Study On Drying Technics Under High Frequency-Hot Air Heating For 120mm Boxed-Heart Larch Lumber

In this study,the box-heart larch with a cross-section of 120×120 mm was dried under combined convection and high frequency with different drying technics.The distribution oftemperature,drying rate,checks and drying strains were investigated.A more appropriate technicsofcombined convection and high frequency drying was obtained through the analysis of these parameters.The findings indicate that:(1)Study on convective heating technics in combined convection and high frequency drying:high-frequency heating with the same power(4kW)and time(ON90s,OFF 1200s)and two different benchmarks(reference ?(preheating 24h)Dry bulb temperature was keptat 50?,wet bulb temperature was dropped 2? every 48h),reference II(preheating 36h,dry bulb temperature kept at 70?,wet bulb temperature dropped 2? every 48h))Convective heat drying(combined heating Drying reference:reference ?-2 is reference ?-1+ high frequency heating,reference ?-2 is reference ?-1+ high frequency heating).The distribution of temperature,drying rate,moisture content distribution and checks in the cross-section were investigated.A more appropriate process of convective drying was obtained through the analysis of these parameters.The results show that:1)The driving force and drying rate of the internal moisture migration of wood,the driving force of hot air convective heating and drying according to the benchmarks ?-1 and ?-1 is positively related to the drying potential(the temperature difference between dry and wet bulbs of drying media).The driving force of the former is much greater than the latter,resulting a faster drying rate in latter;the driving force of drying with double heat sources according to the benchmarks ?-2,?-2 is positively correlated with the drying potential and the temperature gradient generated by high frequency heating.When the temperature of the drying medium was low,the role of the latter was greater than that of the former.When the temperature was high,the former had a greater effect than the latter,and the combined effects of the two under two technics were similar,thus the average drying rate had little difference.2)The moisture content distribution in woodafter drying was uniform when dried in accordance with the benchmark ?-2 and according to the benchmark ?-2.3)The degree of cracking after drying in stack ?-2 is smaller than in ?-2.4)Thus the more appropriate technics processof convective drying was benchmark ?-2.(2)Study on highfrequency drying technics in combined convection and high frequency drying,:The appropriate convective heating technics ?-1 was combined with different high-frequency heating,and thedistribution of temperature in lumber,drying rate,moisture content distribution,cracking,drying strain and other drying quality were investigated.By analyzing the changes of these parameters,the influence of temperature gradient on drying rate and drying quality was analyzed,and then the suitable hybrid heating technics wich high frequency and hot air was determined.1)The effects of drying technics on the temperature gradient,drying rate and moisture content distribution are as followed:? the variation of temperature gradients could be handled by changing the match of high frequency and convection heating.? The increase of the positive temperature gradient accelerated the drying rate and made the distribution of final moisture content evenly.? The influence of positive temperature gradient on the decrease of the moisture content gradient was reflected in the mid and later drying stage,and the effect was in line with the temperature gradients.2 The effect of drying technics on drying strain,?In the early drying stage below fiber saturation point,the surface layer produced a large tensile strain,the middle and the core layer produced a compressive strain,and the tensile and compressive strain increased with increasing temperature gradient;?The stress reversal was occurred in advance for the sample with a large temperature gradient,the surface of the lumber was subjected to compressive strain,and the core was to tensile strain;? In the late drying stage,the temperature gradient had little effect on the instantaneous elastic deformation;4 The effect of temperature gradient on the viscoelastic creep was mainly in the middle of drying.The viscoelastic creep strain was decreased with the increased temperature and improved temperature gradient.5 Drying in the middle and late stage,the compressive mechano-sorptive creep was shown in wood,and the strain value increased with decreasing moisture content and temperature gradient.The more appropriate drying technics for combined convection and high frequency dryingwas as follows:convection heating technics with benchmark II-2,high frequency vibration(ON)90s,and stopover(OFF)1200s.