The work of the Principal Investigator (PI) has encompassed four topics related to the experimental and theoretical study of combustion limits in premixed flames at microgravity, as discussed in the following sections. These topics include: (1) radiation effects on premixed gas flames; (2) flame structure and stability at low Lewis number; (3) flame propagation and extinction is cylindrical tubes; and (4) experimental simulation of combustion processes using autocatalytic chemical reactions

Ronney, Paul D.

English

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STUDIES OF PREMIXED LAMINAR AND TURBULENT FLAMES
,T M,CROGRAVrrY N {) 3" 2 0 1 9 6
Paul D. Ronney
Department of Mechanical and Aerospace Engineering
Princeton University
Princeton, New Jersey 08544
Introduction
The work of the Principal Investigator (PI) has encompassed four topics
related to the experimental and theoretical study of combustion limits in
premixed flames at microgravity, as discussed in the following sections. These
topics include:
• Radiation effects on premixed gas flames
• Flame structure and stability at low Lewis number
• Flame propagation and extinction in cylindrical tubes
• Experimental simulation of combustion processes using autocatalytic
chemical reactions
Radiation effects on premixed qas flames
Our previous work has demonstrated the importance of gas radiation in the
propagation and extinguishment of premixed gas flames [I, 2]. However, gas
radiation is a weak effect and is only a significant influence for very slowly
burning flames. These flames can only be observed at _g due to buoyancy effects
at one-g. Furthermore, it is not clear whether radiation is a fundamental lim/t
because in very large systems (say, tens of meters) emitted radiation could be
reabsorbed within the gas and thus not lost from the system. In this case the
propagation rate could actually increase because radiation would augment the
usual transport by conduction of heat from the flame front to the unburned gas
[3].
To test these hypotheses, we have examined radiation effects by studying
flames at _g in gas mixtures to which small quantities of inert, radiant
particles have been added. Solid particles emit and absorb radiation across a
broad spectrum, unlike gas molecules which are active only in narrow spectral
bands. Thus, appreciably more radiation and absorption can be expected in
particle-laden gases. Our preliminary experiments [4] have shown that at small
particle loadings, burning rates are reduced (Fig. i), peak pressures in
constant-volume combustion are lower (Fig. i) and thermal decay rates in the
burned gases are increased (Fig. 2). This indicates that the significance of
radiative loss is enhanced by the addition of particles to the gas. With
sufficient seeding, the burning rates are practically the same as those found in
particle-free mixtures (Fig. I), the peak pressures are comparable (Fig. i) and
the thermal decay rate is smaller than particle-free mixtures (Fig. 2). All of
these observations are consistent with the hypothesis that at sufficiently high
particle loadings, radiation is reabsorbed within the combustible medium, and
thus may not constitute a fundamental limit in very large systems. In essence,
the presence of particles has made the system "larger" in that the ratio of the
system size to mean absorption length of radiation is increased.
Future work will focus on methods of quantifying the particle density,
mixture optical thickness, and obtaining results for a variety of mixtures and
particle loadings to compare with theory and to determine if flammability limits
can be entirely suppressed when radiation losses are eliminated.
159
PRECEOtNG PaGE BLANK NOT FfI.MED
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Flame structure and stability at low Lewis number
We have found in previous work that the tendency of flames in mixtures with
low Lewis numbers, e.g. hydrogen-air, to break up into cells affects the near-
limit behavior dramatically [5] . In some cases flame propaqation ceases entirely
without flame extinction occurring. In these cases stabler stationary spherical
flames ("flame balls") seem to exist. Stationary spherical flames have been
predicted previously but are predicted to be unstable. Further work has showed
that such phenomena seem to occur in all mixtures with sufficiently low Lewis
number and near flan_nability limits, independent of the chemical mechanism [6].
Experiments on the KC-135A aircraft showed that these structures are stable for
at least the 20 seconds of low-g available [6]. By comparison with analytical
models [7, 8], it has been concluded that radiation from the combustion pro_uct_,
along with dif_fusive-thermal effects in low-Le mixtures, ±s probably the
stabilization _mech=aD_sm whigh allows flame ball_ to exist a£ _g. = _==
At one-g, these flames are entirely buoyancy-dominated. Consequently, the
flammability limits are much leaner at _Ig than one-g, for example 3.35% vsl 4.0%__
H 2 for H,-air mixtures [6]. This result is in very good agreement with a
computational study of flame balls employing detailed chemistry and transport
models [9] in which a limit of 3.5% H, was predicted.
The g-jitter in the KC-135A experiments (-- 0.02g) caused substantial motion
of flame balls. The drift velocity was found [6] to be proportional to gln,
consistent with bubble theory. This drift also led to the formation of two types
of quasi-cylindrical flame structures which we have termed "flame strings" (Fig.
3) [6]. These strings are unstable, in that they eventually break into flame
balls, but live much longer than would be expected based on thermal diffusion
time scales alone. Consequently, they are almost certainly being supported by
chemical reaction. Their existence is curious, because no steadv solution is
possible for cylindrical flames (unlike spherical flames). It seems then that
they would evolve radially given sufficient time, but the axial instability which
leads to their breakup manifests itself before radial evolution occurs. Recent
theory [i0] supports this suggestion_ i
Another type of flame string, not directly related to that discussed above,
has been observed in H2"O2-SF 6 and CH4-O2-SF6 mixtures, particularly at high
pressures [6]. These strings are different in that they are uncorrelated with
buoyancy effects and seem to form much faster than any conventional diffusional
or hydrodynamic timescale would allow. We conjecture that these are a result of
reabsorption of emitted radiation, which is most likely to occur in SF6"diluted
mixtures and at high pressures because the Planck mean absorption length (_) is
much shorter in SF6 than CO2 or H20 and is inversely proportional to pressure; at
300K and 2 atm/ _Lp _or SF 6 iS only 0_13 cm_.
A flight experiment is under development to determine if the apparent
stability of flame balls can be confirmed in experiments where both the duration
of _g is long (unlike drop-tower experiments) and the quality is significantly
better than that in aircraft tests. Also, the presence or absence 0f flame
strings will be studied to determine the role buoyancy plays in their formation.
In addition to addressing development issues in support of the deveiopment:of
flight hardware, future work will stress the role of reabsorption of emitted
radiation on these flame structures using high-pressure tests and particle-laden
mixtures. It is possible that the presence of particles will influence transport
of thermal energy (see above) but should not influence mass transport, ieading
to a radiation-influenced Lewis number which may be higher than the particle-free
value, therebyinhibiting the cellular instability.
Flame propaqatlon and extlnctlon in cyllndr_cal tubes
The standard apparatus for measuring flammability limits of premixed gases
at earth gravity is the Standard Flammability Limit Tube, or SFLT. This is a
tube 5 cm diameter and 200 cm long which is filled with combustible gas and
ignited at one end. The mixture is defined to be flammable if it su_)-ports
propagation throughout the tube. Despite many years of study, the mechanisms of
flame extinction in the SFLT is not well understood. The relative importance of
160
±
F
buoyancy, flame stretch, heat loss to the tube wall, radiation loss, etc. have
not been assessed.
Most theories of flammability limits predict the flame propagation rate at
the limit (Sb.lm). Hence, measurement of Sb. llm enables comparison with theories.
We have conducted such experiments at earth gravity in tubes of varying diameter,
at varying pressures and with mixtures having varying fuels, inerts, and Lewis
numbers (Le). We have found that the characteristics of the limits can be
described in terms of the effect of the Grashof number Gr _ g_/_2 on the limit
Peclet number Pe m Sb.li,d/_ , where g is gravity, d the tube diameter and _ the
thermal diffusivity at room temperature. Figures 4 and 5 show the results for
upward and downward propagation, respectively. In each case, at low Gr the
results are consistent with theoretical predictions [ii] for flame extinguishment
by conduction loss to the tube walls, namely Pe = constant = 40. At higher Gr,
results are consistent with buoyancy-induced extinction mechanisms, in the upward
propagating case Pe - Gr li2 [12] and in the downward case Pe - Gr w3. Because of
the difference in extinction mechanisms, we have found that the flammability
limit can actually be wider for downward propagation than upward propagation
(though for most commonly-studied values of Gr and Le, this is not the case).
In the upward case, at Gr > 2.0 x I0' (corresponding to a pipe-flow
Reynolds number of 2000) turbulent flow is exhibited; the flame behavior can be
either flamelet-like or distributed-like depending on Gr and Le. Downward
turbulent propagation at the limit is also possible, but would require Gr = 40
x 109 according to our scaling analysis, and is beyond the limits of our
apparatus.
Our scaling analyses indicate that radiation losses cannot be significant
at earth gravity for any experimentally realizable conditions; either conduction
losses or buoyancy effects will dominate. Of course, radiation can be the
dominant extinction mechanism in large tubes at _g. We are initiating a study
of this extinction process for comparison to our previous results in a constant-
volume chamber with central ignition (which produces spherically expanding
flames.)
Experimental simulation of premixed flames usinq autocatalytic chemical reactions
The PI has introduced the use of aqueous autocatalytic propagating chemical
fronts (in particular, the arsenous acid-iodate system) for the experimental
simulation of premixed combustion in nonuniform and unsteady flows [13]. These
fronts more nearly match the assumptions made by most relevant theoretical models
that do gaseous flames. These assumptions include that of constant density. The
role of density change (due :to heat r_lease) in gaseous premixed flame
propagation has been a continuing source of controversy, especially in turbulent
flows. It has not even been established whether density changes result in an
increase or decrease in the turbulent flame propagation rate (ST) [13].
Consequently, it is desirable to compare front propagation in gaseous flames and
aqueous autocatalytic fronts to study effects of density change.
A limitation on the utility of aqueous fronts is that even the small
fractional density change across the aqueous front leads to significant buoyancy
influences at one-g because of the very low planar front propagation rate (SL).
Only when the imposed disturbance intensity (u') is much greater than buoyancy-
induced flow velocities can buoyant effects be neglected. Gaseous flames with
u' >> SL cannot be observed because of quenching which results from the
hydrodynamic strain at high U m u,/SL; this makes it impossible to compare the
aqueous and gaseous front propagation at the same U, and thereby assess the role
of density changes. High U is also inaccessible to computational studies because
of numerical difficulties, especially when density changes are included.
This discussion indicates the need for studying aqueous fronts at _g to
eliminate buoyancy influences, enabling the study of front propagation at low U
and thereby allowing comparison of front propagation in aqueous and gaseous
fronts at the same u'/S L.
We have studied these autocatalytic propagating fronts in a Taylor-Couette
flow, i.e. in the annulus between two rotating concentric cylinders. This flow
was chosen because (i) the flow is homogeneous in the direction of front
161
propagation (axially), so that a quasi-steadily propagating front can be studied,
and (2) well-characterized disturbances are generated even at very low Reynolds
numbers (and thus low u'). When only the inner cylinder is rotated, pairs of
counter-rotating toroidal vortex pair (Taylor vortices) fill the annulus. Fig.
4 shows the measured [13] effect of u'/S L on S_/S L in the Taylor-vortex regime and
the predictions of a theoretical model [13]; agreement is good. (Data were taken
up to u'/SL = 10,000, but only the lower values of u'/SL are of interest to this
study in that they may be affected by buoyancy).
Our scaling analysis has shown that in a space experiment, it is possible
to study aqueous fronts with U _ 7 at g = 10-4go (a typical figure for space
experiments) without buoyant convection, whereas U _ 140 is the lowest possible
U at g - go without buoyant convection. It is possible to study U = 7 in gas
combustion without quenching, whereas U = 140 is not possible. Thus, space
experiments would enable us to study the aqueous fronts at values of U accessible
to gas combustion experiments and numerical simulations, enabling us to create
a "bridge" between studies of fronts with and without substantial density
changes.
Re fe rences
"I.
"2.
Ronney, P.D., "On the Mechanisms of Flame Propagation Limits and
Extinction Processes at Microgravity," Twenty Second Symposium (Interna-
tional) on Combustion, Combustion Institute, 1988, pp. 1615-1623.
Abbud-Madrid, A., Ronney, P. D., "Effects of Radiative and Diffusive
Transport Processes on Premixed Flames Near Flammability Limits," Twenty
Third Symposium (International) on Combustion, Combustion Institute, 1990,
pp. 423-431.
''7 .
"'8 •
Joulin, G., Eudier, M. Twenty-Second Symposium (International) on
Combustion, Combustion Institute, 1988, p. 1579.
Abbud-Madrid, A., Ronney, P. D., "Effects of Particulate Radiation on
Premised Gas Flames," AIAA 31st Aerospace Sciences Meeting, Jan. 11-14,
1993, Reno, NV (to be presented.)
Ronney, P. D., "Near-Limit Flame Structures at Low Lewis Number,"
Combustion and Flame 82, 1-14 (1990).
Ronney, P. D., Whaling, K. N., Abbud-Madrid, A., Gatto, J. L. Pisowicz, V.
L., "On Stationary Premixed Flames in Spherical and Cylindrical
Geometries," submitted to Proceedings of the Royal Society (London) Series
A (1992).
Buckmaster, J. D., Joulin, G., Ronney, P. D., "Structure and Stability of
Non-adiabatic Flame Balls: II. Effects of Far-Field Losses," Combustion
and Flame 84, 411-422 (1991).
Buckmaster, J. B., Gessman, R., Ronney, P. D., "The Three-Dimensional
Dynamics of Flame Balls," Twenty-Fourth International Symposium on
Combustion, 1992 (to appear in the Proceedings).
° Buckmaster, J. D. and Smooke, M. "Analytical and Numerical Modelling of
Flame-Balls in Hydrogen-Air Mixtures," submitted to Combustion and Flame.
10. Buckmaster, J. D., Combust. Sci. Tech. 84, 163 (1992).
ii. Spalding, D. B., Proc. Roy. Soc. (London) A240, 83 (1957).
12.
"13.
Levy, A., Proc. Roy. Soc. (London) A283, 134 (1965).
Shy, S. S., Ronney, P. D., Buckley S. G., Yakhot, V., "Experimental
Simulation of Premixed Turbulent Combustion Using Aqueous Autocatalytic
Reactions," Twenty-Fourth International Symposium on Combustion, 1992, (to
appear in the Proceedings).
162
Other work supported by the NASA-Lewis:
"Farmer, J. N., Ronney, P. D., "A Numerical Study of Unsteady Nonadiabatic
Flames," Combustion Science and Technology 73, 555-574 (1990).
"Buckmaster, J. D., Joulin, G., Ronney, P. D., "Effects of Heat Loss on the
Structure and Stability of Flame Balls," Combustion and Flame 79:381-392 (1990).
"'"Effects of Ambient Atmosphere on Flame Spreading and Extinction," Workshop on
Spacecraft Fire Safety Risk Analysis Assessment, Oct. 31 - Nov. I, 1991, Los
Angeles, CA (Invited presentation).
"'"New Premixed Gas Combustion Phenomena," Gordon Research Conference on
Gravitational Effects in Physico-Chemical Systems, June 16-21, 1991, Plymouth,
NH. (Invited Presentation).
"'Ronney, P. D., "An Investigator's Suggestions for Effective use of the NASA-JSC
Reduced Gravity Program KC-135A Aircraft," in: JSC Reduced Gravity Program User's
Guide, NASA JSC-22803, July 1991.
"'Buckmaster, J. D., Lee, C. J., Joulin, G., Ronney, P. D., "Modelling of
Microgravity Ignition Experiments," in: Recent Advances in Combustion Modelling
(B. Larrouturou, ed.), Series in Advances in Mathematics for Applied Sciences,
Vol. 6, pp. 1-18, World Scientific Press, Teaneck, NJ, 1991.
"Work supported by NASA-Lewis under grant NAG3-965
"'Work supported by NASA-Lewis under grant NAG3-1242
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---- Burning velocity ..... Peak P
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1600
1400
1200
1000
800
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400
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I ' ' I ' | ' t ' ' ' t
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_ _l from spark gap t
, , , t , _ _ I . • , I , , j J , , ,
0.20 0.40 0.60 0.80 1.00
Time (seconds)
Figure i. Propagation rates and peak
pressures of particle-laden flames.
Mixture: 5.25% CH 4 in air.
Figure 2. Thermal characteristics of
flames in particle-laden mixtures.
Mixture: 5.25% CH4 in air.
163
b_
Upward Propqstlon
Pz
...................................................... ..................................... o L¢-02
Lz.l
--_ii!ii_tiiitN i i_bf 1iN--!-ii_fl_--_!,i".'_..'.'--!_I:.-.:.-_-;t:[i':_
Figure 3. IR image of "flame string"
at _tg. Mixture: 5.5% H 2 in air.
Figure 4. Limit Peclet number versus
Grashof number for upward-propagating
flames in tubes.
D_wawor_l Prop,pfien
Figure 5. Limit Peclet number versus
Grashof number for downward-
propagating flames in tubes.
O3
O
o
O
80
60
40
2O
0
0
° ° I ! '
Theory
• S L = 0.0023 cm/s
• Su = 0.0051 cm/s
o S L=0.0071 cm/_
j ] , , I , , , , 1 _ _ _ • 1 • , i i
50 100 150 200
Disturbance Intensity (u'/S L)
¥igure 6. Effect of disturbance
intensity on front propagation rate
in aqueous autocatalytic system.
164


Studies of Premixed Laminar and Turbulent Flames at Microgravity

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