Laser-Induced Incandescence (LII)

Several laser-based techniques offer the possibility of soot diagnostic in combustion processes. They allow the assessment of soot concentration, soot particle size distribution, soot structure etc. Laser-induced incandescence (LII) allows particle size measurements and two-dimensional imaging of soot properties. The same strategy can be used for diagnostics on particles of a wide variation of chemical composition. LII, therefore, is not only used for combustion diagnostics but is also applied in material synthesis of nano particles in the gas phase.

Following Planck´s law, hot particles emit thermal radiation. This causes the orange color of sooting (e.g. candle) flames. The particles in these flames can reach temperatures of up to 2000 K. In the laser-induced incandescence technique, the soot particles are additionally heated by an intense laser beam to their vaporization temperature of approx. 4000 K [1]. The heated particles show an emission characteristic ("incandescence") that is strongly different from the non-heated particles (more intense radiation, blue-shifted emission maximum, different temporal emission characteristics). This allows selective detection of the incandescence with fast detectors and image-intensified cameras.

Theoretical considerations [2] and experiments [3] have shown that the LII signal is directly proportional to soot volume fraction. LII, therefore, can give two-dimensional images of the soot concentration distribution in a flame [4].

Moreover, time-resolved measurements provide access to soot paricle size (time-resolved LII, TR-LII) [5]. After laser heating, the particles slowly cool down again until they reach ambient flame temperature some 500 ns after the laser pulse. During this cooling process they change their emission characteristics. Since large particles cool down slower than smaller ones, time-resolved emission measurements after excitation allow the determination of soot particle size distributions.

The simulation program LIISim (www.liisim.com) developed in house simulates LII signal and evaluates measured signal traces of time-resolved measurements.

References:

[1] B. F. Kock, T. Eckhardt, and P. Roth, "In-cylinder sizing of Diesel particles by time-resolved laser-induced incandescence (TR-LII)," Proc. Combust. Inst. 29, 2775-2781 (2002).
[2] M. Hofmann, W. G. Bessler, J. Gronki, C. Schulz, and H. Jander, "Investigations on laser-induced incandescence (LII) for soot diagnostics at high-pressure," in Laser Applications to chemical and environmental analsyis, OSA Technical Digest Series (Optical Society of America, Washington DC, 2002), p. FC1/1-FC1/3.
[3] M. Hofmann, W. G. Bessler, C. Schulz, and H. Jander, "Laser-induced incandescence (LII) for soot diagnostics at high pressure," Appl. Opt. 42, 2052-2062 (2003).
[4] C. Schulz, B. F. Kock, M. Hofmann, H. A. Michelsen, S. Will, B. Bougie, R. Suntz, and G. J. Smallwood, "Laser-induced incandescence: recent trends and current questions," Appl. Phys. B, DOI: 10.1007/s00340-006-2260-8 (2006).
[5] A. Eremin, E. V. Gurentsov, M. Hofmann, B. F. Kock, and C. Schulz, "Nanoparticle formation from supersaturated carbon vapor generated by laser photolysis of carbon suboxide," J. Phys. D., DOI: 10.1007/s00340-006-2199-9 (2006).
[6] B. F. Kock, C. Schulz, and P. Roth, "Time-resolved LII applied to soot particle sizing and concentration measurements in the cylinder of a Diesel engine," Combust. Flame, in press (2006).
[7] A. V. Filippov, M. W. Markus, and P. Roth, "In situ characterization of ultrafine particles by laser-induced incandescence: Sizing and particle structure determination," Journal of Aerosol Science 30, 71-87 (1999).
[8] B. F. Kock and P. Roth, "Two-color TR-LII applied to in-cylinder Diesel particle sizing," in Proc. of the European Combustion Meeting (Orléans, 2003).
[9] B. F. Kock, C. Kayan, J. Knipping, H. R. Orthner, and P. Roth, "Comparison of LII and TEM sizing during synthesis of iron particle chains," Proc. Combust. Inst. 30, 1689-1697 (2005).
[10] B. F. Kock, C. Kayan, J. Knipping, H. R. Orthner, and P. Roth, "Comparison of LII and TEM sizing during synthesis of iron particle chains," Proceedings of the Combustion Institute 30, 1689-1697 (2005).
[11] B. F. Kock, T. Eckhardt, and P. Roth, "In-cylinder sizing of Diesel particles by time-resolved laser-induced incandescence (TR-LII)," Proc. Combust. Inst. 29, 2775-2781 (2002).
[12] B. F. Kock, T. Eckhardt, and P. Roth, "In-cylinder sizing of diesel particles by time-resolved laser-induced incandescence (TR-LII)," Proceedings of the Combustion Institute 29, 2775-2782 (2002).
[13] R. Starke, B. Kock, and P. Roth, "Nano-particle sizing by laser-induced incandescence (LII) in a shock wave reactor," Shock Waves 12, 351-360 (2003).
[14] P. Roth and A. V. Filippov, "In situ ultrafine particle sizing by a combination of pulsed laser heatup and particle thermal emission," Journal of Aerosol Science 27, 95-104 (1996).