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3.3 Drying modelling - Measurement of kiln conditions and wood drying properties We used specialised instruments on test boards during vacuum drying trials to obtain, through continuous ‘real–time’ measurements, a greater understanding of the kiln conditions and wood drying properties. This is essential to validate and provide actual kiln and wood property data for the modelling component of this project and hence optimise the process. As the proposed model is ‘deterministic’, actual wood property and kiln data are required to ‘feed’ the model to obtain the most accurate simulations possible. This section describes the specialised instrumentation used to measure both wood boundary condition properties and internal wood properties we used for model input. It should be noted that a certain level of ‘trial and error’ and ‘prototyping’ of these instruments occurred between drying trials, the account of which is presented in Milestone Report 2. This part of the experimental design is not included in this report as we deemed it unnecessary. Three test boards were selected for each trial to measure internal temperature using thermocouples, MC using load cells and shrinkage using strain gauges. Except for the first trial SPG1, one of the three test boards was modified to perform a ‘flying wood’ test, described later in this section. The flying wood test board utilised the specialised instrumentation in the same manner described below, as the other two test boards. Each test board was loaded onto two load cells, one at each end, to measure the MC of these boards based on board weight. We measured the overall shrinkage for the same three boards using specialised ‘omega’ shaped strain gauges attached to the board edges using a jig developed by DEEDI. Two strain gauges were used for each test board. Internal temperature was measured for the test boards using 1 mm diameter thermocouples. Measurements were made at 1/3 thickness and 1/2 thickness inserted 50 mm into the side of the boards, and at the centre of the board end at a depth of 50 mm. Inlet and outlet airflows were measured using two anemometers placed at approximately 1⁄2 the stack height on the second board in from the stack edge at each side of the stack. The output of these devices was obtained using DataTakerTM data acquisition equipment. 3.3.1 Shrinkage and flying wood Generally, drying stresses are assessed at stages or at the end of drying using prong or cup tests. However, it is very difficult to gather information on the stress field within boards during drying. The flying wood test was developed in the laboratory of Wood Science at École Nationale du Génie Rural des Eaux et des Forêts (ENGREF) in Nancy, France and is a well documented test method for gathering information on the stress field during drying and the phenomenon of stress reversal (Aguiar and Perré, 2000, Brandao and Perré, 1996). The data gathered from this test method, although not used in this report will be used for future modelling work incorporating a mechanical (degrade) component. The flying wood test consists of non-symmetrical drying of small boards. Five surfaces of the sample are sealed, so that the moisture migration occurs only in one direction. This strategy transforms a part of the drying stresses encountered during drying into a global deformation of the sample (Figure 3). 20PDF Image | Evaluation of super-heated steam vacuum drying
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