基于欧拉薄壁模型的陶瓷坯体干燥模拟

通过对砖坯干燥传热传质过程的分析,采用FLUENT流体分析软件中欧拉薄壁模型建立陶瓷坯体三维流体传热传质模型,通入同温度不同湿度的两种干燥介质,在此基础上得到瓷砖坯体干燥温度和湿度场,比对两种不同工况工质对蒸发过程的影响,并简要分析了影响干燥时间的主要因素,以期用于干燥生产过程参考。     

大规格抛光砖干燥过程及控温要点分析

1砖坯的干燥过程及特征 所谓干燥就是指固体物料受热后蒸发出所含水分的过程。湿坯进入干燥窑后，通过与干燥介质交换，坯体表面水分首先向周围介质扩散，并借助干燥介质的不断流动和扩散作用，使蒸发过程连续进行。     

单颗粒褐煤高温烟气干燥过程数值模拟

褐煤干燥对于提高其品质具有重要意义。为了模拟高温烟气干燥这一高温差、变温差非稳态传热传质过程中褐煤内水分蒸发过程,采用有限体积法建立了一维球坐标系下蒸发界面向内迁移的单颗粒褐煤干燥数学模型,并利用该模型分析了初始烟气温度和颗粒粒径对单个褐煤颗粒干燥特性的影响。模型模拟结果与实验结果对比表明二者变化趋势一致,所建模型能较好地反映出高温烟气干燥过程中褐煤内水分蒸发过程。结果表明,初始烟气温度越高,颗粒粒径越小,蒸发界面向内迁移速度越快,水分脱除越快,干燥时间越短;蒸发界面平均迁移速度均与初始烟气温度和颗粒粒径呈线性关系;在初始烟气温度700℃下,较短的停留时间使得颗粒表面温度未达到挥发分析出温度,本研究中不同粒径褐煤颗粒在干燥过程中基本没有挥发分的析出。     

生物质型煤干燥能耗的瞬态分析及预测模型

通过在不同干燥温度(140,160,180℃)、不同风速(0.4,0.8,1.2 m/s)下对生物型煤进行了干燥特性实验,并对其瞬时单位能耗进行了计算和分析,结果表明:生物质型煤干燥过程中,单位能耗曲线分为3个阶段:下降阶段、恒定阶段和上升阶段;当干燥速率处在升速阶段时,单位能耗随干燥温度和风速的提高下降迅速;当干燥速率处在恒速阶段时,单位能耗随干燥温度和风速的提高而降低;当干燥速率处在降速阶段时,单位能耗随干燥温度和风速的提高而快速上升。基于干燥特性数学模型——Sabbet方程,得到了生物质型煤干燥时瞬时单位能耗的预测模型,其可以有效地反映出生物质型煤在干燥过程中单位能耗瞬态变化,为生产和工艺改进提供指导。     

深色釉面地砖喷墨印花缺陷的解决实例

深色釉面砖的喷墨打印难度大,经常出现图案打印和墨水干燥等缺陷。图案打印缺陷可通过"双重墨水通道"解决;而墨水干燥缺陷则因墨水干燥速度太慢引起。生产工艺试验表明,合适的砖坯温度能使墨水快速渗透入釉层,达到较好的干燥效果,降低砖坯水分、提高砖坯吸水率、提高釉浆吸干速度,均可明显加快墨水干燥,从而避免墨水干燥缺陷的发生。     

问：在干燥窑升速阶段和排水阶段，砖坯会过早产生“硬壳”现象，造成排水裂或行走过程的机械应力裂纹，请问该缺陷如何克服？

答：由于翻坯入干燥窑引起的砖坯底裂是一种新的缺陷，也是由许多方面的原因造成的，干燥窑升速阶段和排水阶段温度高、湿度小、排水过快，使砖坯发生底裂现象，就以上缺陷的克服办法有。     

油烧辊道窑预干燥带干燥过程微观数学模型的研究

本文分析了辊道窑内预干燥带传热及干燥过程的特点，引入Morgan的蒸发面后退模型，建立了陶瓷制品干燥过程中温度场分布及湿含量计算的微观数学模型。     

专家门诊

关于干燥后砖坯开裂问题的答疑 我负责一条180m干燥器(3层)的主管工作,近期在生产600规格微粉砖时出现较多的边裂缺陷(在干燥后已经发现),裂纹多在干燥器的窑墙两侧,并平衡于辊棒方向,严重时砖坯分成两半,经对干燥器的温度进行了各种调整仍未见效果,应该采取什么措施进行调整?答:对于干燥后造成砖坯开裂的原因比较多,从以上反映情况分析,造成砖坯开裂有三种可能。第一是压机在成型时,由于压力不平衡或者粉料强度较差,已经有隐性小裂纹或者致密度差异,砖坯再经过传送、干燥后,裂纹被“推波助澜”作用进一步扩大,造成更明显的开裂和断裂。第二…     

污泥干燥及气体释放特性研究

采用污泥薄片模拟分散态污泥干化过程,研究了干化风速、温度对污泥干燥速率的影响,并分析了污泥干燥过程中的形貌变化,采用热红联用分析污泥在(35～700℃)干化过程中气体的释放情况.结果表明:污泥干燥过程中的自由水、空隙水和吸附水干燥速率不同,提高干燥温度和热风风速,污泥干燥速率增大;污泥体积的收缩主要由于自由水的蒸发,粘滞区的存在是由于自由水蒸发完毕而引起干燥速率发生突变;污泥升温过程中释放的气体主要有C02、H2O、NH3、VFA及庚烷,273.75～333.76℃的温度区间为失重速率最大区域;VFA在273.75℃释放量最大,庚烷在333.76℃释放量最大.     

谈粉煤灰空心砖干燥裂纹的消除方法

粉煤灰空心砖坯体干燥过程中常常出现裂纹 ,既影响质量 ,也影响产量。其原因主要是砖机机头 (泥缸 )结构及安装部位不合理。空心砖成型过程 ,如果原料在泥缸中遇到挤压力度不同会导致泥条断面挤出速度不匀 ,坯体边部湿度、密实度与中间的湿度、密实度不一致 ,相同气候条件下 ,因坯体四周密实度低于中间 ,边部温度却高于中间而导致边部水分蒸发快 ,中间水分蒸发慢 ,形成边部脱水收缩率大于中间部的脱水收缩率 ,如果脱水收缩过快 ,砖坯体边部就会开裂并出现形状不同的裂纹。砖机机头 (泥缸 )的芯架由刀架、芯杆、芯头组成 ,如果刀架、芯头、芯…     

Examining the Suitability of the Reaction Engineering Approach (REA) to Modeling Local Evaporation/Condensation Rates of Materials with Various Thicknesses

The reaction engineering approach (REA) is examined here to investigate its suitability as the local evaporation rate to be used in multiphase drying. For this purpose, REA is first implemented to model the convective drying of materials with various thicknesses. The relative activation energy, as the fingerprint of REA, generated from one size of a material is used to model the convective drying of the same material with different thicknesses. Because the results indicate that REA parameters can model the drying of materials with various thicknesses, REA can be scaled down to describe the local evaporation rate (at the microscale as affected by local composition and temperature). The relative activation energy is used to describe the global drying rate in modeling the local evaporation rate. REA is combined with a system of equations of conservation of heat and mass transfer in order to yield the spatial reaction engineering approach (S-REA) as a nonequilibrium multiphase drying model. By using S-REA, the spatial profiles of moisture content, concentration of water vapor, temperature, and local evaporation rate can be generated, which can assist in comprehending the transport phenomena.     

Convective Drying of Wastewater Sludges: Influence of Air Temperature, Superficial Velocity, and Humidity on the Kinetics

The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 3³ factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.     

Multiphase modeling of intermittent drying using the spatial reaction engineering approach (S-REA)

Several schemes of energy minimization of drying process including intermittent drying have been attempted. Intermittent drying is conducted by applying different heat inputs in each drying period. An effective and physically meaningful drying model is useful for process design and product technology. The lumped reaction engineering approach (L-REA) has been shown previously to be accurate to model the intermittent drying In L-REA, the REA (reaction engineering approach) is used to describe the global drying rate. In this study, the REA is used to model the local evaporation/condensation rate and combined with the mechanistic drying models to yield the spatial reaction engineering approach (S-REA), a non-equilibrium multiphase drying model. The accuracy of the S-REA to model the intermittent drying under time-varying drying air temperature is evaluated here. In order to incorporate the effect of time-varying drying air temperature, the equilibrium activation energy and boundary condition of heat balance implement the corresponding drying settings in each drying period. The results of modeling using the S-REA match well with the experimental data. The S-REA can yield the spatial profiles of moisture content, concentration of water vapor, temperature and local evaporation/condensation rate so that better understanding of transport phenomena of intermittent drying can be obtained. It is argued here that the REA can describe the local evaporation rate under time-varying external conditions well. The S-REA is an effective non-equilibrium multiphase approach for modeling of intermittent drying process.     

Convective Drying of Wastewater Sludges: Influence of Air Temperature,Superficial Velocity,and Humidity on the Kinetics

Abstract

The influence of air temperature, velocity, and humidity during convective drying of two different sludges (A and B) is investigated through a 3³ factorial design. For sludge A, a constant drying flux period is observed, while sludge B is characterized by a long decreasing drying flux phase. A sensitivity analysis shows that temperature is the main operating parameter affecting the drying kinetics. Mass and heat transfer coefficients as well as water evaporation capacities are calculated from drying curves. Transfer coefficients are related to operating conditions through dimensionless relations. For both sludges, a linear relation is found between water evaporation capacity and the maximal measured drying flux.     

Steam Drying of Products Containing Solvent Mixtures

Drying experiments with single, porous spheres wetted with mixtures of 2- propanol and water were performed using superheated steam, air, or steam-air mixtures as drying agent. Both the drying rate and the moisture composition were determined experimentally for different temperatures and compositions of the drying agent and for different initial compositions of the moisture. It is shown that evaporation of 2-propanol is enhanced by using superheated steam as drying agent instead of air due to steam condensing on the sample. While the overall drying rate increases with rising steam temperature, the evaporation rate of 2-propanol is hardly affected. When drying samples containing mixtures of 2- propanol and water, internal boiling can occur depending on the vapor-liquid equilibrium. Vapor generated inside the sample may cause mechanical dewatering of the sample which greatly increases the drying rate.     

RELATIONSHIP BETWEEN A SOLAR DRYING MODEL OF RED PEPPER AND THE KINETICS OF PURE WATER EVAPORATION (I)

Drying of red pepper under solar radiation was investigated, and a simple model related to water evaporation was developed. Drying experiments at constant laboratory conditions were undertaken where solar radiation was simulated by a 1000 W lamp.

In this first part of the work, water evaporation under radiation is studied and laboratory experiments are presented with two objectives: to verify Penman's model of evaporation under radiation, and to validate the laboratory experiments. Modifying Penman's model of evaporation by introducing two drying conductances as a function of water content, allows the development of a drying model under Eolar radiation.

In the second part of this paper, the model is validated by applying it to red pepper open air solar drying experiments.     

Steam Drying of Products Containing Solvent Mixtures

ABSTRACT

Drying experiments with single, porous spheres wetted with mixtures of 2- propanol and water were performed using superheated steam, air, or steam-air mixtures as drying agent. Both the drying rate and the moisture composition were determined experimentally for different temperatures and compositions of the drying agent and for different initial compositions of the moisture. It is shown that evaporation of 2-propanol is enhanced by using superheated steam as drying agent instead of air due to steam condensing on the sample. While the overall drying rate increases with rising steam temperature, the evaporation rate of 2-propanol is hardly affected. When drying samples containing mixtures of 2- propanol and water, internal boiling can occur depending on the vapor–liquid equilibrium. Vapor generated inside the sample may cause mechanical dewatering of the sample which greatly increases the drying rate.     

Pore-Scale Simulation of Drying of a Porous Media Saturated with a Sucrose Solution

This work presents results of Monte Carlo simulations of isothermal drying of a nonhygroscopic porous media initially saturated with a sugar solution. The porous media is represented by a two-dimensional network of cubic pores connected by throats with a given radius distribution. The considered network had just one open side (the three other sides were sealed) from which water evaporation occurred. Water evaporation, hydraulic flow, and diffusivity of sucrose in water are considered in the physical model. It was considered that drying occurred under isothermal conditions (low drying rates) and that the capillary forces surpass the viscous forces, as in invasion percolation. It was also considered that water evaporation inside the network of pores and throats causes solution concentration, which remains at the corners, allowing hydraulic connection throughout the whole network. At each simulation step, a single meniscus moves through a particular pore segment with the higher displacing force. As drying progresses, air replaces the solution. Determination of the mechanism prevailing at any given drying stage requires calculation of evaporation. In other words, each step of the simulation involves finding the solution to three systems of equations: the vapor pressure field in the vapor phase, the pressure field in the liquid phase, and the solutes' concentration in the liquid phase. Herein, we report results of drying curves calculated as a function of the sucrose and water saturation and of the distribution of liquid, sucrose, and vapor as drying advances. The results presented in this work showed that network models are a powerful tool for investigating the influence of the main mechanisms controlling drying at its different stages; that is, from liquid saturation condition to very low saturation (end of drying). Despite the applied simplifications, the model can capture the main aspects of drying of liquids and solutions present in porous media.     

RELATIONSHIP BETWEEN A SOLAR DRYING MODEL OF RED PEPPER AND THE KINETICS OF PURE WATER EVAPORATION (I)

ABSTRACT

Drying of red pepper under solar radiation was investigated, and a simple model related to water evaporation was developed. Drying experiments at constant laboratory conditions were undertaken where solar radiation was simulated by a 1000 W lamp.

In this first part of the work, water evaporation under radiation is studied and laboratory experiments are presented with two objectives: to verify Penman's model of evaporation under radiation, and to validate the laboratory experiments. Modifying Penman's model of evaporation by introducing two drying conductances as a function of water content, allows the development of a drying model under Eolar radiation.

In the second part of this paper, the model is validated by applying it to red pepper open air solar drying experiments.