Measurement Techniques

Principle of Laser Doppler Velocimetry (LDV)

Laser Doppler Velocimetry (LDV) is an optical technique used in fluid mechanics for local and instantaneous measurements of the flow velocity. Its principle is based on the crossing of an interference pattern – generated by a couple of coherent laser beams – by fine particles seeded in the flow and the collection and processing of Mie scattering emitted by these particles during this crossing.


Figure 1 - Principle of Laser Doppler Velocimetry (LDV) in backscattering configuration

Figure 1 presents a diagram of the optical setup for LDV measurements. Two coherent laser beams generated by a CW laser are first collimated and then focused by means of a converging lens, to form an interference pattern in the flow. A fine particle seeded in the flow and crossing that pattern is then successively illuminated in the bright fringe and not illuminated in a dark fringe. The resulting burst of Mie scattering signal is modulated by a frequency which is proportional to the instantaneous speed of the particle, i.e. the component of the flow velocity perpendicular to the interference pattern. A Bragg cell is used to determine the velocity direction. In the most usual configuration, two perpendicular interference patterns are generated at two different wavelengths. This enables to measure instantaneously two components of the velocity. The use of fibre optics for laser and signal detection in backscattering geometry minimizes the size of LDV probe and simplifies its use in complex environments.

The measurement volume in the flow corresponds to the interference pattern and has an ellipsoid shape. The spatial resolution is very good in the direction perpendicular to the fringes (see the x-axis in Figure 1). However it can be relatively poor in the direction parallel to the fringes (z-axis) for a long focal length of the converging lens and small distance between the two incident laser beams. At each measurement location, a large number of LDV bursts are recorded in order to obtain statistics on aerodynamic features of the turbulent flow: the mean, root-mean-square, cross-correlations, kinetic energy. To move the measurement volume in the facility the LDV probe is usually set on multi-axis motorized rails.

Principle of Chemiluminescence Imaging

Figure 2 presents a spontaneous emission spectrum of a hydrocarbon gaseous bluish flame. High intensity peaks corresponding to band-heads of rovibronic transitions of the excited radicals OH (around 308 mm), CH (around 430 mm) and C2 (around 516 mm) can be observed. These emission bands are often superimposed on a continuum in the spectral range [300 mm ; 500 mm] corresponding to CO2 spontaneous emission.

 

Figure 2 - Spontaneous emission spectrum of a gaseous flame

All these species are chemically formed in the flame directly in an excited state. The radiative part of their quasi instantaneous de-excitation is called chemiluminescence. In literature, the main chemical reaction suggested for their formations are:


Even if chemiluminescence intensities can be sometimes correlated with equivalent ratio in premixed flames, it is not a technique that enables quantitative temperature, concentrations or heat release measurements. However, chemiluminescence provides evidence of the presence of an exothermic reaction zone. Chemiluminescence imaging is therefore an effective technique to determine instantaneous and mean features of reaction zones in a flame. Moreover, the variation of chemiluminescence signal provides also qualitative information on a local variation of heat release.
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