超临界CO_2在螺旋管中的流动换热特性研究
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摘要
超临界蒸发器应用到核电中,可大幅提高机组的热效率。超临界压力流体的热物性在准临界温度附近变化非常剧烈,会对其流动和换热产生很大的影响。研究超临界压力流体在螺旋管内的流动和换热规律,有利于对超临界螺旋管蒸发器的设计。本文采用RNGk-ε和SSTk-ω模型对超临界CO_2在螺旋管中的流动换热情况进行了数值模拟,发现SSTk-ω模型模拟结果与实验结果符合得更好。基于此模型,分析了不同进口质量流速及不同热流密度对管壁温和换热系数的影响,发现随着质量流速的减小、热流密度的增加,峰值向远离hpc的一侧偏移。最后讨论并分析了周向壁温和换热系数的分布情况,发现壁温在φ=315°处最高,需在实验操作或实际运行中加以监控,以保障螺旋管蒸发器的安全运行。
Applying supercritical evaporator to nuclear power can greatly improve the thermal efficiency of the unit.Due to the significant variations of fluid properties in the pseudo-critical region,it is important to study the flow and heat transfer characteristics of supercritical pressure fluid in helical tubes,which helps to design the supercritical helical tube evaporator.The heat transfers of CO_2 at supercritical pressures in a helical tube under constant heat flux conditions were numerically investigated in the present study.Both RNGk-εturbulence model and SST k-ωturbulence model were applied in the simulations,and the numerical calculation results show that the SSTk-ωturbulence model agrees better with the experimental data.Effects of mass flow and heat flux on wall temperature and heat transfer coefficient were analyzed based on this model.It isfound that with the decrease of mass flow or the increase of heat flux,the peak of heat transfer coefficient is further away fromhpc.Circumferential wall temperature and heat transfer coefficient distributions at different cross sections along the flow direction were also studied.The results show that the highest wall temperature occurs atφof 315°,which should be gained extensive attention of operation in order to ensure the safe operation of helical tube evaporator.
Applying supercritical evaporator to nuclear power can greatly improve the thermal efficiency of the unit.Due to the significant variations of fluid properties in the pseudo-critical region,it is important to study the flow and heat transfer characteristics of supercritical pressure fluid in helical tubes,which helps to design the supercritical helical tube evaporator.The heat transfers of CO_2 at supercritical pressures in a helical tube under constant heat flux conditions were numerically investigated in the present study.Both RNGk-εturbulence model and SST k-ωturbulence model were applied in the simulations,and the numerical calculation results show that the SSTk-ωturbulence model agrees better with the experimental data.Effects of mass flow and heat flux on wall temperature and heat transfer coefficient were analyzed based on this model.It isfound that with the decrease of mass flow or the increase of heat flux,the peak of heat transfer coefficient is further away fromhpc.Circumferential wall temperature and heat transfer coefficient distributions at different cross sections along the flow direction were also studied.The results show that the highest wall temperature occurs atφof 315°,which should be gained extensive attention of operation in order to ensure the safe operation of helical tube evaporator.
引文
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[14]王淑香,牛志愿,张伟,等.超临界压力下CO2在螺旋管中沿程传热的实验研究[J].核动力工程,2014,35(1):28-31.WANG Shuxiang,NIU Zhiyuan,ZHANG Wei,et al.Experimental investigations on heat transfer of supercritical CO2flowing through a helically coiled tube[J].Nuclear Power Engineering,2014,35(1):28-31(in Chinese).
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[2]LI M J,ZHU H H,GUO J Q,et al.The development technology and applications of supercritical CO2 power cycle in nuclear energy,solar energy and other energy industries[J].Applied Thermal Engineering,2017,126:255-275.
[3]JIANG P X,LIU B,ZHAO C R,et al.Convection heat transfer of supercritical pressure carbon dioxide in a vertical micro tube from transition to turbulent flow regime[J].International Journal of Heat and Mass Transfer,2013,56(1):741-749.
[4]JIANG P X,ZHANG Y,ZHAO C R,et al.Convection heat transfer of CO2,at supercritical pressures in a vertical mini tube at relatively low Reynolds numbers[J].Experimental Thermal&Fluid Science,2008,32(8):1 628-1 637.
[5]张震,杨星团,姜培学.超临界水在垂直管内换热及流动不稳定性研究[J].原子能科学技术,2015,49(11):2 011-2 016.ZHANG Zhen,YANG Xingtuan,JIANG Peixue.Study on heat transfer and flow instability of supercritical water in vertical tube[J].Atomic Energy Science and Technology,2015,49(11):2 011-2 016(in Chinese).
[6]赵民富,张国欣,陈玉宙.竖直圆管内超临界水传热特性数值模拟[J].原子能科学技术,2011,45(2):146-149.ZHAO Minfu,ZHANG Guoxin,CHEN Yuzhou.Numerical simulation on heat transfer behavior of supercritical water in vertical pipe[J].Atomic Energy Science and Technology,2011,45(2):146-149(in Chinese).
[7]GUPTA S,SALTANOV E,MOKRY S J,et al.Developing empirical heat-transfer correlations for supercritical CO2flowing in vertical bare tubes[J].Nuclear Engineering and Design,2013,261:116-131.
[8]WANG C,LI H.Evaluation of the heat transfer correlations for supercritical pressure water in vertical tubes[J].Heat Transfer Engineering,2014,35(6-8):685-692.
[9]NICENO B,SHARABI M.Large eddy simulation of turbulent heat transfer at supercritical pressures[J].Nuclear Engineering and Design,2013,261:44-55.
[10]YOO J Y.The turbulent flows of supercritical fluids with heat transfer[J].Annual Review of Fluid Mechanics,2013,45:495-525.
[11]ZHANG W,WANG S,LI C,et al.Mixed convective heat transfer of CO2at supercritical pressures flowing upward through a vertical helically coiled tube[J].Applied Thermal Engineering,2015,88:61-70.
[12]KIM J K,JEON H K,LEE J S.Wall temperature measurement and heat transfer correlation of turbulent supercritical carbon dioxide flow in vertical circular/non-circular tubes[J].Nuclear Engineering and Design,2007,237(15):1 795-1 802.
[13]KIM J K,JEON H K,LEE J S.Wall temperature measurements with turbulent flow in heated vertical circular/non-circular channels of supercritical pressure carbon-dioxide[J].International Journal of Heat and Mass Transfer,2007,50(23):4 908-4 911.
[14]王淑香,牛志愿,张伟,等.超临界压力下CO2在螺旋管中沿程传热的实验研究[J].核动力工程,2014,35(1):28-31.WANG Shuxiang,NIU Zhiyuan,ZHANG Wei,et al.Experimental investigations on heat transfer of supercritical CO2flowing through a helically coiled tube[J].Nuclear Power Engineering,2014,35(1):28-31(in Chinese).
[15]LEMMON E W,HUBER M L,MCLINDEN M O.Reference fluid thermodynamic and transport properties,NIST Standard Reference Database23,Version 8.0[R].[S.l.]:[s.n.],2007.
[16]ORSZAG A,YAKHOT V,FLANNERY W S,et al.Renormalization group modeling and turbulence simulations in near-wall turbulent flows[M].Amsterdam:Elsevier,1993:1 031-1 046.
[17]MENTER F R.Two-equation eddy-viscosity turbulence models for engineering applications[J].AIAA Journal,1994,32(8):1 598-1 605.