No abstract

We demonstrate the potential of infrared degenerate four-wave mixing (IR-DFWM) as a tool for non-intrusive in situ spatially resolved detection of CH 3 Cl in reactive hot gas flows especially feasible for applications to biomass combustion and gasification. IR-DFWM spectra of CH 3 Cl, by probing ro-vibrational transitions belonging to the fundamental stretching modes v 1 and v 4 , have been successfully recorded in gas flows diluted with nitrogen at atmospheric pressure and elevated temperatures up to 820 K. In order to identify the spectral lines of CH 3 Cl, the recorded IR-DFWM spectra are compared with simulations using molecular parameters extracted from the HITRAN database. The potential interference from water vapor is discussed from measurements of H 2 O spectrum at 820 K combined with simulations of H 2 O IR-DFWM spectrum based on the HITEMP database, and it was found that the Q Q 6 line of the v 1 band is relatively free from water interference at elevated temperatures. At atmospheric pressure, the detection limits for temperatures at 296, 550 and 820 K were estimated to be 2.1, 3.1 and 6.2 (×10 15 molecules/cm 3 ), respectively, by scanning the Q Q 6 line of the v 1 band. These results show the potential of interference free detection of CH 3 Cl with IR-DFWM in harsh environments like combustion.

The measurement of pressure using laser-induced thermal grating spectroscopy, LITGS, with improved accuracy and precision is reported. Pressure values are derived from the record of the time-profile of LITGS signals by fitting of modelled signals to experimental data. The procedure is described for accurate modelling of the LIGS signals involving a sequence of calculation steps with appropriate weighting and calibration to determine the best-fit value of pressure-dependent parameters for averaged and single-shot measurements. Results are reported showing application of this model-fitting method to measurements of pressure in static cells using LITGS generated from NO in mixtures containing N 2 at pressures in the range 0.5-5.0 bar with accuracy of 1-3% and single-shot precision of 4-7%. Time-resolved measurements of pressure, using LITGS signals generated in toluene-seeded fuel vapour, during the compression and expansion strokes of a motored optically accessible engine are reported with pressure-dependent accuracy ranging from better than 10 to around 20% over the cycle and single-shot precision in the range 5-15% over the same range. Measurements in the engine under firing conditions were obtained over a limited range and slightly increased uncertainties associated with varying composition resulting from exhaust gas residuals. The method was found to be of limited utility for measurements in high temperature flames at around ambient pressures. List of symbols LITGS Laser-induced thermal grating scattering MFM Model-fitting method P Pressure T Temperature Λ Inter-fringe spacing of laser-induced grating c s Local speed of sound in the gas f osc Oscillation frequency of LIGS signal γ Ratio of specific heats of gas at constant pressure and volume m Mean molecular mass of gas molecules k B Boltzmann's constant µ Viscosity of the gas τ o Time delay offset on LIGS signal relative to zero of time reference τ g The inter-fringe transit time given by Λ/c s Re Reynolds number Q 1 The 'fast' quenching rate Q 2 The 'slow' quenching rate r The branching ratio between the 'slow' and 'fast' quenching channels. BBO Beta barium borate TDC Top dead centre CAD Crank angle degree NIST National institute for standards and technology (USA)

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.