新型复合搅拌桨自吸气过程机理及气液分散特性研究
发布时间：2018-11-11 22:13
【摘要】：本文针对一种表面吸气式长桨短叶片复合搅拌器(LSB搅拌桨),采用实验研究与数值模拟相结合的方法,对该LSB桨在平底搅拌釜内的自吸气过程及气液分散特性进行研究。分别考察了操作条件及搅拌釜结构参数,如液位高度、挡板到釜壁的距离、液体性质、反应釜的几何尺寸等,对临界搅拌转速(NC)的影响,并对搅拌釜内气泡大小及其分布进行测量与分析。首先,用观察法对临界搅拌转速进行定义,当固定釜底间隙(C/T= 0.25),液位高度变化范围为H/T=0.5-1.8时,长桨(LBs)为部分浸没,结果表明,随着液位的升高,NC先降低后升高最后趋于定值,且在H/T=1.0左右出现最小值。这是由于在液位较低时,短叶片(SBs)距离液面较近,短叶片对表面吸气起着重要作用;在液位较高时,短叶片对表面吸气的影响可忽略,长桨控制着表面吸气。针对具有不同尺寸的搅拌釜(T= 200mm,400 mm,600 mm),测量了不同液位下的NC,将NC与临界叶端速度进行关联(Utip,c=πDLNC),结果表明,当搅拌桨的特征尺寸DL180mm时,utip,c随着DL的增加而增加,当DL达到180mm后,DL对Utip,c的影响较小,Utip,c趋于定值(Utip,c= 0.61 m/s)。其次,采用大涡模拟(LES)耦合流体体积法(VOF)对搅拌釜内表面吸气过程进行数值模拟。分别从液面波动、静压分布、湍涡分布、速度场分布及吸气过程对表面临界吸气现象进行分析,数值模拟结果表明,当长桨在液体表面划过后,气液接触面处的液相轴向平均速度(uz)和轴向脉动速度(uz,rms)在径向范围(0.4r/R0.5)内出现最大值(uz,max≈0.5 m/s,uz,rms,max≈0.0/s),证明了在长桨背水面形成液体凹陷区,随后凹陷区上方的液体回填,将气体卷吸入液下。最后,建立了照相法测量气液两相流气泡大小及其分布的方法,对LSB搅拌釜内气-液分散特性进行实验研究,所得图像通过自主开发的图像处理软件进行处理。分别采用Canny算子与Sobel算子进行气泡识别与边缘检测,发现采用Canny算子得到的气泡分布与实验结果较为吻合。从实验结果来看,当LSB搅拌桨的转速N= 100rpm～140rpm时,釜内气泡的直径约为0.5mm～6mm,且随着搅拌转速增加,气泡直径分布趋于均一。
[Abstract]:In this paper, the self-inspiratory process and gas-liquid dispersion characteristics of the LSB impeller in a flat-bottom agitator are studied by means of experimental study and numerical simulation, aiming at a surface suction type long propeller and short blade composite agitator (LSB agitator). The effects of operating conditions and structural parameters of agitator, such as the height of liquid level, the distance from the baffle to the wall of the reactor, the liquid properties, the geometry of the reactor and so on, on the critical stirring speed (NC) were investigated. The size and distribution of bubbles in agitator were measured and analyzed. First of all, the critical stirring speed is defined by observation method. When the fixed bottom clearance (C / T = 0.25) and the liquid level height range is H/T=0.5-1.8, the (LBs) of the long propeller is partially immersed. The results show that, With the increase of the liquid level, the NC decreases first, then increases to a fixed value, and the minimum value appears around H/T=1.0. This is because when the liquid level is low, the short blade (SBs) is close to the liquid surface, and the short blade plays an important role in the surface suction, while at the higher liquid level, the influence of the short blade on the surface suction is negligible, and the long blade controls the surface suction. For a stirred tank with different sizes (T = 200mm / 400 mm,600 mm), NC, at different liquid levels was measured to correlate NC with critical vane end velocity (Utip,c= 蟺 DLNC), results show that when the characteristic size of the agitator is DL180mm, utip,) C increased with the increase of DL. When DL reached 180mm, DL had little effect on Utip,c, and Utip,c tended to be fixed (Utip,c= 0.61 m / s). Secondly, the large eddy simulation (LES) coupled fluid volume method (VOF) is used to simulate the suction process on the inner surface of the stirred tank. The critical inspiratory phenomena on the surface are analyzed from surface wave, static pressure distribution, turbulent vortex distribution, velocity field distribution and suction process, respectively. The numerical simulation results show that when the propeller is drawn on the liquid surface, The axial average velocity (uz) and the axial pulsation velocity (uz,rms) of the liquid phase at the gas-liquid interface are maximum in the radial range (0.4r/R0.5) (uz,max 鈮，
本文编号：2326289
[Abstract]:In this paper, the self-inspiratory process and gas-liquid dispersion characteristics of the LSB impeller in a flat-bottom agitator are studied by means of experimental study and numerical simulation, aiming at a surface suction type long propeller and short blade composite agitator (LSB agitator). The effects of operating conditions and structural parameters of agitator, such as the height of liquid level, the distance from the baffle to the wall of the reactor, the liquid properties, the geometry of the reactor and so on, on the critical stirring speed (NC) were investigated. The size and distribution of bubbles in agitator were measured and analyzed. First of all, the critical stirring speed is defined by observation method. When the fixed bottom clearance (C / T = 0.25) and the liquid level height range is H/T=0.5-1.8, the (LBs) of the long propeller is partially immersed. The results show that, With the increase of the liquid level, the NC decreases first, then increases to a fixed value, and the minimum value appears around H/T=1.0. This is because when the liquid level is low, the short blade (SBs) is close to the liquid surface, and the short blade plays an important role in the surface suction, while at the higher liquid level, the influence of the short blade on the surface suction is negligible, and the long blade controls the surface suction. For a stirred tank with different sizes (T = 200mm / 400 mm,600 mm), NC, at different liquid levels was measured to correlate NC with critical vane end velocity (Utip,c= 蟺 DLNC), results show that when the characteristic size of the agitator is DL180mm, utip,) C increased with the increase of DL. When DL reached 180mm, DL had little effect on Utip,c, and Utip,c tended to be fixed (Utip,c= 0.61 m / s). Secondly, the large eddy simulation (LES) coupled fluid volume method (VOF) is used to simulate the suction process on the inner surface of the stirred tank. The critical inspiratory phenomena on the surface are analyzed from surface wave, static pressure distribution, turbulent vortex distribution, velocity field distribution and suction process, respectively. The numerical simulation results show that when the propeller is drawn on the liquid surface, The axial average velocity (uz) and the axial pulsation velocity (uz,rms) of the liquid phase at the gas-liquid interface are maximum in the radial range (0.4r/R0.5) (uz,max 鈮，
本文编号：2326289
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