Development and Application of Non-linear Mid-infrared Laser Spectroscopy for Combustion Diagnostics

Abstract: The work reported in this thesis involved the development and application of two novel infrared non-linear laser techniques, namely mid-infrared polarization spectroscopy (IRPS) and mid-infrared degenerate four-wave mixing (IR-DFWM) to combustion diagnostics. These two techniques meet the needs of species concentration detection in combustion environments, especially for those species lacking accessible electronic (one-photon) transitions in the ultraviolet (UV) or visible spectral region. These species include many key molecules in combustion such as hydrocarbons (CmHn), H2O, CO2, HX (X = F, Cl and CN). Using IRPS or IR-DFWM, these important species can be in-situ and non-intrusively measured with a high spatial resolution by probing their ro-vibrational transitions in the mid-infrared, around 3 μm. In terms of IRPS, this thesis mainly shows (1) the effective use of IRPS for measuring species (HCl, HCN, C2H2) concentration in atmospheric pressure clean or sooty flames, even in smoke/particles-laden environments during coal/biomass combustion; (2) the development of quantitative IRPS measurements considering collisional and quenching effects and spectral overlap coefficient; (3) the unique ability of species (HF) imaging in flames, working as a complementarity to planar laser-induced fluorescence commonly used in the UV/visible region. As for IR-DFWM, a novel IR-beam-splitter set-up was developed, which provides a forward phase-matching geometry and greatly simplifies the alignment of an IR-DFWM system. This technique is firstly demonstrated for C2H2 and HCl detections in cold gas flow, and then it is applied for flame thermometry by monitoring the intensity ratios of H2O lines. Both IRPS and IR-DFWM hold great potential for quantitative measurements of key molecular species important for combustion research, especially in low pressure flames, where collisional effects are considerably weaker. Moreover, these two techniques can be applied to other gas-phase diagnostics, such as in plasma.