多级闪蒸海水淡化系统中结垢过程的数值模拟
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摘要
以多级闪蒸海水淡化预热器为研究对象,运用Fluent软件对换热管内的析晶污垢形成过程进行数值模拟。通过编写UDF在控制方程中添加组分源项,修正流体组分的浓度分布,同时考虑H_2O分子对析晶污垢生成速率的贡献,使模拟结果更符合实际情况。探讨了流速、壁面温度对污垢形成速率的影响以及管道径向和轴向方向的析晶规律。以CaSO_4·2H_2O污垢在换热管表面的形成过程为例,验证了提出的模型和求解方法。结果表明:污垢的净生成率随管内流速的增大而降低,随壁面温度的增大而显著增大,且流速的适宜取值范围为1.0~1.5m/s;污垢热阻随管内流速的增大而减小。沿径向方向即从管道中心到管壁,污垢生成速率逐渐增大,而沿轴向方向,管子入口处的污垢净生成率最小。
The fouling process of crystallization in the pre-heater of multi-stage flash was simulated using the CFD code FLUENT.The concentration distribution in the heat exchanger tube was modified by editing UDF to add component source terms to flow-controlled equations.At the same time,the contribution of H_2O to the fouling rate was considered to provide more exact description of the complicated fouling process.The effects of velocity and tube wall temperature on the fouling rate were investigated and the distribution rule of the crystallization fouling along the radical and axial direction was discussed.A case study of CaSO_4·2 H_2O crystallization fouling process on the heat surface was adopted to test and verify the proposed model and solving method.The result shows that the fouling net generation rate decreases with the increment of velocity and increased with the increment of the wall temperature.The optimum range of velocity is 1.0~1.5 m/s considering both fouling and energy consumption.The fouling thermal resistance decreases when the velocity increased.In addition,the fouling generation rate increased generally along the radical direction of tube from center to wall,which was usually bigger inlet than inside along the axial direction.
The fouling process of crystallization in the pre-heater of multi-stage flash was simulated using the CFD code FLUENT.The concentration distribution in the heat exchanger tube was modified by editing UDF to add component source terms to flow-controlled equations.At the same time,the contribution of H_2O to the fouling rate was considered to provide more exact description of the complicated fouling process.The effects of velocity and tube wall temperature on the fouling rate were investigated and the distribution rule of the crystallization fouling along the radical and axial direction was discussed.A case study of CaSO_4·2 H_2O crystallization fouling process on the heat surface was adopted to test and verify the proposed model and solving method.The result shows that the fouling net generation rate decreases with the increment of velocity and increased with the increment of the wall temperature.The optimum range of velocity is 1.0~1.5 m/s considering both fouling and energy consumption.The fouling thermal resistance decreases when the velocity increased.In addition,the fouling generation rate increased generally along the radical direction of tube from center to wall,which was usually bigger inlet than inside along the axial direction.
引文
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[14]Hasson D,Avriel M,Resnick W,et al.Mechanism of calcium carbonate scale deposition on heat-transfer surfaces[J].Industrial&Engineering Chemistry Fundamentals,1968,7(1):632-637.
[15]Brahim F,Augustin W,Bohnet M.Numerical Simulation of the Fouling on Structured Heat Transfer Surfaces(Fouling)[C].//Heat Exchanger Fouling and Cleaning:Fundamentals and Applications,New Mexico,USA:Engineering Conference International(ECI),2004.
[16]Krause S.Fouling of heat transfer surfaces by crystallization and sedimentation[J].International Chemical Engineering(A Quarterly Journal of Translations from Russia,Eastern Europe and Asia);(United States),1993,33:3(3):355-401.
[2]Guan B,Yang L,Wu Z.Effect of Mg2+ions on the nucleation kinetics of calcium sulfate in concentrated calcium shloride Solutions[J].Industrial&Engineering Chemistry Research,2010,49(12):5569-5574.
[3]Pkk9nen T M,Riihimki M,Simonson C J,et al.Crystallization fouling of CaCO3Analysis of experimental thermal resistance and its uncertainty[J].International Journal of Heat and Mass Transfer,2012,55(23-24):6927-6937.
[4]Melo L F,Bott T R.Biofouling in water systems[J].Experimental Thermal&Fluid Science,1997,14(4):375-381.
[5]杨大章,柳建华,鄂晓雪,等.海水冷却水结晶污垢分析及其生长模型[J].化工进展,34(8):3179-3182.YANG Dazhang,LIU Jianhua,E Xiaoxue,et al.Composition analysis and kinetic modeling of crystallization fouling in cooling seawater[J].Chemical Industry and Engineering Progress,2015,34(8):3179-3182.
[6]Brahim F,Augustin W,Bohnet M.Numerical simulation of the fouling process[J].International Journal of Thermal Sciences,2003,42(3):323-334.
[7]Nebot E,Casanueva J F.Model for fouling deposition on power plant steam condensers cooled with seawater:Effect of water velocity and tube material[J].International Journal of Heat&Mass Transfer,2007,50(17):3351-3358.
[8]程浩明.CaCO3污垢生长过程的数值模拟[D].吉林:东北电力大学,2009.CHENG Haoming.Numerical Simulation of the CaCO3 Fouling Process[D].Jilin:Northeast Dianli University,2009.
[9]Hasson D,Avriel M,Resnick W,et al.Mechanism of calcium carbonate scale deposition on heat-transfer surfaces[J].Industrial&Engineering Chemistry Fundamentals,1968,7(1):632-637.
[10]Zhang F,Xiao J,Chen X D.Towards predictive modeling of crystallization fouling:apseudo-dynamic approach[J].Food&Bioproducts Processing,2014,93:188-196.
[11]孙卓辉.换热面上结垢过程数值模拟[D].青岛:中国石油大学(华东),2008.SUN Zhuohui.Numerical Simulation of Fouling Process on Heat Transfer Surface[D].Qingdao:China University of Petroleum(East China),2008.
[12]Mwaba M G,Golriz M R,Gu J.A semi-empirical correlation for crystallization fouling on heat exchange surfaces[J].Applied Thermal Engineering,2006,26(4):440-447.
[13]Bansal B,Chen X D,Müller-Steinhagen H.Analysis of'classical'deposition rate law for crystallisation fouling[J].Chemical Engineering&Processing Process Intensification,2008,47(8):1201-1210.
[14]Hasson D,Avriel M,Resnick W,et al.Mechanism of calcium carbonate scale deposition on heat-transfer surfaces[J].Industrial&Engineering Chemistry Fundamentals,1968,7(1):632-637.
[15]Brahim F,Augustin W,Bohnet M.Numerical Simulation of the Fouling on Structured Heat Transfer Surfaces(Fouling)[C].//Heat Exchanger Fouling and Cleaning:Fundamentals and Applications,New Mexico,USA:Engineering Conference International(ECI),2004.
[16]Krause S.Fouling of heat transfer surfaces by crystallization and sedimentation[J].International Chemical Engineering(A Quarterly Journal of Translations from Russia,Eastern Europe and Asia);(United States),1993,33:3(3):355-401.