四斜叶桨搅拌下釜内盘管非稳态对流传热过程的模拟和实验研究
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摘要
搅拌釜内的强放热反应过程常常需采用内置螺旋盘管来增加换热面积,对带盘管搅拌釜内传热过程的数值模拟与盘管外侧对流传热系数的实验测定有重要的应用价值。今使用流体力学软件FLUENT对四斜叶桨搅拌下釜内盘管非稳态对流传热过程进行模拟,得到了搅拌釜内温度场分布和盘管内外两侧平均对流传热系数。模拟采用标准k-ε湍流模型和强化壁面函数,考虑了釜内液体黏度、导热系数随温度的变化关系。在模拟搅拌釜的非稳态冷却过程中,将搅拌釜内温度的变化过程分成四段,模拟得到四个釜温下的拟稳态温度场和对流传热系数,在此基础上计算得到全冷却过程的平均对流传热系数。为了验证模拟的可靠性,对盘管外侧对流传热系数进行了实验测定。模拟获得的盘管外侧对流传热系数值与实验值相比误差为22.5%,盘管内侧对流传热系数模拟值与经验公式计算值相比误差为2.6%,不同釜温下盘管出口温度模拟值与实验测定值偏差在1.52 K以内,表明所用模拟方法在工程上是可行的。
Helical coils are frequently used to enhance heat transfer in the stirred tank with strong exothermicreactions. It is significant in industry applications to simulate the heat transfer process and measure the heat transfer coefficient outside the helical coil in stirred tanks. In this work, the unsteady heat transfer process in a stirred tank equipped with helical coils was investigated using software FLUENT. The temperature field and the heat transfer coefficients in both inside and outside regions of the helical coils were obtained. In the simulation, standard k-ε turbulence model and enhanced wall function were used. Both the relationship between viscosity and temperature as well as the relationship between thermal conductivity and temperature were considered during the simulation. The quasi-steady temperature fields were obtained at four different temperatures separately instead of simulating the whole heat transfer process at a constant temperature. Moreover, the heat transfer experiment was conducted to measure the heat transfer coefficient outside the helical coils. The deviation between the CFD calculation results and experimental results is 22.5%. Also, the obtained simulated heat transfer coefficient in the inside region of the coils agrees well with the empirical formula value with deviation of 2.6%. The difference of the helical coils outlet temperature between the simulation value and experiment value is less than 1.52 K. The results confirm that the precision of the simulation is acceptable and it is feasible in engineering application.
Helical coils are frequently used to enhance heat transfer in the stirred tank with strong exothermicreactions. It is significant in industry applications to simulate the heat transfer process and measure the heat transfer coefficient outside the helical coil in stirred tanks. In this work, the unsteady heat transfer process in a stirred tank equipped with helical coils was investigated using software FLUENT. The temperature field and the heat transfer coefficients in both inside and outside regions of the helical coils were obtained. In the simulation, standard k-ε turbulence model and enhanced wall function were used. Both the relationship between viscosity and temperature as well as the relationship between thermal conductivity and temperature were considered during the simulation. The quasi-steady temperature fields were obtained at four different temperatures separately instead of simulating the whole heat transfer process at a constant temperature. Moreover, the heat transfer experiment was conducted to measure the heat transfer coefficient outside the helical coils. The deviation between the CFD calculation results and experimental results is 22.5%. Also, the obtained simulated heat transfer coefficient in the inside region of the coils agrees well with the empirical formula value with deviation of 2.6%. The difference of the helical coils outlet temperature between the simulation value and experiment value is less than 1.52 K. The results confirm that the precision of the simulation is acceptable and it is feasible in engineering application.
引文
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[3]Zakrzewska B,Jaworski Z.CFD modelling of turbulent jacket heat transfer in a Rushton turbine stirred vessel[J].Chemical Engineering&Technology,2004,27(3):237-242.
[4]WANG Zhi-feng(王志锋),HUANG Xiong-bin(黄雄斌),SHI Li-tian(施力田).Measurement and numerical simulation of temperature distributions in agitated tank equipped with PBT impeller(垂直列管加热的搅拌槽中温度场的测量与数值模拟)[J].J Chem Ind and Eng(China)(化工学报),2002,53(11):1175-1181.
[5]QIAN Xiao-jing(钱小静),WANG Zhi-feng(王志锋),Huang Xiong-bin(黄雄斌).Numerical simulation of external heat transfer coefficient of vertical heating tubes in a stirred tank(搅拌槽中垂直列管外壁表面传热系数的模拟计算)[J].The Chinese Journal of Process Engineering(过程工程学报),2007,7(5):853-858.
[6]Lakghomi B,Kolahchian E,Jalali A,et al.Coil and jacket's effects on internal flow behavior&heat transfer in stirred tanks[J].World Academy of Science,Engineering and Technology,2008,24:873-877.
[7]Jayakumar J S,Mahajani S M,Mandal J C,et al.Experimental and CFD estimation of heat transfer in helically coiled heat exchangers[J].Chemical Engineering Research and Design,2008,86(3):221-232.
[8]Kumar V,Faizee B,Mridha M,et al.Numerical studies of a tube-in-tube helically coiled heat exchanger[J].Chemical Engineering and Processing,2008,47(12):2287-2295.
[9]QIN Wen-jie(覃文洁),HU Chun-guang(胡春光),Guo Liang-ping(郭良平),et al.Effect of near-wall grid size on turbulent flow solutions(近壁面网格尺寸对湍流计算的影响)[J].Transations of Beijing Institute of Technology(北京理工大学学报),2006,26(5):387-392.
[10]ZHANG Shi-shuai(张师帅).Computational fluid dynamics and applications(计算流体动力学及其应用)[M].Wuhan(武汉):Huazhong University Press(华中科技大学出版社),2011.
[11]Kelly W J,Humphrey A E.Computational fluid dynamics model for predicting flow of viscous fluids in a large fermentor with hydrofoil flow impellers and internal cooling coils[J].Biotechnol Prog,1998,14(2):248-258.
[12]STOESSEL F(弗朗西斯·施特塞尔).Thermal safety of chemical process:risk assessment and process design(化工工艺的热安全-风险评估与工艺设计)[M].Beijing(北京):Science Press(科学出版社),2009.
[13]Bourne J R,Buerli M,Regnass,W.Heat transfer and power measurement in stirred tanks using heat flow calorimetry[J].Chem Eng Sci,1981,36(2):347-354.
[14]Wilson E E.A basis for rational design of heat transfer apparatus[J].Trans Am Soc Mech Eng,1915,37:47-53.
[15]HE Chao-hong(何潮洪),FENG Xiao(冯霄).Chemiacal engineering principle(化工原理)[M].Beijing(北京):Beijing Science Press(北京科学出版社),2011.
[16]YANG Shi-ming(杨世铭),TAO Wen-quan(陶文铨).Heat transfer(传热学)[M].Beijing(北京):Higher Education Press(高等教育出版社),2006.