柱形流化床传热特性的数值模拟
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摘要
对Shedid等搭建的圆柱体流化床采用欧拉-欧拉法进行三维数值模拟,考察了颗粒球形度、表观进气速度和床料初始堆积高度对流化床内垂直加热壁面与流动床料之间对流传热特性的影响,采用有效导热系数分别计算气相和固相的对流传热系数。结果表明,随表观进气速度增大,流化床内颗粒物料湍流运动加剧,加热壁面平均温度和流体平均温度下降,壁面流体间传热平均温度差减小,壁面流体间对流传热系数增大;随初始床料高度增加,流化床内颗粒与加热壁面的接触面积增大,导致固相平均对流传热系数增大。
Based on the cylinder fluidized bed built by Shedid and Hassanto, a threedimensional Euler-Euler simulation of the effect on the convective heat transfer characteristics between the fluidized particles and the inner heated surface was carried out under different operation conditions including particles sphericity, superficial gas velocity and initial solid packing height in the vertical fluidized bed. Moreover, the experimental average temperature had been chosen to test the validity of numerical average temperature. Contour plots of transient distribution of solid volume fraction and solid temperature havebeen obtained in fluidized bed on horizontal cross-section in order to understand the effects of hydrodynamic and flow patterns on heat transfer characteristics. The results showed that the solid phase concentration distributed from the initial centrally symmetric annular stratification to the final severely turbulent fluidization by observing contour plots of the solid-phase volume fraction on horizontal cross-section. Solid temperature decreased from center to periphery in the radial direction at initial state since the gas-solid heat exchange rate affected the particle temperature in the entire bed. The temperature distribution of particles was non-uniform on the annular region due to the bed without being fluidized. With the process of fluidization underway, the particles temperature distribution tended to be uniform in horizontal cross-section because the bed material heat transferred from the center's cylindrical heater wall to the bed. The effective thermal conductivity was used to calculate the individual gas and solid phase convective heat transfer coefficient from heater surface to fluidized beds. Not only the average temperature of both heating wall and fluid but the difference of the average temperature between wall and fluid were decreasing with increasing superficial gas velocity. It enhanced turbulence intensity and led to increase the heat transfer coefficient between heater surface and fluid with the same time. The solid average convective heat transfer coefficient growed up with the initial height increasing of the bed material due to the contact area enlargement of particles and the heated surface in the fluidized bed.
Based on the cylinder fluidized bed built by Shedid and Hassanto, a threedimensional Euler-Euler simulation of the effect on the convective heat transfer characteristics between the fluidized particles and the inner heated surface was carried out under different operation conditions including particles sphericity, superficial gas velocity and initial solid packing height in the vertical fluidized bed. Moreover, the experimental average temperature had been chosen to test the validity of numerical average temperature. Contour plots of transient distribution of solid volume fraction and solid temperature havebeen obtained in fluidized bed on horizontal cross-section in order to understand the effects of hydrodynamic and flow patterns on heat transfer characteristics. The results showed that the solid phase concentration distributed from the initial centrally symmetric annular stratification to the final severely turbulent fluidization by observing contour plots of the solid-phase volume fraction on horizontal cross-section. Solid temperature decreased from center to periphery in the radial direction at initial state since the gas-solid heat exchange rate affected the particle temperature in the entire bed. The temperature distribution of particles was non-uniform on the annular region due to the bed without being fluidized. With the process of fluidization underway, the particles temperature distribution tended to be uniform in horizontal cross-section because the bed material heat transferred from the center's cylindrical heater wall to the bed. The effective thermal conductivity was used to calculate the individual gas and solid phase convective heat transfer coefficient from heater surface to fluidized beds. Not only the average temperature of both heating wall and fluid but the difference of the average temperature between wall and fluid were decreasing with increasing superficial gas velocity. It enhanced turbulence intensity and led to increase the heat transfer coefficient between heater surface and fluid with the same time. The solid average convective heat transfer coefficient growed up with the initial height increasing of the bed material due to the contact area enlargement of particles and the heated surface in the fluidized bed.
引文
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[25]Olsson S E,Almstedt A E.Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes:influence of pressure,fluidization velocity and tube-bank geometry[J].Chemical Engineering Science,1995,50(20):3231-3245.
[2]Kalita P,Saha U K,Mahanta P.Parametric study on the hydrodynamics and heat transfer along the riser of a pressurized circulating fluidized bed unit[J].Experimental Thermal and Fluid Science,2013,44(1):620-630.
[3]Dehnavi M A,Shahhosseini S,Hashemabadi S H,et al.CFDsimulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors[J].International Communications in Heat and Mass Transfer,2010,37(4):437-442.
[4]Lim K S,Gururajan V S,Agarwal P K.Mixing of homogeneous solids in bubbling fluidized beds:theoretical modelling and experimental investigation using digital image analysis[J].Chemical Engineering Science,1993,48(12):2251-2265.
[5]Gao J,Lan X,Fan Y,et al.Hydrodynamics of gas-solid fluidized bed of disparately sized binary particles[J].Chemical Engineering Science,2009,64(20):4302-4316.
[6]Yusuf R,Halvorsen B,Melaaen M C.Eulerian-Eulerian simulation of heat transfer between a gas-solid fluidized bed and an immersed tube-bank with horizontal tubes[J].Chemical Engineering Science,2011,66(8):1550-1564.
[7]Wang S Y,Liu G D,Wu Y B,et al.Numerical investigation of gasto-particle cluster convective heat transfer in circulating fluidized beds[J].International Journal of Heat and Mass Transfer,2010,53(15/16):3102-3110.
[8]Abdelmotalib H M,Ko D G,Im I T.A study on wall-to-bed heat transfer in a conical fluidized bed combustor[J].Applied Thermal Engineering,2016,99(7):928-937.
[9]Armstrong L M,Gu S,Luo K H.Study of wall-to-bed heat transfer in a bubbling fluidised bed using the kinetic theory of granular flow[J].International Journal of Heat and Mass Transfer,2010,53(21):4949-4959.
[10]Lu Y,Zhang T,Dong X.Bed to wall heat transfer in supercritical water fluidized bed:comparison with the gas-solid fluidized bed[J].Applied Thermal Engineering,2015,88(2):297-305.
[11]Mostafazadeh M,Rahimzadeh M,Hamzei M.Numerical analysis of the mixing process in a gas-solid fluidized bed reactor[J].Powder Technology,2013,239:422-433.
[12]Gidaspow D.Hydrodynamics of fluidization and heat transfer:supercomputer modeling[J].Applied Mechanics Reviews,1986,39(1):1-23.
[13]Hua L N,Zhao H,Li J,et al.Eulerian-Eulerian simulation of irregular particles in dense gas-solid fluidized beds[J].Powder Technology,2015,284:299-311.
[14]Behjat Y,Shahhosseini S,Hashemabadi S H.CFD modeling of hydrodynamic and heat transfer in fluidized bed reactors[J].International Communications in Heat and Mass Transfer,2008,35(3):357-368.
[15]Schmidt A,Renz U.Eulerian computation of heat transfer in fluidized beds[J].Chemical Engineering Science,1999,54(22):5515-5522.
[16]Armstrong L M,Gu S,Luo K H.The influence of multiple tubes on the tube-to-bed heat transfer in a fluidized bed[J].International Journal of Multiphase Flow,2010,36(11/12):916-929.
[17]Shedid M H,Hassan M A M.Heat transfer characteristics of the fluidized bed through the annulus[J].Heat Mass Transfer,2016,52(9):1943-1952.
[18]Gidaspow D,He Y,Lu H.Hydrodynamic modeling of binary mixture in a gas bubbling fluidized bed using the kinetic theory of granular flow[J].Chemical Engineering Science,2003,58(7):1197-1205.
[19]Wen C Y,Yu Y H.Mechanics of fluidization[J].Chemical Engineering Progress Symposium Series,1966,62(1):100-111.
[20]Ergun S.Fluid flow through packed columns[J].Chemical Engineering Progress,1952,48(2):89-94.
[21]Natale F D,Lancia A,Nigro R.Surface-to-bed heat transfer in fluidised beds:effect of surface shape[J].Powder Technology,2007,174(3):75-81.
[22]El-Behery S M,El-Askary W A,Hamed M H,et al.Hydrodynamic and thermal fields analysis in gas-solid two-phase flow[J].International Journal of Heat and Fluid Flow,2011,32(3):740-754.
[23]Kuipers J A M,Prints W,Van-Swaaij W P M.Numerical calculation of wall-to-bed heat transfer coefficients in gas-fluidized beds[J].AIChE Journal,1992,38(7):1079-1091.
[24]Patil D J,Smit J,Van Sint Annaland M,et al.Wall-to-bed heat transfer in gas-solid bubbling fluidized bed[J].AIChE Journal,2006,52(1):58-74.
[25]Olsson S E,Almstedt A E.Local instantaneous and time-averaged heat transfer in a pressurized fluidized bed with horizontal tubes:influence of pressure,fluidization velocity and tube-bank geometry[J].Chemical Engineering Science,1995,50(20):3231-3245.