摘要
[目的]小通道内的换热已被广泛应用于核反应堆与航天发动机冷却等方面。为保证该系统内热力装置安全、高效运行,需系统、深入研究超临界CO_(2)的传热规律与异常传热机理。[方法]通过CFD数值模拟,对不同质量流速下内径为2 mm的竖直上升管中超临界CO_(2)的换热随热流密度与压力的变化规律展开研究,并对传热强化与传热恶化工况下不同截面处径向物性参数的变化规律进行研究,深入分析异常传热机理。[结果]研究结果表明,当质量流速为500~1000 kg/(㎡·s)时,随着热流密度的升高,超临界CO_(2)由传热强化转变为传热恶化,且在传热恶化工况下,随着热流密度的升高,超临界CO_(2)传热恢复区的换热系数峰值有所降低,直至消失;随着运行压力的升高,超临界CO_(2)的异常传热程度均被削弱。通过机理分析发现,在传热强化工况下,超临界CO_(2)的换热在拟临界点附近达到最强,其主要原因在于管截面上大比热容区流体的份额显著增加;在传热恶化工况下,壁温峰值对应的截面处速度呈“M”型分布,拐点处速度梯度降为0,湍动能也降到最低,导致传热能力变弱。[结论]研究结果对于改善小通道内超临界CO_(2)的传热性能具有一定的指导意义。
[Objectives]The heat transfer techniques of supercritical CO_(2)(S-CO_(2))in small channels have been extensively used in nuclear reactor cooling,aerospace engine cooling and so on.To ensure the safety and high operation efficiency of the thermodynamic devices involved in the S-CO_(2) Brayton cycle system,it is necessary to systematically study the flow and heat transfer characteristics of S-CO_(2),and thoroughly analyze the abnormal heat transfer mechanism.[Methods]Through CFD numerical simulation,the heat transfer characteristics of S-CO_(2) in a vertically upward tube with an internal diameter of 2 mm at different mass flow rates are studied.At the same time,under conditions of heat transfer enhancement and deterioration,the change of radial physical properties at different sections is studied,and the mechanism of abnormal heat transfer is analyzed in depth.[Results]It is found that under conditions with mass flow rates of 500–1000 kg/(㎡·s),the heat transfer of S-CO_(2) moves from heat transfer enhancement to heat transfer deterioration.In the heat transfer deterioration cases,the peak value of the heat transfer coefficient in the recovery region gradually decreases with the increase in heat flux,then finally disappears.As the operating pressure is far from the critical pressure,heat transfer enhancement and deterioration are both weakened.As shown by mechanism investigation,in heat transfer enhancement cases,the heat transfer of the fluid is stronger near the pseudo-critical region than in other regions,and the increased heat transfer coefficient comes from the increased portion of the cross-section area occupied by the fluid with large specific heat.In heat transfer deterioration cases,the velocity profile of the fluid at the cross-section where the wall temperature has its peak value presents an M-shaped curve,the velocity gradient at the inflection point of the velocity profile decreases to 0,the turbulent kinetic energy reaches its lowest level and heat transfer deterioration occurs.[Conclusions]The results of this study are significant for improving the heat transfer characteristics of S-CO_(2) in small channels.
作者
胡娟娟
李增光
高一民
杨建
刘浪
HU Juanjuan;LI Zengguang;GAO Yimin;YANG Jian;LIU Lang(Shanghai Division,China Ship Development and Design Center,Shanghai 201108,China)
出处
《中国舰船研究》
CSCD
北大核心
2021年第3期170-179,共10页
Chinese Journal of Ship Research
关键词
超临界CO_(2)
竖直上升管
异常传热
机理
supercritical CO_(2)
vertically upward tube
abnormal heat transfer
mechanism
