Tasmany Sami, Kaiser Daniel, Woisetschläger Jakob, Gürtler Johannes, Kuschmierz Robert, Czarske Jürgen
Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology, Inffeldgasse 25/A, 8010 Graz, Austria.
Laboratory of Measurement and Sensor System Technique, Faculty of Electrical and Computer Engineering, Technische Universität Dresden, Helmholtzstrasse 18, 01062 Dresden, Germany.
Exp Fluids. 2024;65(10):151. doi: 10.1007/s00348-024-03890-1. Epub 2024 Oct 1.
In aircraft engines, thermoacoustic oscillations in the combustion chamber contribute significantly to noise emissions, which, like all other emissions, must be drastically reduced. Thermoacoustic oscillations are not only a concern, they can also be beneficial in hydrogen combustion. This work demonstrates that thermoacoustic density oscillations with amplitudes at least an order of magnitude smaller than those resulting from density gradients in a turbulent flame can be detected using laser interferometric vibrometry. This improvement was made possible by heterodyning a carrier fringe system in background-oriented schlieren (BOS) recordings, which were subsequently analyzed using techniques commonly used for holographic interferometry. In comparison with other BOS evaluation techniques, the filtering of the individual frames in the Fourier domain offers a more efficient computational approach, as it allows for phase averaging of a high number of single recordings to reduce noise from turbulence. To address fringe pattern distortions and cross talk in the Fourier domain, which both have been observed by other authors, we propose background subtraction methods and an optimized background pattern. Additionally, the procedure provides a visualization tool for marking the high turbulence regions of heat release by the variations in fringe amplitude. Finally, the line-of-sight data are reconstructed using the inverse Abel transform, with the data calibrated by laser interferometric techniques, resulting in local values for density oscillations.
在航空发动机中，燃烧室内的热声振荡对噪声排放有显著影响，与所有其他排放一样，必须大幅降低。热声振荡不仅是一个问题，在氢燃烧中也可能有益。这项工作表明，使用激光干涉振动测量法可以检测到热声密度振荡，其振幅至少比湍流火焰中密度梯度产生的振幅小一个数量级。通过在背景纹影（BOS）记录中对载波条纹系统进行外差处理实现了这一改进，随后使用全息干涉测量常用技术对其进行分析。与其他BOS评估技术相比，在傅里叶域中对单个帧进行滤波提供了一种更高效的计算方法，因为它允许对大量单个记录进行相位平均，以减少湍流噪声。为了解决其他作者观察到的傅里叶域中的条纹图案失真和串扰问题，我们提出了背景减法方法和优化的背景图案。此外，该程序提供了一种可视化工具，通过条纹振幅的变化来标记热释放的高湍流区域。最后，使用逆阿贝尔变换重建视线数据，并通过激光干涉技术对数据进行校准，从而得到密度振荡的局部值。
