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的冷却是热工水力设计的主要成就，因此对工厂的安全和性能具有最大的影响。通常，这种热工水力设计和工厂安全评估依赖于计算模型，该计算模型通过提供物理系统的数学表示来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需要紧急冷却的事故情况下，这通常需要解决气液多相流问题。不幸的是，尽管现在可获得任何复杂程度的计算工具，但是气-液多相流的建模和计算仍然主要限于明确定义的流动条件和/或完全基于非线性。该奖学金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良好的精度，但没有一种技术在没有流动状态的先验知识的情况下成功地对气液多相流的整个频谱进行建模。这极大地限制了现有模型对具有工业利益的流动的适用性，因为这些流动很少表现出相同的良好表征和定义的流动特征。在这个项目中，通过新的数值技术，先进的建模方法将在同一个计算模型中耦合，并根据对当地水流条件的适用性有选择地应用。这将确保准确性和对多相气液流的前所未有的适用性，避免限制性假设，但同时也避免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模拟能力，支持改进当前反应堆群的运行以及未来工厂的设计和评估。可靠的预测将为反应堆的设计和安全极限的评估提供信息，减少保守主义和保守主义。此外，昂贵的实验数量将限于数量较少的模型驱动测试。更安全、价格更便宜、废物足迹更少的反应堆将支持政府到2050年新增16吉瓦至75吉瓦核电装机容量的计划。这一新的能力对于确保英国安全、可持续和低碳的能源未来至关重要，并尊重到2050年将碳排放量减少至少80%的具有法律约束力的承诺。这项工作将在核部门以外的许多利用天然气的工业设备的设计和操作的优化方面有更广泛的应用，液体多相流跨越工程的所有分支（例如，通过泡罩塔中的气泡增强混合，分离设备中的流体分散和传质，石油和天然气的提取、处理和运输中的两相/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。这些将为多相流的许多方面提供新的启示，这些方面仍然缺乏彻底的理解，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（马萨诸塞州理工学院和北卡罗来纳州州立大学）以及核工业计算产品开发和核反应堆热工水力学分析与评估行业领导者（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).