Johan Sjöholm, Mattias Richter and Marcus Aldén
Contact person: Johan Sjöholm
Raman spectroscopy is a technique that has one benefit and one major drawback. The benefit is that you can theoretically measure all different species in a region at once. The drawback is that the signal strength is very low, usually limiting measurements to the major species. In flames this usually means N2, fuel, O2, CO2 and H2O.
The Raman signal depends on a number of parameters. The most important are the so called Raman cross section and the wavelength of the incoming light. The Raman cross section reflects the probability for Raman scattering and is species specific. The wavelength of the incoming light is important since it strongly affects the strength of the Raman signal. Just like for Rayleigh scattering, the signal strength depends on the wavelength raised to the fourth power. This means that if you change laser wavelength from 532 to 266 nm you gain roughly a factor of 16 in signal intensity. Remembering the extremely low signal intensity of Raman scattering, this is very useful. The Raman signal is further temperature and pressure dependent through the number density N that, given the ideal gas law, follows N = pV/RT.
In Fig. 1 is shown an image if the Raman signal from a Bunsen burner. The upper image shows the spatially resolved spectrum acquired just over the burner edge. The y-axis denotes the distance from the center of the burner. The x-axis shows the Ramanshift in cm-1 from the laser wavelength, 266 nm. The lower image shows the Raman signal along three different positions in the flame. The difference in relative species concentrations is evident.
Figure 1. Image of the Raman spectra from a Bunsen burner just over the burner edge. In the upper image the x-axis indicates the Raman shift and the y-axis indicates the distance from the centre of the burner along the laser beam. The lower image shows the Raman signal along three different positions in the flame.
There are three different regions in this image. At the top and bottom is air with only O2 (1556 cm-1), N2 (2331 cm-1) and H2O (3651 cm-1) visible, between 0.5 and 1 cm is the flame zones with lower signal due to the higher temperature and a lot more species. In the middle, around 0 cm there is mixed fuel with the air as indicated by the large CH peak around 2930 cm-1.
Raman spectroscopy has also been applied to a truck-sized Scania engine that has been modified for optical access. In this case the local fuel-air ratio, F, in a fuel spray from a high pressure Diesel injector was studied as a function of different engine parameters. One example of a Raman spectrum is seen in Fig 2. One benefit of working inside an engine is the increased pressure that increases the Raman signal. The drawback is that the windows of the engine limit the laser power that can be used thus limiting the signal.
Figure 2. The left image shows the Raman spectra from a 266 nm laser beam that is crossing a spray of isooctane from a high pressure direct injector in an engine. The x-axis shows the Raman shift and the y-axis indicates the distance along the laser beam. The right image shows the F value (fuel-air-ratio) along the laser beam.