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1.
In this paper we have examined the influence of gravity on the moisture transport process during the isenthalpic drying period, and we have considered the use of diffusion models both to predict saturation pro- files and to extract apparent diffusivities from experi- mental data. For granular or unconsolidated porous media, the one-dimensional moisture transport process can be characterized by two dimensionless groups that account for capillary forces, gravitational forces and viscous forces. Detailed numerical solutions of the saturation transport equation indicate under what circumstances the diffusion model can be used with confidence, and under what circumstances the diffusion model can be used to predict saturation profiles even though it is an incorrect representation of the moisture transport process. In addition to exploring the predictive capabilities of the diffusion model, we have 相似文献
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Stefan Jan Kowalski 《Drying Technology》2013,31(3):453-482
ABSTRACT Aspects of the thermomechanics of fluid-saturated porous solids relating to the drying process of materials are discussed in the paper. From the balance equations and the principles of thermodynamics of irreversible processes, the differential equations describing the drying body deformations and induced stresses, the temperature, and the moisture content distributions are developed. The porous body is assumed to be elastically deformed and the pore space is filled with an immiscible liquid-gas mixture-the moisture. The phase transitions of liquid into gas are taken into account 相似文献
4.
ABSTRACT The diffusion of water vapour through pulp and paper was investigated experimentally in a diffusion cell. The diffusion cell consisted of two chambers differing in relative humidity. Any given sample was placed as a banier between the two chambers. Two pulp qualities and eleven paper grades were included in the investigation. The diffusivities measured ranged from 2.1.10-8m2/s to 5.4.10-6m2/. Effective vapour diffusivity was found to correlate with the density of the sheet. For five of the paper or pulp grades measurements were performed a t different temperatures and relative humidities. In the range of relative humidities investigated, effective vapour diffusivity was found to be independent of relative humidity. 相似文献
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M. Prat 《Drying Technology》2013,31(5):1181-1208
TThis paper is essentially devoted to the study of the interfacial coefficient problem and to a characterization of the falling rate period.By means of a 2D numerical simulation, it is shown that the falling rate period is characterized by strong humidity heterogeneities at the interface. These heterogeneities affect significantly the interfacial mass transfer coefficient to be used in the traditional ID approach of drying. In this paper. the heterogeneities are due to the coupling between external and internal transfer in a boundary layer flow leading edge region.0ther phenomena that may also cause the occurence of humidity heterogeneities at the interface are discussed. 相似文献
6.
M. Prat 《Drying Technology》1991,9(5):1181-1208
TThis paper is essentially devoted to the study of the interfacial coefficient problem and to a characterization of the falling rate period.By means of a 2D numerical simulation, it is shown that the falling rate period is characterized by strong humidity heterogeneities at the interface. These heterogeneities affect significantly the interfacial mass transfer coefficient to be used in the traditional ID approach of drying. In this paper. the heterogeneities are due to the coupling between external and internal transfer in a boundary layer flow leading edge region.0ther phenomena that may also cause the occurence of humidity heterogeneities at the interface are discussed. 相似文献
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ABSTRACT Drying experiments are carried out in two-dimensional etched networks under quasi-isothermal conditions. The evolution of phase distributions within the network are visualized and compared to numerical discrete simulations. The phase evolution determined by numerical simulations agrees very well with experimental results. However, the comparison between numerical and experimental drying kinetics does not show good agreement. The differences are explained by the presence of liquid films on the micromodel walls and edges during drying. These films are not taken into account in the numerical model. 相似文献
8.
《Drying Technology》2013,31(7):1287-1303
The effects of operational conditions on the drying performance in closed superheated steam drying were examined theoretically and experimentally. The vapor generated from the sample was circulated in the drying chamber. In the theoretical analysis, the replacement of air with vapor in drying chamber and the convective vapor transfer in sample were considered. At the start of drying, the drying chamber was filled with air. As the drying proceeded, the air was replaced with the vapor generated from sample. The calculated results explained the characteristics of experimental data. The pore diameter of sample had little effect on the drying characteristics. During the internal evaporation period, the evaporation occurred in the narrow zone, which moved from the surface to the bottom of sample. The convective vapor transfer in sample had a significant influence on the drying performance. The excess increments in temperature and velocity of drying gas hardly contributed to shortening the drying time. 相似文献
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Yuji Tatemoto Yoshiyuki Bando Koji Oyama Keiji Yasuda Masaaki Nakamura Yasuhiro Sugimura Masahito Shibata 《Drying Technology》2001,19(7):1287-1303
The effects of operational conditions on the drying performance in closed superheated steam drying were examined theoretically and experimentally. The vapor generated from the sample was circulated in the drying chamber. In the theoretical analysis, the replacement of air with vapor in drying chamber and the convective vapor transfer in sample were considered. At the start of drying, the drying chamber was filled with air. As the drying proceeded, the air was replaced with the vapor generated from sample. The calculated results explained the characteristics of experimental data. The pore diameter of sample had little effect on the drying characteristics. During the internal evaporation period, the evaporation occurred in the narrow zone, which moved from the surface to the bottom of sample. The convective vapor transfer in sample had a significant influence on the drying performance. The excess increments in temperature and velocity of drying gas hardly contributed to shortening the drying time. 相似文献
10.
S. Pang 《Drying Technology》2013,31(1-2):271-281
ABSTRACT The influence of vapour diffusion on the drying rate of a softwood board has been examined for drying temperatures varying from 60°C to 140°C. It is found that for very low temperature drying a model which considers both vapour convective and diffusion in wood predicts dry-rate curves matching the experimental data closely. For high temperature drying, both of the above drying model and a drying model which considers only vapour convective flow give predictions in agreement with the observed data. This illustrates that the diffusion of vapour and air is important in low temperature drying panicularly during the late stages of drying. However, for high temperature drying, the convective flow of moisture vapour is dominant and the diffusion component is negligible. The observation provides evidence for simplifying a drying model for high temperature drying without reducing its credibility in predicting drying rate curves. 相似文献
11.
本文在尘气传递模型的基础上,开发了在微机上实现的模拟单粒催化剂多组份多反应体系反应扩散过程的通用软件。它可用于横拟速率方程为任意形式的,扩散系数随温度、浓度变化的,不同形状颗粒、粒外有或无扩散效应时等温或非等温的反应扩散过程。本软件具有管理功能强、模型精度高、算法先进以及适用性广等特点,并配有彩色图形输出,是辅助催化剂设计和分析反应器性能的有效工具。 相似文献
12.
ABSTRACT In this work a sophisticated numerical model is presented that describes the drying of porous media. This model, which is known as TransPore, has evolved over the years through the direct inputs of both authors. Nowadays, TransPore can be used to analyse the drying of media that are of completely arbitrary shape and size, under a variety of drying conditions. The engine of the computational model uses a number of state-of-the-art numerical methods that ensure the simulation results describe the particular drying process accurately, whilst guaranteeing the most efficient and effective usage of computer resources. For example, the numerical discretisation method is based on a completely conservative hybrid finite element control volume technique that uses a finite element mesh for its background gradient interpolation. Furthermore, flux limiting is used to reduce numerical dispersion in the drying kinetics and the generated non-linear system is resolved using the full Newton method for the outer iteration coupled together with a preconditioned conjugate gradient technique for the inner iteration. A graphical interface has been linked to the model to enable online visualisation of the drying process. The mathematical model allows both homogeneous and heterogeneous porous media to be simulated. The resultant software is an extremely powerful and effective tool for investigating existing dryer designs and for proposing new and innovative drying schedules that provide optimal drying quality in minimal drying time. 相似文献
13.
Gang Guo Guangli Liu Karsten E. Thompson 《Chemical Engineering Communications》2003,190(12):1641-1660
Many engineering problems require the estimation of mass transfer coefficients in porous materials. In heterogeneous materials or in cases where mass transfer sites are not spatially uniform, empirical equations for mass transfer coefficients vary widely, and the origin of these differences is not well understood. In this article, we use a stochastic algorithm to model mass transfer from single particles in a two-dimensional heterogeneous packed bed. The computed mass transfer coefficients are used to generate a distribution of local Peclet numbers in the bed. Detailed hydrodynamics are then used to interpret variations in the local Peclet number. The results show clear relationships between pore structure, streamline patterns, and mass transfer rates. 相似文献
14.
In this work a sophisticated numerical model is presented that describes the drying of porous media. This model, which is known as TransPore, has evolved over the years through the direct inputs of both authors. Nowadays, TransPore can be used to analyse the drying of media that are of completely arbitrary shape and size, under a variety of drying conditions. The engine of the computational model uses a number of state-of-the-art numerical methods that ensure the simulation results describe the particular drying process accurately, whilst guaranteeing the most efficient and effective usage of computer resources. For example, the numerical discretisation method is based on a completely conservative hybrid finite element control volume technique that uses a finite element mesh for its background gradient interpolation. Furthermore, flux limiting is used to reduce numerical dispersion in the drying kinetics and the generated non-linear system is resolved using the full Newton method for the outer iteration coupled together with a preconditioned conjugate gradient technique for the inner iteration. A graphical interface has been linked to the model to enable online visualisation of the drying process. The mathematical model allows both homogeneous and heterogeneous porous media to be simulated. The resultant software is an extremely powerful and effective tool for investigating existing dryer designs and for proposing new and innovative drying schedules that provide optimal drying quality in minimal drying time. 相似文献
15.
Yuji Tatemoto Yoshiyuki Bando Keiji Yasuda Yoshitaka Senda Masaaki Nakamura 《Drying Technology》2001,19(7):1305-1318
The drying characteristics of porous material in fluidized bed were examined theoretically and experimentally. The brick ball was used as the sample and immersed in the fluidized bed. The glass beads were used as the fluidizing particles and the particle diameters were changed. When the pore diameter of sample was relatively large, the fluidizing particles were adhered on the sample surface. In the theoretical analysis, the heat and mass transfers in adhered particle layer were considered. The fluidizing particles were adhered on the sample surface during the earlier period of drying. The sample temperature largely decreased when the mass of adhered particle decreased. The calculated results are in good agreement with experimental data. The diameter of fluidizing particle had a small effect on the drying time. The excess increments in drying gas temperature hardly contributed to shortening the drying time. 相似文献
16.
From the mathematical formulation presented in part I, a numerical code is developed to simulate heat and: mass transfers in porous media. The aim of this· tool is to understand and to improve each process related to drying. The association of a comprehensive set of equations with a efficient 2-D computer code allows us to predict the comportment of several porous media even if submitted to severe drying conditions. A few runs have been selected with special attention paid to the effect of internal gaseous pressure: Convective drying of softwood at high temperature illustrate the typical two-dimensional transfers that occur in an anisotropic medium. Microwave drying of light concrete pinpoints liquid expulsion of water which is driven by the pressure due to internal heating. Finally, appropriate physical behaviours of a bed of glass spheres allows one to deal with simple processes for which full saturation occurs. 相似文献
17.
PROCESSES RELATED TO DRYING: PART II USE OF THE SAME MODEL TO SOLVE TRANSFERS BOTH IN SATURATED AND UNSATURATED POROUS MEDIA 总被引:1,自引:0,他引:1
From the mathematical formulation presented in part I, a numerical code is developed to simulate heat and: mass transfers in porous media. The aim of this· tool is to understand and to improve each process related to drying. The association of a comprehensive set of equations with a efficient 2-D computer code allows us to predict the comportment of several porous media even if submitted to severe drying conditions. A few runs have been selected with special attention paid to the effect of internal gaseous pressure:
Convective drying of softwood at high temperature illustrate the typical two-dimensional transfers that occur in an anisotropic medium.
Microwave drying of light concrete pinpoints liquid expulsion of water which is driven by the pressure due to internal heating.
Finally, appropriate physical behaviours of a bed of glass spheres allows one to deal with simple processes for which full saturation occurs. 相似文献
Convective drying of softwood at high temperature illustrate the typical two-dimensional transfers that occur in an anisotropic medium.
Microwave drying of light concrete pinpoints liquid expulsion of water which is driven by the pressure due to internal heating.
Finally, appropriate physical behaviours of a bed of glass spheres allows one to deal with simple processes for which full saturation occurs. 相似文献
18.
ABSTRACT Fish crackers are semi-prepared starch-based products which must be expanded just prior to consumption. This work aims at the experimental and theoretical study of the thin-layer drying process of fish crackers in a thin disc geometry. The process was carried out in an insulated cabinet dryer and the experimental kinetic drying data were fitted using the analytical solutions resulting from both a drying model governed by the Fick' s second law and a model based on the lumped system, highlighting the adequacy of each of them. Therefore, it was possible to calculate the mass transfer coefficients for seven different blends offish crackers. Also, the effect of the drying time on the bulk density of the puffed product was evaluated for these seven blends. Both the kinetic drying parameters and the optimal bulk density responses were analyzed using the Simplex-Lattice Design over a mixture triangle within a minimal number of experiments. 相似文献
19.
This work is focused on the numerical solution of steady boundary-layer stagnation-point flow of a polar fluid towards a stretching surface embedded in porous media in the presence of the effects of Soret and Dufour numbers and first-order homogeneous chemical reaction. The governing boundary-layer equations of the problem are formulated and transformed into a self-similar form. The obtained equations are solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method. Both assisting and opposing flow conditions are considered. Comparisons of the present numerical results with previously published work under limiting cases are performed and found to be in excellent agreement. Representative results for the fluid velocity, angular velocity, temperature, and solute concentration profiles as well as the local heat and mass transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms. 相似文献