Mid-infrared laser-induced thermal grating spectroscopy in flames

For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3. μm is demonstrated as a versatile diagnostic tool. This spectral region is of particular interest in combustion diagnostics as many relevant species such as hydrocarbons and water exhibit fundamental vibrational modes and hence can be probed with high sensitivity. Another benefit of the IR-LITGS is that it allows performing spectroscopy in the infrared combined with signal detection in the visible. Hence, the strong thermal radiation inherent in flames does not represent an interference. As the first step, we present the application of IR-LITGS to cold gas flows, where traces of ethylene and water vapor are detected. The time-resolved LITGS signals, which can be acquired in a single laser shot, are rich in information and allow deriving temperature and to some extend chemical composition. In the second step, the IR-LITGS technique is applied to ethylene/air flames stabilized on a flat flame burner. A proof-of-concept study is carried out, in which the temperature is determined in the burned region of flames with systematically varied equivalence ratio (0.72 < Φ <. 2.57). Moreover, in a highly sooty flame, LITGS signals were recorded as a function of height above the burner and allowed the determination of the temperature profile. The proposed IR-LITGS method has the potential for enabling single-shot measurements of several parameters at a time. Its applicability to sooty flame environments opens up new opportunities to study the complex formation of carbonaceous particles in flames.