超临界CO2在螺旋管中的流动换热特性研究

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RNG k-ε和SST k-ω模型对超临界CO₂在螺旋管中的流动换热情况进行了数值模拟，发现SST k-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h_pc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。

Numerical Study on Heat Transfer Characteristic of CO2 in Helical Tube at Supercritical Pressure

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 CO₂ at supercritical pressures in a helical tube under constant heat flux conditions were numerically investigated in the present study. Both RNG k-ε turbulence model and SST k-ω turbulence model were applied in the simulations, and the numerical calculation results show that the SST k-ω turbulence 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 is found that with the decrease of mass flow or the increase of heat flux, the peak of heat transfer coefficient is further away from h_pc. 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 at φ of 315°, which should be gained extensive attention of operation in order to ensure the safe operation of helical tube evaporator.