Infrared Laser-Induced Grating Techniques for Diagnostics in Reactive Flows

Abstract
The work presented in this thesis is mainly focused on the development and
application of infrared laser-induced grating techniques in order to measure the gas
phase temperature and species concentrations in reactive flows. The two techniques
used for this purpose are laser-induced grating spectroscopy (LIGS) and degenerate
four-wave mixing (DFWM). Their coherent nature allows to perform laser-based
diagnostics in the infrared spectral region where fundamental ro-vibrational
transitions of several combustion-related molecular species can be utilized.

Gas phase temperature measurements has been performed with laser-induced
thermal grating spectroscopy, known as LITGS. The single-shot precision and
accuracy of mid-infrared LITGS was investigated in premixed CH4/H2/air flames at
atmospheric 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 atmospheric
C2H4/air flames. Besides the application of mid-IR LITGS, the alignment of the
technique in terms of grating spacing, number of resolved oscillation peaks in the
LITGS signal and its time characteristics has been investigated as well as
misalignment effects. Attention has also been paid to select absorption lines which
results in thermalization and hence are present in a LITGS excitation spectrum. In
addition to LITGS, laser-induced electrostrictive grating spectroscopy (LIEGS) was
investigated for possible application in harsh environments at temperatures up to
700 K. This study was carried out in the spectral band of oxygen at 760 nm and for
two different geometrical alignments of LIEGS/LITGS.

Ammonia (NH3) and hydrogen cyanide (HCN) are two molecular species of
particular interest in the thermochemical conversion of biomass, which have
fundamental or combinational vibrational bands in the mid-IR. Hence, species
concentration measurements were carried out using mid-IR DFWM. A feasibility
study has been performed for application of DFWM for ammonia detection around
2.3 μm. Moreover, DFWM was applied for HCN detection during the
thermochemical conversion of straw pellets, where the HCN release history during
the devolatilization stage was quantified at different flue gas temperatures.