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干燥是大多数农副产品、食品等加工过程中必不可少的环节,干燥即加热去湿过程,伴随着热质的传递与交换。在烟草行业,烟丝干燥是制丝工艺中的一道关键工序,最常使用的设备是滚筒烘丝机。由于传热传质问题本身的复杂性,以及干燥过程受温度、风速等众多因素的影响,关于滚筒内烟丝传热传质特性的认知还不够深入。为了研究烟丝与气流在顺流流动情况下,滚筒烘丝机内烟丝的传热传质特性,本文基于传热传质学、多相流动等相关理论,建立起滚筒内的传热传质数学模型,通过计算机数值模拟方法,分别研究了不同加料量、壁温、气流流速条件下烟丝的干燥过程。研究结果表明:加料量对烟丝含水率、温度的影响比较明显;壁温是影响烟丝传热传质特性的关键因素,壁温大小直接决定着烟丝含水率及温度的高低;气流流速对烟丝传热传质特性也有一定的影响,且流速在0.2~0.6m/s范围内存在一极值点,使得烟丝最终含水率随流速的增加先下降后略有上升。 相似文献
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目前对于固定床中丝状填充颗粒传热传质特性的认识仍处于初始阶段。为了能够从颗粒尺度的微观层面揭示丝状颗粒与气体、颗粒与颗粒之间的热、质传递机理,建立了一种丝状颗粒传热传质数学模型,之后将离散单元法与计算流体力学相结合,对实验中较难获得的床层局部流动及传递信息进行了数值模拟,并着重分析比较了气流入口温度以及表观气速等关键因素对固定床中丝状颗粒温度和含水率变化的影响规律。通过模拟结果与实验数据的对比,验证了所建模型的可行性。研究结果表明:同一时刻,固定床中颗粒温度大体上是随着床层高度的增加而降低,含水率则是随着床层高度的增加而升高;气流入口温度对于固定床中丝状颗粒平均温度的提升起着主导作用,而颗粒的传质速度则受表观气速的影响更为明显。 相似文献
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物料干燥过程—传质传热最优化方法的研究 总被引:1,自引:0,他引:1
论述了多孔介质干燥过程中传质传热的数学模型及干燥过程最优化准则的建立;并且动用控制理论产物料干燥过程传热的动态优化方法--准则约束法,且得到了最优解。 相似文献
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针对单个乙醇溶液液滴在降压环境下蒸发的传热传质过程建立了数学模型。模型基于液相的能量守恒和 传质扩散理论,利用经典拓展模型计算液滴的质量蒸发率,并引入活度系数考虑液滴表面的蒸气分压。采用液 滴悬挂法进行实验,分别记录了乙醇溶液液滴和乙酸溶液液滴在降压蒸发过程中的液滴内温度变化。将实验数 据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴内部温度分布以及浓度分布随时间的变化。 结果表明:快速降压阶段空气流动较快,加之乙醇工质易挥发,液滴表面温度下降迅速,液滴内部温差和乙醇 浓度梯度较大;压力稳定后,空气流速为零,液滴内部温差和乙醇浓度梯度逐渐减小。由于液滴内部的热扩散 速率大于传质扩散系数,内部温度随时间的变化比浓度随时间的变化更快。 相似文献
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应用Luikov理论和有限元方法求取物料在间断加热干燥不同瞬间的温度、湿度及应力分布,并用自编程序试算的结果与文献中的实验值或理论解进行了比较,本文介绍的方法可望用于求取间断加热干燥过程的最佳操作参数. 相似文献
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流化床氛围下多孔物料干燥传热传质的数值模拟 总被引:4,自引:1,他引:4
用有限差分法数值求解一个热、质传递耦合模型,理论研究多孔物料流化床干燥过程。方程离散采用全隐格式的控制容积方法,三对角矩阵法(TDMA)用来求解线性方程组。选用球形的苹果丁作为多孔物料。在典型操作条件下,通过分析温度、饱和度和压力的分布侧形,讨论了物料内部的热、质传递机理。在对比条件下,考察了气体入口温度、气速和床面积因子对干燥过程的影响。结果表明:干燥过程受气、固相间的耦合传热传质的影响十分明显,干燥时间随气体入口温度和气速的提高而减少;随床面积因子的增大而增加。 相似文献
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This article presents a mathematical model of heat and mass transfer for the process of fluidized-bed spray granulation, which can be applied in the analysis of bed temperature profile, temperature and humidity of outlet gas and moisture content of particles. Effects of operation parameters on the batch granulation are investigated. The theoretical calculation agrees reasonably well with the experimental data.Keywords fluidized-bed spray granulation, mathematical model, heat and mass transfer 相似文献
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为揭示在固定床反应器中固体热载体法快速热解褐煤工艺过程中的热、质传递机理,建立了固体热载体法褐煤热解过程中的传质传热模型。模型包括球型颗粒的一维非稳态导热方程和基于分布活化能模型的动力学模块,分别采用有限容积法与Matlab软件中遗传算法工具箱对二者进行数值计算。通过呼伦贝尔褐煤热重实验数据与温度测定实验数据分别验证了预测的动力学参数及颗粒传热模型结果。研究发现,热、质变化在固体热载体法褐煤热解工艺中呈现复杂的耦合特性。此外,考察了在不同初始温度、热载体进料比与煤颗粒半径条件下,褐煤在热解过程中颗粒内部温度场在径向上随时间的变化规律,并分析了产物释放速率与温度场的关联性,结果表明热历程改变是工艺条件对热解产物分布造成影响的根本原因。 相似文献
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This article concerns the modelling and simulation of a deep-bed grain dryer in a large diameter-column. Two-dimensional (2D) models of deep-bed grain dryers were built by considering simultaneously momentum, heat, and mass transfer in the drying phase together with coupled heat and mass balance in the grain phase. The dynamic equations are solved numerically by using finite difference method. The momentum equations are applied to simulate pressure drop and velocity field of the drying air across the bed. The mass and heat balance in the two phases determine the profile of temperature and moisture content in both phases. Further, drying rate curves for various temperature of inlet drying gas together with moisture content of grain were simulated. The simulated profiles are in close agreement with experimental data. 相似文献
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ABSTRACT A mathematical model able to predict solid and drying gas temperature and moisture content axial profiles along a direct contact rotary dryer was developed. The study was focused on the drying kinetics based on phenomenological models. Two different drying mechanisms in the decreasing drying rate period were tested: proponional to the unbound moisture content and moisture diffusion inside the particle. Experimental data collected in a pilot-scale direct contact rotary dryer was used to validate the model. Soya and fish meals were used as drying material. 相似文献
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One of possibilities to extend operating efficiency of spray dryers is to increase turbulence of the drying agent flow. In the literature no quantitative data describing this phenomenon are available.
In the paper results of experimental investigations on the effect of turbulence on heat and mass transfer during atomization are discussed. The scope of experiments covered the analysis of changing evaporation capacity. temperature of gas and atomized material and particle size distribution as a function of distance to the atomizer. It was proven that an increase of air flow turbulence could cause 20-25% increase of evaporation capacity. 相似文献
In the paper results of experimental investigations on the effect of turbulence on heat and mass transfer during atomization are discussed. The scope of experiments covered the analysis of changing evaporation capacity. temperature of gas and atomized material and particle size distribution as a function of distance to the atomizer. It was proven that an increase of air flow turbulence could cause 20-25% increase of evaporation capacity. 相似文献
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多孔介质是大量干燥过程的主体,由于实际多孔介质干燥过程的复杂性,建立通用的干燥过程传热传质模型十分困难。通过分析喷雾干燥过程中高初始含湿多孔介质与干燥介质之间的传热传质机理以及各因素对传热传质的影响,根据马歇尔方程探讨了干燥介质与料雾之间的水蒸汽分压差在干燥过程中的变化情况,反映了多孔湿介质在喷雾干燥操作中的传热传质过程的几种特性,为确定实际生产中喷雾干燥器的操作条件指明了新的出路。 相似文献
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Arun S Mujumdar Professor 《Drying Technology》2013,31(2):399-400
ABSTRACT Results from three experimental studies are reviewed in order to examine the relationship between the heat and mass transfer coefficients in the combined infrared and convective drying process. These results demonstrared that the heat 相似文献
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ABSTRACT Drying curves obtained in a pilot-scale fluidized bed dryer using biological source solids (sawdust, soya and fish meal) were used to estimate the parameters involved in heat and mass transfer phenomenas: heat transfer coefficient and moisture diffusivity coefficient. Parameters involved in mass transfer were estimated from drying models based on diffusional mechanisms and others that in addition consider internal and external resistance to the mass transfer. The estimate ef ective diffusivity coefficient was between 2x10-11 to lx10 (m2/s) for the considered products. Heat transfer coefficient was estimated from drying data points in the constant drying rate period when the external resistance to the mass transfer controls the process. 相似文献
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ABSTRACT A finite difference scheme for solving the problem of natural transport of heat, mass, momentum and species concentration along vertical porous plates is presented. Several drying related problems are numerically solved, by including a gas-injection boundary condition directly into the governing equations. The effect of variable physical properties is investigated by means of direct comparison against experimental data obtained through holographic interferometry. The relative importance of wall diffusive and convective fluxes is examined. Sherwood and Nusselt numbers can be accurately obtained by means of the proposed techniques. 相似文献
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A finite difference scheme for solving the problem of natural transport of heat, mass, momentum and species concentration along vertical porous plates is presented. Several drying related problems are numerically solved, by including a gas-injection boundary condition directly into the governing equations. The effect of variable physical properties is investigated by means of direct comparison against experimental data obtained through holographic interferometry. The relative importance of wall diffusive and convective fluxes is examined. Sherwood and Nusselt numbers can be accurately obtained by means of the proposed techniques. 相似文献