Global NOx Measurements in Turbulent Nitrogen-Diluted Hydrogen Jet Flames Page: 2 of 10
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the basic NOx emission characteristics of these flames, with the intent of providing information
that will be useful in designing a dry, low-NOx hydrogen/nitrogen diffusion flame combustor.
The adiabatic flame temperature of a mixture of equal parts of hydrogen and nitrogen with
stoichiometric air is about 2025 K, indicating that thermal NO is an important NOx formation
pathway in these diffusion flames. One strategy for reducing thermal NOx is to reduce the
residence time in the diffusion flame by increasing the jet velocity and/or reducing the jet
diameter, though this also tends to increase flame strain and results in turbulence-chemistry
interactions that promote NOx formation through the increase of super-equilibrium O-atom
concentrations [2-4]. The effects of flame strain are more apparent when the NOx emission
index is normalized by a characteristic flame residence time, r =L/ uod, where Lfis the flame
length, uo is the jet exit velocity, and do is the jet exit diameter. According to Chen and Driscoll
, the normalized emission index scales as:
EINOX/(L f /uodo )C Re'--2 Da'
where Re is the Reynolds number at the jet exit and Da is the Damkohler number, which is
inversely proportional to the global strain rate, uo/do*. The global strain rate can also be viewed
as a characteristic mixing time, where the jet momentum diameter, do* = do(po/p)2, po is the jet
exit density, and p, is the ambient air density.
In experiments with pure hydrogen and helium-diluted hydrogen [5, 6], the data is very well fit
using m = 1/2 and n = -1/2 in Eq. (1). This well-known scaling relation has been verified by
various modeling studies [3, 4, 7, 8], and shows that normalized NOx emissions increase with
respect to flame strain due to turbulence-chemistry interactions that increase O-atom
concentrations to higher super-equilibrium levels. In the absence of flame strain effects an
equilibrium chemistry assumption should yield n = 0, however, subsequent studies by Chen and
co-workers show that for hydrogen diluted by 60% argon or carbon dioxide, n ~ 2/3, meaning
that flame strain helps reduce NOx emissions .
This result is attributed to low fuel Lewis number (LeF), which is defined here as the ratio of the
thermal diffusivity of the hydrogen/diluent
mixture to the mass diffusivity of the
hydrogen in the diluent. Values of LeF for
various dilution gases and levels of dilution
are given in Table 1. Gabriel and co-workers
hypothesize that the value of n increases as
the fuel Lewis number decreases below 0.9
with increasing dilution of the hydrogen .
A secondary goal of the present study is to
test this hypothesis, as increasing nitrogen
dilution from 10% to 60% will decrease the
fuel Lewis number from 1.66 to 0.68, the
region in which n is predicted to exhibit a
positive slope and flame strain and residence
time effect are both expected to help reduce
Table 1: Fuel Lewis number as a function of
type and level of diluent in hydrogen .
Entries denoted by * were tested by Gabriel
et. al. .
Diluent mole N2 He Ar CO2
0.1 1.661 0.994 1.626 1.830
0.2 1.387 0.998* 1.389* 1.438*
0.3 1.162 1.004 1.187 1.139
0.4 0.974 1.012* 1.012* 0.903*
0.5 0.814 1.022 0.857 0.715
0.6 0.677 1.035* 0.718* 0.563*
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Weiland, N.T. & Strakey, P.A. Global NOx Measurements in Turbulent Nitrogen-Diluted Hydrogen Jet Flames, article, March 1, 2007; (digital.library.unt.edu/ark:/67531/metadc882648/m1/2/: accessed December 9, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.