Study On Characteristics Of Water Movement And Transverse Relaxationon Wood During Different Drying Methods

This paper selects Elm sawn timber as an object of study, using four drying methods such as air-drying, conventional kiln, high temperature drying and microwave drying, to conduct drying experiment for test materials. This experiment includes four stages to make the initial moisture content from about50%dring to about40%,30%,20%,10%respectively, using NMR techniques to study characteristics of water movement and transverse relaxation on wood during different drying methods, to establish the relationship among moisture content, transverse relaxation time (T2) and peak area of total integral (S). The main conclusions are as follows:(l)The affect of four kinds of drying methods on wood drying uniformity:for the material with initial moisture content about50%, final moisture content of the center position of the test piece is slightly higher than that of sides, left and right sides of the test material in a consistent. Throughout the test materials is in a uniform state. For the air drying, conventional drying and high temperature drying, the final moisture content of center of specimens showed higher than that of both sides, left and right sides of moisture content tend to be consistent during every process. During microwave drying process, the final moisture content of the specimen showed a bilateral below middle or side above the center.(2)Wood drying rate and drying curve of four kinds of drying methods:for conventional drying, high-temperature drying and microwave drying, general average drying rate of timber increases with drying temperature (microwave power). During every drying temperature (power), each stage of the drying speed increases at first and then decreases. For conventional drying and high temperature drying, the drying curve was divided into three stages such as the preheating, acceleration and deceleration. Microwave drying process was divided into preheating, constant velocity drying and deceleration drying.(3) Wood internal temperature field distribution during different drying methods:for conventional drying and high temperature drying, the change of wood interior temperature is divided into heating phase and constant phase two stages. In the constant phase, there is not obvious temperature gradient. For microwave drying, wood internal temperature can be divided into three stages:heating phase including slow heating and rapid heating, the constant temperature drying period and post heating phase. In the constant temperature drying period, the temperature distribution of wood internal every layer is uniform.(4)The effect of four different drying methods on layer moisture content of wood can be summarized as follows:samples both in the side and the center have the same rule about the distribution of layer moisture content. During the air-drying and conventional kiln process, moisture gradient of each layer increased when the moisture content above FSP, meanwhile water movement and evaporation mainly occurred under the action of moisture gradient which decreased when the moisture content below FSP.The samples of free water overall rapidly discharged under high-thermal function in the high-temperature drying process, and the moisture gradient of each layer reached minimum values. Because of the short drying time, the moisture content deviation of each layer increased with decreased moisture content. The movement of water based on the moisture content gradient for air-drying, conventional kiln and high-temperature drying process. Both the moisture content deviation and gradient are reduced when MC below FSP in the microwave drying, especial moisture content distribution present outside the high-low rule in low power, and the rule became less obvious with the increment of power. Moisture content of each layer was uniformity through the MC below FSP. So the main drying power for water movement was not the moisture content gradient but the vapor-pressure.(5)Four kinds of drying methods about transverse relaxation characteristics of the wood were not only related to the moisture content of samples, but with the specimen location. There were two kinds state of water at the edge of untreated wood while three kinds state in the center. The T2valus of core was more than other layers which included free water of two kinds state in the center of samples, besides the free water2of each layer was greater than others which were at edge of wood. Along the thickness direction of wood, free water T2values of the edge of the surface was more than the other layers, and the core T2value of the center was greater than the edge of wood, while the surface and subsurface values were less than the edge.(6) T2values of two states water which included free water and bound water in the wood were increased when the water content was near40%or30%. Then with the depletion of free water, movement and evaporation of bound water, T2values were reducted with the moisture content decreased. When MC above the FSP, transverse relaxation properties of various water states were related to the drying characteristics, moisture content and the location of samples. The variation of transverse relaxation time about bound water accompany with all stages of drying process, representing more regularly than movement of free water.(7) Through the established linear relationship among moisture content, T2and S, the initial moisture content of samples about MC below FSP and the different scope moisture contents were respectively calculated by NMR. So NMR technology can be used as a new kind of method to test the moisture content.