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Test apparatus The need to carry out measurements of permeability to air of various species, along the three material directions and at different pressure levels requires an accurate and reliable device capable of measurement over a wide range of permeability values (from 10-10 down to 10-18 m2). For this purpose, the team at AgroParisTech-ENGREF developed a system called ALU- CHA at LERMAB. The originality of this medium lies in a high reliability of lateral air tightness, the rapid operation and the possibility to measure permeability with the same geometry, thus on the same samples as those employed for the diffusion measurement (Figure 6). This device allows us to apply an accurate and constant pressure difference between the two faces of the sample. We can determine the corresponding gas flux by the measurement of the thermal perturbation due to the air flux when it passes through a capillary tube. Figure 6. Schematic of sample support with ALU-CHA system. Diagram courtesy of (Rousset et al., 2004) Note the presence of a rubber tube around the sample. Partial vacuum phase is applied in the chamber between the rubber tube and the aluminum tube to easily place or remove the sample. Then, during measurement, a typical pressure of 2–3 bar is applied in this chamber to press the rubber joint against the lateral face of the sample for air tightness. In addition, before placing the specimen in the ALU-CHA system, a silicone-based grease of high viscosity (vacuum grease) was applied on the specimen’s lateral surfaces to inhibit surface flow in the micro-porosity layer formed between the epoxy resin and rubber surfaces. The measurement was carried out by applying a controlled and constant pressure difference (ΔP) between two faces of the specimen, and then measuring the corresponding airflow (Q). All measurements were performed in a temperature-controlled room with minimum temperature fluctuations. Due to the effect of temperature on the air viscosity, the room temperature was also recorded for each measurement. The air flux permeating through the specimen was recorded when the flowmeter exhibited a nearly constant airflow. The air permeability of each sample was then calculated by Darcy’s law (Dullien, 1992): K = Q.μ.e.P A.∆P.P (38) Where K is intrinsic permeability (m2), Q is flux (m2 s-1), μ is dynamic viscosity of the air (Pa. s), e is sample thickness (m), P is pressure at the flux Q (Pa), A is sample area (m2), ΔP is 29 Evaluation of super–heated steam vacuum drying viability and development of a predictive drying model for Australian hardwood species – Final reportPDF Image | Evaluation of super-heated steam vacuum drying
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