Mid-infrared laser-induced thermal grating spectroscopy in flames

Abstract

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.

Cycle-to-Cycle Variation Analysis of Two-Colour PLIF Temperature Measurements Calibrated with Laser Induced Grating Spectroscopy in a Firing GDI Engine

“…In-depth theoretical discussions of LIGS grating formation [49][50][51][52] and its application for measurements in combustion relevant environments including flames [56][57][58][59], IC engines [44,60], flashboiling sprays [61], shock tubes [62], and rapid compression 01/14/2019 machines [63] are available in the literature. A brief description of the LIGS technique is provided here for the convenience of the reader.…”

Quantitative measurement of temperature in oxygen enriched CH4/O2/N2 premixed flames using Laser Induced Thermal Grating Spectroscopy (LITGS) up to 1.0 MPa

“…[5]), Laser Induced Electrostrictive Gratings (LIEGS) (e.g. [6]), and Laser Induced Thermal Grating Spectroscopy (LITGS) [7][8][9][10][11][12][13][14][15][16][17][18][19]. The high pressures prevalent in gas turbines and rocket systems create significant challenges for optical diagnostics.…”

“…As detailed further on, this is because the signal intensity is proportional to the square of the induced refractive index fluctuation [15], which increases with the density of the mixture. A number of studies using LITGS have been used in nonreacting [8][9][10][11][12][13] and reacting [14][15][16][17][18][19] flows. Although LITGS can only be used in the presence of an absorbing species, LITGS also offers the advantage of significantly higher signal relative to LIEGS, and less stringent requirements on optical alignment relative to CARS.…”

“…Absorption by the hydroxyl radical OH around 307 nm was used in measurements of product temperature over a flat-flame burner stabilized using H2/O2 at atmospheric pressures [14], and CH4/air mixtures at pressures from 1.0 to 4.0 MPa [16]. More recently, Sahlberg et al [18] obtained LITGS signals over an atmospheric pressure ethylene flame using infrared absorption around 3.2 µm for both the fuel itself and H2O to generate the thermal grating. Since fuel in unburnt mixture and water vapor in product gas are major components, a stronger LITGS signal than for OH is expected even at low pressure.…”

Flame thermometry using laser-induced-grating spectroscopy of nitric oxide

“…Finally, Sahlberg et al have demonstrated LITGS thermometry of flames using mid-IR pumps around 3000 nm and a cw probe laser at 457 nm to generate LITGS signals from combustion-generated H 2 O [28].…”