Experimental and numerical investigations on mixing processes in compressible nozzle flows

涉及两种或更多种气体组分的混合过程对于许多技术应用是必不可少的。它们的实际实现通常采用将反应物平行注入快速共流的中心注入器。混合物最理想的品质是其成分的均匀分布和达到这种状态所花费的时间。混合层的特征，从喷射器后缘演变而来，取决于喷射器的几何形状和并流条件。对于不可压缩（亚音速）和超音速尾流的情况，已经确定并详细研究了在不同同向流条件下占主导地位的不同流动结构。然而，跨音速尾流很少被讨论。该项目的目的是填补这一知识空白，为此，在第一个资助期内设计并制造了一个模块化跨音速流道。应用激光诱导荧光（LIF）成像技术，系统地研究了两种不同类型的喷射器在不同压力梯度下尾流中示踪剂的浓度分布。结果表明，归一化的轮廓表现出自相似性，既不依赖于后缘位置，也不依赖于喷嘴下游的压力梯度。此外，这可能是第一次表明，一个区域与不完全的分子混合存在。这是通过利用甲苯在氧气存在下的荧光信号的红移来实现的。进一步的测量技术，激光诱导热声（LITA）和纹影摄影，被应用到与LIF得到的结果的基础。实验结果与数值模拟（2D和3D URANS）进行了比较。第二个资助期的目的是确定跨音速流的自相似性是否普遍存在。因此，温度，密度和速度分布将进行实验分析，应用改进的LIF设置，粒子图像测速（PIV），并通过数值模拟（URANS，DES）的自相似行为。此外，几个新的注射器的几何形状将被设计和制造，以探索自相似性的限制。收集的知识将用于设计和实验验证一个优化的injector.In第一个funding期间，两个实验设计来评估示踪剂的荧光特性：一个小的超音速流动通道和一个冷却的荧光池。后者现在将得到增强，以允许在从室温到“大”流动通道中存在的低温度的条件下进行测量。结果将用于与文献数据进行比较，并将已知的荧光模型扩展到远低于室温的温度，在第二个资助期结束时，将免费提供关于跨音速流中混合行为的完整数据集。

Mixing processes that involve two or more gaseous components are essential for many technical applications. Their practical realization often employs a central injector that injects the reactants parallel into a fast co-flow. The most desired qualities of the mixture are a homogeneous distribution of its constituents and the time elapsed to reach this state. The character of the mixing layer, which evolves from the injector trailing edge, depends on the injector geometry and on the co-flow conditions. Different flow structures that are dominant at distinct co-flow conditions have been identified and excessively studied for the cases of incompressible (subsonic) and supersonic wake flows. However, transonic wake flows have only rarely been addressed. The aim of this project is to close this knowledge gap.To this end, a modular transonic flow channel was designed and manufactured in the first funding period. Laser-induced fluorescence (LIF) imaging was applied to systematically investigate the concentration distribution of the injected tracer in the wake of two different injector types and under various pressure gradients. It was shown that the normalized profiles behave self-similar and neither depend on the trailing edge position nor on the pressure gradient downstream of the nozzle. In addition, it could be shown for the first time that a zone with incomplete molecular mixing exists. This was achieved by exploiting the red-shift of the fluorescence signal of toluene in the presence of oxygen. Further measurement techniques, laser induced thermal acoustics (LITA) and schlieren photography, were applied to underpin the results obtained with LIF. The experimental results were compared with numerical simulations (2D and 3D URANS). The aim of the second funding period is to determine if self-similarity is universal for transonic flows. Consequently, the temperature, density, and velocity profiles will be analyzed experimentally applying an improved LIF setup, particle image velocimetry (PIV), and by numerical simulations (URANS, DES) for self-similar behaviour. In addition, several new injector geometries will be designed and manufactured to explore the limits of self-similarity. The gathered knowledge will be used to design and experimentally validate an optimized injector.In the first funding period, two experiments were designed to evaluate the tracer’s fluorescence characteristics: a small supersonic flow channel and a chilled fluorescence cell. Latter will be now enhanced to allow for measurements at conditions ranging from room temperature to temperatures as low as present in the “large” flow channel. The results will be used to compare with literature data and to expand known fluorescence models to temperatures significantly below room temperature.At the end of the second funding period, a complete data set on mixing behaviour in transonic flows will be made freely available.