Combustion Research Program: chapter from Energy and Environment Division annual report 1977 Page: 13 of 28
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173
CATALYZED COMBUSTION IN A BOUNDARY LAYER
R. Schefer, F. Robben, R. K. Cheng, and I. NamerINTRODUCTION
Studies have indicated that considerable
promise exists in the use of surface catalysis
in the combustion process to reduce the associa-
ted pollution.1 Results have shown that cata-
lytically supported combustion makes possible
the efficient burning of a variety of fuels
under fuel lean pre-mixed conditions with a
substantial reduction in thermal NOx levels.
Surface catalysis is very effective in this
application by initiating and speeding up the
combustion reactions so that complete combus-
tion can be attained under conditions where
stable combustion is difficult or impossible
to achieve using conventional combustors,
The principal objective of the present
research is to improve the understanding of
high temperature heterogeneous catalysis of
combustion reactions and the coupling with
the homogeneous reactions and the fluid me-
chanics. Combustion in the laminar boundary
layer of a heated flat plate with a free stream
flow of premixed fuel and air involves most
of the important physical and chemical proces-
ses of catalytic combustion systems while pro-
viding a geometry which facilitates experimental
study and numerical modeling. The experimental
configuration used in the present study consists
of a thin quartz plate with vacuum deposited
platinum heating strips mounted over an open,
atmospheric pressure jet of premixed hydrogen
and air. Electrical heating of the platinum
strips is used to heat the plate surface to
temperatures approaching 14000K. The plate
may be used to study catalysis of combustion
by a platinum surface, or may be coated with
various other catalytic and noncatalytic materi-
als. Current diagnostic techniques include
laser-Doppler velocimetry for measurement of
velocity, Rayleigh scattering for the measurement
of density, and optical pyrometry for measurement
of temperature. In addition, a variant of the
Schlieren technique known as deflection mapping
has given considerable insight into the boundary
layer behavior under combustion conditions.
Future measurements may include species concentra-
tion profiles utilizing Raman scattering of
laser light and gas sample probing in conjunction
with a gas chromatograph-mass spectrometer system.
ACCOMPLISHMENTS DURING 1977
Detailed density measurements were obtained
over SiO and Si02 "noncatalytic" surfaces and
with a platinum catalytic surface for equivalence
ratios from 0.0 to 0.3 and plate temperatures
up to 12500K.2 Measurements with the silicon
dioxide surface indicated no gas phase combustion
of lean hydrogen-air mixtures at surface tempera-
tures up to 12500K. This was determined by
comparison of experimental and calculated thermal
boundary layer profiles and boundary layer
thicknesses. The numerical results show a very
marked thickening of the thermal boundary layer
in the zone of maximum energy release rate, aswell as a peak in the temperature profile.
Neither of these was present in the experimental
results. In Fig. 1 thermal boundary layer
thickness is plotted as a function of distance
from the plate leading edge for equivalence
ratios from 0 to 0.1. As shown, the measure-
ments agree well with numerical calculations
for no combustion, and thus it is concluded that
no gas phase reactions were occurring under
these conditions.
Similar results were found with a catalytic
platinum plate where no gas phase reaction was
apparent up to a surface temperature of 12000K.
However, significant surface combustion was
measured at plate temperatures less than 9000K.
Preliminary measurements indicate that surface
heat release rates can be determined from the
change in power input to the plate heating
strips as fuel is added. Such measurements will
be useful in evaluating the effectiveness of
various catalytic surfaces.
The numerical calculations were carried out
based on a numerical finite difference solution
of the governing differential equations for
laminar boundary layer flow and combustion over
a heated flat plate.3 A detailed hydrogen-air
reaction mechanism with 13 reactions and 8 chemical
species was used. Results for a lean ($ = 0.1)
H2/air mixture over a heated constant temperature
(T = 11000K) noncatalytic plate indicate the
existence of several stages of combustion. These
include an initial induction period during which2,0
E
-
vO1,5
1.0
0.50 5 10
15 20
25
Distance from leading edge (mm)
Fig. 1. Comparison of measured thermal boundary
layer thickness (T/TO = 2.0) at hydrogen-air
equivalence ratios of 0.0, 0.05, and 0.10.
Silicon dioxide coated plate with surface
temperature of 12500K and free stream velo-
ocity 3.17 m/s. Also shown is the numerical
result for combustion. (XBL 779-2011)Numericalno combustion- -
SYMBOL
6 0
L 0 (2pt)
v 0.05
o 0,01
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Budnitz, R. J. Combustion Research Program: chapter from Energy and Environment Division annual report 1977, report, January 1, 1978; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc1188304/m1/13/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.