Infrared Laser-Induced Grating Techniques for Diagnostics in Reactive Flows

Abstract: AbstractThe work presented in this thesis is mainly focused on the development andapplication of infrared laser-induced grating techniques in order to measure the gasphase temperature and species concentrations in reactive flows. The two techniquesused for this purpose are laser-induced grating spectroscopy (LIGS) and degeneratefour-wave mixing (DFWM). Their coherent nature allows to perform laser-baseddiagnostics in the infrared spectral region where fundamental ro-vibrationaltransitions of several combustion-related molecular species can be utilized.Gas phase temperature measurements has been performed with laser-inducedthermal grating spectroscopy, known as LITGS. The single-shot precision andaccuracy of mid-infrared LITGS was investigated in premixed CH4/H2/air flames atatmospheric pressure by probing the hot water absorption lines around 3.1 μm,resulting in a single-shot precision better than 1 % and an accuracy of 2.5 %.Furthermore, the technique has been applied in sooty premixed atmosphericC2H4/air flames. Besides the application of mid-IR LITGS, the alignment of thetechnique in terms of grating spacing, number of resolved oscillation peaks in theLITGS signal and its time characteristics has been investigated as well asmisalignment effects. Attention has also been paid to select absorption lines whichresults in thermalization and hence are present in a LITGS excitation spectrum. Inaddition to LITGS, laser-induced electrostrictive grating spectroscopy (LIEGS) wasinvestigated for possible application in harsh environments at temperatures up to700 K. This study was carried out in the spectral band of oxygen at 760 nm and fortwo different geometrical alignments of LIEGS/LITGS.Ammonia (NH3) and hydrogen cyanide (HCN) are two molecular species ofparticular interest in the thermochemical conversion of biomass, which havefundamental or combinational vibrational bands in the mid-IR. Hence, speciesconcentration measurements were carried out using mid-IR DFWM. A feasibilitystudy has been performed for application of DFWM for ammonia detection around2.3 μm. Moreover, DFWM was applied for HCN detection during thethermochemical conversion of straw pellets, where the HCN release history duringthe devolatilization stage was quantified at different flue gas temperatures.