Towards comprehensive multiphase flow modelling for nuclear reactor thermal hydraulics

在任何核反应堆中，确保核燃料始终保持适当冷却是热水力设计的主要成就，因此对工厂的安全和性能有最大的影响。通常，这种热水力设计和工厂安全评估依赖于计算模型，通过提供物理系统的数学表示，预测冷却剂的流体动力学行为和系统中的传热速率。在核电站中，在正常运行条件下或在需要紧急冷却的事故场景中，这通常需要解决气液多相流问题。不幸的是，尽管现在有各种复杂程度的计算工具，但气液多相流的建模和计算仍然主要局限于定义良好的流动条件和/或完全基于经验主义。这项研究的目的是开发一种先进的计算模型，克服这些限制，并远远超出目前可用的能力。目前，不同的技术在不同的和明确定义的流动条件下达到了很高的精度，但没有一种技术在没有先验流态知识的情况下成功地模拟了气液多相流动的整个谱。这极大地限制了可用模型对工业感兴趣的流的适用性，因为这些模型很少表现出相同的良好特征和定义的流特征。在这个项目中，通过新颖的数值技术，先进的建模方法将在同一个计算模型中耦合，并根据对局部流动条件的适用性有选择地应用。这将确保对多相气液流动的准确性和前所未有的适用性，避免了限制性假设，同时也避免了不切实际的计算要求。在核领域，这种模型将提供领先的建模和仿真能力，为改进当前反应堆群的运行以及未来工厂的设计和评估提供基础。自信的预测将为反应堆设计和安全限度评估提供信息，减少经验主义和保守主义。此外，昂贵的实验数量将被限制在较少数量的模型驱动测试中。更安全、发电价格更低、废物排放量更少的反应堆，将支撑政府到2050年新增核电装机容量16至75吉瓦的计划。这一新产能对于确保英国安全、可持续和低碳能源的未来至关重要，并尊重具有法律约束力的承诺，即到2050年将碳排放量在1990年的基础上至少减少80%。此外，这项工作将有更广泛的应用于核部门以外的许多工业设备的设计和运行优化，这些设备利用气液多相流在所有的工程分支中（例如，通过气泡柱中的气泡增强混合，分离设备中的流体分散和传质，石油和天然气的提取、处理和运输中的两/三相流）。同时，空间和时间尺度以及大多数界面细节的精细分辨率将允许进行更多的基础研究。这将为多相流的许多方面提供新的视角，这些方面仍然缺乏透彻的理解，这对多相设备的设计和操作产生了负面影响。该项目将受益于与英国和海外知名学者（麻省理工学院和北卡罗莱纳州立大学）以及核工业计算产品开发和核反应堆热工液压分析和评估方面的行业领导者（西门子工业软件有限公司和弗雷泽-纳什咨询公司）的密切合作。

In any nuclear reactor, ensuring that the nuclear fuel always remains properly cooled is the main achievement of the thermal hydraulic design, which thus has utmost impact on the safety and the performance of the plant. Often, this thermal hydraulic design and the plant safety assessment rely on computational models that, by providing a mathematical representation of the physical system, predict the fluid dynamic behaviour of the coolant and the rate of heat transfer in the system. In a nuclear plant, in normal operating conditions or in accident scenarios that require emergency cooling, this often requires solving gas-liquid multiphase flow problems. Unfortunately, although computational tools of any degree of complexity are now available, modelling and computation of gas-liquid multiphase flows is still mainly limited to well-defined flow conditions and/or entirely based on empiricism. The aim of this fellowship is to develop an advanced computational model that overcomes these limitations and goes well-beyond currently available capabilities. At the present time, different techniques reach good accuracy in distinct and well-defined flow conditions, but none has been successful in modelling the entire spectrum of gas-liquid multiphase flows without a priori knowledge of the flow regime. This strongly limits the applicability of available models to flows that are of industrial interest, since these rarely exhibit the same well-characterized and defined flow features. In this project, by means of novel numerical techniques, advanced modelling methods will be coupled in the same computational model and selectively applied based on suitability to the local flow conditions. This will ensure accuracy and unprecedented applicability to multiphase gas-liquid flows, avoiding limiting assumptions but at the same time unrealistic computational requirements.In the nuclear sector, such a model will provide leading edge modelling and simulation capabilities, underpinning improved operation of the current reactor fleet and design and assessment of future plants. Confident predictions will inform the reactor design and the assessment of safety limits, reducing empiricism and conservatism. In addition, the number of costly experiments will be limited to a smaller number of model-driven tests. Reactors that are safer and produce electricity at a cheaper price and with a reduced waste footprint will underpin Government's plan for between 16 GW and 75 GW of new nuclear generation capacity by 2050. This new capacity will be essential to ensure a secure, sustainable and low-carbon energy future to the UK and respect the legally binding commitment to reduce carbon emission by 2050 of at least 80% with respect to 1990.In addition, the work will have wider application outside the nuclear sector in the optimization of the design and operation of the numerous industrial equipment exploiting gas-liquid multiphase flows across all branches of engineering (e.g. enhanced mixing by bubbles in bubble columns, fluid dispersion and mass transfer in separation equipment, two/three phase flow streams in extraction, treatment and transportation of oil and gas). At the same time, the fine resolution of spatial and temporal scales as well as of the majority of the interfacial details will allow more fundamental studies to be made. These will shed new light on the many aspects of multiphase flows that still miss thorough understanding, which negatively affects the design and operation of multiphase equipment. The project will benefit from close collaboration with esteemed academics within the UK and overseas (Massachusetts Institute of Technology and North Carolina State University) and industrial leaders in the development of computational products for the nuclear industry and in the analysis and assessment of nuclear reactor thermal hydraulics (Siemens Industry Software Ltd and Frazer-Nash Consultancy).