Abstract:
This thesis describes the development and evaluation of a high-temperature combustion
standard. This comprises a McKenna burner premixed flame, together with a full
assessment of its temperature, stability and reproducibility. I have evaluated three
techniques for high-accuracy flame thermometry: Modulated Emission in Gases
(MEG), Rayleigh scattering thermometry and photo-acoustic thermometry.
MEG: Analysis shows that MEG is not usable in this application because the sharp
spectral features of the absorption coefficient of gases are represented within MEG
theory as an average absorption coefficient over the optical detection bandwidth. A
secondary difficulty arises from the lack of high power lasers operating at wavelengths
that coincides with molecular absorption lines in the hot gas.
Rayleigh Scattering: Applying corrections for the temperature-dependence of the
scattering cross-section, it has been possible to determine the temperature of the
combustion standard with an uncertainty of approximately 1%. The temperature
dependence of the scattering cross-section arises from changes in the mean molecular
polarisability and anisotropy and can amount to 2% between flame and room
temperatures. Using a pulse Nd-YAG laser operating at 532 nm and high linearity
silicon detectors, the Rayleigh scattering experimental system has been optimised.
Temperatures measured over a three-month interval are shown to be reproducible to
better than 0.4% demonstrating the suitability of the McKenna burner as a combustion
standard.
Photo-Acoustic: By measuring the transit time of a spark-induced sound wave past two
parallel probe beams, the temperature has been determined with an uncertainty of
approximate 1%.
Flame temperatures measured by the photo-acoustic and Rayleigh scattering
thermometry system show good agreement. For high airflow rates the agreement is
better than 1% of temperature, but for low airflow rates, photo-acoustic temperatures are
approximately 3.6% higher than the Rayleigh temperatures. Further work is needed to
understand this discrepancy.
