Spectroscopic Methods and Visualization Applied to Combustion Diagnoses - Novel Combustion Concepts for Sustainable Energy Development - page 173

1.1.1 Spectroscopic Temperatures

Regarding with heat transfer and toxic emission, temperature is among the most
important physical properties for combustion science. Temperature distributions in
combustion flames have conventionally been measured by thermocouples placed in

different positions. However, several disadvantages are encountered in measure-
ment, such as disturbance of combustion

fl

flows, errors due to gray body emission,
low spatial resolution, due to the limited number of thermocouples etc. In addition,
thermocouples have no capability to discriminate temperatures of combustion and
non-combustion

fl

flames and surrounding air.
From those viewpoints, spectroscopic measurements have advantages over ther-
mocouple measurements, as well as no limited temperature ranges. However, careful
selection of spectral lines and/or bands should be made to obtain accurate results.
In this chapter, spectroscopic measurements of soot temperature and molecular
temperature of an intermediate species, C 2 , during combustion will be described.

fields, e.g., inside fl

1.1.2 Soot Temperature

The temperature on the surfaces of soot particles is determined by the solid lattice
vibration, or graphite lattice typically, and the gray body emission intensity at a
wavelength

λ is well known to be expressed as a function of soot temperature, T s by

'

the Planck s law as follows:

8 p hc eð T s ; kÞk 5 I k ¼ ð Þ=f exp ð ch =k kT s Þ 1 g ð 1 Þ

where h is the Planck ' is constant, c the light velocity, e the emissivity, and k the

Boltzmann s constant. Taking a ratio of the intensities at two different wavelengths,

or two colors, we obtain the expression for well-known two-color thermometry.
I k 1 . I k 2 ¼ eð T s ; k 1 Þ=eð T s ; k 2 Þ

'

. exp

5

ð 2 Þ

f gðk 2 =k 1 Þ f exp ð ch =k 2 KT s Þ 1 g f ð ch =k 1 kT s Þ 1 g

The temperature is calculated from this equation.
The issue here is to cancel the emissivity term, eð T ; k 1 Þ = eð T ; k 2 Þ . If the differ-

ence in the wavelength between the two colors, the two emissivity is not identical in
magnitude and the ratio is variable dependent on the temperature, leading to the
errors in the obtained temperature. If the wavelengths are too close, the measure-
ment precision is low since the exponential terms becomes insensitive to the
temperature. Thus, optimized selection should be made, also avoiding the structured
spectra from chemical species, such as molecular bands from OH, CH, and C 2 by
using narrowband optical

filters. We named this method taking these issues into
account, as a narrowband two-color method (Kubota et al. 1998 ). The filter pair

should be selected after careful examination of structured spectra from combustion

fields. Soot temperature measurement is suitable for radiant fl flames of high equiv-

alence ratios, and/or high-carbon fuels such as heavy oil.