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转速对内浸螺旋管组合桨搅拌釜内流动与传热特性的影响

Effect of rotational speed on flow and heat transfer characteristics in internal immersed spiral tube stirred tank with combined impeller

  • 摘要: 搅拌釜通过机械搅拌实现釜内介质的混合、加热、冷却或化学反应,对釜内介质的流动和传热性能有着较高的要求。该研究以典型轴向流和径向流桨叶组成的四斜叶-Rushton组合桨叶为例,通过结构优化设计,将现有搅拌釜桨叶数量由6个改为4个,并在搅拌釜内加螺旋管和挡板,使釜内换热努塞尔数提升约20%。进一步研究了转速对内浸螺旋管搅拌釜内流动和传热特性的影响规律。结果表明:经相似准则缩比处理的数值模型与原型间对流换热系数及功率最大偏差仅为3.75%,证实弗劳德相似准则具有工程适用性。转速增加虽未显著改变螺旋管搅拌釜内流体的轴向循环流态特征,但有效增强了流体湍动能强度。转速从30 r/min增大到90 r/min,釜内轴向速度峰值增加约68.93%,提高了釜内流体的轴向循环,同时管外对流传热系数增大了64.66%,而功率消耗增长27.09倍。研究获得了不同工况下的功率准数,可为高效桨叶的优化设计提供了理论支撑与数据参考。

     

    Abstract: A stirred tank is one of the most key process equipment for the synthesis of energy compounds. It is highly required for the flow and heat transfer performance of the stirred tank, due to the harsh synthetic reaction with the exothermic heat. Therefore, the spiral tube is frequently used to enhance the heat transfer in the stirred tank. Different types of blades can also play a decisive role in the flow fields inside the stirred tank. This study aims to explore the flow and heat transfer behavior of the internal spiral tube in the stirred tank with the four-pitched blade-Rushton impeller. The standard k-ε turbulence model and wall functions were also adopted for the numerical simulation. Typical axial and radial flow blades were selected to form the four-pitched blade-Rushton impeller. A systematic investigation was also made to clarify the influence of the rotational speeds on the flow and heat transfer performance of the internally immersed spiral tube inside the stirred tank. The fluid flow and heat transfer were determined using similarity criteria and multiple reference frames. The general applicability of the simulation was obtained for the flow distribution, turbulence kinetic energy pattern, and stirrer power in the stirred tank. The similarity criterion was used to reduce the calculation volume for the high efficiency of the model. The maximum deviation of 3.75% was achieved to simulate the internal immersed spiral tube in the stirred tank, compared with the prototype. The high accuracy was verified for the Froude similarity criterion during the simulation. The rotational speed increased the fluid turbulent kinetic energy, indicating little influence on the formation of the axial circulation. The optimal model achieved a 20% enhancement in the Nusselt number, demonstrating the superior performance of the structural modifications. The higher the rotational speed was, the higher the mixing and diffusion efficiency in the tank were. According to the velocity observation line graph, the axial velocity was symmetrically distributed about the central axis x=0. The radial and tangential velocities were centrally symmetric about x=0. Each velocity component also increased with the increase of rotational speed. The peak axial velocity in the tank increased by approximately 68.93% with the rotational speeds ranging from 30 to 90 r/min. As such, the axial circulation efficiency was improved inside the stirred tank. The maximum scope of peak velocity was 24.3%~33.33% at 15 r/min. At the same time, the extra-tube convection heat transfer coefficient increased by 64.66%, the magnitude of which in the tube basically remained unchanged with the change of rotational speed. There was a constant amplitude of the fluid flow inside the spiral tube, indicating a stable structure. The higher the rotational speed was, the greater the resistance of the paddles was. The power also increased by 27.09 times with the rotational speeds ranging from 30 to 90 r/min. The power numbers remained consistent across different rotational speeds. The convective heat transfer was fitted to compare for the mixer optimization. Furthermore, the power number exhibited a low sensitivity to the Reynolds number under identical impeller configurations. The maximum deviation of 0.58% was achieved in the combined impeller, indicating the Reynolds number suitable for the power characteristics.

     

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