Analysis of Heat and Mass Transfer in Drying Process of Polymer Solutions Using Infrared Radiation Heating
|關鍵字:||熱輻射;溶液乾燥;頻譜效應;熱毛細流;參數逆運算;光學常數;thermal radiation;solution drying;spectral effect;thermocapillary flow;estimation of parameters;optical constants|
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.
|Appears in Collections:||Thesis|