Fundamental experiments for model development
A laser pulse with a duration of approx. 100 nanoseconds is sent into the system to be investigated (at the Institute of Technical Thermodynamics this is, for example, a turbulent natural gas/air flame at a semi-technical scale). The Raman effect results in a scattered light signal away from the laser beam’s axis, which can be detected outside the flame.
This Raman scattered light has several properties that are of great value for the measurements: its wavelength is specific to the type of molecule that had caused the scattering. The gases present in the flame can be detected separately from each other through spectral decomposition, therefore a quantitatively precise measurement of species concentrations becomes possible. Since the signal comes from the focal point of a laser, which can be made very small using suitable optics (a few hundredths of a micrometer), very good spatial resolution is achieved. The measurement takes place within fractions of a microsecond, such that the requirement for good temporal resolution is also met to a high degree.
These advantages, however, are accompanied by several decisive drawbacks that result directly from the very low signal strength in Raman scattering, and that limit the applicability of the technique to a few special cases. The method’s application requires very clean conditions without dust, soot particles or fuel droplets, and free access to the system without solid obstacles such as e.g. the cylinder or combustion chamber’s walls. The technique can be implemented only at very high technical and financial costs for the laser and detection instruments. In certain respects, Raman experiments are the experimental analogue to direct numerical simulations: the quality and predictive power of the results is higher than with other known methods, but the cost is very high and applicability very limited.
The Raman experiment is not, therefore, normally used for the investigation of technical combustion systems, but in order to obtain detailed information from systems with precisely defined boundary conditions. This information is used in order to verify existing computational models, develop new ones and discover new combustion phenomena.
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Raman measurement of a turbulent flame (blue glow). The horizontal green lines are the laser beams used for exciting the Raman signal. The vertical green line in the flame comes from fuel droplets whose combustion in the flame is being investigated using the Raman technique.
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Result of a Raman measurement in a turbulent methane/air premixed flame. The Raman signal can be seen along the laser beam (direction from top to bottom), spectrally resolved (wavelength shift of the Raman scattered light, from left to right). The spectral signatures of some chemical species in the flame are visible. Colour scale: red indicates high intensities, blue is low intensities. The bright regions at 1-2 mm show unburned zones with high fractions of methane and oxygen, the darker zones above and below correspond to burned zones (higher fraction of carbon dioxide and water). |
Two-dimensional laser diagnostic
Many laser diagnostic techniques are imaging methods, in which the laser light is introduced into the measurement volume not as one-dimensional beam but rather as a planar light section. The resulting laser-induced signal forms a thin, uniform zone within the investigated object; this imaging signal can be recorded using suitable detectors (electronic cameras). Here too, pulsed lasers are used typically such that very rapid processes can be investigated by means of ultrashort snapshots.
One example of the application of such an imaging method is research into the technically important process of spark ignition with laser-induced fluorescence (LIF). An experiment conducted at the Institute of Technical Thermodynamics in this area consists of a gas-filled cell (Fig. 9), into which laser light is introduced through large glass windows and in which the laser-induced signal can be detected. The cell is equipped with a device for generating precisely defined, reproducible ignition sparks at its centre. Fans for generating a precisely defined turbulent flow are also built into the cell. This allows technically important phenomena associated with spark ignition, such as e.g. the well-known missfiring in Otto engines, to be simulated and investigated.
The main advantage of an ignition cell as an experimental set-up compared with a real engine is that many phenomena such as the flow field and the mixture formation, which overlap in the engine in complicated ways, can be varied and investigated in isolation from each other. The investigation often relies on laser diagnostic methods, which permit a detailed elucidation of the structure and temporal course of the ignition nucleus.
Fig. 9 shows on the right two time sequences. The top row is the light emission of the ignition nucleus caused by chemical reactions (chemiluminescence), the bottom row shows the laser-induced fluorescence of the OH radical measured at the same time. The distribution of this radical (see Elementary chemical kinetics) shows clearly that the turbulent flow field distorts the ignition nucleus considerably. In some regions, a significantly decreased OH concentration is visible at the edge of the nucleus, which suggests that the system here is close to extinction, i.e. in some places the combustion is, in a sense, being ‘blown out’ by the strongly turbulent flow. In the chemiluminescence images, this phenomenon is no longer exhibited or less clearly, since these images integrate all the light emitted in the direction of observation and the information ends up ‘blurred’. In other words, unlike the laser-based methods, the chemiluminescence images cannot provide satisfactory spatial separation of a narrow region of observation.
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An ignition cell fitted with glass windows for investigating spark ignition in turbulent gas mixtures. Four electric motors for turbulence generation in the cell are visible.
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A high-velocity image sequence of an ignition process in a turbulent flow, recorded in this cell.
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