SIMULATED DEHYDRATION OF WEDGE-SHAPED SPECIMENS IN TURBULENT FLOW OF SUPERHEATED STEAM AND AIR

Numerical methods were used to study the effectiveness of superheated steam, humid air and dry air as dehydrating media in turbulent flow for the drying of wedge-shaped specimens. The Couette flow assumption was used to formulate the coupled heat and mass transfer in the turbulent boundary layer. Superheated steam was found to have highest evaporation rates at free stream temperatures above the inversion point.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Abstract

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

Characteristic modes and flow structure of non-premixed flame in humid-air combustion

An experimental study has been performed in order to determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame. Two-dimensional Particle Image Velocimetry (PIV) measurements were made to quantify the velocity field, with and without steam injected. The results indicate the addition of steam decreases the recirculation flow and reduces the distance between the forward and aft stagnation points. The detailed stabilization regimes show that the critical fuel-to-air velocity ratios of the central fuel penetration in the humid air case are 16% to 22% lower, and the partially quenching limits are at least 25% lower. The decreased penetration limit is due to a reduction in momentum of the humid air. An analysis of flamelet concepts reveals that increased chemical reaction time leads to lower partially quenching limits in the humid air combustion.     

THE CONVECTIVE VELOCITY OF HEAVY PARTICLES IN TURBULENT AIR FLOWS

This paper examines the influence of the “crossing trajectories” effect on the convective velocity for heavy particles suspended in a turbulent air flow. The fluid energy spectrum “experienced” by a falling particle is deduced from experimental data and is used to evaluate the turbulent component of the convective velocity. The results indicate a significant increase in the rms value of the turbulent component due to the “crossing trajectories” effect.     

高压煤粉锅炉过热器爆管原因分析

针对2台WG240-10.3/2型高压煤粉锅炉过热器多次爆管的问题,对爆管原因进行分析,认为四角切圆燃烧器残余旋流、蒸汽流量偏差、管内汽流阻力不均及三次风带粉过多是促使过热器爆管的重要原因。     

EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-?) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

EFFECT OF NEAR-WALL MODELLING ON PREDICTION OF IMPINGEMENT HEAT TRANSFER

Due to enhanced transport characteristics, impinging jets are widely used in industry to dry large surface area products such as paper and textiles. The present numerical study concerns the modelling of convective heat transfer for impingement drying. Flow and heat transfer under a confined two dimensional turbulent air jet impinging on a flat surface were modelled by solution of two-dimensional Navier-Stokes and energy equations. The turbulence model used was the high-Re number version of the well known two-equation (κ-ε) model and numerical solution was by the upwind finite difference scheme. The specific objective was to evaluate the accuracy of schemes for modelling the near-wall turbulent flow.

The mean flow properties such as centerline velocity decay and the pressure distribution at the impingement surface show no dependence on the near-wall model used. Heat transfer predictions were found to be quite sensitive to the choice of near-wall model. Best agreement between predictions and experiments was obtained for a Chieng-Launder type model with a new modification, use of k_p instead of k_v in the calculation of τ_w.     

Simulation of Superheated Steam Turbulent Flows and Heat Transfer in an Impingement Dryer

Numerical simulations are presented for two-dimensional flow field and heat transfer characteristics due to a turbulent single slot jet of superheated steam discharging tangentially into a confined cylinder. A finite volumes method was used to solve the equations that describe the problem. Calculations were performed for steady state turbulent flow and unsteady state heat transfer. Constant velocities and superheated steam temperature are imposed at the inflow. Particle properties were assumed to be the same of a cubic particle of carrot. Numerical tests were performed to ensure that the model solutions were “grid independent” and also independent of the turbulence intensity of stated as boundary condition at the inlet slot. The solution procedure developed is fast and that convergence is reached after a few iterations. The results obtained are relevant to flow and heat transfer behaviors of the impingement dryer and it will be useful future studies considering particles interactions.     

固定床煤气化炉的模拟与优化

借助过程模拟软件Aspen Plus建立了固定床煤气化炉的数学模型,模拟计算了固定床煤气化炉的制气过程,并利用现场数据对模型进行考核,结果表明该模型与实际生产运行数据吻合较好,最大误差低于7.05%;利用该模型研究了空气和蒸汽的通量、空气和蒸汽的预热温度对评价指标的影响,以及在体积流量相同时,夏季和冬季实际空气通量对评...     

水力旋流器湍流场数值模拟

<正> 引言 水力旋流器作为一种简便、易行和高效率的分离、分级和离心沉降设备,已被广泛应用于化工、冶金、石油等众多工业领域中.以往的水力旋流器设计主要是根据大量物理模型试验得出的经验准数方程来求出旋流器的几何结构参数和操作参数.然而,随着水力旋流器应用范围的迅速扩大和人们对其分离(级)性能指标的要求日益提高,传统的按经验或半经验公式进行旋流器设计方法的局限性越来越明显,以及物模试验的耗时费钱,已促使人们开始采用数值模拟的方法,通过对旋流器内部流体运动的深入研究,弄清旋流器的分离机理,以便为提高水力旋流器的分离效率和分级准确度予以理论指导.本文采用了适于水力旋流器液相(水)流场的K-ε湍流数学模型,对水力旋流器内的湍流运动规律进行了数值模拟并根据激光实测结果对部分模型常数进行了修正.     

Micro-climatic environmental conditions inside a greenhousewith a built-in solar distillation system

A numerical study was carried out to investigate the micro-climatic environmental conditions inside a greenhouse-distillation system, self-sufficient for irrigating water. The greenhouse consists of the planting cavity, circulating air channels and roof solar distiller for the production of a rather modest rate of irrigating water. A turbulent, steady-state flow, energy and humidity concentration equations were solved using the numerical code FLUENT 6.1. Velocity vectors, steam function, isotherms and temperature and humidity distribution inside the greenhouse present the resultant: micro-climatic environmental conditions. The results are presented for hot days where cold and humid air (from the evaporative cooler) enters the greenhouse from one side and is circulated through the partially porous cavity (representing the plants) and flows through air flow channels and leaves from a vertical thermal chimney. The results show that, with the selected inlet flow conditions, the flow velocity, temperature, and relative humidity can be within the comfort values for plant growth. The effect of some important environmental, design, and operational parameters on greenhouse micro-climatic conditions has also been highlighted.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

COMPUTATIONAL FLUID DYNAMICS OF INDUSTRIAL BATCH DRYERS OF FRUITS

An industrial batch-type, tray air dryer constructed for drying of several fruits, was investigated. Momentum, heat and mass transfer regarding both gaseous and solid phases was simulated using computational fluid dynamics. A mathematical model of the dryer for predicting the turbulent, three-dimensional transfer phenomena inside the industrial batch dryer equipment was developed and analyzed. The model consists of the full set of partial differential equations that describe the conservation of mass, momentum and heat inside the dryer. The standard k-ε model was used to describe turbulence in addition to the governing conservation equations. The simulated profiles of flow field, temperature and humidity of the air phase revealed a lack of spatial homogeneity of air conditions above the product. The situation was found to greatly influence the distribution of material moisture content of the dried product for the various stages of drying. The investigation was carried out for the representative case of sultana raisins.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

ABSTRACT

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

涡流空气分级机熵产与分级性能分析

利用热力学第二定律中的熵产理论对涡流空气分级机各不可逆因素引起的熵产进行分析,通过粉料分级试验对其分级性能进行验证,获得了黏性熵产、湍流熵产和壁面熵产分布特点及操作参数对熵产和分级精度的影响规律。熵产分析结果表明,涡流空气分级机内湍流熵产和壁面熵产占总熵产的比例高达56.41%和43.11%,湍流熵产主要产生于转笼叶片间和转笼内部,进风口和细粉出口壁面剪切引起较大壁面熵产;此外,转笼转速和进口风速变化分别仅对转笼区域和切向进风口区域内气流运动熵产影响较大,进口风速-转笼转速处于8.6m/s、 800r/min和18m/s、1200r/min操作工况附近时,涡流空气分级机内总熵产/总能变化率较小,分级流场稳定性较高,对粗、细颗粒分离有利,该工况下分级机的粉料分级试验效果较好,说明熵产理论可用于涡流分级机内流动分析及其操作参数的优化匹配。     

汽轮机高压反动级叶栅内流动损失的数值分析

采用FLUENT软件对某300MW汽轮机高压反动级叶栅的内部流动损失进行数值分析.结果表明,随着级进汽流量的减少,同一通流部分截面内的熵函数值呈逐渐上升趋势;叶栅压力横向梯度加剧,导致端壁二次涡区影响范围增大,损失逐渐增大;动叶尾部流场紊乱程度增大,湍流动能增大;压力系数沿流动方向逐渐增大,高损失区首先在前缘部分产生.     

在役涡轮搅拌桨混合过程流热耦合有限元分析

化工搅拌装置内的流体流动伴随着显著的传热过程,由于搅拌流动的复杂性以及温度的分布难以测量,采用试验法具有一定的局限性。利用计算流体力学(CFD)软件,采用标准k-ξ紊流模型,分析了非稳态非等温情况下,涡轮搅拌桨混合过程中搅拌槽中流体的三维紊流流场和温度场的分布。结果表明,大型搅拌反应装置采用搅拌桨通蒸汽加热方式,能够使搅拌槽温度分布更加均匀,热量扩散更加迅速。     

LASER-BASED FLOW MEASUREMENTS OF DILUTE VERTICAL PNEUMATIC TRANSPORT

Dilute vertical pneumatic transport in a vertical lifter was studied using the sophisticated measurement techniques of laser Doppler anemometry (LDA) and phase Doppler anemometry (PDA). The vertical lifter consisted of a lower fluidized silo, an upper receiving tank, and a connecting vertical transport pipe made of clear glass. The experimental study was performed in order to get detailed information of the complex gas-particle flow behavior in a dilute vertical conveying system. Particle diameter, axial particle, and tangential particle velocities, as well as root mean square velocities, were measured simultaneously for different flow conditions. In addition, overall solid mass fluxes were obtained using weighing cells. Smooth and spherical zirconium oxide (ZrO₂) solids were applied with two different particle size distributions. Measurements were performed using different flow rates of air. The air inlet condition was varied in order to study its effect on the flow behavior. The particle diameter measurements show that no axial or radial segregation by size occurs for this transport condition. The results show that the particle velocity is independent of the particle size as well. The axial velocity profiles at different heights are almost identical and flat, which indicates fully developed turbulent pipe flow. The turbulent velocity measurements show that turbulence is mainly caused by the velocity gradients, and not by particle-particle collisions in dilute flow. The solid mass flux measurements show the importance of optimum inlet condition and how this influences the mass flux.