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Laser spectroscopy in the visible and near infrared is widely used as a diagnostic tool for combustion devices, but this approach is difficult at high pressures within a sooty flame itself. High soot concentrations render flames opaque to visible light, but they remain transparent to far-infrared or terahertz (THz) radiation. The first farinfrared absorption spectra, to the best of our knowledge, of sooty, non-premixed, ethylene high-pressure flames covering the region of 0.2-2.5 THz is presented. A specially designed high-pressure burner which is optically accessible to THz radiation has been built allowing flame transmission measurements up to pressures of 1.6 MPa. Calculations of the theoretical combustion species absorption spectra in the 0.2-3 THz range have shown that almost all the observable features arise from H 2 O. A few OH (1.84 and 2.51 THz), CH (2.58 THz), and NH 3 (1.77 and 2.95 THz) absorption lines are also observable in principle. A large number of H 2 O absorption lines are observed in the ground vibrational in a laminar non-premixed, sooty flame (ethylene) at pressures up to 1.6 MPa.

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Laser spectroscopy in the visible and NIR is widely used to study flame behaviour in internal combustion engines and turbines, but is inapplicable at high pressures because high soot concentration renders flames opaque; however they remain transparent to THz radiation. We use THz time-domain transmission spectroscopy to characterise gaseous species in flames. A specially designed high-pressure burner vessel has been built which allows spectroscopy to be carried out at pressures above 5 bar. IntroductionAn understanding of the complex soot formation processes occurring in combustion systems is required for the study of technical factors associated with sooty combustion, which have implications on combustion efficiency. Therefore there is a need to develop advanced diagnostic techniques for the very harsh environment of gas turbine combustor burning liquid fuels. However, optical diagnostic laser-based techniques such as laser induced fluorescence (LIF), polarization spectroscopy, CARS and DFWM [1-5] are difficult or impossible to apply in strongly sooted combustion environments, owing to strong absorption, spectral interference from particulate scattering, and fluorescence from large molecules. THz spectroscopy, therefore, offers a new tool for the study and visualisation of heavily sooted flames, because they are transparent at THz frequencies, whilst combustion products have absorption lines in the THz range. Although the spatial resolution THz imaging and spectroscopy is limited by its longer wavelength, and is therefore lower than comparable optical diagnostic methods, it is judged to be sufficient for practical applications owing to the large size of the combustion device. The first THz absorption measurements of a premixed laminar hydrocarbon-air flame were carried out by Cheville and Grischkowsky using the technique of terahertz timedomain spectroscopy [6]. They observed a large number of absorption lines, including those of water, CH and NH3, establishing the relative abundance of combustion products.