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(dǎo)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(duì)木材進(jìn)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(jì)效果也越來越被人們所認(rèn)識(shí)、所實(shí)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(duì)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(dòng)和表面蒸發(fā)強(qiáng)度的外部因素中，周圍空氣的壓力也同樣是一個(gè)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)。它與傳統(tǒng)的常規(guī)蒸汽干燥

2、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(kuò)展到改變壓力并使其浮動(dòng)。這種環(huán)境壓力的波動(dòng)可大大加快木材內(nèi)部水分的移動(dòng)速率，而且在整個(gè)浮壓干燥過程中，木材表面始終處于濕潤(rùn)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(dǎo)致木材干燥應(yīng)力的增加，進(jìn)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(diǎn)，日益受到木材干燥界的廣泛重視，被國(guó)際上認(rèn)為是最具發(fā)展前景的木材干燥新工藝。在目前我國(guó)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(duì)提高干燥質(zhì)量和干燥效率，節(jié)約木材具有重要意義。浮壓干燥技術(shù)盡管在德國(guó)、丹麥、法國(guó)

3、、加拿大等國(guó)有小規(guī)模的工業(yè)應(yīng)用，但諸多工藝問題并未從機(jī)理上得到根本的解決。目前，國(guó)內(nèi)外對(duì)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(duì)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(shí)用性試驗(yàn)，將具有重要的理論意義和實(shí)用價(jià)值。本篇論文來源于國(guó)家自然基金課題（ 59876005），其研究?jī)?nèi)容與實(shí)際生產(chǎn)需要密切相關(guān)，著眼于當(dāng)前急需解決的進(jìn)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(shí)用價(jià)值最大的壓力區(qū)段即負(fù)壓區(qū)段進(jìn)行試驗(yàn)研究。文中首先從浮壓干燥的干燥介質(zhì)特性入手，通過對(duì)木材浮

4、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(xué)模型，進(jìn)而為浮壓干燥工藝的優(yōu)化設(shè)計(jì)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(diǎn)如下：1.從干燥介質(zhì)的熱力學(xué)特性出發(fā)，對(duì)真空狀態(tài)下過熱蒸汽和空氣干燥“逆轉(zhuǎn)溫度”的存在進(jìn)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(duì)比試驗(yàn)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(duì)流換熱的條件下，當(dāng)環(huán)境絕對(duì)壓力為 0.02MPa 時(shí)，“逆轉(zhuǎn)溫度”的理論值為 90左右，試

5、驗(yàn)值在 80 85之間。出現(xiàn)偏差的主要原因是由于理論分析時(shí)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(shí)存在一個(gè)溫度點(diǎn)，稱為“逆轉(zhuǎn)溫度” ，當(dāng)過熱蒸汽的溫度高于“逆轉(zhuǎn)溫度”的溫度時(shí)，過熱蒸汽干燥速率比空氣要快，低于逆轉(zhuǎn)溫度時(shí)則正好相反。由于木材浮壓干燥的工業(yè)化應(yīng)用是在負(fù)壓區(qū)實(shí)現(xiàn)的，因此對(duì)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(shí)存在，那么在制定木材浮壓干燥工藝時(shí)，就可使干燥介質(zhì)溫度始終高于“逆轉(zhuǎn)溫度”，這樣即可實(shí)現(xiàn)在保持較快干燥速率的基礎(chǔ)之上，充分發(fā)揮過熱蒸汽干燥時(shí)木材表面對(duì)流換熱系數(shù)大、傳質(zhì)阻力和干燥品質(zhì)好等一系列

6、的優(yōu)勢(shì)，這對(duì)于優(yōu)化木材浮壓干燥工藝具有重要的理論意義和實(shí)用價(jià)值。2.首次通過大量的試驗(yàn)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(duì)浮壓和常規(guī)干燥過程中預(yù)熱階段的對(duì)比試驗(yàn)研究表明：木材浮壓干燥預(yù)熱階段進(jìn)行的非常迅速。在絕對(duì)壓力為0.02MPa，介質(zhì)溫度分別為60、 80時(shí)，以蒸汽為干燥介質(zhì)時(shí)的平均升溫速率分別是以空氣為干燥介質(zhì)時(shí)的 1.55、 1.64 倍。這說明隨著干燥介質(zhì)溫度的提高，從預(yù)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(shí)試驗(yàn)中還發(fā)現(xiàn)以蒸汽為干燥介質(zhì)時(shí)，在預(yù)熱階段結(jié)束后的木材的含水率比預(yù)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

7、分僅位于木材表面且預(yù)熱時(shí)間很短，因此對(duì)整個(gè)干燥過程影響很小。通過試驗(yàn)及理論分析， 探討了試件尺寸 （厚度和長(zhǎng)度） 對(duì)浮壓干燥速率的影響規(guī)律。試驗(yàn)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(zhǎng)度的影響。然而，有一點(diǎn)應(yīng)當(dāng)特別關(guān)注， 即縱向水分移動(dòng)的速率隨試件長(zhǎng)度的增加衰減迅速，隨著試件長(zhǎng)度的增加木材內(nèi)水分從側(cè)面的移動(dòng)對(duì)干燥速率的貢獻(xiàn)率將逐漸占據(jù)主導(dǎo)地位。因此，在浮壓干燥過程中，預(yù)見有一個(gè)“臨界尺寸”的存在。通過浮壓干燥介質(zhì)條件對(duì)水分移動(dòng)速率的影響規(guī)律的研究表明，木材的浮壓干燥速率隨著干燥介質(zhì)溫度的增加、絕對(duì)壓力的減小和浮動(dòng)頻率的加大而增加。但在上述三因素中對(duì)干燥速率的影響

8、程度各不相同，從大到小的排列順序?yàn)椋航橘|(zhì)溫度（T） >>壓力浮動(dòng)頻率（ Hz）>絕對(duì)壓力（ P）。3. 首次對(duì)浮壓干燥過程中木材的濕遷移特性進(jìn)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(dòng)力及其遷移特性，并推導(dǎo)出計(jì)算方程。通過對(duì)浮壓干燥的干燥速率和干燥過程中木材內(nèi)部溫、濕度場(chǎng)變化的研究表明，在溫度為 80、壓力浮動(dòng)范圍為 0.02 0.1MPa 的條件下，當(dāng)浮動(dòng)頻率分別取0.75次/h 和 0.375次 /h 時(shí)，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(shí)的 3.24 和 2.69 倍，這足見壓力浮動(dòng)幅度和頻率對(duì)干燥速率的巨大影響。而且在同一溫度水平，壓力浮動(dòng)幅度

9、越大和浮動(dòng)頻率越快，則木材內(nèi)的溫、濕度場(chǎng)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(chǎng)的變化可見，木材內(nèi)部各層的含水率變化幅度大致相當(dāng)，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(dòng)的主要驅(qū)動(dòng)力。在浮壓干燥過程中自由水的移動(dòng)由兩部分組成，其一是在毛細(xì)管壓力下液體的團(tuán)塊遷移；其二是在壓力梯度下，由于壓力波動(dòng)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(dǎo)地位。自由水在毛細(xì)管中的蒸發(fā)或沸騰實(shí)質(zhì)上是復(fù)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(dòng)和換熱受管道尺寸、形狀、表面結(jié)構(gòu)等因素的影響很大。在木材為溫度為 80，環(huán)境絕對(duì)壓力為

10、 0.02MPa 的條件下，理論計(jì)算表明此時(shí)的汽泡平衡態(tài)半徑為 4.68 m，而針葉樹材管胞的內(nèi)徑為 2030 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(dāng)木材中大毛細(xì)管水達(dá)到沸點(diǎn)以后，在細(xì)胞腔中液體內(nèi)部的強(qiáng)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(dāng)液態(tài)水處于紋孔口處時(shí)，可能就會(huì)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(dòng)是吸著水移動(dòng)的主要驅(qū)動(dòng)力。FSP以下時(shí)水分的擴(kuò)散移動(dòng)分成兩個(gè)部分。其一是在水蒸汽壓力梯度下的擴(kuò)散遷移；其二是由于干燥介質(zhì)壓力的波動(dòng)而引起的浮動(dòng)壓力下的遷移。采用擴(kuò)散杯法對(duì)水

11、分?jǐn)U散系數(shù)和浮壓系數(shù)進(jìn)行了測(cè)定。確定出不同壓力下與常壓(0.1MPa)下水分?jǐn)U散系數(shù)的數(shù)學(xué)表達(dá)式，以及不同壓力變化速率下與壓力固定不變時(shí)浮壓系數(shù)的數(shù)學(xué)表達(dá)式。運(yùn)用上述方程，再對(duì)常壓下含水率和溫度與水分?jǐn)U散系數(shù)之間的關(guān)系式進(jìn)行修正后，即可確定出浮壓干燥過程中吸著水遷移量的計(jì)算方程。對(duì)試驗(yàn)數(shù)據(jù)回歸分析表明：木材水分?jǐn)U散系數(shù)隨絕對(duì)壓力的增加呈線性減小；隨壓力變化速率的增加呈對(duì)數(shù)曲線增大。4. 在借鑒常壓下鋸材干燥數(shù)學(xué)模型的基礎(chǔ)之上，以水蒸汽壓力梯度為水分移動(dòng)的主要驅(qū)動(dòng)力，首次建立了木材浮壓干燥中熱質(zhì)傳遞的數(shù)學(xué)模型。通過對(duì)理論模擬與試驗(yàn)結(jié)果的比較分析可知：計(jì)算機(jī)模擬的曲線和試驗(yàn)曲線除在高含水率區(qū)域有一

12、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(jì)算機(jī)模擬值為 2.95%/h，試驗(yàn)值為 3.04%/h，兩者的誤差不超過 5%。數(shù)學(xué)模型組成了一個(gè)木材干燥的動(dòng)態(tài)機(jī)制，通過實(shí)驗(yàn)證明，模型在較寬的條件范圍內(nèi)能較為準(zhǔn)確預(yù)測(cè)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(dòng)Characteristics of Heat and Mass Transfer in WoodDrying Process under Floating Pressure（ Yi SonglinDirected by Prof

13、. 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 consumption. Therefore proper drying is of importance

14、 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 the conventional drying methods take advantage of

15、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 neglected. Wood drying under floating pressure is a

16、n 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 steam. As a result, the fluctuation of ambient p

17、ressure 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 hasbeen universally recognized as a novel wood drying method l

18、ighted with color future. R&D ofthe innovative drying processing and technology to improve the quality of products andefficiency, especially in response to solving the conflicts between supplies of wood failure tosatisfy the demands of wood, has an important strategic meaning.Although drying und

19、er 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 under floating pressure both in domestic and overse

20、a are few,dontmention 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 is supported by the Natural Science Foundation o

21、f 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 pressure, experimental program is carried out when the

22、 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 established to optimize design and control the dr

23、ying 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 temperature between the condition of superheated st

24、eam 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 temperature really exist under the condition of vacuum through comparable e

25、xperience 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 90 , and the experimental value, however, is 80 85. The reason for this difference is that we neglect

26、ed the effect of radiation.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 temperature, the drying rate of superheated steam is faster than

27、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 industries is operated under vacuum, so it is of importance

28、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 of these credits of superheatedsteam drying such as the

29、 bigger convection heat coefficient, lowerresistance 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 out to explore the fundamentals of wood drying under flo

30、ating 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 are 60 and 80 respectively, the average rates of rising

31、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 average temperature increment with the drying medium of superheated steam to that of air illustrates that coupled with the drying medium temperature

32、 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 by the condensation of water. Thus this phenomenon is fa

33、vorable 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 within a certain range makes no difference on the drying rate

34、, 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 length; simultaneously moisture movement along lateral directi

35、on 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 drying rate of floating pressure drying increases with the i

36、ncreasing 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 pressure frequency, and the last is absolute pressure.3. The dr

37、iving 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 rate and the changes of temperature plateau and moisture pla

38、teau under floating pressure, results demonstrate that when the temperatureof drying medium is 80 , the fluctuating frequency are 0.75 times per hour and 0.375 time per hour respectively, the drying rates of wood drying under floating pressure are 3.24 times and 2.69 time than that of air drying, th

39、e 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 the change of temperature and moisture plateau, as a result,

40、 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 parts during the process of drying underfloating pressure, one

41、 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 free water.Essentially the evaporation or boiling of free

42、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 easily. When the temperature of wood is 80 , the absolute press

43、ure of ambient is 0.02Mpa, the equilibrium radius of bubble calculated theoretically is 4.68 m, however, the tracheid of softwood internal diameter is 2030 m，obviously there is sufficient space to form bubble in liquid. When the water in bigger capillary reaches its boiling point, it is possible tha

44、t 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 entranceto 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 pressurecoefficient by the means