Research progress on soot measurement by laser induced incandescence

Laser induced incandescence is a non-contact optical diagnosis method. With this method, we can obtain the two-dimensional spatial distribution of instantaneous soot within the thin layer of incoming sheet laser. This method has become an important measurement technology of soot due to its high spatial and temporal resolution. This paper first introduces the development of LII technology and basic theory. Then the technical methods of LII and research progress at home and abroad are summarized detailedly from three aspects, that numerical simulation, qualitative and quantitative measurement. The LII mathematical model mainly includes Melton model, Liu model and Michelsen model. These models can be used to predict the change rule of LII signal and also lay a foundation for the test of soot particle size. To realize quantitative measurement, the calibration of LII signal is necessary. This is also one of the difficulties in LII measurement. There are mainly three methods for calibration, that are sampling technique, light extinction method(LEM) and 2-color laser induced incandescence(2C-LII). The sampling technique is less used because it will disturb the combustion process and mix impurities. The LII-LEM needs two laser systems and uses the measuring result of LEM to calibrate the LII signal, so its operation is complicated. Nevertheless, 2C-LII does not need other measurement technology and can realize online real-time calibration. Since this method is relatively simple, it develops rapidly and has achieved many significant results. Naturally, LII technology still needs improvement, such as optimizing the incident laser wavelength and energy, controlling the uniformity of laser sheet, perfecting the LII mathematical model, and extending the application in complex environment. Through summarizing the research achievements of LII technology, this paper aims to emphasize the research status and the importance of this method in understanding the soot formation and oxidation mechanism, and providing some references for its future development.