木材浮壓干燥過程的傳熱傳質(zhì)

1、木材浮壓干燥過程的傳熱傳質(zhì)(作者：伊松林 導師：張璧光教授、常建民教授 )摘要 木材干燥是木材加工與利用的基礎(chǔ)環(huán)節(jié)，其能耗約占木制品生產(chǎn)總能耗的40%70%。對木材進行正確合理的干燥處理，既是保證木制品質(zhì)量的關(guān)鍵，又是木材合 理利用和節(jié)約木材的重要手段，其重要性和經(jīng)濟效果也越來越被人們所認識、所實踐。傳 統(tǒng)的干燥方法通常是以濕空氣作為干燥介質(zhì)，將空氣溫度、相對濕度和氣流速率作為控制 木材干燥過程的“三要素” 。然而，在影響木材內(nèi)部水分移動和表面蒸發(fā)強度的外部因素 中，周圍空氣的壓力也同樣是一個決定性的因素。木材的浮壓干燥就是在充分考慮了外界 壓力影響的基礎(chǔ)之上，出現(xiàn)的一種新型的干燥技術(shù)。它與傳

2、統(tǒng)的常規(guī)蒸汽干燥以及通常意 義上的真空干燥相比，最主要的區(qū)別在于是以過熱蒸汽作為干燥介質(zhì)，且將壓力因素從基 本保持壓力不變擴展到改變壓力并使其浮動。這種環(huán)境壓力的波動可大大加快木材內(nèi)部水 分的移動速率，而且在整個浮壓干燥過程中，木材表面始終處于濕潤狀態(tài)，從而可以有效 地防止因表面水分蒸發(fā)過程快而導致木材干燥應力的增加，進而防止開裂和變形的產(chǎn)生。 木材的浮壓干燥以其干燥速率快、質(zhì)量好等一系列突出的優(yōu)點，日益受到木材干燥界的廣 泛重視，被國際上認為是最具發(fā)展前景的木材干燥新工藝。在目前我國木材供需矛盾日益 突出的今天，研究開發(fā)這種干燥工藝和技術(shù)，對提高干燥質(zhì)量和干燥效率，節(jié)約木材具有 重要意義。浮

3、壓干燥技術(shù)盡管在德國、丹麥、法國、加拿大等國有小規(guī)模的工業(yè)應用，但諸多工 藝問題并未從機理上得到根本的解決。目前，國內(nèi)外對木材浮壓干燥的理論研究還比較少 見，關(guān)于浮壓干燥的干燥特性和干燥規(guī)律等方面缺乏系統(tǒng)深入地研究，特別是對浮壓干燥 過程中木材內(nèi)部熱質(zhì)傳遞規(guī)律的研究尚屬空白。因此，開展浮壓干燥的基礎(chǔ)研究與實用性 試驗，將具有重要的理論意義和實用價值。本篇論文來源于國家自然基金課題( 59876005)，其研究內(nèi)容與實際生產(chǎn)需要密切相 關(guān)，著眼于當前急需解決的進口硬闊葉材等難干材以及速生材和幼齡材的干燥問題。本論 文以木材的浮壓干燥為背景，在其實用價值最大的壓力區(qū)段即負壓區(qū)段進行試驗研究。文 中

4、首先從浮壓干燥的干燥介質(zhì)特性入手，通過對木材浮壓干燥的基本規(guī)律以及干燥過程中 木材內(nèi)部熱質(zhì)傳遞規(guī)律等方面的研究，建立了木材浮壓干燥熱質(zhì)傳遞的數(shù)學模型，進而為 浮壓干燥工藝的優(yōu)化設(shè)計和過程控制提供理論依據(jù)。這一探索性的研究，以全新的理念為 木材干燥技術(shù)的創(chuàng)新和發(fā)展開辟了一條嶄新的道路。本研究主要成果與創(chuàng)新點如下：1. 從干燥介質(zhì)的熱力學特性出發(fā)，對真空狀態(tài)下過熱蒸汽和空氣干燥“逆轉(zhuǎn)溫度”的 存在進行了深入的理論分析，求解了真空狀態(tài)下過熱蒸汽干燥 “逆轉(zhuǎn)溫度”的理論模型， 并首次通過空氣與過熱蒸汽干燥的對比試驗發(fā)現(xiàn)了真空狀態(tài)下過熱蒸汽干燥“逆轉(zhuǎn)溫度” 的存在。在僅有對流換熱的條件下，當環(huán)境絕對壓力

5、為 0.02MPa 時，“逆轉(zhuǎn)溫度”的理論 值為90C左右，試驗值在80C85C之間。出現(xiàn)偏差的主要原因是由于理論分析時未考 慮真空過熱蒸汽的輻射特性。已有的研究表明，在（常壓）過熱蒸汽干燥時存在一個溫度點，稱為“逆轉(zhuǎn)溫度” ， 當過熱蒸汽的溫度高于“逆轉(zhuǎn)溫度”的溫度時，過熱蒸汽干燥速率比空氣要快，低于逆轉(zhuǎn) 溫度時則正好相反。由于木材浮壓干燥的工業(yè)化應用是在負壓區(qū)實現(xiàn)的，因此對真空狀態(tài) 下過熱蒸汽干燥是否也存在“逆轉(zhuǎn)溫度”問題的研究非常重要。如果真空狀態(tài)下的“逆轉(zhuǎn) 溫度”確實存在，那么在制定木材浮壓干燥工藝時，就可使干燥介質(zhì)溫度始終高于“逆轉(zhuǎn) 溫度”，這樣即可實現(xiàn)在保持較快干燥速率的基礎(chǔ)之上，

6、充分發(fā)揮過熱蒸汽干燥時木材表 面對流換熱系數(shù)大、傳質(zhì)阻力和干燥品質(zhì)好等一系列的優(yōu)勢，這對于優(yōu)化木材浮壓干燥工 藝具有重要的理論意義和實用價值。2. 首次通過大量的試驗和理論分析較為系統(tǒng)深入地研究了木材浮壓干燥的基本規(guī)律。 通過對浮壓和常規(guī)干燥過程中預熱階段的對比試驗研究表明：木材浮壓干燥預熱階段進行的非常迅速。在絕對壓力為 0.02MPa,介質(zhì)溫度分別為60C、80C時，以蒸汽為干 燥介質(zhì)時的平均升溫速率分別是以空氣為干燥介質(zhì)時的1.55、 1.64倍。這說明隨著干燥介質(zhì)溫度的提高， 從預熱階段的升溫速率來講， 以蒸汽為干燥介質(zhì)要優(yōu)于以空氣為干燥介質(zhì)； 同時試驗中還發(fā)現(xiàn)以蒸汽為干燥介質(zhì)時，在預

7、熱階段結(jié)束后的木材的含水率比預熱之前的 初含水率要大。這主要是因水分凝結(jié)所致，由于凝結(jié)的水分僅位于木材表面且預熱時間很 短，因此對整個干燥過程影響很小。 通過試驗及理論分析， 探討了試件尺寸（厚度和長度） 對浮壓干燥速率的影響規(guī)律。 試驗表明，在一定尺寸范圍內(nèi)，浮壓干燥的干燥速率不受試件厚度的影響，但受試件長度 的影響。然而，有一點應當特別關(guān)注， 即縱向水分移動的速率隨試件長度的增加衰減迅速， 隨著試件長度的增加木材內(nèi)水分從側(cè)面的移動對干燥速率的貢獻率將逐漸占據(jù)主導地位。 因此，在浮壓干燥過程中，預見有一個“臨界尺寸”的存在。 通過浮壓干燥介質(zhì)條件對水分移動速率的影響規(guī)律的研究表明，木材的浮壓

8、干燥速 率隨著干燥介質(zhì)溫度的增加、絕對壓力的減小和浮動頻率的加大而增加。但在上述三因素中對干燥速率的影響程度各不相同，從大到小的排列順序為：介質(zhì)溫度（T） 壓力浮動頻率（Hz） 絕對壓力（P）。3. 首次對浮壓干燥過程中木材的濕遷移特性進行了深入研究， 揭示了浮壓下自由水和 吸著水遷移的驅(qū)動力及其遷移特性，并推導出計算方程。 通過對浮壓干燥的干燥速率和干燥過程中木材內(nèi)部溫、濕度場變化的研究表明，在溫度為80C、壓力浮動范圍為0.02O.IMPa的條件下，當浮動頻率分別取0.75次/h和0.375 次/h時，馬尾松試材的浮壓干燥速率分別為常壓下相同干燥溫度時的3.24和2.69倍，這足見壓力浮動

9、幅度和頻率對干燥速率的巨大影響。而且在同一溫度水平，壓力浮動幅度越 大和浮動頻率越快，則木材內(nèi)的溫、濕度場的變化越快，它表明木材內(nèi)水分遷移和蒸發(fā)速 率加快。從濕度場的變化可見，木材內(nèi)部各層的含水率變化幅度大致相當，表、心層含水 率差值不大，這說明含水率梯度不是浮壓干燥過程中水分移動的主要驅(qū)動力。 在浮壓干燥過程中自由水的移動由兩部分組成，其一是在毛細管壓力下液體的團塊 遷移；其二是在壓力梯度下，由于壓力波動而引起自由水的蒸發(fā)或沸騰后所產(chǎn)生的水蒸汽 的遷移，且后者在自由水的遷移過程中占主導地位。自由水在毛細管中的蒸發(fā)或沸騰實質(zhì)上是復雜的微尺度傳熱傳質(zhì)過程。由于尺度的微 小化，使管道中流動和換熱受

10、管道尺寸、形狀、表面結(jié)構(gòu)等因素的影響很大。在木材為溫 度為80T，環(huán)境絕對壓力為0.02MPa的條件下，理論計算表明此時的汽泡平衡態(tài)半徑為 4.68卩m，而針葉樹材管胞的內(nèi)徑為 2030卩m。由此可見，此狀態(tài)下管胞內(nèi)腔有足夠的 空間用于液體內(nèi)部汽泡的生成。因此當木材中大毛細管水達到沸點以后，在細胞腔中液體 內(nèi)部的強烈汽化是可能發(fā)生的。在微尺度傳熱傳質(zhì)中的“汽化空間”與“擬沸騰”的概念 同樣適用于木材，可以設(shè)想當液態(tài)水處于紋孔口處時，可能就會出現(xiàn)“擬沸騰”狀態(tài)。 在浮壓干燥過程中， 水蒸汽壓力梯度和壓力的浮動是吸著水移動的主要驅(qū)動力。 FSP 以下時水分的擴散移動分成兩個部分。其一是在水蒸汽壓力

11、梯度下的擴散遷移；其二是由 于干燥介質(zhì)壓力的波動而引起的浮動壓力下的遷移。 采用擴散杯法對水分擴散系數(shù)和浮壓系數(shù)進行了測定。確定出不同壓力下與常壓 （0.1MPa）下水分擴散系數(shù)的數(shù)學表達式，以及不同壓力變化速率下與壓力固定不變時浮壓 系數(shù)的數(shù)學表達式。運用上述方程，再對常壓下含水率和溫度與水分擴散系數(shù)之間的關(guān)系 式進行修正后，即可確定出浮壓干燥過程中吸著水遷移量的計算方程。對試驗數(shù)據(jù)回歸分 析表明：木材水分擴散系數(shù)隨絕對壓力的增加呈線性減??；隨壓力變化速率的增加呈對數(shù) 曲線增大。4. 在借鑒常壓下鋸材干燥數(shù)學模型的基礎(chǔ)之上，以水蒸汽壓力梯度為水分移動的主要 驅(qū)動力，首次建立了木材浮壓干燥中熱

12、質(zhì)傳遞的數(shù)學模型。通過對理論模擬與試驗結(jié)果的比較分析可知：計算機模擬的曲線和試驗曲線除在高含 水率區(qū)域有一定偏差外 (含水率最大偏差值為 8.12%)，其余區(qū)域均可很好地吻合。 特別是 從初含水率90%到終含水率12%的平均干燥速率來看，計算機模擬值為2.95%/h，試驗值為3.04%/h，兩者的誤差不超過5%。數(shù)學模型組成了一個木材干燥的動態(tài)機制，通過實驗 證明，模型在較寬的條件范圍內(nèi)能較為準確預測浮壓干燥下馬尾松的平均含水率。關(guān)鍵詞 木材、真空干燥、浮壓、水分移動Characteristics of Heat and Mass Transfer in WoodDrying Process

13、under Floating Pressure( Yi Songlin Directed by Prof. Zhang Biguang & Prof. Chang Jianming)Abstract Wood drying is generally regarded as the key item in wood processing and utilization, the drying process accounts for approximately 40% to 70% percent of total wood-products manufacturing energy consu

14、mption. Therefore proper drying is of importance to improve the quality of forest products and is playing an essential role in using and saving wood reasonably, and its contributions to the improved performance to products and cost reduction have been drawing an even increasing attention. Usually th

15、e conventional drying methods take advantage of the temperature of air, relative humidity and air velocity to control drying process with the medium of wet-air, however, the fact that the role of ambient pressure in determining the moisture transfer in wood and its evaporation intensity has been neg

16、lected. Wood drying under floating pressure is an innovative technology, which considerably takes the effects of ambient pressure into account. Compared with the traditional drying and vacuum drying methods, its ambient pressure varies to form floating pressure with the drying medium of superheated

17、steam. As a result, the fluctuation of ambient pressure accelerates the rate of moisture transfer while keeps wood surface wet, which effectively protect the occurrences of drying defects. With the performance of short drying-cycle and good quality, wood drying under floating pressure has been unive

18、rsally recognized as a novel wood drying method lighted with color future. R&D of the innovative drying processing and technology to improve the quality of products and efficiency, especially in response to solving the conflicts between supplies of wood failure to satisfy the demands of wood, has an

19、 important strategic meaning.Although drying under floating pressure has a limited application to industries in Germany, Danish, and France as well as Canada recently, a number of fundamentals with respect to processing are far from satisfying. At present, researches on the theory of wood drying und

20、er floating pressure both in domestic and oversea are fewd, onmtention the heat and mass transfer of drying under floating pressure. Therefore, carrying out the theoretical and experimental research on drying under floating pressure has an important implication both in theory and practice.This paper

21、 is supported by the Natural Science Foundation of China (Grant No. 59876005), its content is closely related to the requirements of practice, aiming at the problem that imported hardwood is difficult to dry and how to dry faster grow-tree wood. Based on the background of drying under floating press

22、ure, experimental program is carried out when the ambient is under an atmospheric pressure which is the most useful. This paper began with the characteristics of drying medium, and explored the fundamentals of heat and mass transfer, then a mathematical model based on the heat and mass transfer was

23、established tooptimize design and control the drying process. The research developed an innovative way for wood drying technology with new conception.The conclusions and originalities are:1. based on the characteristics of drying medium, theoretical analysis concerning the existence of inversion tem

24、perature between the condition of superheated steam under vacuum-drying and air drying has been carried out, developed the theoretical model of inversion temperature of superheated steam under vacuum, and for the first time it has been proven to be true that the inversion temperaturereally exist und

25、er the condition of vacuum through comparable experience of air and superheat steam. If we consider that there is only convection, when the absolute pressure of ambient is 0.02Mpa, the theoretical value of inversion temperature is approximate to 90C , and the experimental value, however, is 80C 85C

26、. The reas on for this differe nee is that we n eglected the effect of radiati on.The previous literatures indicate the inversion temperature exists under superheated steam drying with an atmospheric pressure, when the temperature of superheated steam is higher than the point of inversion temperatur

27、e, the drying rate of superheated steam is faster than that of air drying, but if the temperature of superheatedsteam is lower than the point of inversion temperature, the drying rate of superheated steam is slower than that of air drying. Owing to the fact that the floating pressure drying in indus

28、tries is operated under vacuum, so it is of importance to reveal whether the inversion temperature of superheated steam drying exists or not under vacuum is important, if it does, we can make the temperature of drying medium is always higher than the inversion temperature, thus we can make full use

29、of these credits of superheatedsteam drying such as the bigger convection heat coefficient, lower resistance to mass transfer and good quality, finally to optimize wood drying processing under floating pressure.2. For the first time a number of experiences and theoretical analysis have been carried

30、out to explore the fundamentals of wood drying under floating pressure.The comparable experiments from the preheating process of floating pressure drying and conventional drying indicated that heat transfer increases sharply. When the absolute pressure is 0.02Mpa, the temperatures of drying mediums

31、are 60S and 80C respectively, the average rates of rising temperature with the drying medium of superheated steam is 1.55 and 1.64 times than that of using air for drying medium. The ration of the rate of wood averagetemperature increment with the drying medium of superheated steam to that of air il

32、lustrates that coupled with the drying medium temperature rising, the rising rate of temperature of superheated is superior to that of air during the stage of preheating; meanwhile the moisture content of wood treated with superheatedsteam is higher that of its initial moisture content, which caused

33、 by the condensation of water. Thus this phenomenon is favorable in the whole drying process. The effects of sample size including thickness and length on the drying rate under floating pressure have been analyzed through experiments and theoretical analysis. The results showed sample thickness with

34、in a certain range makes no difference on the drying rate, but the length do. The fact illustrates that moisture transfer mainly along longitudinal direction. However, one thing need to mention is the rate of moisture movement along longitudinal direction reduces sharply with the increasing of lengt

35、h; simultaneously moisture movement along lateral direction will be dominant in contribution to the drying rate. Therefore, we can draw a conclusion that a critical sample size exists.Results from analyzing the effects of drying medium under floating pressure on moisture movements rate show: the dry

36、ing rate of floating pressure drying increases with the increasing of drying medium temperature; the reducing of absolute pressure and the increasing of fluctuating frequency. Their contributions to the drying rate list as followings sequence: the first is temperature, followed by fluctuating pressu

37、re frequency, and the last is absolute pressure.3. The driving force and the characteristics of free water and absorbed water movements under floating pressure have been revealed for the first time, and their mathematic equations have also been established.Through the research of both the drying rat

38、e and the changes of temperature plateau and moisture plateau under floating pressure, results demonstrate that when the temperature of drying medium is 80C, the fluctuating frequency are 0.75 times per hour and 0.375 time per hour respectively, the drying rates of wood drying under floating pressur

39、e are 3.24 times and 2.69 time than that of air drying, the fact show that the range of pressure fluctuation and its frequency play important roles in determining the drying rate. When the temperature is fixed, the expanding range of fluctuating pressure and faster fluctuation frequency increases th

40、e change of temperature and moisture plateau, as a result, the time required is shortened. However the surface moisture content is approximate to the core moisture content, that is to say moisture content is not the dominant force under floating pressure.The movement of free water comprises two part

41、s during the process of drying under floating pressure, one is the mass transfer driven by capillary pressure, and the other is steam transfer due to the evaporation and boiling of free water caused by the pressure fluctuation under pressure gradient. Furthermore, the later dominates the removing of

42、 free water.Essentially the evaporation or boiling of free water in capillary can be regarded as the heat and mass transfer with micro-dimension. the minimization of dimension make the heat and mass transfer affected by the flow, size of pipeline, and its shape as well as surface structure more easi

43、ly. When the temperature of wood is 80C , the absolute pressure of ambient is 0.02Mpa, the equilibrium radius of bubble calculated theoretically is 4.68 卩 m, however, the tracheid of softwood internal diameter is 2030 卩 m, obviously there is sufficient space to form bubble in liquid. When the water

44、in bigger capillary reaches its boiling point, it is possible that the intensive evaporation in cell cavity occurs. The conception of vapor space and boiling can be also used in wood drying. We can image that boiling will begin when the liquid water lies in the entrance to pits.The moisture diffusion below fiber saturation point can be divided into two parts: the transfers caused by steam gradient and the pressure fluctuation respectively. Measuring respectively the moisture diffusion coefficient and floating pressure coefficient by the means