涡流空气分级机环形区流场的实验研究

为优化涡流空气分级机内的流场,提高分级机性能,研究了涡流空气分级机蜗壳内加水平隔板和不加水平隔板2种结构对分级精度的影响。2种结构分级机环形区流场的激光多普勒测速计测量结果表明:在涡流空气分级机蜗壳内加隔板后,减小了环形区气流的轴向速度,沿转笼轴线方向,切向速度分布趋于均匀一致,流场稳定。重质CaCO3物料实验表明:在所试验的风速和转速范围内,采用较高转速分级,加隔板分级精度降低。环形区内适当的湍流能够提高物料的分散性,对提高分级精度有利。     

涡流空气分级机转笼结构对其分级性能的影响

摘要：研究了涡流空气分级机底盘开口与封闭两种型式的转笼对分级指标的影响。重质CaCO₃物料试验表明：采用底盘开口转笼时,切割粒径小,分级精度低;随着分级转速增加,切割粒径对风速的敏感性下降。另外,用激光多普勒测速计测量了上述两种转笼结构的分级机环形区的流场特性,结果表明：转笼底盘开口,环形区气流出现旁路,进入转笼径向风速减小,造成分级物料切割粒径减小;底盘封闭的分级机环形区内靠近转笼处,切向风速突变增大,特别是轴向上湍流度的增大,有利于团聚物料的分散和分级精度的提高。     

气流预分散对涡流空气分级机分级性能的影响

在物料进入涡流空气分级机前采用了气流预分散装置,使物料悬浮分散并输送到分级机中。以滑石粉和石灰石粉料为原料进行分级实验,研究不同条件下气流预分散对分级性能的影响。采用激光多普勒测速仪对分级机内环形区流场进行速度测定,分析预分散气流对流场的影响。结果表明,当预分散气速大于1.1 m/s时,细粉产率、牛顿分级效率和切割粒径随预分散气速的增大而增加;在不同的系统进口风速条件下,物料预分散后,细粉产率、牛顿分级效率增加;平均粒径小的石灰石粉料其中含有的超细颗粒(<4μm)不易实现气流预分散,预分散气流可以明显地减小对分级不利的轴向速度。     

涡流空气分级机转笼转速对其分级精度的影响

引言随着科学技术的不断发展,建材、化工、矿业等行业都需要特定粒径的超细粉体,涡流空气分级机作为一种重要的分级设备被广泛应用。同时,各行业对分级机的分级效率和分级精度提出了更高的     

涡流分级机环形区流场的激光多普勒测速研究

采用激光多普勒测速计研究了涡流分级机环形区气流的切向速度分布。测量了蜗壳进口风速18 m/s时,从330 r/min到1800 r/min之间6个不同转笼转速条件下的环形区气流切向速度。结果表明:当转笼转速小于1 200 r/min时,转笼转速的变化对环形区气流平均切向速度的数值影响很小;转笼高速旋转,环形区气流切向速度数值大小与转速有关。用系数F表示转笼附近环形区气流切向速度波动程度与转笼转速之间的关系,当转笼低速旋转时F大,极小值点出现在转笼旋转速度与当地气流速度一致时,随着转速的增加,趋于平稳。     

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。     

涡流分级机的分级轮研究

在分级系统中,分级轮对于物料的分散、分级起着非常重要的作用。通过分析气体在分级轮中所受的作用力,推导出分级粒径公式,并得出分级轮的风量、转速与分级粒度及分级精度的关系。     

涡流分级机蜗壳内水平隔板对其流场的影响

为了探究蜗壳结构对涡流空气分级机流场特性的影响,文中利用Fluent软件对蜗壳内增加不同数目水平隔板的涡流空气分级机流场进行数值模拟。研究表明:蜗壳内水平隔板数目对流场速度以及环形区湍流耗散率有不同影响,存在临界隔板数使流场轴向速度分布均匀,在本文条件下的临界隔板数为3;在蜗壳中增加水平隔板可以对转笼外柱面处切向速度产生不同影响,在文中条件下增加1—3层水平隔板可以在一定程度上减小转笼切向速度与气流切向速度的差值,从而改善转笼叶片间的惯性反漩涡;随着隔板数的增加,湍流耗散率得到显著提高,物料分散性变好;涡流空气分级机的结构设计中应当考虑水平隔板对分级流场的影响。     

涡流分级机流场特性分析及分级过程

利用流体运动方程和平面强旋转流的流动特性分析了涡流分级机内空气流动特性和气固两相流运动特性。指出涡流分级机内主气流作二维运动，主气流速度场就时均值而言是一个点汇和一个Rankine涡的叠加，这将引起Tailer柱效应，形成均匀力场，有利于分级效率和精度提高。分析了涡流分级机的分级过程，并用“空气动力筛”概念说明分级原理。提出了改散物料分散性、改进流场均匀性的措施。这将对理解和改进涡流分级机分级性能具有指导作用。     

几何相似涡流空气分级机环形区流场变化规律研究及预测

为探究几何相似涡流空气分级机环形区流场的分布特性,建立不同尺寸的涡流空气分级机模型,提取关键结构的几何参数,通过数值模拟分析尺寸比例因数对分级机环形区流场分布的影响。模拟结果表明,在相同进口风速下,几何相似分级机模型环形区内切向速度呈准自由涡的分布。模型尺寸增大,环形区内柱面面积加权平均切向速度增大,环形区切向速度分布不均匀。转笼内外缘切向速度越接近,环形区内近转笼外缘处切向速度产生的波动越小。几何相似分级机模型环形区内径向速度分布符合点汇流动的规律,除了在靠近转笼外缘处径向速度数值有较大差异外,环形区其余位置柱面面积加权平均径向速度不随比例因数变化而变化。以几何相似分级机模型数值模拟数据作为训练样本拟合分级机环形区内柱面面积加权平均切向速度和径向速度预测公式,并建立测试样本分级机模型对预测公式进行验证。柱面面积加权平均切向速度和径向速度预测值与模拟值的最大误差分别为3.5%和1.8%。此外,通过对几何相似涡流空气分级机模型环形区流场运动相似和动力相似分析,得出几何相似分级机模型流场具有相似性。     

涡流分级机撒料盘的试验研究

研究了涡流分级机撒料盘上凸棱高度对分级精度和分割粒径的影响。试验得出,随着径向凸棱高度的增加,物料分散变差,分割粒径ｄ５０减小,分级精度指数降低。在试验范围内,当撒料盘凸棱高度与内径之比为００３７时,分级精度指数最高。     

Effect of die gap width on annular extrudates by the annular extrudate swell simulation in steady-states

We calculated the steady-state annular extrudate swell of polymer melts through flow geometries encountered in processes used to control parison thickness. A streamline-upwinding finite element method with an under-relaxation for the rate of deformation tensor was used. The Giesekus model was employed as the constitutive equation. An operation that widens the die gap is appropriate for the control of parison thickness corresponding to the change of die gap width. However, a control process that decreases the die gap width is not useful, because the parison thickness does not correspond to the die gap width. Furthermore, thickness swells change strikingly with the Weissenberg number. It is difficult to control the parison outer diameter in the case of a converging die, because the change of the outer diameter swell becomes large with increasing Weissenberg number. In the case of a diverging die, the changing value of the outer diameter swell is smaller than that in the case of a converging die.     

Mass transfer in the entrance region for axial and swirling annular flow

Rates of mass transfer to the inner core of an annular flow system have been determined for the mass transfer entry region using the limiting current density method. Both in laminar and turbulent flow, the hydrodynamic and concentration boundary layers were not fully developed. The variation of the mass transfer coefficients with length of core section has been demonstrated, and the data for swirling flow correlated by the equation for 1500<Re < 14000, 1500 < Sc < 6200 and 1.75 < L/D^e < 10.14. The data for axial flow in the entry region have been correlated by the equation. for 1800 < Re < 12500 and Sc = 2604. Mass transfer enhancement in the entry region due to swirl when compared to axial flow-systems with a jetting transverse inlet is only noticeable for Re > 6000, but is very significant compared to systems with fully developed boundary layers in axial or swirling flow.     

Overall mass transfer in swirling decaying flow in annular electrochemical cells

Mass transfer coefficients between an electrolyte solution and the inner cylinder of an annulus were determined experimentally, using the electrochemical technique, for both laminar and turbulent swirling flows of a liquid induced by means of a tangential inlet in the annular gap. The average mass transfer coefficients were determined as a function of the axial distance from the entrance, for various diameters of the inlet duct and thicknesses of the annular space, for a Reynolds number range of 100 to 5900. Enhancement of mass transfer up to 400% was achieved in comparison to that obtained in fully developed axial flow. Correlations of the experimental data, taking into account the geometrical and hydrodynamic parameters influencing the overall mass transfer, are presented.Nomenclature A Cathode surface area - C Potassium ferricyanide concentration - D Ferricyanide ion diffusion coefficient - d Tube diameter - e=R ₂–R₁ Thickness of the annular gap - ERR Mean square error between experimental and calculated data - F Faraday's constant - f(N) Function of the geometrical parameters - h Local heat transfer coefficient - l ₁ Limiting diffusional current - k Mass transfer coefficient - L,L ₁,L ₂ Axial distances from the inlet - L/2e Reduced axial coordinate - N=R ₁ /R ₂ Radii ratio of the annular gap - n Number of electrons involved in the electrochemical reaction - Nu _x =hx/ Local Nusselt number - Q Volumic flow rate - R ₁ External radius of the inner cylinder - R ₂ Internal radius of the outer cylinder - Re=2eU/ Reynolds number - Re _x =Ux/ Local Reynolds number - S ₀=U /U Initial swirl intensity - Sc=/D Schmidt number - Sh=2ek/D Sherwood number obtained in swirling flow - Sh_a=2ek/D Sherwood number obtained in fully developed axial flow - Sh _x Local Sherwood number - U Mean axial velocity in the annular gap - U Axial velocity in the inlet duct - x Axial coordinate Greek letters , , , , Correlation parameters - Diameter of the tangential inlet - Thermal conductivity - Kinematic viscosity of the electrolyte - Density of the working solution     

一种新型分级机分级效果的研究

介绍了一种新型分级机和由其组成的闭路磨粉系统,指出该系统中影响分级效果的重要因素,并着重对分级机转速和分级叶片处的通流面积两个因素进行了实验研究.结果表明:粉磨方解石时,分级机转速为600 r/min左右、通流面积为0.766 m2时,分级效果较好.     

基于CFD的选粉机分级过程数值模拟

为研究颗粒在选粉机内的分级过程,在对颗粒受力进行理论分析的基础上,对涡流空气选粉机SLK5500分级室平面进行了稳态和非稳态的CFD数值模拟,在稳态模拟中考察了颗粒运动轨迹和分级效率,在非稳态模拟中考察了颗粒的实时运动规律。最后得出,颗粒在选粉机内的分级过程是一个概率事件,粒径较小的颗粒进入细粉的概率大,进入粗粉的概率小,而粒径较大的颗粒进入细粉的概率小,进入粗粉的概率大。通过数值模拟对颗粒的实时运动进行了仿真,为选粉机结构改造和操作参数优化提供了模拟方法和数据基础。     

进料方式对超细分级机分级性能的影响

采用数值模拟方法对两种不同进料方式的分级机进行了气固两相流研究,分析了内部压降、气相速度、固相浓度分布等特性;对两种进料方式的分级机进行了应用研究,对比分析了上下进料方式下的特征粒径和细粉粒径分布特点。结果表明:采用上进料方式由于增加了机械撒料盘,进出口压差更大,同时转子外沿圆柱面上的切向和径向速度分布更为均匀,有利于减小分级粒径的波动,得到粒径分布窄的细粉;采用上进料方式有利于提高颗粒浓度分布的均匀性,但受转速的影响较大;采用上进料和下进料两种方式均能取得较好的分级效果,分级后细粉粒径小且分布窄,主要集中在1~30 μm,30 μm以下的颗粒含量达到90%以上。研究结果对分级机进料方式的设计和结构优化有一定指导作用。     

Investigations in an external-loop airlift photobioreactor with annular light chambers and swirling flow

Photosynthetic microorganisms could serve as valuable compounds, but also for environmental applications. Their production under controlled conditions implies to design specific reactors, named photobioreactors, in which light supply is the main constraint. This paper was devoted to an original external-loop airlift photobioreactor (PBR) with annular light chambers in which a swirling motion was induced. The aim was to characterize this novel geometrical configuration in terms of gas-liquid hydrodynamics, and to test its potentiality for algal cultures. This PBR consisted of two identical columns connected by flanges defining tangential inlets, each column being made of two transparent concentric tubes (6 L in liquid volume, 50 m⁻¹ in specific illuminated area). Firstly, the global flow characteristics (circulation and mixing times) were determined by a tracer method and modelled by an axial dispersed plug flow with complete recirculation (Péclet number). By means of a double optical probe, both local and global time-averaged parameters of the gas phase were measured, namely void fraction, bubble velocity, frequency and size. The gas-liquid mass transfer were also characterized, in tap water and in culture medium, by measuring overall volumetric mass transfer coefficients. In a second time, cultures of the microalga Chlamydomonas reinhardtii were run in batch mode. The variations of biomass concentration and pigment content with time from inoculation were successfully obtained. All these findings highlighted: (i) some significant differences in terms of gas-liquid hydrodynamics between the present PBR and the usual airlift systems, (ii) the interest of this configuration for algal cultures, even if complementary studies and technological improvements are still required for definitively validating its scale-up.     

Experimental studies on a novel swirling fluidized-bed combustor using an annular spiral air distributor

This work reports studies on hydrodynamics as well as combustion and emission characteristics of a conical swirling fluidized-bed combustor (SFBC) using an annular spiral air distributor as the swirl generator. In the experimental study on a ‘cold’ SFBC model, hydrodynamic regimes and characteristics of an air-sand bed were investigated for variable bed particle size and static bed height. Depending on the superficial air velocity, the bed exhibited four operational regimes. Based on the results from the ‘cold’ hydrodynamic study, optimum bed characteristics (sand particle size and bed height) and the range of primary air were determined prior to the combustion tests. In the second part of this work, a conical SFBC was tested for firing 80 kg/h rice husk. During the combustion tests, swirl motion of a fluidized bed was induced by primary air injected into the bed through the air distributor and, also, sustained by tangential injection of secondary air into the bed splash zone. Radial and axial temperature and gas (O₂, CO, NO) concentration profiles in the reactor were obtained for 20-80% excess air. Effects of operating conditions on formation and decomposition of major gaseous pollutants (CO and NO) in the reactor are discussed. Both CO and NO were found to be reduced significantly in the bed splash zone, resulting in quite low CO and moderate NO emissions from the reactor. High combustion efficiency, 99.4-99.5%, is achievable when burning rice husk in the proposed conical SFBC at 80 kg/h feed rate and excess air of 40-80%.