Over the past decade, the need to supplement system-scale simulations of reactor transients with the results of finer simulations (subchannel or CFD) has increased continuously. In many cases, the local phenomena predicted at these scales (such as flow patterns in the core or within inlet/outlet plenums) can affect the overall transient: in that case, then all codes should be run concurrently in a consistent manner in order to obtain a single, “multi-scale” simulation of the transient of interest.

Because their subchannel/CFD components tend to require meshes beyond the capabilities of the 3D modules present in modern system codes, most multiscale simulations can only be performed by coupling different codes together. The strategy used to implement this coupling can have a crucial impact on both the solution accuracy and on the numerical cost of the calculation: in particular, algorithms which require small time steps or large number of iterations between the codes can multiply the numerical cost of multiscale compared to an (already expensive) standalone CFD simulation.

This paper discusses a range of algorithms suitable for coupling thermal-hydraulics codes at either thermal or hydraulic boundaries. These algorithms are grouped into four broad classes of increasing complexity (fixed-point, improved fixed-point, quasi-Newton and Newton). The more complex variants are more difficult to implement, but have been observed to significantly decrease the numerical overhead of multi-scale coupling.

在过去的十年中，用更精细的模拟（子通道或 CFD）的结果来补充反应堆瞬变的系统规模模拟的需求不断增加。在许多情况下，在这些尺度上预测的局部现象（例如核心或入口/出口压力通风系统内的流动模式）会影响整体瞬态：在这种情况下，所有代码都应以一致的方式同时运行，以便获得感兴趣的瞬态的单个“多尺度”模拟。

由于它们的子通道/CFD 组件往往需要超出现代系统代码中存在的 3D 模块功能的网格，因此大多数多尺度模拟只能通过将不同的代码耦合在一起来执行。用于实现这种耦合的策略可以对求解精度和计算的数值成本产生至关重要的影响：特别是，需要小时间步长或代码之间大量迭代的算法可以乘以多尺度的数值成本与（已经很昂贵的）独立 CFD 模拟相比。

本文讨论了一系列适用于在热力或水力边界耦合热工水力代码的算法。这些算法分为四大类，复杂度不断增加（定点、改进定点、拟牛顿和牛顿）。更复杂的变体更难实现，但已经观察到显着降低了多尺度耦合的数值开销。