Filtered Rayleigh Scattering (FRS)

The non-intrusive Filtered Rayleigh Scattering (FRS) method is well suited to measure the temperature, velocity, density and pressure in unseeded gas flows. It was developed and proved by DLR. The light source is a high power cw-Laser with a precisely adjustable wavelength. The Laser is used to create a light sheet with a homogenous intensity distribution over the entire cross-section. The FRS method is based on the acquisiton of the intensity of the Rayleigh scattered light from gas molecules in a laser light sheet spectrally filtered by a molecular filter, a iodine cell. The absorption spectrum of iodine is used to eliminate the scattered light of walls and Mie scattered light. The intensity signal in the light sheet is then focused on a sCMOS chip behind the iodine cell.

The shape of the frequency spectrum is influenced by flow field parameters temperature, pressure, Doppler shift and the composition of the gas. Knowing the composition of the gas and the exact shape of the iodine absorption spectrum therefore enables determining the parameters temperature, pressure, velocity and density by using the Tenti model. To gain the information needed for the model to reconstruct the flow parameters the laser frequency is varied systematically in equidistant steps (Fig.3), which is called Frequency Scanning-FRS (FSM-FRS).

Figure 1 shows the FRS measurement setup of a heated free jet. Main objective of the measurement was to determine the temperature distribution in the stream and to compare the FRS measured temperature with that of a calibrated PT100 temperature probe. The results are shown in Figure 2. The temperature distribution in the FRS graph on the left is spatially highly resolved  whereas the PT100 graph shows an averaged temperature distribution due to the interpolation of 147 individual measuring points distributed over the entire measurement range. The temperature averaged over 10 pixels in the core stream and in the surrounding area show very similar results in both measurements with less then 0,4% deviation.

The FRS method uses the elastic light scattered from molecules and does not need additional particles in its setup. This fact makes it interesting for environments where seeding particles cannot be added easily or where there are few particles in principle (e.g. cryogenic wind tunnels). Other applications are small regions of interest where a measuring probe would negatively interfere with the gas flow, turbulent and high temperature flows (100-2000K) and of course measurements where the various flow quantities should be acquired with only one simple setup. Furthermore the proportional dependence of the intensity in the Tenti model on pressure makes FRS especially suited for high pressure environments (e.g. combustion chambers, turbomachinery).

The ILA R&D FRS system consists of the high power laser source, a wavemeter, lightsheet optics, the FRS camera module with the iodine filter (Fig. 4), the INDIO Controller and an FRS PC. All hardware signals come together in the INDIO Controller, which controls i.a. the wavelength, light sheet, laser power and iodine cell temperature. All of this is incorporated in the ARES-Software,  which offers a comprehensible design, full control of all settings and easy-to-follow steps that are necessary for performing measurements.

The accuracy of the measurable parameters proved to be as follows:

•Temperature: ± 1 %
•Pressure: ± 3 %
•Density: ± 3 %
•Velocity: ± 1 m/s

Fig. 1: FRS mesurement setup with Laser and Camera Module.
Fig. 2: Comparison of the FRS temperature measurement (high spatial resolution) with a averaged PT100 measurement in a free jet.
Fig. 3: Convolution of the intensity of the rayleigh scattered light with the filter curve of the iodine cell.
Fig. 4: Camera modul with iodine cell.
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