engleski

Determination of Thermo-modified Oak Wood Emissivity Properties in the Infrared Spectral Wavelength Range 7.5–13 μm

Infrared (IR) thermography is a widely applicable non-destructive testing (NDT) method for measuring the temperature distribution of an observed object surface. Through thermographic analysis, the IR radiation intensity of the electromagnetic spectrum is detected and recorded on the obtained thermograms. IR thermography has its advantages and disadvantages but it is a promising technique, which is easily applicable in the field and has no harmful impacts on the operator or equipment. In wood science and related fields, IR thermography can be applied in production quality control, monitoring of manufacturing equipment and plants for hydrothermal wood processing, wood properties characterization, defects detection (particularly holes, rot and drying cracks), heat flux in wood, and more [1]. During the thermographic analysis, temperature is not measured directly, but indirectly. Therefore, many factors can affect the accuracy of temperature measurement during quantitative thermography analysis, and one such factor is the emissivity of the observed object. Emissivity of the material is the ability of the surface to emit radiant energy expressed as the ratio of the radiant energy emitted by a surface to that emitted by a blackbody at the same temperature and under the same conditions. Emissivity depends on wavelength, temperature, object surface (opacity, roughness, orientation), the material from which the object is made, angle of measurement. Although important, those associated parameters often are not entirely specified along with the emissivity value in the literature. Therefore the experiment for establishing thermo-modified oak wood emissivity values within spectral range 7.5 – 13 μm was conducted. Also the emissivity of innate oak wood was determined for the comparison. For the experiment, an FLIR infrared system with the following characteristics was used. Imaging and optical data were as follows : field of view 25° x 25°, minimum focus distance 0.1 m, focal length 10 mm, instantaneous field of view (spatial resolution) 2.42 mrad, F-number 1.5, NETD 100 mK, image frequency 9 Hz. Detector type: FPA, uncooled microbolometer, with spectral range 7.5–13 μm (LWIR, ISO–MIR), IRR 180 x 180 pixels. Oak wood (Quercus robur L.) specimens used in the experiment were radial texture elements clear of defects, with approximately the same three ring width. There were 60 innate oak wood specimens and 60 thermo-modified oak wood specimens with the final dimensions 270 mm (length) by 65 mm (width) by 25 mm (thickness). Innate oak wood elements were kiln dried and then four sides planed. The moisture content of the specimens was 9 %. Thermo- modified wood elements were kiln dried, then thermally treated and afterwards four sides planed. The moisture content of the specimens was 5 %. The moisture content of specimens was determined according to EN 13183-1:2002/AC:2003 Moisture content of a piece of sawn timber – Part 1: Determination by oven dry method. A polyvinyl chloride material with known emissivity in long wavelength infrared (LWIR) spectral range was applied to the surface of all specimens. The emissivity of the polyvinyl chloride material was verified and the oak wood specimens were heated to 50 °C. During the experiment, the ambient characteristics were as follows: air temperature 21 °C, relative humidity 60 %, reflected apparent temperature 20.2 °C, the distance of oak wood specimens surface from the lens was 120 mm, angle of measurement in relation to the object normal was 15°, airflow and dotted reflections were inhibited. Emissivity values were determined via FLIR software with an area box tool with following dimensions 70×30 pixels. There was a significant difference in the results for innate and thermo- modified oak wood emissivity values (two-sample t(df) = -15.6, p = 0.00).

Infrared Thermography; Emissivity; Oak Wood; LWIR Spectral Waveband

nije evidentirano