Numerical simulation of infrared radiation absorption for diagnostics
of gas-aerosol medium by remote sensing data
O.K. Voitsekhovskayaa, O.V. Egorova,b, D.E. Kashirskii*b, O.V. Sheferc
Department of Quantum Electronics & Photonics, Radiophysics faculty, National Research Tomsk
State University, Tomsk, Russia;
b
Academician V.D. Kuznetsov’s Siberian Physical-Technical R&D Institute, Tomsk, Russia
c
National Research Tomsk Polytechnic University, Tomsk, Russia
a
ABSTRACT
Calculated absorption spectra of the mixture of gases (H2O, CO, CO2, NO, NO2, and SO2) and aerosol (soot and Al2O3),
contained in the exhausts of aircraft and rocket engines are demonstrated. Based on the model of gas-aerosol medium, a
numerical study of the spectral dependence of the absorptance for different ratios of gas and aerosol components was
carried out. The influence of microphysical and optical properties of the components of the mixture on the spectral
features of absorption of gas-aerosol medium was established.
Engine exhaust, combustion product, gas, aerosol, absorption spectrum
1. INTRODUCTION
Currently, to prevent anthropogenic disasters, a relevance of basic research in improving vehicle safety, energy, military
and civilian rockets increased. Particular attention is paid to the influence of combustion products of various fuels on the
gas composition of the Earth's atmosphere, as well as the study of fires and volcanic emissions [1, 2]. Many of the
compounds contained in exhaust, directly involved in the cycle of ozone destruction in the atmosphere. H2O, H2SO4,
HNO3, ions and soot particles contribute to the formation of aerosols influencing the radiation balance of the Earth. This
paper presents the results of modeling the spectral characteristics of the combustion products of fuels in the most suitable
laser sensing spectral regions to assess the operating condition of the engine.
Optical methods, including active (measuring attenuation of radiation of external source such as a laser) and passive
(registration of an exhaust radiation) sensing methods, are among the most promising non-invasive methods of diagnosis
of the exhaust jet engines. Their development requires simultaneous studies of the spectral characteristics of both
molecular and aerosol components of the medium. Quantitative analysis of the attenuation of the infrared radiation
passed through the gas-aerosol heterogeneous medium allows to set the boundary values of microphysical parameters of
the scattering particles, which determines the feasibility of the joint account of the molecular absorption and aerosol
scattering.
2. METHODS
To perform the numerical simulation of absorption spectra of the gas-aerosol media the following equation was used
A(ν 0 ) =
1
∫ (1 − T (ν ) )dν ,
Δν Δν
(1)
where A(ν 0 ) is the absorptance at wavenumber ν 0 , Δν = 0.01 cm-1, T (ν) is transmittance defined by the relation
− α ( ν ) +α aer ( ν ) )l
T (ν) = e ( sel
,
* [email protected]
21st International Symposium on Atmospheric and Ocean Optics: Atmospheric Physics,
edited by G. G. Matvienko, O. A. Romanovskii, Proc. of SPIE Vol. 9680, 968054
© 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2206009
Proc. of SPIE Vol. 9680 968054-1
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(2)
where α sel (ν ) is the selective absorption coefficient of gases, α aer (ν ) is aerosol extinction coefficient, and
l = 100 cm-1 is the length of the optical path.
Calculation of the selective absorption coefficient is performed using the most accurate line-by-line method
α sel (ν) =
∑ ρi ∑ Sij ⋅ g (ν − νij ) ,
i
(3)
j
where ρi is the partial pressure of i gas, Sij is the strength of j spectral line of i gas, g (ν − νij ) is the normalized line
shape function. In this work we use the well known Lorentz line shape with halfwidth δ
g (ν − νij ) =
δ
1
⋅
.
2
π δ + ν−ν 2
ij
(
)
(4)
The main gas combustion products of fuels are oxides of: carbon COx, hydrogen HOx, nitrogen NOx and sulfur SOx.
Water vapor (H2O), carbon oxides (CO and CO2), nitrogen oxides (NO and NO2) and sulfur dioxide (SO2) was
considered as components of the exhaust for which spectral characteristics was simulated.
To calculate the transmission spectra of the atmosphere (Summer, mid-latitude) spectral line parameters (SLP) from the
database HITRAN2012 [3] were used. To simulate the high temperature spectra the database HITEMP [4], as well as
developed by us SLP databases for molecules SO2 and NO2 [5, 6, 7] were applied.
The combustion process is usually accompanied by the emission of soot aerosols, in the case of metal fuels the
dialuminum trioxide (Al2O3) added. Calculation of the aerosol extinction is carried out for the case of spherical particles
according to the Mie theory [8]
α aer (ν) = N ⋅
2π ∞
∑ (2i + 1) Re(ai + bi ) ,
k 2 i =1
(5)
where N is the volume concentration of particles, k = 2π λ , λ is wavelength of the incident radiation, and ai and bi
can be expressed in terms of Riccati-Bessel functions. The optical properties of the particles characterized by a complex
refractive index n% = n + iχ , where n describes the refraction, and χ is the absorption coefficient.
3. RESULTS
Figs. 1-4 show the values of molecular absorptances and the transmittance spectra of the ten meter atmosphere.
Figure 1. Spectral lines suitable for SO2 and H2O detecting. Solid line is atmospheric transmission spectrum.
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Figure 2. Spectral lines suitable for NO and H2O detecting. Solid line is atmospheric transmission spectrum.
Figure 3. Spectral lines suitable for NO2 detecting. Solid line is atmospheric transmission spectrum.
Figure 4. Spectral lines suitable for CO2 and CO detecting. Solid line is atmospheric transmission spectrum.
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We considered the exhaust gases of the engine at three temperatures: 400 K, 700 K, and 1000 K. Size of plume
was 100 cm. The spectral lines of gas absorptances were selected in transparency windows of the atmosphere. It should
be noted that the absorption lines of water vapor give contribute practically in all spectral ranges taken into
consideration.
Analysis of a number of experimental and theoretical studies shown that microphysical properties of the particles vary
widely [9, 10]. Absorption spectra of soot and dialuminum trioxide particles with different sizes are demonstrated at
Figs. 5 and 6.
Figure 5. The absorption spectra of soot particles with different sizes (top) and their integral absorption spectrum (bottom).
Figure 6. The absorption spectra of Al2O3 particles with different sizes (top) and their integral absorption spectrum (bottom).
The maximum absorption of the soot particles correspond to size 100 nm. The growth of the Al2O3 particle’s size leads
to increasing the absorption. For particles with sizes greater than 5 micrometers absorption decreases. The calculations
can be said that the refractive index n and the particles size determine the spectral behavior of the absorptance.
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4. CONCLUSION
To illustrate the effect of various factors (the characteristics of the gas compounds, microphysical and optical properties
of aerosols and thermodynamic properties of the medium) on the absorption coefficients of the engine exhaust, the
numerical experiment was carried out. This information is necessary for the accurate determination of the
thermodynamic parameters and the composition of the gas-aerosol media from measured absorption spectra of the
combustion products.
ACKNOWLEDGEMENTS
The study presented was conducted with the support from Ministry of Education and Science of the Russian Federation
within the framework of the State Program № 16.1032.2014/K (2014-2016).
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