The NASA Glenn Research Center is currently engaged in characterizing the solid and volatile aerosol emissions from gas turbine combustors. A pressure-reduction chamber was fabricated at Glenn as a part of the aerosol sampling system, and a computational fluid dynamics code developed at Glenn was used to predict the evolution of the aerosol in the device.
To understand the impact of aircraft emissions on the environment, it is necessary to understand the formation and subsequent development of gaseous and aerosol emissions in the exhaust gas of jet engines. For gas and particle emissions to be measured accurately, the elevated pressure of the exhaust gas from the aircraft combustor must be brought down to the applicable range of the measuring devices.
Pressure-reduction chamber. OD, outside diameter of tube.
Long description of figure 1.
A pressure reducer is a cylinderlike device that has an inlet tube at the top and an exit tube at the bottom. The schematic shows a typical pressure-reduction device. The portion of the inlet tube inside the pressure reducer may be expanded up to 5° to slow down the drop of the incoming pressure and temperature. In this study, the internal diameter (ID) of the inlet tube was 0.09 in., and the diameter at the exit of the inlet tube was 0.2646 in. because of a 5° expansion. The gap distance was 2 in., the ID of the sample extraction tube was 0.34 in., and the diameter of the reducer was 3 in. The overall length of the domain that includes the inlet tube, the chamber, and the sample extraction tube was 35 in. (0.889 m). A chemistry mechanism having 29 species and 73 reactions was used, and the aerosol size distribution was divided into 12 bins. The size distribution of the soot particle was prescribed to be log-normal, with a median radius of 40 nm, modal widths of 1.5, and a total number density of 1013 particles/m3.
Two cases are presented to show the chemical conversion and particle evolution in the device. The total pressures of the incoming gas were 65 and 255 psi, respectively. The total temperature was set to be 485 K for both cases. The exit pressure was set at 19.7 psi, and the pressure at the bleeding location was 14.7 psi. The wall temperature of the inlet tube entering the chamber was set to 477 K (400 °F); the rest of the wall was either at 450 K or was insulated. For the 65-psi case, the temperature was above 450 K from the inlet to the exit, hence the H2SO4-H2O droplets1 could not be nucleated. For the 255-psi case, the temperature dropped to near 250 K at the end of the inlet tube. The distributions of H2SO4 mass fraction along the 0.018-in. line are shown in the following graph, which shows that the mass fraction increased in the straight portion of the inlet tube outside of the chamber region. This graph also indicates that the chamber had little influence on the evolution of H2SO4.
H2SO4 distribution close to the centerline of the device (0.018 in. above the axis) for the 400 °F pressure-reduction chamber study.
The next graph shows the number density distribution, in log10 scale, of the soot particles for the 255-psi case. The influence of the chamber on the size distribution of the soot particles is noticeable.
Number density distribution of soot particles 0.018 in. above the axis for the 400 °F pressure-reduction chamber study.
The final graph shows that the nucleation of H2SO4-H2O droplets is significant for the 255-psi case, so is the coagulation of droplets. Work to implement more aerosol microphysics models is in progress.
Number density distribution of H2SO4-H2O droplets 0.018-in. above the axis for the 400 °F pressure-reduction chamber study.
Wey, Thomas; and Liu, Nan-Suey: Modeling of Aerosols in Post-Combustor Flow Path and Sampling System. NASA/TM--2006-214397, 2006. http://gltrs.grc.nasa.gov/Citations.aspx?id=159
Taitech, Inc. contact: Dr. Thomas Wey, 216-433-2934, Changju.T.Wey@nasa.govLast updated: December 14, 2007
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