Combustion Physics

Lund University

Rayleigh and Filtered Rayleigh scattering

Johan Zetterberg, Zhongshan Li and Marcus Aldén

Contact person: Johan Zetterberg

Temperature is one of the most important parameters in combustion. It needs to be measured with high accuracy, high spatial resolution and if possible in two dimensions. There are only a few techniques that can fulfil all these criteria, Rayleigh scattering and its sister-technique filtered Rayleigh scattering being among them, where gas density and the Doppler/collisional broadening (due to the thermal motion of the molecules), are utilized respectively.

The light scattered by molecules and particles that are much smaller than the wavelength of the incident light it is called Rayleigh scattering, named after Lord Rayleigh, who first explained the phenomenon in the nineteenth century. Rayleigh scattering is a phenomenon one comes across in everyday life. It is responsible for the blue sky and the colorful sunset. The reason for the blue color is that the cross section, or “probability”, for light scattering scales as the frequency of the light to the fourth power. Rayleigh scattering is a non-resonant, elastic scattering meaning that no energy is exchanged between the energy levels in the molecule – the scattered light has the same wavelength as the incident laser light.

Rayleigh scattering is dependent on the number of scatterers in a given volume and the signal is given as the sum of the scattered light from each individual scatterer, meaning that the signal increases linearly with pressure, something that can be made use of in certain applications. Through the dependence of the signal on density the temperature of a gas may be determined, other things that can be measured with Rayleigh scattering is density and pressure.

The Rayleigh scattering process is relatively strong in comparison to e.g. Raman scattering, and can therefore be applied in two dimensions. The technique has been used for decades in combustion diagnostics, due to its strong signal, relatively simple setup (see Figure 1) and ease of implementation.

Figur 1. A typical experimental setup for Rayleigh and filtered Rayleigh scattering. The difference between the two techniques is that the filtered Rayleigh scattering requires a narrow-linewidth single-mode laser and an atomic mercury filter, as indicated in the figure.

Conventional Rayleigh scattering has one strong drawback – the scattered light has the same wavelength as the incident light, making it extremely hard to separate the two spectrally, see figure 2a. The unwanted spurious scattering from particles in the air as well as from surfaces and optics usually means that the technique is limited to controlled environments. This image is un-processed raw-data from a jet flame, low signal corresponds to high temperature and all the bright spots are dust particles in the air.

Figur 2. a) shows an un-processed raw-data image obtained using conventional Rayleigh scattering. The image is not converted to temperature (low signal corresponds to high temperature and all the bright spots are dust particles in the air). The principle of filtered Rayleigh scattering is shown in b). The mercury filter blocks the un-broadened elastically scattered light from particles and from surfaces, whereas the Doppler-broadened wings of the Rayleigh-Brillouin scattering are transmitted. The modelled temperature in the cell is 0°C and the temperature of the scattering medium is 500K.

At the Division of Combustion Physics Filtered Rayleigh scattering (FRS), is under development. In filtered Rayleigh scattering a single-mode laser (~100 MHz) is utilized together with an extremely-narrow atomic mercury-vapor filter (~3-4 GHz) in order to filter out the unwanted scattered light from the incident laser, a typical setup is shown in Fig. 1. This is made possible due to the Doppler broadening of the Rayleigh-scattered light, which originates from the thermal motion of the molecules (500 m/s at room temperature), the principle of filtered Rayleigh scattering can be found in Fig. 2b. This means that the line profile of the scattered light is of utmost importance for an accurate result. An example of a filtered Rayleigh scattering measurement is shown in Fig. 3.

Filtered Rayleigh scattering has become an important tool in order to pursue two-dimensional measurements in reaction flows, close to surfaces (~250 µm) and has even been applied in a Diesel engine with some success. As you read this the development of filtered Rayleigh scattering continues where other filter candidates, measurements of fuel/air ratios and measurements at elevated pressures are being pursued.

Figur 3. a) shows a single shot two-dimensional temperature measurement conducted in a Wolfhard-Parker burner and b) shows a 10-shot average-temperature profile of the temperature at a level 9 mm above the burner. The circles indicate calibrated thermocouple measurements made by Smyth and the crosses filtered Rayleigh scattering measurements made at Lund University.

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