Characteristics Of Heat And Mass Transfer In Wood During Radio-Frequency Vacuum Drying

Radio-frequency vacuum(RF/V) drying is an advanced drying technology.The detection of parameters such as wood temperature and moisture is difficult due to the special RF environment, and it restricts the research of heat and mass transfer,and reliable practices of drying schedule.In order to hold the above parameters better,study the theory of heat and mass transfer further,and practice drying schedule credibility during RF/V drying.In this article,the characteristics of heat and mass transfer during RF/V drying were studied preliminarily by using larch.The contents and results were as follows:(1) Factors influencing relative humidity and the phenomenon of evaporation from the wood surface during the vacuum field were studied.The results showed that:the relative humidity in the vacuum chamber was determined by temperature and was not affected by ambient pressure in the chamber.Relative humidity during RF/V drying should be measured after heating stopped completely for the relative humidity was affected by radiating of RF.Moisture in wood not only moved in the form of vapor,but also evaporated from the wood surface and diffused through wood during RF/V drying and vacuum drying.(2) In order to estimate the moisture content of wood during RF/V drying,small timber specimens were used to investigated the influential factors of equilibrium moisture content(EMC) under negative pressure;the estimated moisture content was revised by the desorption law of wood and the relationship of pressure and EMC,then compared with the moisture value by weighted.The results showed that:EMC decreased as temperature increased but increased as the relative humidity and pressure increased;moisture content with different temperature,pressure and relative humidity exhibited exponential decrease in the first 1～2 hours,and nearly reached the level of EMC;the value of revised moisture content was 0.45%less than the value by weighted.(3) The change of wood temperature and the on and off time of RF radiated to maintain the setting temperature during RF/V drying were investigated,results showed that:the temperature and time during the process of wood heating had a linear or polynomial relations,which can be used to calculated the needed time to reach the setting temperature;on the present experimental conditions, the maintenance of RF radiated 30s and made a 10 minutes interval can ensure that temperature fluctuate slightly around the setting temperature.(4) The change of and the influential factors temperature field in wood during RF/V drying was studied.The results showed that:in the direction of thickness,moisture content had no effect on the temperature,and in the direction of the length and width,temperature decreased slightly as the moisture content decreased.The temperature of both side sealed and control samples(side unsealed samples) decreased closing to the surface,the temperature of the surface closing to earth-plate was lower than that of the surface closing power plate,and the places closing to the opening port and ground plate had the lowest temperature.The influential factors of temperature distributing were latent heat of water,heat exchange and heat radiation.The temperature of side sealed sample closing to the opening port was lower,compared with that of the control samples,which was caused by the difference of moisture evaporation and absorption from wood port.The drying rate of side sealed samples was significantly lower than that of the control one,which explained the sides of larch with poor permeability played great role in water movement during RF/V drying.