Heat and Mass Transfer Processes: New Developments and Applications


doi: 10.4028/www.scientific.net/DF.3

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作者: E.M.A. Pereira, J.V. Silva, T.H.F. Andrade, S.R. de Farias Neto, A.G. Barbosa de Lima
摘要: The objective of this study was to investigate numerically heat and mass transport during drying of grains with particular reference to bean and rough rice. The proposed mathematical models based on the Fick’s and Fourier’s Laws consider constant physical properties and convective boundary condition at the surface of the solid. The solutions of the governing equations were performed using ANSYS CFX® software. The grains were regarded as an ellipsoid of revolution. Results of the drying and heating kinetics and moisture content and temperature distributions in the grains along the drying process are presented and analyzed.
作者: J.M.P.Q. Delgado
摘要: A method is presented for determining the coefficient of transverse dispersion in flow through packed beds, which is based on the measurement of the rate of dissolution of planar or cylindrical surfaces, buried in the bed and aligned with the flow direction. The underlying theory is initially explained and experiments are then described in which more than three hundred new data points were obtained. These data are for the flow of water, at interstitial velocities between 0.1 and 1.5 mm/s, through beds of silica sand with average particle sizes between 0.219 and 0.496 mm. The experiments were performed at a range of temperatures, between 20oC and 35oC, and this yielded dispersion data for values of the Schmidt number (Sc=μ /ρDm) between 1170 and 570. For all the data reported, the ratio between the coefficient of transverse dispersion and the coefficient of molecular diffusion was shown to correlate well with the Reynolds number (Re=udρ /μ), both for beds with narrow and with wide particle size distributions.
作者: João M.P.Q. Delgado, M. Vázquez da Silva
摘要: In this work, consideration is given to the problem of dissolution of a buried solid sphere in the liquid flowing uniformly through the packed bed around it. The differential equations describing fluid flow and mass transfer by advection and diffusion in the interstices of the bed are presented and the method for obtaining their numerical solution is indicated.From the surface concentration fields, given by the numerical solution, the concentration boundary layer thickness as a function of the relevant parameters were undertaken. Mathematical expressions that relate the dependence with the Peclet number and d/d1 ratio of an immersed active sphere are proposed to describe the approximate size of the concentration boundary layer thickness.
作者: I. Malico, P.J.S.A. Ferreira de Sousa
摘要: This work presents the extension of a compact finite difference immersed boundary method for the detailed calculation of fluid flow and heat transfer in porous media. The unsteady incompressible Navier-Stokes and energy conservation equations are solved with fourth-order Runge-Kutta temporal discretization and fourth-order compact schemes for spatial discretization, which allows achieving highly accurate calculations. Verification proves that the method is higher than third-order accurate. Three test cases were used for the validation of the method: (i) isothermal flow around a square cylinder in a plane parallel channel, (ii) isothermal flow through an infinite row of square cylinders and iii) flow and heat transfer around a square cylinder in a plane parallel channel. The validation tests establish confidence in the application of the method to porous media. As an example of such an application, direct numerical simulations are conducted for a staggered array of equal size square cylinders. Although the problem is rather complex from the geometrical point of view, a Cartesian grid is employed, with all its advantages. The potential of applying an immersed boundary method to the solution of a multiphase problem with complex internal boundaries is demonstrated.
作者: V.C. de Almeida Cruz, J.M.P.Q. Delgado, A.G. Barbosa de Lima, M.M. Silva Nóbrega, L.H. de Carvalho, W.S. Cavalcanti
摘要: This paper presents a theoretical and experimental study about water absorption in unsaturated polyester polymer composites reinforced with vegetable fibers, with particular reference to macambira fiber. A mathematical modeling based on the liquid diffusion theory has been proposed and numerical procedures using the finite volume technique are presented and discussed. Results of the water absorption kinetic and moisture content distribution for the polymer composites are shown and analyzed. The knowledge of moisture distribution inside the composite is essential for determination of areas that may show delamination problems (moisture induced degradation) due to the weakness of the fiber-matrix interface and consequently reduction in the mechanical properties of the composites.
作者: Jozef Kačur, Jozef Minár
摘要: In this paper we present a method for the determination of the hydraulic permeability for flow in partially saturated porous media. The dependence of hydraulic permeability on effective saturation is not assumed to be a member of any specific finite dimensional class of functions (e.g. vanGenuchten-Mualem, Burdin-Mualem, Brook-Corey). Instead, an infinite dimensional space of functions with limited a priori assumptions (e.g. smoothness, monotonicity) is considered. Consequently, we face a more challenging problem compared to the finite-dimensional case, in which only few tuning parameters need to be determined. We consider the case of 1D unsaturated flow and assume that the data are collected at the outflow of the sample. The hydraulic permeability is determined in an iterative way. We minimize the cost functional reflecting the discrepancy between the measured and computed data. In doing so, we use the Gateaux differential to obtain the direction of the descent.
作者: E. Barreira, J.M.P.Q. Delgado, V.P. de Freitas
摘要: Moisture is one of the most deteriorating factors of buildings. The moisture content depends on hygroscopic equilibrium between buildings materials and environment, which is determined by the drying and wetting rate of masonry. So, the moisture content is not only determined by the water that is absorbed by the material, but also by the amount of water that is evaporated under favourable conditions, which is described by the drying process.This work presents the results of an experimental evaluation of the drying kinetics of solid red brick, considering the bulk moisture transfer. The drying kinetics was assessed considering different environment conditions (air temperature and humidity). The experimental results showed that the drying flux is extremely dependent of temperature and relative humidity in the first stage of the drying process. The values obtained were between 0.025 kg/m2h (T=15oC and RH=80%) and 0.135 kg/m2h (T=30oC and RH=50%), for the worst and better drying ambient conditions.Different first-order kinetics models, available in the literature, were adjusted to describe the drying process and estimate the equilibrium moisture content of the samples. The results point that Midilli et al. model allows the best fit and that the drying time constant is strongly affected by the drying air conditions. It was also estimated the apparent molecular diffusion coefficient for solid red brick samples and its variation with temperature.
作者: R.M.S.F. Almeida, P. Paula, A. Santos, N.M.M. Ramos, J.M.P.Q. Delgado, V.P. de Freitas
摘要: This work consisted of two main lines of research: one of a literature review and other of simulation. In the first part, a modular constructive system based on gypsum blocks is presented. This constructive system reduces the manpower considerably, the time of execution and the final costs of construction. However, the durability assessment should be analysed but first its implementation and performance along the country must analysed.In this work a preliminary experimental characterization of the thermal conductivity, vapour permeability and water absorption coefficient of two different samples of gypsum plasters from the northeast Brazil (plaster pole of Araripe, Pernambuco) was performed. The experimental and analytical procedures followed either international standards or well-established methodologies, supporting the analysis of gypsum walls hygrothermal behaviour. The measured properties were adopted as inputs to hygrothermal simulation software and the analysis of gypsum based exterior walls was conducted for different climate zones and exposure conditions. The study demonstrates the durability issues that may arise in each location, due to differences in hygrothermal action.The other simulation part included hygrothermal advanced modelling, using the EnergyPlus software. The numerical results carried out allow a discussion over the interior comfort and durability of this modular constructive system. In this paper, the computer simulation model and the results of a parametric analysis of the dwelling performance on the eight climatic regions defined in Brazilian regulations are presented. The model was validated using the in situ measurements of air temperature.
作者: Antonio F. Miguel
摘要: Locomotion consists of cyclic events controlled by the neuronal activity of networks called central pattern generators. For a correct management of pedestrian flows, under regular or safety-critical situations, a deep understanding of individual and crowd dynamics is crucial. Here, we examine the emergence of walking and running forms of human gait. Individual locomotion and its interaction with other pedestrians is studied. Another key aspect examined is the self-organization experienced by a group of individuals which is a key concept to understand crowd dynamics. Self-organization leads to emergent properties, meaning that the whole system has characteristics that differ qualitatively from those of the component parts. The mechanisms behind the emergence of self-organized pattern of motion are also studied.