Predicting Engine Exhaust Plume Spectral Radiance & Transmittance

Engineering ProjectMANE 6980 – Spring 2010 Wilson Braz

IR Radiation Introduction Infrared (a.k.a. thermal) portion of

electromagnetic spectrum spans approximately 0.5m to 1000m

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

1

12,

5

2

kThce

hcTi

Planck’s Law

800K

700K

600K

500K

400K

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

srm

W

2

srm

W2

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

Sae

TT ,,,,,, SS ,1, and

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

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

Chemical Reactions of Combustion Fuel (CH2) combines with Oxygen (O2) and

results in water (H2O), carbon dioxide (CO2), and heat energy 2 CH2 + 3 O2 = 2 H2O + 2 CO2

Given mass flow of air and fuel, and the temperatures of plume, we can calculate the particle concentrations using ideal gas law

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)’

GE-T700 – 100%MC100% Burn Efficiency (No soot)

MODTRAN Inputs (ppmv) H2O = 40874 CO2 = 39334 O3 = 0.0686 N2O = 0.0 CO = 176.7 CH4 = 4.284 O2 = 144863 NO = 0.0 SO2 = 0.0 NO2 = 148.54 NH3 = 0.001 HNO = 0.0

Temp = 533°K Path = 1 meter Soot = 0 Integrated Radiance = 0.0141 W/cm2 sr (1 – 12

Modeling particulate

Several techniques have been devised for particulate modeling

Proper usage depends upon particle size parameter where D is particle diameter and m is wavelength in the particle fluid.

mD

Turbine engine exhaust soot

For soot, is generally < 0.3 therefore Mie Scattering Theory is used.

Mie equation yield:

mD

22222 42

36

nn

n

C

a

Plot of C

a

a/C = 5/

From Mie equation using properties of propane combustion

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

8101.2 C m3 soot per m3 plume

Results using published values for soot particle density

N = 4.24x106 cm-3 N = 4x1011 cm-3

Results

MODTRAN and additional procedure to calculate soot radiant emissions

Published measured values Predicted values

Project TODO

Vary soot concentration to see effects of soot; plot results

Show attenuation plots Plots as a function of distance from

emitting plume Soot may become more important

Discussion of Conclusions