多孔材料非穩(wěn)態(tài)物性參數(shù)及干燥特性研究
發(fā)布時間：2018-08-19 13:17
【摘要】：多孔材料在干燥過程中由于溫度梯度和濕度梯度的驅動,導致物料內(nèi)部水分子以汽化和遷移的方式擴散到表面,再以對流的方式將水分蒸發(fā)到熱空氣中,在此過程中隨著內(nèi)部水分的減少,從而導致物料體積、各相百分比、孔隙率和密度發(fā)生變化,表現(xiàn)出了極大的非穩(wěn)態(tài)特性；而上述變化又對物料下一步的干燥產(chǎn)生影響。因此,多孔材料的干燥過程涉及傳熱傳質和力學特性等問題。在多孔材料干燥過程中傳熱傳質和力學特性等問題的研究中,物性參數(shù)如水分有效擴散系數(shù)、有效導熱系數(shù)的求解是問題的關鍵。因此對其變化規(guī)律的研究,對掌握物料的干燥特性,保證其干燥品質,制定干燥工藝以及為相關干燥設備的設計提供理論依據(jù)。本文針對多孔材料干燥過程中表現(xiàn)出來的非穩(wěn)態(tài)特性,對球狀玉米籽粒、塊狀胡蘿卜顆粒和柱狀蘋果顆粒的水分有效擴散系數(shù)和有效導熱系數(shù)的非穩(wěn)態(tài)變化規(guī)律及非穩(wěn)態(tài)干燥特性進行了系統(tǒng)研究,主要研究內(nèi)容有以下幾個方面：(1)基于Fick第二擴散定律建立了多孔材料熱風干燥過程中非穩(wěn)態(tài)水分有效擴散系數(shù)數(shù)學模型和非穩(wěn)態(tài)收縮模型,對不同形狀(球狀、圓柱狀、片狀)、不同物料熱風干燥過程中內(nèi)部各單元層非穩(wěn)態(tài)水分有效擴散系數(shù)變化規(guī)律及組織收縮特性進行了研究。研究結果表明：在熱風干燥初期由于物料內(nèi)部水分多以自由水的形式存在,蒸發(fā)擴散較容易,水分有效擴散系數(shù)幾乎不發(fā)生變化；隨著干燥過程的進行,物料內(nèi)部自由水分變少,結合水分所占比例增大,水分蒸發(fā)擴散難度增加,水分有效擴散系數(shù)隨著水分比的變小而變�。蛔詈笪锪蟽�(nèi)部水分達到平衡干基含水率后不再發(fā)生變化,阻礙水分擴散的阻力和水分擴散驅動力達到平衡,水分有效擴散系數(shù)不再發(fā)生變化：在整個干燥過程中水分有效擴散系數(shù)是一個逐漸減小的非穩(wěn)態(tài)變化過程,而不是一個穩(wěn)態(tài)定值。先失水的外單元層先收縮,內(nèi)單元層滯后收縮,而不是同時收縮,外單元層收縮量大于內(nèi)單元層。(2)基于Fourier熱平衡定律建立了多孔材料熱風干燥過程中非穩(wěn)態(tài)有效導熱系數(shù)數(shù)學模型,對不同形狀(球狀、圓柱狀、片狀)、不同物料熱風干燥過程中內(nèi)部各單元層有效導熱系數(shù)非穩(wěn)態(tài)變化規(guī)律及傳熱特性進行了研究。研究結果表明：在整個干燥過程中有效導熱系數(shù)是一個逐漸減小的非穩(wěn)態(tài)變化過程,而不是一個穩(wěn)態(tài)定值,這是因為物料隨著水分向外部擴散蒸發(fā),水分減小所留下的孔隙一部分轉化為收縮體積,一部分轉化為內(nèi)部孔隙,因此在干燥過程中水分所占體積百分比逐漸減小,孔隙所占百分比逐漸增大,而水的導熱系數(shù)大于空氣的導熱系數(shù)。干燥初期物料內(nèi)部干燥速率較大,水分迅速減少,物料有效導熱系數(shù)迅速減小,當干燥速率進入減速干燥階段后,由于干燥速率減小進而水分擴散較慢,物料內(nèi)部導熱系數(shù)變化曲線趨于平緩。(3)基于上述多孔材料非穩(wěn)態(tài)傳熱和傳質特性,將其視為粘彈性體建立了球狀物料內(nèi)部各單元層的質熱傳遞耦合作用下的非穩(wěn)態(tài)粘彈性應力模型,結合球狀幾何形狀的特性,在徑向應力的基礎上推導出了切向應力分布和非穩(wěn)態(tài)變化規(guī)律,并將粘彈性應力模型分離為溫度梯度引起的熱應力部分和濕度梯度引起的濕應力部分。研究結果表明：在整個干燥過程中球狀物料的切向應力和徑向應力出現(xiàn)兩個應力峰值,且第一次峰值大于第二次。徑向應力在干燥初期各單元層受到的力為壓應力,在應力反向后變?yōu)槔瓚?在整個干燥過程中物料內(nèi)部呈現(xiàn)拉應力和壓應力兩種應力類型,而不是只呈現(xiàn)一種應力類型；切向應力干燥初期是外拉內(nèi)壓,在應力反向后變?yōu)閮?nèi)拉外壓；熱應力和濕應力隨時間的曲線變化趨勢和總應力變化趨勢一致,在同一時刻濕應力值大于熱應力值,表明在干燥過程中濕度梯度對干燥應力的影響大于溫度梯度。(4)最后結合多孔材料非穩(wěn)態(tài)物性參數(shù)模型、非穩(wěn)態(tài)干燥特性模型,研究了物料收縮、擴散、熱量傳遞和力學特性之間的耦合關系,研究結果表明：干燥工藝對物料干燥特性的影響是通過改變干燥過程中物性參數(shù)來實現(xiàn)的,水分有效擴散系數(shù)和有效導熱系數(shù)隨熱風溫度、速率的增大而增大,干燥速率和干燥應力隨水分有效擴散系數(shù)增大而增大,熱量傳遞速率隨有效導熱系數(shù)增大而增大；考慮收縮情況下的物性參數(shù)小于不考慮收縮的物性參數(shù),這是由于考慮收縮情況下物料失去水分的體積一部分轉化為物料內(nèi)部孔隙,一部分轉化為物料收縮的體積,而不考慮收縮情況下物料失去水分的體積全部轉化為孔隙,而物料內(nèi)部孔隙越大水分擴散和熱量傳遞所受到的阻力越小。
[Abstract]:Driven by temperature gradient and humidity gradient, porous materials diffuse water molecules to the surface by vaporization and migration, and then evaporate water into hot air by convection. In this process, with the decrease of internal moisture, the volume of materials, the percentage of each phase, porosity and density are caused. Therefore, the drying process of porous materials involves heat and mass transfer and mechanical properties. In the study of the heat and mass transfer and mechanical properties in the drying process of porous materials, physical parameters such as effective diffusion of water are studied. The key to the problem is to find out the coefficient and the effective thermal conductivity.Therefore, the research on the variation of the coefficient can provide theoretical basis for grasping the drying characteristics of materials, ensuring their drying quality, formulating drying technology and designing relevant drying equipment. The unsteady-state variation and drying characteristics of water effective diffusion coefficient and effective thermal conductivity of grain, block carrot particle and columnar apple particle were systematically studied. The main research contents are as follows: (1) Based on Fick's second diffusion law, the unsteady-state water content in hot air drying process of porous materials was established. The mathematical model of effective diffusivity and unsteady shrinkage model were used to study the variation law of unsteady water effective diffusivity and tissue shrinkage characteristics of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying. It is easy to evaporate and diffuse in the form of free water, and the effective diffusivity of water hardly changes; with the drying process, the free water in the material becomes less, the proportion of combined water increases, the difficulty of water evaporation and diffusivity increases, and the effective diffusivity of water decreases with the decrease of water ratio; finally, the effective diffusivity of water in the material becomes smaller. The effective diffusion coefficient of water does not change any more. The effective diffusion coefficient of water does not change any more. During the whole drying process, the effective diffusion coefficient of water is a gradually decreasing unsteady state change process, not a steady state constant value. (2) Based on Fourier heat balance law, a mathematical model of unsteady effective thermal conductivity of porous materials during hot air drying was established, and the effective thermal conductivity of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying were calculated. The results show that the effective thermal conductivity in the whole drying process is a gradually decreasing unsteady state change process, not a steady state constant value, because the material evaporates with the diffusion of water to the outside, and the water decreases. In the initial stage of drying, the drying rate of the material is higher, the moisture content decreases rapidly, and the effective thermal conductivity of the material is higher than that of the air. When the drying rate enters the stage of deceleration drying, the change curve of thermal conductivity becomes gentle because of the decrease of drying rate and the slow diffusion of moisture. (3) Based on the unsteady heat and mass transfer characteristics of the porous materials, the coupling of mass and heat transfer among the cell layers in the spherical materials is established as a viscoelastic body. The unsteady viscoelastic stress model under the action of temperature gradient and humidity gradient are separated from the viscoelastic stress model. The results show that the viscoelastic stress model is composed of the thermal stress part caused by temperature gradient and the wet stress part caused by humidity gradient. The tangential stress and radial stress of the spherical material appeared two peaks during the drying process, and the first peak value was greater than the second one. In the initial stage of shear stress drying, the tensile and internal pressures change into the tensile and external pressures after the stress reverses; the curves of thermal stress and wet stress change with time are consistent with the total stress, and the wet stress value is greater than the thermal stress value at the same time, indicating that the humidity gradient is related to the drying stress during the drying process. (4) Finally, the coupling relationship between material shrinkage, diffusion, heat transfer and mechanical properties was studied by combining the unsteady physical parameters model of porous materials and the unsteady drying characteristics model. The results show that the influence of drying process on material drying characteristics is through changing physical parameters during drying process. The effective diffusion coefficient and effective thermal conductivity of water increase with the increase of temperature and rate of hot air, drying rate and stress increase with the increase of effective diffusion coefficient of water, and heat transfer rate increases with the increase of effective thermal conductivity. It is due to the fact that the volume of the material losing moisture is partly converted into the internal pores of the material and partly into the volume of the material contracting under the consideration of shrinkage, while the volume of the material losing moisture is completely converted into the pores without considering shrinkage, and the greater the internal pores of the material, the smaller the resistance of water diffusion and heat transfer.
【學位授予單位】：昆明理工大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TB383.4
本文編號：2191764
[Abstract]:Driven by temperature gradient and humidity gradient, porous materials diffuse water molecules to the surface by vaporization and migration, and then evaporate water into hot air by convection. In this process, with the decrease of internal moisture, the volume of materials, the percentage of each phase, porosity and density are caused. Therefore, the drying process of porous materials involves heat and mass transfer and mechanical properties. In the study of the heat and mass transfer and mechanical properties in the drying process of porous materials, physical parameters such as effective diffusion of water are studied. The key to the problem is to find out the coefficient and the effective thermal conductivity.Therefore, the research on the variation of the coefficient can provide theoretical basis for grasping the drying characteristics of materials, ensuring their drying quality, formulating drying technology and designing relevant drying equipment. The unsteady-state variation and drying characteristics of water effective diffusion coefficient and effective thermal conductivity of grain, block carrot particle and columnar apple particle were systematically studied. The main research contents are as follows: (1) Based on Fick's second diffusion law, the unsteady-state water content in hot air drying process of porous materials was established. The mathematical model of effective diffusivity and unsteady shrinkage model were used to study the variation law of unsteady water effective diffusivity and tissue shrinkage characteristics of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying. It is easy to evaporate and diffuse in the form of free water, and the effective diffusivity of water hardly changes; with the drying process, the free water in the material becomes less, the proportion of combined water increases, the difficulty of water evaporation and diffusivity increases, and the effective diffusivity of water decreases with the decrease of water ratio; finally, the effective diffusivity of water in the material becomes smaller. The effective diffusion coefficient of water does not change any more. The effective diffusion coefficient of water does not change any more. During the whole drying process, the effective diffusion coefficient of water is a gradually decreasing unsteady state change process, not a steady state constant value. (2) Based on Fourier heat balance law, a mathematical model of unsteady effective thermal conductivity of porous materials during hot air drying was established, and the effective thermal conductivity of different shapes (spherical, cylindrical, flaky) and different materials during hot air drying were calculated. The results show that the effective thermal conductivity in the whole drying process is a gradually decreasing unsteady state change process, not a steady state constant value, because the material evaporates with the diffusion of water to the outside, and the water decreases. In the initial stage of drying, the drying rate of the material is higher, the moisture content decreases rapidly, and the effective thermal conductivity of the material is higher than that of the air. When the drying rate enters the stage of deceleration drying, the change curve of thermal conductivity becomes gentle because of the decrease of drying rate and the slow diffusion of moisture. (3) Based on the unsteady heat and mass transfer characteristics of the porous materials, the coupling of mass and heat transfer among the cell layers in the spherical materials is established as a viscoelastic body. The unsteady viscoelastic stress model under the action of temperature gradient and humidity gradient are separated from the viscoelastic stress model. The results show that the viscoelastic stress model is composed of the thermal stress part caused by temperature gradient and the wet stress part caused by humidity gradient. The tangential stress and radial stress of the spherical material appeared two peaks during the drying process, and the first peak value was greater than the second one. In the initial stage of shear stress drying, the tensile and internal pressures change into the tensile and external pressures after the stress reverses; the curves of thermal stress and wet stress change with time are consistent with the total stress, and the wet stress value is greater than the thermal stress value at the same time, indicating that the humidity gradient is related to the drying stress during the drying process. (4) Finally, the coupling relationship between material shrinkage, diffusion, heat transfer and mechanical properties was studied by combining the unsteady physical parameters model of porous materials and the unsteady drying characteristics model. The results show that the influence of drying process on material drying characteristics is through changing physical parameters during drying process. The effective diffusion coefficient and effective thermal conductivity of water increase with the increase of temperature and rate of hot air, drying rate and stress increase with the increase of effective diffusion coefficient of water, and heat transfer rate increases with the increase of effective thermal conductivity. It is due to the fact that the volume of the material losing moisture is partly converted into the internal pores of the material and partly into the volume of the material contracting under the consideration of shrinkage, while the volume of the material losing moisture is completely converted into the pores without considering shrinkage, and the greater the internal pores of the material, the smaller the resistance of water diffusion and heat transfer.
【學位授予單位】：昆明理工大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TB383.4
【相似文獻】
相關期刊論文 前10條
1 孫師白;非穩(wěn)態(tài)轉化和沸騰轉化的比較[J];硫酸工業(yè);1992年01期
2 彭明華;非穩(wěn)態(tài)氧化法在前蘇聯(lián)硫酸生產(chǎn)中的應用[J];硫酸工業(yè);1992年03期
3 宋金甌,彭寶成,解茂昭,趙志廣;汽油機催化轉化器非穩(wěn)態(tài)模型的試驗研究[J];河北科技大學學報;2000年04期
4 肖文德,項紅,袁渭康,張瑞生;二氧化硫轉化的非穩(wěn)態(tài)操作研究[J];硫酸工業(yè);1991年02期
5 褚治德,白峰,劉嘉智,孟憲玲;非穩(wěn)態(tài)干燥動力學優(yōu)化實驗[J];天津大學學報;1993年03期
6 ＫａｚｕｈｉｒｏＡｒａｉ,王淑玲;鹿島煉鋼廠非穩(wěn)態(tài)板坯生產(chǎn)采用的先進技術[J];武鋼技術;1998年08期
7 殷德洪;蘇聯(lián)非穩(wěn)態(tài)氧化制酸法的新進展[J];硫酸工業(yè);1987年05期
8 王靜康,衛(wèi)宏遠,張遠謀;連續(xù)結晶過程非穩(wěn)態(tài)特性的研究[J];化工學報;1993年05期
9 ;非線性開環(huán)非穩(wěn)態(tài)聚合反應器的預報控制[J];化學反應工程與工藝;1992年01期
10 張海燕,邵善根,邵慶;半無限大介質中非穩(wěn)態(tài)擴散模型的精確解[J];化學工業(yè)與工程;2003年02期
相關會議論文 前8條
1 李浩明;謝晶;徐世瓊;趙蘭萍;;積分法用于非穩(wěn)態(tài)測定冷庫圍護結構傳熱系數(shù)的研究[A];上海市制冷學會一九九七年學術年會論文集[C];1997年
2 史敏;鐘瑜;張秀平;李炅;;多聯(lián)式空調(熱泵)機組非穩(wěn)態(tài)制熱性能試驗方法和要求的探討[A];第二屆中國制冷空調專業(yè)產(chǎn)學研論壇論文集[C];2013年
3 李紅霞;田保紅;林陽明;康布熙;劉平;;基于氧在固態(tài)銅中進行非穩(wěn)態(tài)擴散的動力學研究[A];第三屆中國熱處理活動周暨第六次全國熱處理生產(chǎn)技術改造會議論文專輯[C];2005年
4 賀運紅;陳兒同;王芳;;降溫過程中隔熱層非穩(wěn)態(tài)放熱試驗研究[A];上海市制冷學會二○○一年學術年會論文集[C];2001年
5 李中付;華宏星;宋漢文;傅志方;;一種非穩(wěn)態(tài)環(huán)境激勵下線性結構的模態(tài)參數(shù)辨識方法[A];第八屆全國振動理論及應用學術會議論文集摘要[C];2003年
6 陳生;;基于邊界元方法求解非穩(wěn)態(tài)熱傳導/擴散正問題[A];2008鐵路暖通空調學術年會論文集[C];2008年
7 李鎮(zhèn);劉承軍;孫群;姜茂發(fā);;非穩(wěn)態(tài)澆鑄條件下連鑄板坯潔凈度及夾雜物的控制研究[A];第十八屆（2014年）全國煉鋼學術會議論文集——S07：非金屬夾雜物控制[C];2014年
8 于曉菲;陳景文;喬顯亮;王震;楊鳳林;;非穩(wěn)態(tài)多介質環(huán)境模型在突發(fā)性環(huán)境污染事故環(huán)境影響預測中的應用——以松花江硝基苯污染事故為例[A];中國環(huán)境科學學會2006年學術年會優(yōu)秀論文集（中卷）[C];2006年
相關博士學位論文 前1條
1 劉國紅;多孔材料非穩(wěn)態(tài)物性參數(shù)及干燥特性研究[D];昆明理工大學;2015年
相關碩士學位論文 前6條
1 王玉娟;非穩(wěn)態(tài)紡織復合保溫材料導熱性能的研究[D];西安工程大學;2015年
2 張瑤;織物非穩(wěn)態(tài)隔熱性能測試方法的研究[D];東華大學;2006年
3 周瑤;輪胎復合滑移非穩(wěn)態(tài)半經(jīng)驗模型研究[D];吉林大學;2014年
4 何媛媛;三維非穩(wěn)態(tài)熱傳導方程的虛邊界元解法[D];重慶大學;2009年
5 王紅曼;非穩(wěn)態(tài)及泛傅立葉效應傳熱計算新模型及其程序化[D];燕山大學;2014年
6 李慧;三維非穩(wěn)態(tài)熱傳導邊界元方法研究及數(shù)值系統(tǒng)開發(fā)[D];山東科技大學;2011年
，本文編號：2191764
本文鏈接：http://sikaile.net/kejilunwen/cailiaohuaxuelunwen/2191764.html
最近更新
教材專著
