喷气涡流纺喷嘴内气流流动的二维数值模拟

 为探讨喷嘴内部气流流动特征对纤维加捻成纱的作用,建立计算流体动力学(CFD)模型,对喷气涡流纺喷嘴中的气流流动进行二维数值模拟,并根据计算结果分析喷嘴内高速气流的速度场和压力场分布。模拟结果表明:气流的最大速度可达超音速,由于切向速度所产生的离心力和加捻作用,纤维的尾端从纤维束中分离出来,并旋转加捻到纤维束上,形成具有真捻的纱线;喷嘴入口区域的压力小于外界气压,这有利于由前罗拉钳口处输出的纤维束吸入喷嘴。研究还表明CFD技术是研究喷气涡流纺喷嘴内气流流动的重要手段。     

基于CFD流场分析的反应釜搅拌器结构改进

承压反应釜在固液混合过程中往往伴随强烈的热传递效应,而密度较低的粉末状固相介质与液体混合的难度相对较大,投料溶入效果不佳和物料结团漂浮等问题较常见。针对此类反应釜的搅拌不充分等问题,采用CFD技术指导反应釜内搅拌器结构进行设计和改进,对反应釜内的固液搅拌混合过程进行了三维数值模拟。数值计算采用多重参考系法(MRF),κ-ε湍流模式以及Mixture多相流模型,模拟了反应釜中的流场形态,通过混合过程的两相流计算得到固相体积分数的分布情况和变化规律,并在此基础上预测了混合时间。结果表明:改进后的搅拌器效果明显增强,该混合过程的CFD模拟可得到相对准确的流场分布及各项特性参数预测结果,可为该类反应釜搅拌器的设计和优化提供参考。     

CFD modelling of flow and scalar exchange of spherical food products: Turbulence and boundary-layer modelling

The performance of several steady Reynolds-averaged Navier–Stokes turbulence models and boundary-layer modelling approaches is evaluated for a single sphere, by comparison with empirical data for a Reynolds number range of 10–3.2 × 10⁴. A sphere serves here as a representative model for many spherical food products. The shear stress transport (SST) k–ω turbulence model performs exceptionally well when combined with low-Reynolds number modelling (LRNM) of the boundary layer, which confirms that the turbulence model characteristics are particularly suitable to deal with this specific flow problem. Especially the k–ε turbulence models are less accurate at higher Reynolds numbers (>10²). Boundary-layer modelling with wall functions (WFs) leads to inaccurate flow-field and scalar transfer predictions, compared to LRNM. However, LRNM grids and their inherently higher computational cost are often not practically feasible, leaving WFs as the only option. It is shown that using cell sizes on the sphere surface of a few millimetres, typical for CFD studies on food products, can compromise accuracy, and grids with smaller cell sizes are actually required.     

稀疏软性磨粒流的磨粒群分布及其动力学特性研究

利用CFD技术和PIV测量研究了结构化表面约束流道内的流场和磨粒群分布。基于液-固两相流体耦合理论,利用欧拉-拉格朗日模型中的DPM模型和标准κ-ε湍流模型对V型纹理矩形截面流道内颗粒运动进行了数值模拟,计算了流场中的湍流性能等参数。结果表明:V型纹理矩形截面约束流道内磨粒群的分布总体均匀,各速度矢量紊乱无序,靠近底部壁面区域颗粒相对比较密集,这些都有利于磨粒对壁面的微力微量切削。     

Determination of Local Heat-Transfer Coefficients Around a Circular Cylinder Under an Impinging Air Jet

Heat transfer coefficients around a model food shaped as a circular cylinder placed on a flat surface and impinged by a slot air jet has been determined using an inverse heat transfer method. The determination was based on time-temperature data measured with a thermocouple in the cylinder and in the air jet. The cylinder was rotated around its horizontal axis to determine the heat-transfer coefficients at different locations around the cylinder. A sensitivity analysis using Monte Carlo simulations was also performed. The local heat-transfer coefficients determined, were compared to computational fluid dynamics (CFD) simulations using the k-ω SST and RSM models. The heat-transfer coefficients determined from temperature measurements was larger than predicted by the CFD simulations. The heat-transfer rates were in better agreement on the upper part of the cylinder, including the decrease along the cylinder due to flow separation, than on the lower part close to the wake recirculation area. The SST model predicted in general a slightly higher heat-transfer rate on the upper part of the cylinder and slightly lower on the lower part of the cylinder, as compared to the RSM model.     

COMPUTATIONAL FLUID DYNAMICS MODELING OF FLUID FLOW IN HELICAL TUBES

The effect of different processing parameters on the degree of mixing and axial and radial pressure drops, during single‐phase flow in helical tubes was investigated by using CFD software. Correlations were developed to calculate axial and radial pressure drops, and also the ratio of maximum to average fluid velocities. All of these quantities were found to be dependent on curvature ratio (ratio of tube diameter to coil diameter). Flow visualization experiments were performed to assess the degree of mixing in different configurations. At identical conditions, the degree of mixing was higher in the system with the large curvature ratio, which is in agreement with the simulation results. A minimum ratio of maximum to average fluid velocities of 1.61 was achieved, representing a 20% reduction in hold tube length for Newtonian fluid in laminar flow.     

Physico-chemical studies on agricultural sprays VIII.—Viscosity and spray drop size of water-in-oil emulsions

The effect of the viscosity of the spray fluid on the size of drops produced from fan-jet nozzles is complex; an increase in viscosity can lead to either an increase or a decrease in the drop size. With water-in-oil emulsions the value of the viscosity governing spray behaviour is the limiting value measured under conditions of very high shear. The factors which influence the limiting value of the viscosity include the disperse phase ratio, the method of mixing and the degree to which the two phases are mixed, the nature and concentration of the emulsifying agents used and the type of toxicant incorporated in the emulsion. Within certain limits, it is shown that the disperse phase ratio provides a convenient means of varying the viscosity of the emulsions and that the viscosity may also be varied by variations in the emulsifying agents and toxicants. Although the method of mixing has a considerable effect on the viscosity of the emulsion, the further mixing that takes place in the spray nozzle governs the effective viscosity of the spray fluid that is emitted from the nozzle. The practical application of the theoretical expressions which describe the formation of drops from fanjet nozzles has been discussed and the relationship between disperse phase ratio, viscosity, nozzle size and operating pressures has been illustrated for several typical water-in-oil emulsions.     

易腐食品冷藏运输车内温度场影响因素仿真研究

运用k-ε湍流模型,对不同厚度的车厢隔热材料、不同送风口位置、不同送风温度及不同送风速度条件下冷藏运输车厢内温度场进行数值模拟。结果表明:冷藏运输车车厢聚氨酯隔热材料合理厚度为100～120mm;制冷设备送风口位于车厢内正前下方位置时,车厢内各个不同典型断面温度均匀性最好,并且厢内各处温度更接近于设定值,送风口位于车厢中间位置次之,送风口位于车厢内正前上方最差;冷藏运输温度要求不同,对制冷设备的最佳送风速度要求也不同,冷藏运输车厢内温度要求高,其合理的送风速度也相应要求高,并且每种冷藏运输温度条件下都有与之对应的最佳风速。     

The role of computational fluid dynamics (CFD) in hair science

The use of computational fluid dynamics (CFD) as a virtual prototyping tool is widespread in the consumer packaged goods industry. CFD refers to the calculation on a computer of the velocity, pressure, and temperature and chemical species concentrations within a flowing liquid or gas. Because the performance of manufacturing equipment and product designs can be simulated on the computer, the benefit of using CFD is significant time and cost savings when compared to traditional physical testing methods. CFD has been used to design, scale-up and troubleshoot mixing tanks, spray dryers, heat exchangers and other process equipment. Recently, computer models of the capillary wicking process inside fibrous structures have been added to CFD software. These models have been used to gain a better understanding of the absorbent performance of diapers and feminine protection products. The same models can also be used to represent the movement of shampoo, conditioner, colorants and other products through the hair and scalp. In this paper, we provide an introduction to CFD and show some examples of its application to the manufacture of consumer products. We also provide sonic examples to show the potential of CFD for understanding the performance of products applied to the hair and scalp.     

空气冷却器开缝翅片传热与流动特性的数值模拟研究

文章采用CFD软件FLUENT对空压机中间冷却器用开缝翅片和平直翅片的流动和传热进行了三维数值模拟。通过建立管翅结构的局部模型,利用标准κ-ε湍流方程,研究了温度、速度等在翅片附近的分布,并对比了平直翅片和开缝翅片的模拟结果。结果显示:开缝翅片对空气流动有较大扰动作用,在开缝结构附近出现了大量涡流,强化了空气和冷却水之间的传热过程。在同样条件下,开缝翅片空气侧换热系数比平直翅片大8%,而开缝翅片的压力降比平直翅片也相应增大。     

叶片形状对旋涡泵性能的影响

在分析旋涡泵叶轮设计方法的基础上应用改进的经验系数法设计了4种不同形状的叶轮。采用基于雷诺时均方程和RNG k-ε湍流模型对不同结构的旋涡泵内部流场进行了数值模拟。结果表明转角叶片性能最好,后倾叶片性能最差,并同时验证了横向旋涡泵和纵向旋涡的存在。结合模拟结果对4种叶片进行特性试验,试验结果与模拟结果基本吻合,由于实验中存在摩擦及泄漏损失,使实验所得的扬程较小。通过对不同形状的叶片的研究,指出了采用转角叶片叶轮的旋涡泵效率比较高的观点,为旋涡泵的设计提供了理论依据和参考价值。     

计算流体力学技术在粮食储藏中的应用

计算流体动力学(computational Fluid Dynamics,简称CFD)是通过计算机数值计算和图像显示,对包含有流体流动、热量和质量传递等相关物理现象所做的分析、计算和优化的数值模拟工具。通过CFD技术在储粮通风工程中的应用实例,对CFD技术进行了介绍,并对采用CFD技术模拟储粮中的通风过程中流动、热湿传递过程和生态系统的研究现状进行了综述,探讨了采用CFD对储粮中的流动、传热传质过程进行数值模拟的优势以及未来发展趋势。     

仿生非光滑表面纺杯减阻性能的数值模拟

基于仿生微小非光滑表面具有减黏降阻特性的基本思想,将高速纺杯表面布置不同形状和尺寸的非光滑沟槽。采用RNG k ～ ε模型对其三维流场进行模拟,分别计算光滑纺杯与非光滑纺杯壁面阻力系数。对比二者壁面剪应力大小与周围速度流场可知,将微小非光滑仿生沟槽表面布置于高速纺杯表面,可降低纺杯在纺纱器内高速气流对壁面的空气阻力,从而降低转杯纺纱机的动力消耗,并且沟槽形状和深度均对纺杯壁面阻力产生不同影响。与光滑纺杯相比,三角形、半圆形、矩形3种沟槽纺杯最大减阻率分别为1.271%、1.261%和1.385%。     

具有仿生表面结构转杯减阻性能的数值模拟研究

基于微小非光滑沟槽表面具有湍流减阻的特性的基本思想,将高速转杯表面布置不同尺寸的三角形沟槽。采用RNGk-ε模型对其三维流场进行模拟,分别计算光滑转杯与非光滑转杯总阻力系数,对比二者壁面剪应力大小,得出转杯壁面布有微小非光滑沟槽能够起到一定的减阻效果,沟槽深度和间隔均对总阻力产生影响,与光滑转杯相比最大减阻率为1．281％。     

计算流体力学在食品传热传质过程中的应用

对计算流体力学(CFD)技术进行了简单介绍，并对采用CFD技术模拟食品加工过程中的传热传质过程的研究现状进行了综述，探讨了采用CFD对这些传热传质过程进行数值模拟的优势。CFD模拟的结果详细、直观，模拟得到的流体流动和各种热力学参数的分布信息，可以为装置的设计、传热过程的优化提供参考。     

An investigation of bread-baking process in a pilot-scale electrical heating oven using computational fluid dynamics

A computational fluid dynamics (CFD) model was developed for bread-baking process in a pilot-scale baking oven to find out the effect of hot air distribution and placement of bread on temperature and starch gelatinization index of bread. In this study, product (bread) simulation was carried out with different placements of bread. Simulation results were validated with experimental measurements of bread temperature. This study showed that nonuniform air flow pattern inside the oven cavity leads to uneven temperature distribution. The study with respect to placement of bread showed that baking of bread in upper trays required shorter baking time and gelatinization index compared to those in the bottom tray. The upper tray bread center reached 100 °C at 1200 s, whereas starch gelatinization completed within 900 s, which was the minimum baking index. Moreover, the heat penetration and starch gelatinization were higher along the sides of the bread as compared to the top and bottom portions of the bread.     

熔盐密度对熔盐泵性能的影响

基于标准κ-ε湍流模型、多重参考坐标系和SIMPLE算法,采用FLUENT数值模拟软件,对3台不同比转速的熔盐泵,针对4种密度不同的介质在5种不同流量下分别进行数值模拟,预测了相应工况下熔盐泵的水力性能。结果表明,泵的扬程与密度变化无关,但相同流量下同一泵的轴功率和效率会随着密度的增加相应上升,泵内静压与速度分布规律基本一致。研究结果对熔盐泵的设计具有一定的参考价值。     

不同孔分布孔板的水力空化效果的数值模拟

基于大型流场计算软件FLUENT,选用标准κ-ε双方程湍流模型,针对多孔孔板水力空化器中圆形阵列、圆形均布和水平均布3种孔分布方式,对孔板汽含率、湍动能及流线速度矢量分布图等水力空化效果进行了数值模拟计算。结果表明,水平均布多孔孔板的空化效果优于圆形阵列和圆形均布多孔孔板,圆形均布孔板空化效果好于圆形阵列孔板;孔的分布方式对流速影响不大,但对孔板后临近壁面处漩涡产生的位置及数量有影响;空化大部分发生在孔板小孔内流域中,且高汽含率区域发生在孔板末端。     

A 3-D computational fluid dynamics model for forced air cooling of eggs placed in trays

Freshly laid shell eggs must be cooled quickly for controlling Salmonella Enteritidis (SE) growth. To fulfill a research need identified by Food Safety and Inspection Service (FSIS), a 3-D computational fluid dynamics (CFD) model was developed to predict the temperature of eggs placed on a tray (6 rows × 5 columns) under forced air cooling. The continuity, momentum, and energy equations were solved along with standard k − ε turbulence model using PHOENICS software. The model was validated by conducting experiments in a wind tunnel at various air temperatures (7-11 °C) and velocities (0.3-0.7 m/s). Root mean square error for predicting yolk temperatures was within 1 °C. Finally, the CFD model was integrated with a microbial growth model to estimate the risk of SE growth during cooling. This model can be incorporated into the FSIS risk assessment model for more accurate estimation of SE risk in shell eggs.