Jets in cross flow
To completely characterize flow conditions in thermal turbo machines, at least three physical quantities have to be known: Flow speed, temperature and density, if possible as time-resolved functions of space. All so far available measurement systems are not entirely able to fulfill these requirements. The most advanced and widely established methods are known in the field of flow speed detection, that is to say LDA and PIV. Only limited and insufficient tools are available for non-intrusive temperature and density measurements. For most non-intrusive temperature or density measurement systems, especially the system-inherited accuracy is by far not adequate for the needs of the researcher. For example, it is still not possible to non-intrusively determine the local temperature distribution within turbines and compressors even tough this information is necessary to identify regions where energy losses occur.
The proposed Helmholtz-University Young Investigators Group therefore concentrates on this widely pristine area of research. The focus of attention is laid on two comparatively new techniques for temperature and density measurements, the Transient Grating Spectroscopy (TGS) and the Filtered Rayleigh Scattering (FRS). The physical fundaments of these techniques are well understood, but their application potential for technical measurement tasks is still unexploited. TGS offers the possibility to laser-optically determine the local speed of sound, which allows calculating the temperature. With FRS, the proportionality of the intensity of the Rayleigh scattering to density is used to determine the latter.
In the area of turbomachinery, modern combustion chambers are facing difficult demands: They were developed to produce less pollutants and less noise and - at the same time – have increased life cycles and higher operational reliability. With regards to the legal limit values of NO and NO2, the trend is to build lean, premixed combustors. Unfortunately, such combustors tend to show very strong combustion oscillations. At operating pressures of 35 bar, their amplitudes become as big as 1 bar, which shows the tremendous destructive power combustion instabilities may have. Avoiding such oscillations is therefore a key factor in the development of modern combustors. Most combustion oscillations are generated by several kinds of feed-back loops. Among other effects, important mechanisms are acoustic feed-back loops and the propagation of entropy waves. The term “entropy waves” denotes the movement of hot spots within the flow, the convection of large temperature and density in-homogeneities. The development of FRS will in particular be aimed to detect these entropy waves.
For acoustic experiments in oscillating combustion, most often microphones with electric or electro-magnetic conversion are used to characterise pressure fluctuations. Since this kind of pressure transducer is very sensible to heat and since their size can not be minimized sufficiently, their use in combustion chambers is often restricted, if not impossible. Therefore, a further important goal of the Helmholtz-University Young Investigators Group is the development of a microphone based on glass fibre, which optically detects the oscillation of a metal membrane. These microphones are by far more heat resistant and probably also smaller than conventional ones. In spite of such promising features, this concept has not been realised until today.