Development of cooling strategies and advanced numerical approaches for heat transfer in nuclear fusion reactor components under extreme heat loads

世界能源需求正以每年约2%的速度增长。其中87%的需求由化石燃料来满足，同时产生二氧化碳和污染气体。虽然减少对化石燃料的依赖势在必行，但必须找到新的无碳能源来应对气候变化并满足日益增长的需求。核聚变是零排放的，不会产生持久的放射性分数，并且可以产生高功率密度。然而，虽然净能量增益似乎并不太遥远，但在技术上需要保护墙壁和组件免受聚变反应堆产生的极端热负荷（百万度和20兆瓦/平方米的热流）的影响。在这种极端的热负荷下，传统的水冷却是非常有限的，并且会导致诸如空化、局部蒸发和防止这些问题所需的强加压等问题。因此，能够提取和重新分配这些热量的新技术的发展对于未来在工业循环中使用核聚变至关重要。在这个项目中，将评估液态金属的可行性。液态金属具有强传热系数和扩散性的优点，因此原则上它们可以提取更多的热量并更快地重新分配。此外，它们不依赖高压保持液态，它们的流动可以由核反应堆内的磁场驱动。然而，由于缺乏实验，它们在强热负荷和强磁场下的行为是复杂的，而且还没有得到很好的理解。此外，需要更高的温度来保持金属的液态，这可以抵消高传热系数的影响。最近一项使用液态锂进行的初步共轭传热分析表明，只有在极端热负荷下，液态金属的冷却性能才优于水。没有考虑蒸发和磁效应。该博士项目将采用高保真、大涡模拟来进一步分析液态金属在极端热负荷下的性能。高保真度模拟方法将涉及共轭传热，以有意义的方式评估冷却剂对结构的影响，模拟磁场对液态金属的不稳定影响以及热峰区域的局部蒸发（相变）。这些都是相对未被探索的现象，调查将利用在UKAEA进行的实验活动，以便获得模型验证的数据。

World's energy demand is increasing at a rate of about 2% per annum. 87% of this demand is met by fossil fuels, with production of CO2 and polluting gases. While it is imperative to reduce the dependence on fossil fuels, new, carbon-free energy sources have to be found to tackle the climate change and meet the increasing demand. Nuclear fusion is emission-free, produces no long-lasting radioactive scores, and can generate high power densities. However, while a net energy gain seems not-too-far to be proved, there is a technological need to protect the walls and components from the extreme heat loads generated in a fusion reactor (millions degrees and heat fluxes of order of 20 MW/m2). Conventional water-cooling is very limited under such extreme heat loads and leads to issues like cavitation, local evaporation and strong pressurization needed to prevent these. The development of new technology that allows to extract and redistribute this heat is thus fundamental for the future employment of nuclear fusion in industrial cycles.In this project the viability of liquid metals will be assessed. Liquid metals have the advantage to have strong heat transfer coefficients and diffusivity, so they can in principle extract more heat and redistribute it faster. Moreover, they do not rely on high pressure to remain in liquid form and their flow can be driven by the magnetic field within the nuclear reactor. Nevertheless, their behaviour under strong heat loads and magnetic field is complex and not well understood, also due to the lack of experiments. Also, higher temperatures are needed to keep the metal in liquid form, which can counteract the effect of the high heat transfer coefficient. A recent preliminary conjugate heat transfer analysis conducted using liquid lithium has revealed that only under extreme heat loads the liquid metal outperforms water for cooling purposes. Evaporation and magnetic effects were not considered. This PhD project will employ high fidelity, large eddy simulation to further analyse the performance of liquid metals under extreme heat loads. The high-fidelity simulation approach will involve conjugate heat transfer to assess the effect of the coolant on the structure in a meaningful way, the modelling of the unsteady effect of the magnetic field on the liquid metal and the localised evaporation (phase change) in regions of heat peaks. These are relatively unexplored phenomena and the investigation will take advantage from experimental campaign to be run at UKAEA in order to obtain data for model validation.