【作者】 袁明豪；

【导师】 杨燕华；

【作者基本信息】 上海交通大学 ， 核能科学与工程， 2008， 博士

【摘要】 蒸汽爆炸是核反应堆发生严重事故时,堆芯熔化后燃料与冷却剂相互作用可能产生的一系列后果之一,对于核电站来说,这就带来了将放射性裂变产物释放到环境中去的风险。因此,蒸汽爆炸在核安全领域是一个重要的研究课题。蒸汽爆炸是一个综合的多相流现象,包括粗混合、触发、传播和膨胀四个阶段,目前蒸汽爆炸过程中的机理还未被完全了解,这阻碍了对蒸汽爆炸现象进行准确预测。本文主要采用数值模拟手段,对蒸汽爆炸过程中的现象机理进行研究,弥补了以往传统的理论与实验研究的不足。由于蒸汽爆炸是一个极复杂的热流体相互作用动力学过程,它对算法的适用性与健壮性都提出了较大挑战。为此,本文使用VOF方法追踪自由界面,并将VOF方法推广到复杂计算区域,存在相变及流体可压缩的情况,独立开发了多相流计算程序。本文通过数值模拟,确定了粗混合阶段膜态沸腾条件下高温颗粒的运动阻力变化规律及主要影响因素,结合理论分析建立了新的阻力模型。该模型包含压差阻力和摩擦阻力,考虑了表面张力及汽液界面形状对压差阻力的影响。与实验数据的对比表明该模型能成功描述高温颗粒在冷却剂中的运动。根据数值模拟结果,本文还对各种基于理论分析与实验数据的圆球表面膜态沸腾传热关系式进行了分析评价。在对单个熔融液滴在压力脉冲作用下热碎化的研究中,发现了Richtmyer-Meshkov不稳定性对蒸汽-冷却剂界面上的初始扰动的增长起了决定作用。数值模拟结果表明,当蒸汽膜坍塌时,冷却剂与熔融液滴表面的接触是非均匀的,直接接触点上由于快速蒸发,会形成高压蒸汽包。蒸汽包在液滴表面形成凹陷,在两个凹陷之间则产生尖峰,尖峰上方蒸汽膜的坍塌不足以阻止尖峰的产生,而是会使尖峰出现分叉。计算结果还表明,压力波通过液滴时的衍射效应是触发热碎化的重要因素。对于水力碎化的数值模拟表明,液滴变形与液滴表面的水力不稳定性是液滴碎化的主要机理。在压力波通过后的初期,没有发生流动分离,液滴后滞止点的压力几乎能够完全恢复,液滴在前后挤压下从中间向外拉伸,在液-液系统中,周围流体与液滴的密度比远大于气-液系统,因此液滴变形在整个碎化过程中尤其重要,对液滴的碎化时间有较大影响。更多还原

【Abstract】 Vapor explosion is one of the consequences of fuel-coolant interactions in a severe accident of a nuclear reactor. In nuclear plants, there is then a risk of release of radioactive fission products into the environment. So vapor explosion is an important topic to investigate in nuclear safety. Vapor explosion is a compositive multiphase phenomenon including four stages: coarse premixing, triggering, propagation and expansion. The mechanisms involved in the vapor explosion process have not been understood, which prevents predicting the explosion precisely.The mechanisms of the phenomena involved in vapor explosions are studied with numerical simulation. Vapor explosion is a very complex dynamic progress with heat and mass transfer, which challenges the robustness and applicability of the numerical algorithms. In this study Volume of Fluid (VOF) method is used to track interface and is extended to complex domain, phase change and compressible flow. A computational fluid dynamics code for multiphase flow is developed for this purpose. A hot particle moving in water with film boiling is simulated, and the drag force on the particle is investigated. Based on the simulation results, a new drag model including pressure drag and friction drag is established with the help of theoretical analysis. In this model, the pressure drag force is affected by the surface tension and the shape of the liquid-vapor interface in the vicinity of the particle. The model is validated with experimental data. In addition, different heat transfer correlations are evaluated with the simulation results.In simulation of thermal fragmentation of a melt drop under a pressure pulse, it is found that Richtmyer-Meshkov instability determines the growth of the initial disturbances on the coolant-vapor interface. As the vapor film collapses, the coolant will contact the drop at a series of local points. The local generation of high-pressure vapor at the surface will cause a small crater. A spike appears between two craters. The collapse of the vapor film above the spike could not suppress the formation of spike, but will bifurcate the spike. It is also indicated that the diffraction of the pressure wave is important to trigger a thermal fragmentation.It is shown in simulation that the drop deformation and surface instability are the main mechanisms in hydrodynamic fragmentation. After passing of shock wave, flow separation does not occur in the early time and the pressure is nearly recovered at the back stagnation point. The drop is squeezed by these pressures and pulled out at the equator. In a liquid-liquid system, the density ratio is much larger than that in a gas-liquid system, so the drop deformation is especially important, and affects the fragmentation time evidently.更多还原