反应堆严重事故条件下纳米流体稳定性、传热传质特性及CHF机理研究

Since nanofluids can enhance the heat transfer significantly , it is worthy to study the potential of nanofluid used as coolant in the strategy of In-Vessel Retention for melt under the severe accident condition of nuclear reactor. The special heat transfer conditions under severe accident of nuclear reactor include boric acid, lithium hydrate, sodium phosphate and irradiation etc. However, there is almost no research on the stability, heat and mass transfer and critical heat flux of nanofluids under this special conditions. Thus, the present work just focus on these items based on theoretical and experimental methods. In theoretical study, the migration of nanoparticles and its influence on the bubble behaviors during nanofluid boiling could be investigated using DPM-VOF method. The characteristics of nanofluid boiling on the downward-facing surface can be studied by MPS method with the consideration of the effect of nanoparticles. Moreover, the CHF of nanofluid flow boiling on the downward-facing heating surface based on the liquid sub-layer dry out model will be studied. In experimental study, nanofluid stability can be observed by transmission electron microscope. The characteristics of bubble dynamics in nanofluid can be visualized and studied experimentally. Furthermore, the characteristics of nanofluid flow boiling and CHF in the inclined tube will be investigated experimentally. This work will lay a theoretical and engineered foundation for the application of nanofluid in IVR strategy.

由于纳米流体能显著提高换热能力，将纳米流体作为反应堆严重事故后熔融物堆内滞留策略的冷却工质的潜力有很大的研究价值。在反应堆严重事故情况下，换热环境的特殊性包括硼酸、氢氧化锂、磷酸三钠以及辐照等。然而，目前还没有这种特殊环境下的纳米流体的稳态性、传热传质特性以及临界热流密度相关研究。为此，本项目通过理论和实验研究该特殊环境下纳米流体的稳定性、传热传质及CHF特性。理论研究中利用DPM-VOF方法探索颗粒的迁移以及对气泡的影响；在考虑颗粒影响下，利用MPS方法从气泡动力学角度研究加热面朝下时纳米流体沸腾换热特性；并基于微液层蒸干模型，研究加热面朝下时纳米流体的CHF。在实验研究中通过电镜（TEM）观测研究纳米流体的稳定性；利用可视化实验观测纳米流体内的气泡动力学特性；并通过实验研究倾斜通道内纳米流体流动沸腾及CHF特性。本项目的研究可为纳米流体在IVR策略中的应用提供理论支撑及工程指导。

按照计划书的目标和内容，搭建了相关实验平台，通过实验和理论方法开展了纳米流体的稳定性、混合纳米流体热物性、加热面朝下纳米流体池式沸腾换热及 CHF特性相关研究。主要结果包括：1）添加0.05vol.%PVP表面活性剂、采用合适的超声震荡频率以及时间配制纳米流体可有效提高纳米流体的稳定性；2）在Al2O3 /H2O纳米流体中添加少量CuO或C纳米颗粒制成的混合纳米流体稳定性好、且拥有高的导热系数和低粘性系数，具有工程应用潜力；3）纳米颗粒在基液中的弥散性越好，纳米流体导热系数越高；4）对于换热面朝下的饱和池式沸腾换热，加热面倾角对泡核沸腾换热能力无明显的影响，但加热面倾角越大，CHF 值越大；5）Al2O3/H2O 纳米流体的泡核沸腾换热强度与加热面倾角及壁面过热度有关，当加热面倾角小于30°时，浓度越高，沸腾换热强度越弱。当加热面倾角超过30°后，Al2O3/H2O 纳米流体的沸腾换热强度随浓度的提高有所增强，尤其是在壁面过热度较大的条件下；6）相对于去离子水，0.01vol%的Al2O3/H2O纳米流体在倾角60°的加热面（面积4cm×4cm）上的CHF增幅达 55.5%；7）纳米流体沸腾过程中伴随纳米颗粒在加热壁面上沉积，所形成的多孔介质微层具有更大的毛细吸力，在毛细力梯度的驱动下，液体会朝蒸发更快的点流动，显著改变壁面的润湿特性，加热面不易烧干，CHF得以提高；8）基于去离子水和纳米流体在面朝下加热面上的CHF实验数据，拟合出了考虑加热面倾角影响的CHF关系式。本课题关于提高纳米流体稳定性、加热面朝下的纳米流体池式沸腾CHF 的研究成果，可为纳米流体冷却压力容器下封头的工程应用设计提供参考。