Sooting Flames: Temperature measurement in sooting flames
The relative population of different states (translational, rotational, vibrational and electronic) of an atom or molecule dependents on temperature. Therefore, every measurement method capable of probing the population of one or multiple states can in principle be used for temperature measurements. For the practical application, apart from the experimental realization, the probed state must have an energy that corresponds to the temperature range that should be measured. Laser-induced fluorescence (LIF) is a flexible method for probing state populations. A number of methods has been developed for temperature measurements . Depending on the number of states probed, we distinguish between one-, two-, and multi-line methods. Our group is developing methods for temperature measurements in sooting and non-sooting flames using the nitric oxide (NO) molecule as temperature probe . Two- as well as multi-line approaches are investigated. In the two-line approach, the populations N1 and N2 of two different vibrational states are measured using laser-induced fluorescence. The temperature dependence of the populations depends on the energy difference Δε of the states and is given by a Boltzmann expression
with k, Boltzmann constant.
The advantage of this two-line method is the independence of the local NO concentrations through the ratio in the given equation. We applied the method in lean and sooting laminar ethylene/air flames (see figures). The NO molecule is an excellent temperature probe for experimental as well as spectroscopic reasons. Due to its stability at high temperatures, it is naturally formed in most combustion systems or can be added to the flame. It has a relatively large absorption cross section, high fluorescence efficiency. Furthermore, the molecule is spectroscopically well characterized which is important for interpretation of the LIF signals.
Figure 1: Temperature field in a cross-section through a sooting ethylene-air flame
 Laurendeau N. M., Temperature Measurements by Light-Scattering Methods, Prog. Energy Combust. Sci. 14, 147-170 (1988).
 W.G. Bessler, F. Hildenbrand, C. Schulz, Two-line laser-induced fluorescence imaging of vibrational temperatures of seeded NO, Appl. Opt. 40, 748-756 (2001).
 W. G. Bessler and C. Schulz, "Quantitative multi-line NO-LIF temperature imaging," Appl. Phys. B 78, 519-533 (2004).
 T. Lee, J. B. Jeffries, R. K. Hanson, W. G. Bessler, and C. Schulz, "Quantitative NO-LIF Temperature Imaging in High-Pressure Flames," in 41st AIAA Aerospace Sciences Meeting and Exhibit, January 6-9 (Reno, NV, 2003), Paper No. 2003-0583.
 T. Lee, W. G. Bessler, H. Kronemayer, C. Schulz, and J. B. Jeffries, "Quantitative temperature measurements in high-pressure flames with multi-line nitric oxide (NO)-LIF thermometry," Appl. Opt. 31, 6718-6728 (2005).
 H. Kronemayer, W. Bessler, and C. Schulz, "Gas-phase temperature measurements in evaporating sprays and spray flames based on NO multiline LIF," Appl. Phys. B 81, 1071-1074 (2005).
 H. Kronemayer, I. Düwel, and C. Schulz, "Temperature imaging in spray flames," in European Combustion Meeting (Louvain-la-Neuve, 2005).