紅外線熱輻射加熱高分子/水溶液乾燥過程的熱質傳分析

Abstract

本文針對高分子/水溶液受高強度紅外線熱輻射及/或空氣流加熱乾燥時，其熱質傳現象進行理論分析。首先針對溶液被覆於一光學厚度極大的基材上，受高強度紅外線熱輻射及/或空氣流加熱時，分析不同熱輻射和熱對流參數對乾燥速率的影響，並藉由乾燥速率、熱傳遞和濃度場分佈表示此結果。數值結果顯示實際被溶液和基材吸收的熱輻射能量和溶液中的濃度場分佈，在乾燥過程中佔很重要的地位。為考慮紅外線熱源頻譜以及受熱介質之頻譜輻射熱性質的效應，本文引用「有效輻射性質」使簡化輻射傳遞方程式的數值計算過程以獲得正確數值解；文中高分子/水溶液的非灰體性質由普朗克和羅森連平均吸收係數的幾何平均數表示。當考慮二維高分子/水溶液乾燥問題時，主要在探討非均勻的自由表面溫度梯度所引起熱毛細流對乾燥特徵的影響。針對此一問題，本文利用理論方法分析高分子/水溶液受不均勻的紅外線輻射加熱乾燥時，溶液熱毛細現象對乾燥速率的影響；另外將不同參數對乾燥速率和熱、流和濃度場的分佈之影響做一詳細的分析。結果顯示：熱毛細流效應可將溶液底部的水分帶到液面的附近，提升液面的水濃度，則可減短溶液乾燥過程所需的時間；相較於溶液限制於固體壁面之間以熱輻射加熱液面的情況，當考慮溶液的左右邊界皆為對稱面時，減少了固體壁面減低熱毛細流強度的效應，乾燥速率則較大。 由於水溶液輻射熱性質對熱質傳的影響非常大，而其輻射熱性質又可由其光學折射率直接表示，所以瞭解高分子薄膜光學折射率的大小，是分析紅外線乾燥過程的先決條件。由於文獻上欠缺PVA完整的相關資料，本文針對熱輻射吸收性的薄膜被覆在一熱輻射吸收性的基材上，建立理論分析模式和實驗量測方法，以同時獲得此薄膜的複數型光學折射率和其厚度。探討薄膜受一傾斜入射之非極化光源照射下，不同參數變化對其穿透率和反射率的影響；進而建議最佳的實驗量測組合。結果顯示：利用正向穿透率、大角度和接近正向的反射率做參數逆運算，可避免錯誤的數值解，並提升逆運算參數值之精確度。當增加一任意角度的反射率實驗值做逆運算時，則可能避免遭遇到多重解的問題。針對薄膜紅外線光學性質之量測建立量測理論，量測方法中利用了最小平方誤差法以逆運算得到薄膜在不同頻譜下的複數型光學折射率和其厚度。並實際以實驗方法運用高分子材質PVA研製薄膜，量測不同角度下薄膜的穿透率和反射率，並配合量測理論進行光學常數及厚度等參數之估算；另外，此實驗之完成使PVA之頻譜光學常數之資料更臻完整。

A theoretical study is performed that describes the heat transfer and moisture variation while the polymer solution is exposed to high-intensity infrared radiation flux and/or an airflow. Firstly, we analyze the heat and mass transfer of a polymer solution on an optically thick substrate under infrared radiant heating. During tempering stage in the intermittent heating process, the convective mass transfer is included to simulate the ambient air in reality. The effects of radiation and convection parameters on the transfer processes are presented in terms of the rate of water content removal, heat transfer and the moisture distributions. Numerical results show that both the amount of absorbed radiant energy and the distributions of water mass fraction in the polymer solution dominate on the rate of water removal of polymer solution during the processes. In regard to the drying processes by radiation heating, accurate estimation of radiative transfer is not possible unless the spectral variation of radiation properties is taken into account. To simplify the solving for obtaining the accurate numerical results, appropriate values of effective properties may be utilized. Radiating nongrayness of the PVA solution is considered and is prescribed by the square root of products of the Planck and Rosseland mean absorption coefficients of both water and PVA. The effective absorption coefficient of PVA solution is then introduced and incorporated into the radiative transfer analysis. Results are compared to those of drying process by convective heat. It is shown that the use of thermal radiation combined with convective heat would improve the drying rate. Pure radiation heating, although is straightforward, is not efficient. While focusing on the two-dimensional problem of polymer-solution drying, we consider the thermocapillary effect on drying characteristics, which is induced by a non-uniform temperature distribution along the free surface. Secondly, we analyze the two-dimensional heat and mass transfer characteristics arising from the non-uniform radiant incidence in polymer solutions whose motion is governed by combined thermocapillary/buoyancy laminar flow. In addition, the various radiation inputs, external convective parameters, and important physical properties of solutions that affect drying characteristics are investigated in terms of flow, thermal and concentration fields, and rate of water removal. Thermocapillary flow carries more water content from the interior region to the free surface and thus leads to a shorter drying time required for solution. Furthermore, removing the solid wall effect reduces the drying time required for a solution film with left and right surfaces of symmetry, which is irradiated by an infrared radiant heaters. To analyze heat and mass transfer processes of polymer solutions by infrared radiant heating, one is most interested in the absorption of energy by polymer solutions and also in the effect of the reflection at interface; and the simplest way to examine absorption is to know the complex refractive index of polymer. We describe a technique and also illustrate an application of the technique for simultaneously measuring both the spectral complex refractive index and the thickness of an absorbing thin solid layer. The contours of constant reflectance and transmittance in the , , and planes are examined for a thin layer on a thick substrate, which is exposed to oblique unpolarized radiant incidence, under various conditions in order to facilitate an optimal choice of the combination of measured quantities for inverse estimation of parameters. Theoretical analysis illustrates that optimal choices would include measurement of at a large angle of incidence, combined with measurements of normal and near normal so as to reduce erroneous solutions or nonconvergence. Any additional measurement at any angle of incidence may be used to prevent the multiple solutions. For inverse estimation of parameters, we also present a technique in association of the least squares method to extract the optical constants and thickness characterizing the thin absorbing film from measurements of and . The method is then applied for experimental measurement of the radiant properties and thickness of a polyvinylalcohol (PVA) film placed upon a substrate of ZnSe.