低温透平膨胀机内非平衡自发凝结两相流动的数值研究
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摘要
针对低温透平膨胀机内复杂自发凝结三元两相流动问题,基于非等温修正成核模型及Gyarmathy液滴生长模型,采用非平衡凝结数学模型,准确预测了气体快速膨胀中凝结成核和液滴生长的过程,基于数值模拟结果,研究了进口过热度和旋转作用对工作轮流道内自发凝结过程的影响,并分析了流道内液滴数目密度、液滴直径及带液量的分布规律。模拟结果表明:当进口过热度不够低时,快速成核发生在工作轮内导流段并发展缓慢,致使气体始终保持在较高的过冷度状态且成核区域较大;非平衡自发凝结与二次涡流的相互作用产生的逆温梯度会带来附加热力学损失,同时增大的逆压梯度会造成更强的边界层分离,从而引起附加流动损失;对于径-轴式工作轮,在二次涡流区域、吸力面壁面附近及尾迹涡流区域内容易发生液滴的聚集,这很容易引起二次成核并形成大尺寸二次液滴,进而带来附加机械损失。
In view of 3-D two-phase flow with spontaneous condensation in cryogenic turboexpanders,a mathematical model of non-equilibrium condensation is employed to predict the condensation nucleation and droplet growth in gas repaid expansion,where the nucleation model corrected for non-isothermal effect and Gyarmathy droplet growth model are considered.Following the analysis for simulation results,the effects of inlet superheat degree and rotation on the spontaneous condensation in impeller flow passage are investigated,and the distributions of droplet number density,droplet diameter and wetness are obtained.A higher inlet superheat degree results in that repaid nucleation occurs in guiding section and consequently the process develops slowly,thus the gas remains in a higher supercooled state and the nucleation region is enlarged.Because of the interaction of condensation and secondary flow in impeller flow passage,the adverse temperature gradient causes an additional thermodynamic loss. Moreover,thestrength of adverse pressure temperature gradient promotes the separation of boundary layer and further causes additional flow loss.For radial and mixed-flow turbines,the small droplets very likely gather and form bigger ones in the secondary flow region,wall of suction side and wake flow region,which leads to secondary nucleation,secondary droplets on a large-scale and the further additional mechanical loss.
In view of 3-D two-phase flow with spontaneous condensation in cryogenic turboexpanders,a mathematical model of non-equilibrium condensation is employed to predict the condensation nucleation and droplet growth in gas repaid expansion,where the nucleation model corrected for non-isothermal effect and Gyarmathy droplet growth model are considered.Following the analysis for simulation results,the effects of inlet superheat degree and rotation on the spontaneous condensation in impeller flow passage are investigated,and the distributions of droplet number density,droplet diameter and wetness are obtained.A higher inlet superheat degree results in that repaid nucleation occurs in guiding section and consequently the process develops slowly,thus the gas remains in a higher supercooled state and the nucleation region is enlarged.Because of the interaction of condensation and secondary flow in impeller flow passage,the adverse temperature gradient causes an additional thermodynamic loss. Moreover,thestrength of adverse pressure temperature gradient promotes the separation of boundary layer and further causes additional flow loss.For radial and mixed-flow turbines,the small droplets very likely gather and form bigger ones in the secondary flow region,wall of suction side and wake flow region,which leads to secondary nucleation,secondary droplets on a large-scale and the further additional mechanical loss.
引文
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[2]YAMAMOTO S,SASAO Y,KATO H,et al.Numerical and experimental investigation of unsteady 3-D wet-steam flows through two-stage stator-rotor cascade channels[C]∥Proceedings of ASME Turbo Expo2010:Power for Land,Sea and Air.New York,NY,USA:ASME,2010:2257-2265.
[3]STARZMANN J,SCHATZ M,CASEY M V,et al.Modelling and validation of wet steam flow in a low pressure steam turbine[C]∥Proceedings of the ASME Turbo Expo 2011.New York,NY,USA:ASME,2011:2335-2346.
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[5]SRIRAM A T,MISTRY H,MORAGA F,et al.Numerical sensitivity studies on nucleation of droplets in steam turbine[C]∥ASME 2011 Power Conference.New York,NY,USA:ASME,2012:559-565.
[6]孙皖,牛璐,赖天伟,等.氮气在缩放喷管中非平衡自发凝结的数值研究[J].西安交通大学学报,2015,49(4):130-133.SUN Wan,NIU Lu,LAI Tianwei,et al.Numerical research on nitrogen spontaneous condensation in converging diverging nozzle[J].Journal of Xi’an Jiaotong University,2015,49(4):130-133.
[7]NIU Lu,HOU Yu,SUN Wan,et al.The measurement of thermodynamic performance in cryogenic twophase turbo-expander[J].Cryogenics,2015,70:76-84.
[8]SUN Wan,NIU Lu,CHEN Shuangtao,et al.Numerical investigation of nitrogen spontaneous condensation flow in cryogenic nozzles using varying nucleation theories[J].Cryogenics,2015,68:19-29.
[9]GYARMATHY G.Grundlagen einer theorie der nassdampfturbine[D].Zürich,Swiss:Swiss Federal Institute of Technology in Zúrich,1962:206-225.
[10]ANSYS Inc.ANSYS CFX-solver theory guide[M].Canonsburg,USA:ANSYS Inc.,2010:210-211.
[11]SUN Wan,NIU Lu,CHEN Liang,et al.Numerical study of spontaneous condensation flow in an air cryogenic turbo-expander using equilibrium and nonequilibrium models[J].Cryogenics,2016,7:42-52.
[12]孙皖,马斌,牛璐,等.低温透平膨胀机内平衡凝结两相流动数值模拟[J].西安交通大学学报,2013,47(7):36-39.SUN Wan,MA Bin,NIU Lu,et al.Numerical simulation of equilibrium condensation two-phase flow in cryogenic turboexpander[J].Journal of Xi’an Jiaotong University,2013,47(7):36-39.