杨树活立木生理干燥过程中水分传输和散失机理研究

发布时间：2018-05-24 00:26

本文选题：新疆杨 + 生理干燥　；参考：《内蒙古农业大学》2016年博士论文

【摘要】：木材是一种天然绿色可再生生物质材料,其性能与含水率密切相关。合理干燥可使其具有良好的加工和使用性能。然而常规干燥耗能巨大,新型节能干燥技术,如除湿干燥、太阳能干燥、热泵干燥等,由于多种原因尚未形成规模化应用,因此,本文利用树木蒸腾作用这一生理特性,提出基于蒸腾作用降低木材水分的预干燥技术——生理干燥,对降低干燥能耗具有重要的理论意义和现实意义。本论文通过对新疆杨进行边材切断处理,切断其自根部的水分来源,同时对其进行保留树冠、移除树冠、垂直放置、倾斜45°放置和平躺放置等几种处理,测定了不同处理后新疆杨的含水率、树叶蒸腾特性变化情况,水分横向弛豫时间特性、水分存在状态及变化以及水相关孔隙结构及变化情况,分析了树干边材水分散失的主要动力,影响水分散失的影响因素,并通过分析生理干燥过程中水分状态及水相关孔隙结构变化情况,探讨了水分迁移规律,最后提出了杨树活立木生理干燥过程中水分传输和散失的机理。本论文的主要研究内容和结论为：(1)通过对新疆杨立木进行底部水分切断及移除／保留树冠处理,测定了杨树活立木生理干燥过程中的含水率变化情况和树叶蒸腾特性,研究表明：含水率在9天内由60.54%下降到41.32%,平均下降速率为2.30% MC/天,但边材含水率的快速下降主要发生在前3-5天；树叶蒸腾作用是水分下降的主要动力和原因；树叶蒸腾速率同木材含水率的下降呈正相关关系；正常生长的新疆杨和经水分切断处理后的新疆杨,其蒸腾速率和气孔导度均随相对湿度的增加而增加,随蒸汽压差和空气温度的增加而降低,而且水分散失主要在空气-叶面温差为1°C-3°C之间发生。(2)在新疆杨进行水分切断处理后,分别使其保持直立、倾斜45°和平躺于地面三种位置,测定了新疆杨木材水分降低过程中木材含水率、树叶蒸腾特性的变化情况,研究表明：水分切断处理会使新疆杨的树叶蒸腾速率显著下降,但不同放置方式对树叶蒸腾作用的影响不大；不同放置方式对新疆杨木材含水率下降或水分散失的影响不大,水分的快速散失主要发生在水分切断处理后的3-5天；随着相对湿度的增加或者蒸汽压差的降低,蒸腾速率越大,水分散失速率越快。(3)为了探讨生理干燥过程中的水分存在状态及变化,提出用低温和室温低场核磁共振方法测定了新疆杨边材的自由水／结合水横向弛豫时间截止时间T2cutoff,并根据该截止时间,计算了生理干燥过程中自由水、结合水的含量、自由水／结合水的比值以及它们的变化情况,研究表明：杨木心边材的自由水／结合水截止时间T2cutoff存在较明显的差异,心材的T2cutoff值约为91.07±15.07 ms,边材的T2cutoff值约为36.82±7.12 ms；在常规干燥过程中,新疆杨边材中的水分主要存在自由水和结合水两种状态；切断边材处理的各组新疆杨,在切断水分至树叶完全枯萎期间,随着含水率的下降,自由水和结合水均呈下降的趋势,自由水降至5%左右,结合水下降至30%左右,而在树叶枯萎后,自由水会部分转化成结合水,使结合水上升至35%左右；在水分降低过程中,对照组新疆杨的Ω值一直呈上升趋势,新陈代谢的能力不断增强,而切断边材处理的各组新疆杨的Ω值整体都呈下降趋势,新陈代谢水平不断降低。(4)为了探讨生理干燥过程中水分的迁移规律,从孔隙结构与水分移动关系的角度出发,提出通过表面弛豫率这一参数将木材水分的横向弛豫时间分布谱转化成木材内部与水相关的孔隙分布情况,分析了水分降低过程中各处理组新疆杨边材的水相关孔隙结构及其变化情况,研究表明：本试验所用新疆杨的水相关孔隙结构呈纳米级至微米级多尺度分布,在各级孔隙中,介孔孔隙(2 nmr250 nm)所占的比例最大,平均约为51.88%,其次为纳米级大孔(50nmr3100nm),约占23.99%,中微米级大孔(1μmr515μm)和小微米级大孔(100 nmr41μm),分别占10.62%和8.04%,大微米级大孔(15μmr640μm)约占4.59%,微孔孔隙(r12nm)和超大微米级大孔(r740μm)所占比例都很小,不超过1%；对照组新疆杨边材的水相关孔隙总体积在试验过程中呈增加趋势,切断水分后不同放置方式新疆杨的水相关孔隙总体积均呈下降趋势；在水分降低过程中,介孔(r2)孔隙体积的变化趋势与对照组的相同,呈增加趋势,其余各级孔隙(r3,,r4,r5,r6)结构的孔体积均呈下降趋势。其中,纳米级大孔(r3)和小微米级大孔(r4)的孔体积都呈较明显的下降趋势。(5)在综合分析上述研究结果的基础上,提出了生理干燥过程中水分传输和散失机理：叶内水分在叶片与周围环境蒸汽压差的作用下扩散到大气中,在木质部细胞壁上形成负压；叶脉木质部产生的负压在细胞壁孔隙(r2,r3,微毛细管系统)和导管(r6)或木纤维细胞腔(r5)等大毛细管中交替传递,实现水分的长距离传输。
[Abstract]:Wood is a kind of natural green renewable biomass material , its performance is closely related with water content . It has important theoretical and practical significance to reduce the water content , leaf transpiration characteristic , moisture content and water content in the process of physiological drying .
The transpiration of leaves is the main power and cause of the decline of water .
The transpiration rate of leaves is positively correlated with the decline of wood moisture content .
The transpiration rate and stomatal conductance increased with the increase of relative humidity , and the transpiration rate and stomatal conductance decreased with the increase of relative humidity .
The effect of different placement methods on the water content and loss of water in poplar wood in Xinjiang is not large , and the rapid loss of water mainly occurs 3 - 5 days after the water cut - off treatment ;
( 3 ) In order to study the state and change of water content in the process of physiological drying , it is suggested that the free water / binding water transverse relaxation time T2cutoff of poplar sapwood in Xinjiang is determined by low temperature and room temperature low field nuclear magnetic resonance method , and the T2cutoff value of the heartwood is about 91.07 卤 15.07 ms , and the T2cutoff value of heartwood is about 36.82 卤 7.12 ms .
In the conventional drying process , there are two states of free water and water - binding water in poplar sapwood of Xinjiang .
As the water content decreases , the free water decreases to about 5 % , and the free water is reduced to about 30 % . After the leaves are wilted , the free water will be partially converted into the combined water to raise the combined water to about 35 % .
In order to study the water - related pore structure and its change of poplar sapwood in Xinjiang , the distribution of pore structure and water - related pore structure of poplar sapwood in Xinjiang was studied .
The total volume of water - related pore volume of poplar sapwood in the control group increased during the test , and the total volume of water - related pore in Populus alba was decreased after cutting off water .
On the basis of comprehensive analysis of the results of the above research , the mechanism of water transmission and loss of water in the process of physiological drying was put forward : the moisture of the leaves diffused into the atmosphere under the action of the difference between the blades and the surrounding environment , and the negative pressure was formed on the cell wall of xylem .
The negative pressure generated by the xylem of the vein is transmitted alternately in the large capillary tubes , such as cell wall pores ( r2 , r3 , micro capillary system ) and the catheter ( 6 ) or the wood fiber cavity ( r5 ) , so as to realize long - distance transmission of moisture .
【学位授予单位】：内蒙古农业大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：S792.11

【参考文献】