AN ANALYSIS OF HEAT AND MASS TRANSFERPHENOMENON IN WOOD DURING DRYING

According to modeling simulation and experiments study, the heat and mass transtfer phenomenon.in wood during dring processes was analyzed. The results indicate: at initial stage of drying, moisture movement in wood is due to capillary force, heat transfer is major effect, at end stage, moisture movement in wood is due to diffusive transport, heat transfer is less

Lie group analysis for thermophoretic and radiative augmentation of heat and mass transfer in a Brinkman-Darcy flow over a flat surface with heat generation

A boundary layer analysis is presented to investigate numerically the effects of radiation, thermophoresis and the dimensionless heat generation or absorption on hydromagnetic flow with heat and mass transfer over a flat surface in a porous medium. The boundary layer equations are transformed to non-linear ordinary differential equations using scaling group of transformations and they are solved numerically by using the fourth order Runge-Kutta method with shooting technique for some values of physical parameters. Comparisons with previously published work are performed and the results are found to be in very good agreement. Many results are obtained and a representative set is displayed graphically to illustrate the influence of the various parameters on the dimensionless velocity, temperature and concentration profiles as well as the local skin-friction coefficient, wall heat transfer, particle deposition rate and wall thermophoretic deposition velocity. The results show that the magnetic field induces acceleration of the flow, rather than deceleration （as in classical magnetohydrodynamics （MHD） boundary layer flow） but to reduce temperature and increase concentration of particles in boundary layer. Also, there is a strong dependency of the concentration in the boundary layer on both the Schmidt number and mass transfer parameter.

External Heat Transfer in Moist Air and Superheated Steam for Softwood Drying

In kiln drying of softwood timber, external heat and moisture mass transfercoefficients are important in defining boundary temperature and moisture content at the woodsurface. In addition, superheated steam drying of wood is a promising technology but this has notbeen widely accepted commercially, partially due to the lack of understanding of the dryingphenomena occurred during drying. In this work, experimental investigation was performed to quantifythe heat transfer between wood surface and surrounding moist air or superheated steam. In theexperiment, saturated radiata pine sapwood samples were dried using dry-bulb/wet-bulb temperaturesof 60℃/50℃, 90℃/60℃, 120℃/70℃, 140℃/90℃, 160℃/90℃, 140℃/100℃ and 160℃/100℃. The lasttwo schedules were for superheated steam drying as the wet-bulb temperature was set at 100℃. Thecirculation velocity over the board surface was controlled at 4.2m·s^(-1). Two additional runs(90℃/60℃) using air velocities of 2.4 m·s^(-1) and 4.8 m·s^(-1) were performed to check theeffect of the circulation velocity. During drying, sample weight and temperatures at wood surfaceand different depths were continuously measured. Prom these measurements, changes in woodtemperature and moisture content were calculated and external heat-transfer coefficient wasdetermined for both the moist air and the superheated steam drying.

Asymptotic Solution for Coupled Heat and Mass Transfer During the Solidification of High Water Content Materials

This paper focuses on obtaining an asymptotic solution for coupled heat and mass transfer problem during the solidification of high water content materials. It is found that a complicated function involved in governing equations can be approached by Taylor polynomials unlimitedly, which leads to the simplification of governing equations. The unknown functions involved in governing equations can then be approximated by Chebyshev polynomials. The coefficients of Chebyshev polynomials are determined and an asymptotic solution is obtained. With the asymptotic solution, the dehydration and freezing fronts of materials are evaluated easily, and are consistent with numerical results obtained by using an explicit finite difference method.

Theoretical Modeling and Experimental Analysis of Drying Process in Electromagnetic Field

The effects of electromagnetic waves in drying processes of solid materials are investigated theoretically and experimentally. Modified model of mass transfer being constituted by the effect of electromagnetic waves which have different frequency has been obtained. Modeling of the drying process with a two-port electric circuit for the determination of diffusion coefficients is designed. The frequency limits of electromagnetic wave which will be able to hasten the drying process are determined. The effects of the electromagnetic wave in the potato slice drying process by the influence of different frequencies and temperatures are experimentally examined. The results obtained are compared with theoretical calculations. Moisture concentration curves in drying process have been commented by drawing. Theoretical and experimental results which have been obtained are identified as a well adaptation.

Second-order slip MHD flow and heat transfer of nanofluids with thermal radiation and chemical reaction

The effects of the second-order velocity slip and temperature jump boundary conditions on the magnetohydrodynamic （MHD） flow and heat transfer in the presence of nanoparticle fractions are investigated. In the modeling of the water-based nanofluids containing Cu and A1203, the effects of the Brownian motion, thermophoresis, and thermal radiation are considered. The governing boundary layer equations are transformed into a system of nonlinear differential equations, and the analytical approximations of the solutions axe derived by the homotopy analysis method （HAM）. The reliability and efficiency of the HAM solutions are verified by the residual errors and the numerical results in the literature. Moreover, the effects of the physical factors on the flow and heat transfer are discussed graphically.

Research on vacuum drying process and internal heat conduction of Li-ion battery core

In this research, an innovative cylindrical automatic battery core oven was designed to avoid the structural deformation that frequently occurs in traditional ovens. The oven could be automatically connected with the electrolyte injection process after baking, achieving improvement in a battery's baking consistency. This contributed to the feasibility of studying the internal heat conduction process of batch battery cores during actual baking processes. A mathematical model of a certain plate battery cell during the baking process was established. The simulation results of the temperature change inside the battery core during the baking process were consistent with the calculation results of the mathematical model. The temperature distribution at each point inside the battery core could be fitted through the thermal conductivity at different temperatures and the temperature distribution between the layers of the battery core.Finally, based on the thermodynamic balance energy conservation method, the relationship between the temperature change inside the battery core and the entire baking process was established. A feasible algorithm for studying the thermal conduction of complex material and internal structure objects in the baking process was obtained.

An empiric equation for the latent heat of vaporization of moisture in bananas during its isothermal drying

In order to determine the energy needed to artificially dry a product, the latent heat of vaporization of moisture in the product, hfg, must be known. Generally, the expressions for hfg reported in the literature are of the form hfg = h(T)f(M), where h(T) is the latent heat of vaporization of free water, and f(M) is a function of the equilibrium moisture content, M. But expressions of this type contain a simplification because, in this case, the ratio hfg/h would only depend to the moisture content. In this article a more general expression for the latent heat of vaporization, namely hfg = g(M,T), is used to determine hfg for banana. To this end, a computer program was developed which fits automatically about 500 functions, with one or two independent variables, imbedded in its library to experimental data. The program uses nonlinear regression, and classifies the best functions according to the least reduced chi-square. A set of executed statistical tests shows that the generalized expression used in this work given by hfg = g(M,T) produces better results of hfg for bananas than other equations found in the literature.

螺旋金属燃料多物理耦合分析方法与概念设计研究

螺旋金属燃料具有导热系数高、导热路径短、强制旋流交混的特点,可实现更高的堆芯功率密度,进而减小堆芯体积,提高反应堆的安全性和经济性。本文介绍了上海交通大学反应堆热工水力实验室建立的螺旋金属燃料热工水力、中子物理、力学特性分析方法及多物理耦合分析框架。在热工水力方面,基于自研仪器实现了交混及沸腾临界行为精细化测量,建立了三维及精细化子通道分析方法;在中子物理方面,建立了适用于特殊能谱、复杂几何的截面及稳瞬态中子物理特性的分析方法;在力学方面,基于分子动力学方法建立了U-Zr合金燃料基础热物性模型,并开展了辐照条件下螺旋棒宏观力学特性研究。基于热工-物理-力学多物理分析和优化,提出了螺旋金属燃料组件及堆芯设计,具有无硼化、堆芯功率密度高、体积小、换料周期长的特点。

钢轨铝热焊接过程传热传质行为计算流体力学数值分析

铝热焊在道岔焊接、既有线钢轨维修与断轨抢修中有着广泛的应用。由于铝热焊过程的封闭性,其温度与材料流动难以通过试验的方式进行测量。为此,运用计算流体力学的方法,采用k-ε湍流模型,结合流体体积法,并考虑液态金属的重力、浮力、表面张力与马兰戈尼效应,建立数值分析模型,对浇注和冷却过程不同时刻的传热传质行为进行定量分析,通过相分布特征研究凝固顺序,为后期优化浇注体系、构建新焊剂成分和砂型型式提供基础数据。结果表明:浇注过程中空气被裹挟进入熔池,浇注时间约为11 s,接头冷却至固相线1641 K以下的时间约为58 s;浇注开始后,不同区域最大温差约为1400 K,高温区位于轨底,低温区位于轨头;随着浇注的进行,轨头温度升高,轨腰温度先升后降,轨底温度降低;在浇注的最后阶段,不同区域温差缩小,最大约为400 K,高温区位于轨头,低温区位于轨底;在冷却过程中,轨头温度始终最高,轨底温度始终最低,按照轨底、轨腰、轨头的顺序发生凝固。

椭圆型套管-管翅式蒸发式冷凝器传热传质性能及实验研究

介绍了一种椭圆型套管-管翅式蒸发式冷凝器,通过搭建的蒸发式冷凝器性能测试实验系统,研究该蒸发式冷凝器在不同循环水配比、不同地区环境以及“风冷”“水冷”“蒸发冷凝”和“全工况”4种不同运行模式下的传热传质性能与运行特性的变化规律。结果表明:在相同条件下“全工况”模式的热流密度高于“风冷”和“蒸发冷凝”两种模式,而“水冷”模式由于其传热面积远小于其它模式,所以其热流密度最高;“全工况”模式在保持配风量及循环水总量一定的条件下,热流密度随循环水配比的增大而先增大后减小,喷淋水与冷却水的流量最佳配比为2∶5;“全工况”模式在严寒干燥地区、寒冷地区以及夏热冬暖地区的总热流密度分别为5879.64、3964.28及2933.05 W/m^(2),因此不同地区环境运行的传热性能相差较大,该冷凝器“全工况”模式在空气干燥的地区,喷淋冷却水温度范围为13~18℃运行时传热性能较优。在相同的条件下,“蒸发冷凝”模式的水膜传质系数高于“全工况”模式,且两者均随喷淋密度的增大先增大后减小。

Intensification of the Banana Drying Processes by Using EFL Electromagnetic Waves

Drying of the banana in the hot water has a negative impact on the quality of the product and drying effect. The purposes of this study are increasing the drying rate, using the relatively low temperature to improve the quality (40℃, 50℃ and 60℃) and investigate the use of electromagnetic waves to increase the drying speed. Therefore, experiments are performed using 5 kHz,10 kHz and 15 kHz low frequency electromagnetic waves in the air velocity values of 0.5 m/s, 1 m/s, 1.5 m/s and 2 m/s and 40℃, 50℃, 60℃ centigrade degrees of air blast in a special compartment. Mathematical model of the drying process has been created as using the electrical circuits methods and experimental results. As a result, a simple equation describing the drying process has been obtained. Nonlinear expression of the diffusion coefficient for a different situation in this equation has been identified for the first time. The obtained theoretical results and experimental results have been provided a good agreement. This study is considered to be useful for all studies in the drying area.

Research on Constant-Speed Rapeseed Drying Mechanism

This paper presents research on constantspeed rapeseed drying mechanism and the mathematical model established for constant speed drying period.

Effect of bio-tissue deformation behavior due to intratumoral injection on magnetic hyperthermia

Thermal damage of malignant tissue is generally determined not only by the characteristics of bio-tissues and nanoparticles but also the nanofluid concentration distributions due to different injection methods during magnetic hyperthermia.The latter has more advantages in improving the therapeutic effect with respect to the former since it is a determining factor for the uniformity of nanofluid concentration distribution inside the tumor region.This study investigates the effect of bio-tissue deformation due to intratumoral injection on the thermal damage behavior and treatment temperature distribution during magnetic hyperthermia,in which both the bio-tissue deformation due to nanofluid injection and the mass diffusion after injection behavior are taken into consideration.The nanofluid flow behavior is illustrated by two different theoretical models in this study,which are Navier–Stokes equation inside syringe needle and modified Darcy’s law inside bio-tissue.The diffusion behavior after nanofluid injection is expressed by a modified convection–diffusion equation.A proposed three-dimensional liver model based on the angiographic data is set to be the research object in this study,in which all bio-tissues are assumed to be deformable porous media.Simulation results demonstrate that the injection point for syringe needle can generally achieve the maximum value in the tissue pressure,deformation degree,and interstitial flow velocity during the injection process,all of which then drop sharply with the distance away from the injection center.In addition to the bio-tissue deformation due to injection behavior,the treatment temperature is also highly relevant to determine both the diffusion duration and blood perfusion rate due to the thermal damage during the therapy.

Experimental and numerical study on the shrinkage-deformation of carrot slices during hot air drying

In order to further understand the mechanism of material volume change in the drying process,numerical simulations(considering or neglecting shrinkage)of heat and mass transfer during convective drying of carrot slices under constant and controlled temperature and relative humidity were carried out.Simulated results were validated with experimental data.The results of the simulation show that the Quadratic model fitted well to the moisture ratio and the material temperature data trend with average relative errors of 5.9%and 8.1%,respectively.Additionally,the results of the simulation considering shrinkage show that the moisture and temperature distributions during drying are closer to the experimental data than the results of the simulation disregarding shrinkage.The material moisture content was significantly related to the shrinkage of dried tissue.Temperature and relative humidity significantly affected the volume shrinkage of carrot slices.The volume shrinkage increased with the rising of the constant temperature and the decline of relative humidity.This model can be used to provide more information on the dynamics of heat and mass transfer during drying and can also be adapted to other products and dryers devices.

基于NSGA-Ⅱ算法的木材干燥能耗多目标优化

通过计算流体力学(CFD),研究了在常规蒸汽干燥中,空气温度、相对湿度、风速及其相互作用对桦木干燥时间和能耗的影响,以期得到干燥时间短、且能耗低的桦木干燥工艺参数。应用最优拉丁超立方(Opt LHD)采样确定了设计空间中的样本点,并进行CFD数值模拟;采用三阶响应面方法构建近似代理模型,分析了干燥条件对干燥时间和能耗的影响;基于快速非支配排序遗传算法(NSGA-II)多目标优化的Pareto解集,获得最佳木材干燥条件参数。结果表明:该方法可以有效缩短干燥时间,并降低能耗,可为木材干燥工艺的优化提供参考依据。

船体简谐振动下PEMFC气体扩散-电化学反应的动态响应

针对质子交换膜燃料电池(PEMFC)船舶适用性问题,建立船体简谐振动条件下三维单流道PEMFC计算模型,并以计算流体力学为主要研究方法,仿真研究PEMFC在船体简谐振动情况下的动态性能变化,并通过组分运输过程以及电化学反应对振动条件下PEMFC的动态响应进行内在机制解析。结果表明:简谐振动下PEMFC在经过初始阶段响应后,呈周期性输出规律;船体振动会显著影响PEMFC内部组分传热传质,导致电化学反应无法正常进行,PEMFC性能出现衰变趋势,甚至造成电池饥饿现象;当振幅不变时,振动频率的增大会加剧该现象。

Behaviour of a Composite Concrete-Trapezoidal Steel Plate Slab in Fire

The present paper investigates fire resistance of a simply-supported composite concrete-trapezoidal steel sheet slab. The objective is to find out if a steel sheet, as a moisture diffusion barrier, may substantially effect the hydro-thermal situation in the concrete part of the cross-section. The numerical integration of the equations of a coupled hygro-thermal boundary-value problem (Tenchev, R.T., Li, L.Y. and Purkiss, J.A. (2001) Num. Heat Transfer Part A, 39(7), 685-710), with and without considering the barrier, shows that the barrier does not really effect the magnitude and the development of temperatures over the cross-section, while there is a significant effect on the pattern of moisture transport and the magnitude of vapour pressure. Particularly high magnitudes of vapour pressure (about 4.5 MPa) were shown in cases where the steel sheet was considered in analyses, which indicates a possible micro damage of concrete in the web of the section, although spalling probably cannot take place due to the steel sheet cover. As the typical composite slab investigated here is not sufficiently fire resistant without any additional reinforcement bars placed in the web, further in-vestigations are directed to finding an optimal position and area of these bars. Following a simplified procedure given in EC2 (Eurocode 2, Design of Concrete Structures, Part 1.2 (2004) Structural fire design, European Committee for Standardization) and assuming that the present composite slab is subject to the uniform traction , yields that placing one bar with the area 1.153 cm2 4 cm away from the edge suffices for the 60 min fire resistance of the slab.

木材干燥过程的热质迁移及其耦合关系

应用不可逆过程热力学研究木材干燥过程热质迁移及其耦合效应。根据不可逆过程的熵的产生率推导出含有耦合效应的传热传质关系式 ,提出湿度梯度不仅引起水分物质流 ,也引起热流 ,以及驱动水分物质流的力应是扩散势梯度。

自然通风湿式冷却塔传热传质的三维数值分析

基于CFD软件Fluent和传热传质理论,建立了自然通风逆流湿式冷却塔空气三维运动控制方程、液相冷却水运动控制方程以及气水两相间传热传质的理论模型.采用标准k-ε湍流模型进行应力封闭,对塔内传热传质过程进行了三维数值计算.计算分析了塔内外空气的速度场、温度场和含湿量场,给出了塔内冷却水温度分布场,指出塔内雨区外侧部分区域空气和冷却水温度均低于环境干球温度,并指出进风口上沿存在纵向漩涡影响气水两相间的局部传热传质强度.计算了塔内各区冷却水蒸发量,给出塔内不同高度处淋水密度的径向分布曲线和塔内传热传质区的气水比分布场,指出传热传质主要发生在填料区.