruce was measured on one specimen only. Table 1 Density of Spurs, particleboard and LDF Material Wet density [kg/m 3 ] Dry density [kg/m 3 ] Spruce Particleboard LDF (3) 2.2 Measurements on anisotropic materials The TPS has different toolboxes for measurements of the thermal properties. For the measurements on spruce has the biaxial anisotropic toolbox been used. In this toolbox the properties of the material in one plane are assumed to be isotropic. In order to use this toolbox the specific heat of the material must be known. Thus two separate measurements must be made for anisotropic materials, one for determination of the specific heat, and one for determination of conductivity. Volumetric specific heat of wood is determined with TPS by using a special sensor. The sample is,

3 during the measurement, kept inside a thermally insulated holder or enclosure made of a material with high thermal conductivity. The holder is exposed to a constant output of power from a sensor, which is permanently attached to the holder. During the heating the temperature is continuously recorded by following the resistance of the sensor. 3. Test results 3.1 Particleboard and low density fibre board Particleboard and low density fibre board (LDF) have in the present study been assumed as isotropic materials. This assumption may seems to be a strong simplification concerning density variation over cross section of the boards and influence of the manufacturing parameters as the difference on the properties in machine direction and cross machine direction. To find out the influence of these parameters on the thermal properties can be the aim of further investigations. Thermal properties of an isotropic material can be measured directly with the standard toolbox in the TPS software. Hence, the thermal conductivity, the thermal diffusivity as well as the volumetric specific heat is determined from the measurements. The results of the measurements are presented in table 2. Table 2 Thermal properties of dry particleboard and dry LDF at different temperatures. Temperature C Conductivity (W/mK) Diffusivity (mm 2 /s) Specific heat (MJ/m 3 K) Particleboard Low density fibre board Heat capacity (kj/kgk) In order to determine the effect of moisture content on the thermal properties specimens were tested unconditioned and thereafter dried and tested once more. The drying was performed in an excavator, i.e. the samples were not heated. The moisture content of the unconditioned specimens were u = 7 %. The results from the tests are shown in table 3. Table 3 Thermal properties of particleboard and LDF at different moisture content. Moisture content [%] Conductivity (W/mK) Diffusivity (mm 2 /s) Specific heat (MJ/m 3 K) Heat capacity (kj/kgk) Particleboard Difference [%] Low density fibre board Difference [%] Wood Volumetric specific heat Volumetric specific was measured with the special sensor for specific heat. The specific heat was 1003

4 determined on both a dried sample and on a sample with a moisture content of u = 9.5 %. The volumetric specific heat was determined to C = 0.52 MJ/m 3 K for the dry sample and to C = 0.74 MJ/m 3 K for the sample with moisture content u = 9.5 % Thermal conductivity and thermal diffusivity The conductivity and diffusivity were measured by using two different measuring set-ups. In the first set-up, specimens were prepared so that the surface of specimen that should be in connection with the sensor was cut in different orientations of the wood specimen. Three combinations of the contact surface were applied. I. The contact surface is in the radial - tangential plane II. The contact surface is in the radial - longitudinal plane III. The contact surface is in the tangential - longitudinal plane In the second set-up conductivity and diffusivity were measured in different temperatures by assuming that the difference of the thermal properties in tangential and radial direction are negligible. Thus, in these measurements wood is assumed to be an orthotropic material with two principal directions, the longitudinal direction (fibre direction) and the cross fibre direction. The anisotropic toolbox in TPS was used for determination of thermal properties. Since this toolbox require the specific heat as input the measured value C = 0.74 MJ/m 3 K was used. The results of these measurements are presented in tables 4 and 5. The orientation of the sensor is according to these results essential for performing a correct measurement. The TPS method can handle orthotropic materials by placing the sensor in the radial - tangential plane of the specimen. The two other cases does not generate correct values. Table 4 Thermal conductivity and thermal diffusivity with different orientation of surface contact Orientation of sensor Conductivity [W/mK] Diffusivity [mm 2 /s] I II III Table 5 Thermal properties at different temperatures. The presented values are mean value of three to five measurements for each specimen. Temperature [ C] Conductivity [W/mK] Diffusivity [mm 2 /s] Specific heat used as input [MJ/m 3 K] Comparison of results with literature 4.1 Particleboard The measured thermal conductivity at 20 C of particleboard was compared with reported values in NIST databases [7]. The comparison show that the thermal conductivity measured with TPS is slightly higher, about 10 %. Although, the moisture content of the boards is not given in the NIST database. Concerning thermal diffusivity and specific heat reference data were not available. Thus, the increasing of specific heat of particleboard by temperature is compared with equation (1). The trend line of the measured specific heat of dry particleboard is expressed in equation (6). c T (6) Equation (6) can be compared with equation (1) given by [3].

5 4.2 Wood Volumetric specific heat Using equation (1) and (2), volumetric specific heat can be calculated according to [3]. The measurement results and calculated results as well as data from the Wood Handbook [4] are presented in table 2. Table 6 Specific heat of dry wood and wood with moisture content u = 9.5 %. Specific heat (dry) [MJ/m 3 K] Specific heat (wet) [MJ/m 3 K] Present study Kollmann and Cote [3] Wood Handbook [4] Figure 1 show the effect of temperature on the specific heat. Measurements on spruce, particleboard and LDF are shown as well as the relation given by Kollmann and Cote [3]. The slope is the same for particleboard and the theoretical expression from [3]. The comparison indicates that the TPS method can be a reliable method for determination of specific heat of the wood and wood based materials. Specific heat (MJ/m 3 K) Kollmann and Cote [3] Particle board Spruce Low density fibre board Temperature (C) Figure 1 Measure specific heat of spruce, LDF and particleboard as well as from Kollmann and Cote [3] Thermal conductivity Table 7 show a comparison on measurements made on two specimens with values found in literature. Both conductivity and diffusivity is higher in the longitudinal direction compared with Kollmann and Cote [3]. In the perpendicular direction the measured values coincides well with the literature. 1005

6 Table 7 Comparison of thermal conductivity and thermal diffusivity of two wood specimens and values found in literature. Conductivity [W/mK] Diffusivity [mm 2 /s] Specimen A Specimen B Kollmann and Cote [3] * 0.149* Wood Handbook [4] * Calculated values 5. Conclusions The TPS method is a fast, cost effective method for measurement of the thermal properties of materials. The method can handle measurements on anisotropic materials at temperatures up to 700 C. Hence it is a method suitable for wood and wood based materials. The objective with the present study was to see if the method is applicable to wooden materials, and to see whether there are any limitations. From the present study can the following conclusions be drawn: Thermal conductivity, diffusivity and specific heat can be measured with TPS on wood and wood based materials. For anisotropic materials such as wood must a toolbox for anisotropic materials be used which means that the specific heat must be determined separately. When measurements are made on wood the sensor shall be placed in the radial - tangential plane. Values on thermal properties found in literature vary to a large extent. This may be due to factors such as species, density and moisture content that generally not are given. The measured values are of the same magnitude as the ones found in literature. The measured conductivity and diffusivity for wood in the radial - tangential plane is almost equal to the values found in literature. The measured conductivity and diffusivity for wood in the longitudinal direction is higher compared to the values found in literature. The measured effect of temperature on the thermal properties where approximately the same as shown in literature. 6. References [1] Gustafsson, S.E. & Long, T., Transient Plane Source (TPS) Technique for Measuring Thermal Properties of Building Materials, Fire and Material, Vol. 19, 1995, pp [2] Gustafsson, S.E., Transient Plane Source (TPS) Technique for Thermal Conductivity and Thermal Diffusivity Measurements of Solid Materials, Rev. Sci. Instrum, 62(3),1991, pp [3] Franz F.P. Kollman, Wilfred A.Cote, Principles of Wood science and Technology,1968 [4] Forest Products Laboratory, Wood handbook: Wood as an engineering material. Gen. Tech. Rep. FPL-GTR-113. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, [5] Frank P. Incropera, Fundamentals of Heat and Mass Transfer, John Wiely&Sons, 1981 [6] Eurocode 5 - Design of Timber Structures, Part 1-2, General Rules-Structural Fire Design, Version 8, Annex B(informative) Thermal and Mechanical properties, 2001 [7] Detail Summary of a Result from NIST Building Materials Database, Version 1.0, Summary for ID 1833