Spectroscopic flame temperature measurements and their physical significance—I. Theoretical concepts—A critical review

Abstract

In this review the basic theoretical principles of temperature measurements of completely uniform flames (with no temperature or concentration gradients of the thermometric species) by the line reversal, emission-absorption, slope, and two-line Spectroscopic techniques are first presented. Since there have been no unifying accounts on the extension of the basic theoretical concepts to more realistic non-isothermal, non-homogeneous flames, a major portion of this review is devoted to a systematic examination of the validity of temperature data obtained by Spectroscopic techniques for these flames. This examination reaffirms several well known, as well as several less familiar, limitations of the temperature values obtained for real flames. Thus, the values obtained depend on: (a) the measurement method employed; (b) the energy of the quantum states involved in the line producing transition (s); (c) the particular temperature gradient in the flame; and (d) the concentration distribution of the thermometric species. Moreover, the values observed do not represent either the average or a weighted average flame temperature, but correspond to an apparent temperature that yields the ratio of the total number of particles in the relevant quantum states found within the viewing field of the spectrometer, i.e., ▪ where T app is the apparent temperature, q and p refer to the two quantum states, g is the degeneracy, n x is the particle density and E is the energy of the quantum level. This result is quite different from the Boltzmann equation ▪ which defines the temperature T x at the point x in the flame.