發(fā)布時間：2018-09-13 11:49

【摘要】：高水分植物細胞物料的干燥過程中,物料失去的水分主要來源于封閉的細胞�，F(xiàn)有模型和理論對細胞封閉結構在整個干燥過程中的變化以及細胞封閉結構在這個過程中扮演的角色缺乏清晰完整的描述。本文從亞細胞水分傳輸出發(fā),建立了關于高水分植物細胞物料低溫對流干燥的等溫水分傳輸模型,模型中表觀水分傳輸參數(shù)由組織微觀參數(shù)算得。最后利用模型對干燥過程進行了研究。通過對低溫對流干燥過程中細胞結構變化的分析,認為整個干燥過程中都可以把組織看成是由細胞構成的,并提出了對整個干燥過程都適用的模型細胞,模型細胞保留了真實細胞中必要的、參數(shù)可測的亞細胞結構。模型細胞構成了植物細胞組織概念模型。利用場發(fā)射冷凍電鏡觀察得到了馬鈴薯細胞的幾何尺寸參數(shù)。給出了組織模型各部分水分狀態(tài)和水分傳輸規(guī)律。為獲得組織表觀傳輸參數(shù)與微觀參數(shù)之間的關系,建立了理想化立方體薄壁細胞串上細胞腔相和細胞壁相的水勢傳輸方程。假設細胞腔內水勢分布為線性,得到了水分從細胞中心到細胞壁傳輸?shù)囊约皬募毎坏郊毎粋鬏數(shù)陌毎粌葦U散效應的水導系數(shù)。通過兩相方程的尺度分析以及對細胞串上微觀水分傳輸?shù)哪M,得到了以下結論：薄壁細胞的結構特點使得細胞腔和細胞壁之間水勢趨于平衡,干燥過程中細胞腔溶液水分擴散系數(shù)的減小會破壞局部平衡；當馬鈴薯細胞干基含水率不低于1時,細胞的細胞腔和細胞壁之間水勢平衡,存在細胞腔到細胞腔的水分傳輸；干基含水率從1至0.32的過程中,局部平衡狀態(tài)逐漸不再成立,細胞腔到細胞腔的水分傳輸終止。干燥過程中若細胞腔和細胞壁之間局部水勢平衡,則細胞間隙和細胞也保持局部水勢平衡。在局部平衡的條件下得到了組織表觀水導系數(shù)和組織微觀參數(shù)之間的關系模型。假設整個干燥過程中細胞腔、細胞壁和細胞間隙之間局部水勢平衡,利用上文得到的組織表觀水導系數(shù)模型建立了平板狀物料干燥的一維等溫水分傳輸和收縮模型。討論了組織一維收縮效應,認為可以假設垂直于主流方向的截面上各相面積分數(shù)在干燥過程中保持不變,并獲得了迂曲度系數(shù)和收縮系數(shù)的關系。把組織水分傳輸方程轉化為由參考坐標描述的水分擴散方程,得到了等效擴散系數(shù)與組織收縮、組織水導系數(shù)以及組織水容的關系式。為驗證組織水分傳輸模型的正確性,進行了熱風溫度為40℃的平板狀馬鈴薯組織的干燥實驗,并利用建立的擴散方程對干燥過程進行了模擬。模擬和實驗結果表明：模型預測的干燥曲線在高水分段(平均含水率高于1)與實驗吻合得較好,當組織平均含水率小于1.5時,模型預測的干燥速率明顯比實驗值要大。該結果表明假設整個干燥過程中細胞局部水勢平衡使得模型高估了細胞腔到細胞腔水分傳輸存在的時間,進而高估了組織的干燥速率。模型預測表明：馬鈴薯干燥過程中物料內部的水分傳輸主要是液態(tài)擴散、跨膜傳輸和細胞壁中的毛細水分流動,細胞間隙中的蒸氣擴散在高水分段可以忽略不計；影響干燥過程的主要微觀參數(shù)是細胞各部分的水分傳輸參數(shù),幾何尺寸的影響相對來說較小。因此正確測定細胞各部分水分傳輸參數(shù)對揭示干燥過程中組織內水分傳輸機理至關重要。為完整描述整個干燥過程,需提出能考慮非局部平衡狀態(tài)下水分傳輸?shù)哪Ｐ汀?br/>[Abstract]:In the drying process of high-moisture plant cell materials, the loss of water mainly comes from the closed cells. The present models and theories lack a clear and complete description of the changes of the closed cell structure and the role of the closed cell structure in the whole drying process. An isothermal moisture transfer model for high moisture plant cell materials during low temperature convective drying was established. The apparent moisture transfer parameters in the model were calculated from the microstructure parameters. Finally, the drying process was studied by using the model. Tissue is considered to be composed of cells, and a model cell suitable for the whole drying process is proposed. The model cell retains the necessary and measurable subcellular structure of the real cell. The model cell constitutes the conceptual model of plant cell tissue. The geometric size parameters of potato cells are obtained by field emission freezing electron microscopy. In order to obtain the relationship between the apparent transport parameters and the microscopic parameters of the tissue, the water potential transfer equations of the cell lumen and cell wall phases on the idealized cubic parenchyma cell string are established. Assuming the water potential distribution in the cell lumen is linear, the water flow from the cell center to the cell wall is obtained. The water conductivity of cell wall transport and that of cell wall transport from cell cavity to cell cavity including intracellular diffusion effect. Through the scale analysis of two-phase equation and the simulation of micro-water transport on cell string, the following conclusions are obtained: the structural characteristics of parenchyma cells make the water potential between cell cavity and cell wall tend to balance, and the drying process. When the water content in the cell lumen is not less than 1, the water potential between the cell lumen and cell wall is balanced and there is water transfer from the cell lumen to the cell lumen. The water transport in the cell lumen terminates. If the local water potential between the cell lumen and cell wall is balanced during drying, the cell gap and cell also maintain the local water potential equilibrium. A one-dimensional isothermal water transport and shrinkage model for plate material drying is established by using the model of apparent water conductivity obtained above. The effect of one-dimensional shrinkage of tissue is discussed. It is assumed that the area fraction of each phase on the cross-section perpendicular to the main direction remains unchanged during drying. The relationship between tissue water transfer equation and tissue shrinkage, tissue water conductivity coefficient and tissue water capacity was obtained by transforming tissue water transfer equation into water diffusion equation described by reference coordinates. The results of simulation and experiment show that the drying curve predicted by the model is in good agreement with the experiment in the high moisture section (the average moisture content is higher than 1). When the average moisture content of the tissue is less than 1.5, the drying rate predicted by the model is obviously higher than the actual drying rate. The results show that the model overestimates the time of water transport from the cell cavity to the cell cavity, and then overestimates the drying rate of the tissue. The model predicts that the water transport in the material during potato drying is mainly liquid diffusion, transmembrane transport and fineness. The capillary water flow in the cell wall and the vapor diffusion in the intercellular space can be neglected in the high water section; the main microscopic parameters affecting the drying process are the water transport parameters of the cell parts, and the geometric size has relatively small influence. In order to describe the whole drying process completely, it is necessary to propose a model which can consider the moisture transfer in non-local equilibrium state.
【學位授予單位】：中國農業(yè)大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：S375

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本文編號：2241089