Terahertz time-domain spectroscopy of high-pressure flames

Abstract

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 far-infrared 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 H₂O. A few OH (1.84 and 2.51 THz), CH (2.58 THz), and NH₃ (1.77 and 2.95 THz) absorption lines are also observable in principle. A large number of H₂O absorption lines are observed in the ground vibrational in a laminar non-premixed, sooty flame (ethylene) at pressures up to 1.6 MPa.

Key words： terahertz time-domain spectroscopy; high-pressure flames; H₂O absorption lines

Acknowledgements

This work was supported by EPSRC in the frame of basic technology. Thanks go to Dr W Truscott, the project coordinator, at the University of Manchester.

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