散裝穀倉於通風過程之熱質傳分析

Abstract

本研究目的為建立一數學模式模型探討散裝穀倉於通風過程中的熱質傳現象，利用有限元素分析軟體COMSOL Multiphysics 4.2a電腦模擬軟體，考慮空氣流動由於有重力作用，空氣阻力常數為非等向性，且稻穀於貯藏時進行的呼吸作用等條件下，模擬通風過程下散裝穀倉空氣的溫濕度變化。目的在探討散裝穀倉於貯藏時實施通風作業時之風速、溫度、濕度分佈情形，找尋適當的通風時間以及通風量大小，並配合過去文獻實驗數據進行模型的驗證。綜合本研究的結果分析，於通風過程中，穀倉內空氣流速大小於入風口附近變化較劇烈，在一定高度後其風速幾乎維持一定值，受穀物高度影響較小。越接近入風口位置的空氣溫濕度變化在短時間內下降越明顯，且中上層穀物通風時因為下層穀物的熱量被帶上來導致在通風初期產生升溫現象，由模擬結果圖也可看出若穀倉內溫溼度與入風口溫溼度溫差越大，溫度與稻穀含水率的下降速率也越大。相同的入風溫溼度條件下，一段時間後空氣溫溼度的下降速率由於空氣流體有空氣對流項的關係會比稻穀溫溼度的下降速率來的快。本研究建立的數學模式可以配合國內現有的散裝穀倉形狀尺寸，模擬分析於實際通風操作下空氣的流動速度分布、倉內空氣的溫度、相對濕度與稻穀溫度、含水率等性質的變化，期盼本模型能提供農會一參考依據，使其更能有效掌握散裝穀倉內稻穀品質的變化。

The purpose of this study is to develop a mathematical model of heat and mass transfer in grain silo under aeration cooling and during storage for applications in grain silo storage operations. In order to predict the variations of temperature and moisture content of paddy in grain silo and surrounding air quality under process, FEM software COMSOL Multiphysics 4.2a was used in this study. Brinkman equation was used to module air flow with anisotropic resistance in porous of grain. Four conjugated convection-conduction equations were used to model heat and mass transfer process in air as well as in grain respectively. The heat and water generation in grain due to respiration and transport process of heat and water between grain and air were also considered. The model depicted both natural convection and force convection of energy and water in the silo during different operations. Simulation data of the model were validated by paddy aeration cooling experimental data from previous studies in our laboratory with a very good consistence. Various cooling aeration scenarios were investigated in the studies to find a better cooling operation protocols. That included various cooling air temperatures and humanities, hot spots cooling procedure, high humidity aeration, aeration after long term storage. In practice, grain moisture loss was one of a major drawback in aeration cooling, since grain temperature was always higher than air, air temperature increased and relative humidity decreased during flow through grain, in the mean time more water was absorbed by air. The preliminary simulation results showed air humidity control was essential. Through simulation a better humidity of cooling air controlled procedure was found, that would result in less weight loss of grain after cooling. Also, it is feasible to pump in saturated cooling air during grain aeration, that will benefit less grain moisture loss and better moisture uniformity. The model developed in the study will provide a good management tool for grain silo aeration cooling operations.