1. Predicting Engine Exhaust Plume Spectral Radiance & Transmittance
Engineering Project
MANE 6980 – Spring 2010
Wilson Braz

2. IR Radiation Introduction
Infrared (a.k.a. thermal) portion of electromagnetic spectrum spans approximately 0.5mm to 1000mm

3. IR Radiation Introduction (cont.)
All matter emits energy
Perfect emitters are called ‘blackbodies’
Max Planck, in 1900, was the first to derive the equation describing ‘spectral’ radiation emission from a blackbody
Planck unwittingly revolutionized physics with the introduction of the Planck constant h which describes the size of ‘quanta’ in quantum mechanics
800K
700K
600K
500K
400K
Planck’s Law

4. Definitions Radiance - energy flux per unit solid angle
Spectral – modifier that denotes units are given as a function of wavelength (or frequency)
E.g. ‘spectral radiance’ = radiance per unit wavelength
Transmissivity – Fractional amount of energy pass
Absorptivity – Fractional amount of energy absorbed by a medium

5. Exhaust Plume Radiation
Exhaust plume is gaseous and opaque
Radiation is absorbed, emitted, and transmitted through media at different wavelengths
Molecular resonances cause different behaviors at varying wavelengths, so spectral analysis is of interest – CO2 and H2O predominant elements in IR of plume
Beer’s Law describes transmissivity
Kirchoff’s Law describes emissions
and

6. Effects of Soot in plume
Combustion process is never 100% efficient
A small portion of fuel does not completely combust, and carbon molecules coalesce into small particles
Carbon particles, or soot, emit and absorb too
Absorption varies significantly with size and particle density
Effects of soot is considered in this project

7. Plume IR problem break-down
The method of calculating plume emissions broken down into 2 major steps
Gaseous spectral radiance and transmissivity calculations, dominated by CO2 and H2O
Soot

8. Chemical Reactions of Combustion
Fuel (CH2) combines with Oxygen (O2) and results in water (H2O), carbon dioxide (CO2), and heat energy
C12H O2 = 12 H2O + 12 CO2
Given mass flow of air and fuel, and the temperatures of plume, we can calculate the particle concentrations using ideal gas law

9. MODTRAN for Radiance and Transmittance due to CO2 and H2O
MODTRAN uses various techniques for calculating CO2 and H20 radiance.
Leverage these methods to obtain solutions for C02 and H2O
‘Standard Atmosphere’
‘Plume (no soot)’

10. GE-T700 – 100%MC 100% Burn Efficiency (No soot)
MODTRAN Inputs (ppmv)
H2O = 40874
CO2 = 39334
O3 =
N2O = 0.0
CO = 176.7
CH4 = 4.284
O2 =
NO = 0.0
SO2 = 0.0
NO2 =
NH3 = 0.001
HNO = 0.0
Temp = 533°K
Path = 1 meter
Soot = 0
Integrated Radiance = W/cm2 sr (1m – 12m)

11. Modeling particulate
Several techniques have been devised for particulate modeling
Proper usage depends upon particle size parameter where D is particle diameter and lm is wavelength in the particle fluid.

12. Turbine engine exhaust soot
For soot, is generally < 0.3 therefore Mie Scattering Theory is used.
Mie equation yield:

13. Plot of From Mie equation using properties of propane combustion
a/C = 5/l

14. Concentration of Soot in turbine engines from literature
A study performed on the sooting properties of various jet fuels in jet turbines yielded very small soot concentrations.
These values may, or may not be indicative of actual soot concentration in turbo-shaft engines.
Results will be presented for increasing levels of soot concentration
Recommend correlating results with measurements of plume radiance and transmittance
m3 soot per m3 plume

15. Results using published values for soot particle density
N = 4.24x106 cm-3
N = 4x1011 cm-3

16. Results
MODTRAN and additional procedure to calculate soot radiant emissions

17. Conclusions

18. Results
Low resolution of published measured values can account for some error
Test setup for measuring not described… Measurement technique may account for some errors.
13% Error across 2-20mm spectrum
25% Error across 3-5mm spectrum
3.4mm in measured data is probably a measurement artifact
Path attenuation from sensor positioned a shot distance away.
Difference in spectral lobe width in CO2 band ( mm)
MODTRAN may be under predicting spectral broadening of high temperature gas

19. Conclusions Large error in spectral predictions in 3-5mm
MODTRAN may not be the correct tool for modeling high temperature gases
More accurate measured data necessary

20. Results with increasing particle number density
N = 4x107 cm-3
N = 4x108 cm-3

21. Results with increasing particle number density (continued)
N = 4x109 cm-3
N = 4x1010 cm-3

22. Results with increasing particle number density (continued)
N = 4x1011 cm-3
N = 8x1011 cm-3