流化床传热的快速模拟
Extremely fast simulations of heat transfer in fluidized beds
摘要：
除了其巨大的技术重要性之外，流化床还吸引了大量的研究，因为它们是研究高动态颗粒流动的理想场所。它们的整体行为是由短暂的粒子碰撞和固相与气相之间的相互作用决定的。结合计算流体动力学（CFD）和离散元方法（DEM）的现代模拟技术能够描述它们的演变，并提供有关颗粒尺度上发生的详细信息。然而，这些方法受到小时间步长和大数值成本的限制，这抑制了对较慢的长期过程（如传热或化学转化）的研究。在最近的一项研究（Lichtenegger和Pirker，2016）中，我们引入了递归CFD（rCFD）作为一种方法，用于在具有递归模式的系统中将快速自由度与慢速自由度解耦：进行常规模拟以捕获此类相干结构。它们的重现具有重现性，使我们能够推断它们的演化远远超出模拟时间。在这些预测的流场之上，任何被动或弱耦合过程都可以以原始计算成本的几分之一进行研究。在这里，我们提出了应用rCFD在实验室规模的流化床传热。最初，热颗粒与冷空气流化，并记录其温度演变。在传统的CFD-DEM相比，我们观察到的速度约两个数量级的因素，在非常好的精度与最近的测量。
英文摘要：
Besides their huge technological importance, fluidized beds have attracted a large amount of research because they are perfect playgrounds to investigate highly dynamic particulate flows. Their over-all behavior is determined by short-lasting particle collisions and the interaction between solid and gas phase. Modern simulation techniques that combine computational fluid dynamics (CFD) and discrete element methods (DEM) are capable of describing their evolution and provide detailed information on what is happening on the particle scale. However, these approaches are limited by small time steps and large numerical costs, which inhibits the investigation of slower long-term processes like heat transfer or chemical conversion. In a recent study (Lichtenegger and Pirker, 2016), we have introduced recurrence CFD (rCFD) as a way to decouple fast from slow degrees of freedom in systems with recurring patterns: A conventional simulation is carried out to capture such coherent structures. Their re-appearance is characterized with recurrence plots that allow us to extrapolate their evolution far beyond the simulated time. On top of these predicted flow fields, any passive or weakly coupled process can then be investigated at fractions of the original computational costs. Here, we present the application of rCFD to heat transfer in a lab-scale fluidized bed. Initially hot particles are fluidized with cool air and their temperature evolution is recorded. In comparison to conventional CFD-DEM, we observe speed-up factors of about two orders of magnitude at very good accuracy with regard to recent measurements.
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