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Visible absorption spectroscopy of dark zones in solid-propellant flames
UV/Visible Absorption Spectroscopy of Dark Zones in Solid-Propellant Flames YEU-CHERNG LU,* TODD M. FREYMAN, and KENNETH K. KUO Department of Mechanical Engineeving, The Pennsylvania State University, UniversityPark, PA 16802 UV/Visible absorption spectroscopy, coupled with multiparameter least-squares analysis, was used to determine profiles of gas-phase temperature and NO concentration in the dark-zone region of two RDX-based solid propellants (XM39 and M43) at pressure up to 3.55 MPa. At low pressures (e.g., P < 1.6 MPa for M43 and P < 2.17 MPa for XM39), a uniform dark zone was observed between the burning surface and the luminous flame, with intermittent flamelet attachment to the burning surface. As pressure increased, flamelet attachment became very pronounced, and the uniform dark zone was rarely seen. Temperature and NO concentration profiles in the dark zone of both propellants were very uniform when a uniform dark zone existed. Under this situation, M43 and XM39 were found to have similar dark-zone temperatures (around 1050 K for M43 and 1180 K for XM39); however, M43 has about 5% more NO mole fraction than XM39 in the dark-zone region (20% vs 15%). On the other hand, when luminous flamelets were attached to the burning surface, line-of-sight-averaged NO concentrations (where a fixed path length is used in data analysis) exhibited a rapid reduction with increased distance from the burning surface. This reduction results from the shorter intercepted lengthy by the UV light beam in the dark zone as the vertical distance from the burning surface increases; the deduced NO concentration is thereby decreased if the whole propellant diameter is used as the path length in data analysis. According to video records, M43 has a smaller dark zone than XM39 at the same pressure, and its dark zone decreases more rapidly with an increase of pressure than XM39 in the pressure range studied. M43 was found to be able to maintain a stable "flame-burning" mode combustion at a lower pressure than XM39. © 1997 by The Combustion Institute
INTRODUCTION D e t e r m i n a t i o n o f t e m p e r a t u r e a n d speciesconcentration profiles of solid-propellant flames u n d e r v a r i o u s o p e r a t i n g c o n d i t i o n s is t h e first step in u n d e r s t a n d i n g the d e t a i l e d c o m b u s t i o n - w a v e s t r u c t u r e o f solid p r o p e l l a n t s a n d assessing t h e effects o f individual ingredients on the p r o p e l l a n t c o m b u s t i o n process. T h e s e d a t a a r e also i m p o r t a n t in establishing and validating solid-propellant combustion m o d e l s which i n c o r p o r a t e d e t a i l e d c h e m i c a l r e a c t i o n m e c h a n i s m s . H o w e v e r , d u e to the hostile c o m b u s t i o n e n v i r o n m e n t , m o s t o f the p r e v i o u s e x p e r i m e n t a l studies for s o l i d - p r o p e l lant c o m b u s t i o n f o c u s e d o n b u r n i n g - r a t e m e a s u r e m e n t s a n d qualitative o b s e r v a t i o n s o f flame-zone and near-surface phenomena. Only a l i m i t e d n u m b e r o f t h e s e studies q u a n t i t a tively m e a s u r e d the profiles o f g a s - p h a s e t e m p e r a t u r e s o r species c o n c e n t r a t i o n s o f solidp r o p e l l a n t flames using e i t h e r intrusive p r o b ing m e t h o d s (such as t h e r m o c o u p l e s a n d mi-
* Corresponding author. 0010-2180/97/$17.00 PII S0010-2180(96)00095-8
croprobe mass spectrometry) or nonintrusive d i a g n o s t i c t e c h n i q u e s (such as l a s e r - i n d u c e d f l u o r e s c e n c e a n d a b s o r p t i o n s p e c t r o s c o p y ) (e.g., see Refs. 1-9). A b s o r p t i o n s p e c t r o s c o p y is a well-established m e t h o d for s i m u l t a n e o u s l y d e t e r m i n i n g temperature and absolute concentration of c h e m i c a l species f r o m o n e m e a s u r e d s p e c t r u m . R e c e n t l y , this t e c h n i q u e was successfully u s e d for s o l i d - p r o p e l l a n t flame diagnostics at p r e s sure u p to 6.99 M P a [3, 5-7]. Since a b s o r p t i o n s p e c t r o s c o p y is a n o n i n t r u s i v e d i a g n o s t i c m e t h o d , it can b e u s e d to d e t e r m i n e t e m p e r a tures in a r a n g e b e y o n d the capability o f conv e n t i o n a l t h e r m o c o u p l e s . This w o r k e x t e n d e d t h e t e c h n i q u e p r e s e n t e d in Ref. 7 to d e t e r m i n e profiles o f g a s - p h a s e t e m p e r a t u r e a n d N O conc e n t r a t i o n in the d a r k - z o n e r e g i o n o f two n i t r a m i n e c o m p o s i t e s o l i d - p r o p e l l a n t flames u n d e r e l e v a t e d p r e s s u r e s by m e a n s o f U V / Visible a b s o r p t i o n spectroscopy. T h e m a j o r ing r e d i e n t s for t h e solid p r o p e l l a n t s s t u d i e d in this w o r k are: . XM39: cyclotrimethylene trinitramine ( R D X , 76% by weight), cellulose a c e t a t e COMBUSTIONAND FLAME 109:342-352 (1997) © 1997 by The Combustion Institute Published by Elsevier Science Inc.
DARK ZONES IN SOLID-PROPELLANT FLAMES butyrate (CAB, 12%), nitrocellulose (NC (nitration level 12.6%), 4.0%), acetyl triethyl citrate (ATEC,7.6%), and ethyl centralite (EC, 0.4%); 2. M43: similar to XM39, except that ATEC is replaced by an energetic plasticizer. EXPERIMENTAL SETUP The experimental setup used in this work is very similar to that reported in Ref. 7 for OH measurements. Therefore, to avoid duplication, only a brief description of the test facility is given below, with emphasis on the modifications and improvements made to the orevious setup in order to allow NO measurements. Absorption transitions of NO molecules in solid-propellant flames were accessed by passing a high-intensity Xenon arc-lamp (ORIEL 6262, 450-W UV-enhanced lamp) light beam across the flame zone. This lamp is different from the one used in Ref. 7 since the previous 1000-W arc lamp did not produce enough light intensity at UV wavelengths pertinent to NO absorption. The present light source produces a spectral irradiance about four to ten times greater than the 1000-W light in the 230-250 nm range. A portion of the light energy is absorbed by NO molecules in the propellant flame, and the remaining light is focused onto a 100-/xm pinhole and then onto the entrance slit of the spectrometer by means of fused-silica lenses. The pinhole minimizes collection of emission signals from the flame and dictates the spatial resolution of measurements. The focal length ( f ) of the fused-silica lenses is 25.40 cm for the light at the wavelength of 589.3 nm. It varies significantly when the wavelength decreases to the low UV range. Based on the specifications in the manufacturer's catalog, the focal length at any other wavelength ;t (in unit of nm) can be determined by the following equation; (n589. 3 -- 1) fA ---~A89.3
(n~ - 1)
(1)
where na is the index of refraction at any particular wavelength A. For example, n589.3 1.4584 and n240 = 1.5133 for fused silica, and f5893 = 25.4 cm in this case; thus, the focal =
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length at 240 nm is reduced to 22.68 cm. The distance on the focusing side of the lenses was thus adjusted based on Eq. 1. The slit opening was set at 80 /xm for NO measurement, yielding a spectral resolution of around 0.9 A. The spectrometer has a focal length of 75 cm, equipped with a 1200groove/mm grating (Instruments S.A., Inc., 12001-250 M). The grating is blazed at 250 nm, with about 65% efficiency at 230 nm and about 73% at 250 nm. The dispersed light was detected by a thermoelectrically cooled CCD camera and stored in a microcomputer for further processing. The CCD camera covered a 28-nm band of light when the grating was operated at first order. Propellant samples were cylindrical strands 0.64 cm in diameter and 3-5 cm in length. Propellant samples were ignited by a COe laser, and burned inside a windowed high-pressure strand burner under well-controlled operating conditions. During the propellant burning, the propellant sample was fed by a linear actuator system at a prespecified speed so that the propellant surface could be kept at a fixed position or slowly moved downward. Thus, it was possible to conduct either multiple samplings at a fixed position or scanning through a certain range in the flame zone in a single test. Continuous nitrogen purge flow was supplied during tests to carry combustion product gases out of the test chamber. The sampling time was 0.04 s, with 20-30 samplings per test. The chamber was also equipped with an in situ propellant-strand heating unit for investigating the effect of propellant initial temperature on the combustion process and for stretching the propellant flame zone at high pressures. DATA ANALYSIS The data-reduction program is exactly the same as that used in Ref. 7. This program, based on Beer's law, was used to extract temperature and concentration from the measured absorption spectrum. Since the spectrometer's bandwidth is larger than the linewidth of a typical transition line, convolution between molecular absorption and instrument function is required and considered. Due to scattering and absorption by soot particulates and large molecules,
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Y . C . LU ET AL.
strong attenuation of the transmitted light was observed; the attenuation effect was displayed by shifting the baseline away from unity in either a linear or nonlinear fashion. By assuming the Boltzmann distribution, the observed transmittance r at any frequency v0 cm-1 can be described by the following equation [5-7]:
7"(Vo) = (A o + AlVo + A2Vo2)
is given below [7]:
r(v0) = (Ao + A~oo + A~,o ~) 1
x/1 /
x 1-exp
Es%,-o) J
q(T~Bjgje-e#kr
[ hvb N~ × -~S(v' vo)eXp c q(T)
f
× EBgjej-EJkVP(vj)],
dv
(3)
(2)
where h is the Planck constant, b the path length, c the speed of light, k the Boltzmann constant, T the absolute temperature, Bj the Einstein coefficient for the jth transition in energy density units [m3/j s2], gj the degeneracy of the jth energy level, Ej the energy of the jth level, Pj(v) the transition line shape, Nr the total number density, q(T) the partition function, and S the normalized instrument function. The summation j is performed over all of the energy transitions which contribute to the light absorption at frequency v 0. The second-order polynomial A o + Air o + A2vo2 (sometimes a different order polynomial or an exponential function) was used to account for the baseline shift. Each transitional line was treated as a Delta function to improve the efficiency of data analysis. As demonstrated in Ref. 7, line broadening at pressures as high as 6.99 MPa does not change the characteristics of absorption signals nor deteriorate the quality of the curve fit; thus, the Delta-function treatment for transitional line shape is acceptable in the pressure range considered in this paper (up to 3.55 MPa). Since the calibration between wavelength and CCD pixel using an Hg lamp was only an approximation, adjustment of wavelengths in measured spectra was considered in the data analysis. To further simplify data reduction, an average instrument bandwidth was used in each test for the wavelength range considered. The final equation for curve fitting the observed transmittance at any frequency v0
Measured absorption spectra were used as input files of the data analysis program (which was based on Eq. 3). The program analyzed the measured spectrum and adjusted a set of parameters (including temperature, number density, etc.) to best fit the data by the least-squares method. The Einstein coefficients, degeneracies, and energy levels for NO were obtained from Kotlar [10], who obtained the ground state constants from Henry et al. [11], the upper state constants from Engleman and Rouse [12], and the radiative lifetimes from McDermid and Laudenslager [13]. 2125 transitions in the A2E-XZlI(O, 1) and (0,2)vibrational band systems were considered in the data analysis. Other molecule-specific parameters were obtained from Huber and Herzberg [14]. Levenberg-Marquardt's algorithm [15] was used to perform the least-squares analysis for the measured absorption spectra. Absolute number density as converted to mole fraction and molar concentration using the perfect gas law. It should be noted that the propellant diameter was used as the path length in all data analyses in this study. RESULTS AND DISCUSSION Figure 1 presents an image of the M43 propellant flame at 1.6 MPa (217 psig). A uniform visible dark zone was seen to separate the burning surface and the luminous-flame region. Because the lower edge of the luminous flame was not stationary, the size of the dark zone was not constant, and ranged from 2.9 to 4.3 mm at this pressure. Numerous narrow, bright streaks were always observed in the luminous-flame region. According to video im-
DARK ZONES IN SOLID-PROPELLANT FLAMES
the measured data and the solid line the fitted result from data analysis. A series of analyses similar to that shown in Fig. 2 was performed to determine temperature and NO concentration profiles in the dark zone of M43 propellant flame at 1.6 MPa; the results are shown in Fig. 3. It is evident that temperature and NO concentration are quite uniform (i.e., plateau) in the dark-zone region, which is very similar to the characteristics of double-base propellants. NO reduction near the luminous-flame zone is accompanied by energy release; therefore, temperature tends to increase gradually, while NO concentration tends to decrease as the vertical distance approaches the luminousflame region. The deduced averaged dark-zone temperature is about 1050 K, and the NO mole fraction is about 20%. Vanderhoff et al. [16] reported a dark-zone temperature of 1200K and an NO mole fraction of 22% for M43 propellant (in a slightly lower pressure range), which is not far from that reported in the current study. However, it should be noted that even though Ref. 16 reported the above results, many of its absorption measurements did not show the plateau feature of the dark zone, and it did not explain why the NO concentra-
M43 propellant
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ages, these streaks were carbonaceous residues ejected from the burning surface. The residues burned in the flame region and were strongly luminous. Figure 2 shows a typical NO absorption spectrum or deducing one pair of temperature and NO number density. The dots represent
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Fig. 3. Deduced gas-phase temperature and N O propellant in the dark-zone region at 1.6 MPa.
mole fraction profiles of M43
346
Y . C . LU ET AL.
tion showed a decreasing trend in the dark-zone region. These issues will be explored in detail later. The spatially uniform dark zone shown in Fig. 1 did not occur in all of the tests. In fact, only a limited number of tests exhibited the feature of a quasi-steady luminous flame above a dark zone; in many tests, the luminous flame was attached to the burning surface through several flamelets (see Fig. 4). The size of the dark zone decreased drastically, and the flamelet attachment became more pronounced with an increase of pressure. At a higher pressure of 2.17 MPa (300 psig), the size of the visible dark zone (when it indeed existed during the test) was only about 1-1.5 mm. The uniform dark zone only appeared for very short periods of time during the propellant burn; the luminous flame almost always attached to the burning surface through many conical shape flamelets, as shown in Fig. 5. Note that as pressure increases, both the number of illuminating streaks in the flame region and the flame luminosity decrease, in addition to the very pronounced flamelet attachment. It is interesting to note that the change of visual flame characteristics mentioned above occurs
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in the same pressure range where the burning-rate curve of M43 propellant shows the slope variation reported in Ref. 17. NO absorption measurements were also conducted for XM39 propellant at different pressures. Compared to M43 propellant flame at 2.17 MPa, the luminous flame of XM39 propellant is much smaller and nearly indistinguishable from the ambient environment (see Fig. 6 in which a dark zone exists between the burning surface and luminous flame). The size of the visual dark zone is about 1.7-2.2 mm for tests at 2.17 MPa, which is slightly larger than that of M43 propellant at the same pressure. The results of NO absorption measurements conducted in the dark-zone region to deduce temperature and NO concentration profiles for XM39 propellant flame at 2.17 MPa are shown in Fig. 7. Similar to Fig. 3, the temperature and NO concentration are quite uniform in the dark-zone region. The deduced averaged temperature is about 1180 K, and the NO mole fraction is 15% in the dark-zone region of XM39 propellant. Vanderhoff et al. [16] reported a dark-zone temperature of 1275 K and an NO mole fraction of 15% for XM39 propellant. Note that XM39 and M43 have similar
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D i s t a n c e a b o v e s u r f a c e (ram) Fig. 7. Deduced gas-phase temperature and NO mole fraction profiles of XM39 propellant in the dark-zone region at 2.17 MPa.
Fig. 1. An image of M43 propellant flame with a uniform dark zone at 1.6 MPa.
Fig. 5. An image of M43 propellant flame showing Aamelet attachment at 2.17 MPa.
Fig. 4. An image of M43 propellant flame showing tlamelet attachment at 1.6 MPa.
Fig. 6. An image of XM3Y propellant flame with a uniform dark zone at 2.17 MPa.
Fig. 8. An image of XM3Y propellant flamelet attachment at 2.17 MPa.
flame showing
DARK ZONES IN SOLID-PROPELLANT FLAMES
correlates well with the video records for the boundary between the dark zone and the luminous flame. Similar to M43 propellant, the luminous flame of XM39 has the tendency to attach to the burning surface, especially at high pressures. Figure 8 shows an image of XM39 propellant flame with flamelet attachment at 2.17 MPa. A series of NO absorption measurements were conducted at different vertical distance above the burning surface to deduce temperatures and line-of-sight averaged NO concentrations in the dark zone of XM39 with flamelet attachment; the results are shown in Fig. 9. Based on this series of measurements, the following two observations were made:
dark-zone temperatures; however, M43 has about 5% more NO mole fraction than XM39 in the dark zone. This difference is due mainly to the 7.6% energetic plasticizer used in M43, which is known to produce a significant amount of NO 2 upon decomposition [18]. NO 2 then reacts with other chemical species such as CH20 in the primary reaction zone to form NO and other products. Therefore, NO is more abundant in the dark zone of M43 than in XM39. At a distance about 1.6 mm above the burning surface (see Fig. 7), the NO concentration begins to decrease. NO reduction is generally considered to be the major exothermic chemical reactions responsible for heat release to produce the high-temperature environment in the final flame region. Thus, the NO mole faction is expected to be very low in the luminous-flame region, and higher temperatures are expected and were detected at larger vertical distances. The temperature will continue to increase until it reaches the final flame temperature. Note that the position with rapid NO reduction (between 1.6 and 2.0 mm) indeed
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347
1. The temperature profile is quite uniform, while the line-of-sight averaged NO concentration profile exhibits a decreasing trend in the dark-zone region. 2. The deduced temperatures are still very close to the "true" dark-zone temperatures (see Fig. 7) even when the flamelet attachment occurs.
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1200 Z
t~
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v
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X M 3 9 @ 2.17 M P a (Flamelet a t t a c h m e n t a Temperature
A
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•
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348
Y . C . LU ET AL.
A similar decreasing trend of NO concentration in the dark zone was also exhibited in many of the NO concentration profiles reported by Vanderhoff et al. in Ref. 16. At first glance, there seems to be two unusual scenarios in the deduced results associated with the situation of flamelet attachment (e.g., Fig. 9): 1. The uniform temperature profile conflicts with the decreasing NO concentration profile since both of them occur in the same spatial range. 2. The decreasing NO concentration profile in the dark zone violates the energy conservation law since it shows that NO reduction occurs throughout the dark zone (in which the temperature profile is uniform) and is completed at the end of the dark zone. However, these two scenarios can be explained by thorough examination of the flame structure and the nature of the line-of-sight average technique. The concurrence of a uniform temperature profile and a decreasing NO concentration profile can also be constructed through numerical simulation for the observed flame structure with flamelet attachment. The details are given below. Considering a very idealized flame at 2.17 MPa as shown in Fig. 10, there are two distinct regions intercepted by the UV light beam: 1) Region 1 simulates the luminous-flame zone which has a temperature of 2400 K and a NO mole fraction of 1% (or NO number density 6.547 × 1017 molecules/cm3), and 2) Region 2
simulates the dark zone which has a temperature of 1200 K and a NO mole fraction 20% (or NO n u m b e r density 2.619 × 1019 molecules/cm3). These assumed flame properties have certain similarities with those obtained from either UV/Visible absorption measurements or chemical equilibrium calculations for both XM39 and M43 propellants. Consider three particular vertical positions where the UV light beam intercepts the flame zone" 1. Position I, the dark-zone region intercepted by the light beam (Lo.z.) has a length of 0.9D (D is the propellant diameter); 2. Position II, Ld.z.= 0.5D; and 3. Position III, Ld.z.= 0.1D. Figure 11 shows the simulated absorption spectra for Position I. The short dash represents the absorption spectrum caused by NO molecules in the luminous-flame zone, the long dash the absorption spectrum caused by NO molecules in the dark zone, and the circles the "overall" absorption spectrum caused by NO molecules in both the luminous-flame and dark zones (combined effect). As can be seen, the characteristics of the overall absorption spectrum are dominated by absorption due to NO molecules in the dark zone. Similar numerical simulation was also conducted for Positions II and III. It was found that the effect of absorption due to NO molecules in the luminous
i
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Consider {~ 300 psig
0.8 ~ ~ ~ ~ t L~
LuminousFlameZone: Tiff2,400K Nrj.t"6.547x10t7#/CC
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spectrum
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x~=o.2o 0.2
0.0
Fig. 10. Flame conditions considered for calculating simulated absorption spectra.
I
232
236
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1
t
240
244
248
Wavelength (nm) Fig. 11. Simulated absorption spectra for Position I shown in Fig. 10.
DARK ZONES IN SOLID-PROPELLANT FLAMES flame is nearly negligible on the overall absorption spectrum, except in the Position III case. The simulated overall absorption spectra (e.g., circles in Fig. 11) for these three positions were then used as input files for the data analysis program to deduce line-of-sight averaged temperatures and NO concentrations in the dark-zone region as if they were truly "measured" absorption spectra. The propellant diameter was used as the path length (i.e., the parameter "b" in Eqs. 2 and 3) in the data analysis. The calculated results are summarized in Table 1. It can be seen that the deduced temperatures are very close to the "exact" dark-zone temperature, except for the case where the majority of the UV light beam is in the luminous-flame zone (Position III). The deduced number density decreases as the intercepted length in the dark-zone region decreases. It should be noted that if the assumed NO concentration in the luminous flame is lower, its effect on the simulated overall absorption spectra and deduced temperatures is even smaller. Based upon this set of simulations, the following explanations can be made for the two observations mentioned earlier. When the UV excitation light beam travels any particular distance above the surface, it passes through both the dark-zone and luminous-flame regions when flamelet attachment occurs. In the luminous-flame region, the temperature is high, but NO concentration is very low. In the dark-zone region, the temperature is lower, but the NO concentration is very high. As can be seen from Figs. 4, 5, and 8, the dark-zone region intersected by the light beam becomes smaller as the distance from the surface increases. If the true light path length is known and used in
349
the data analysis, the NO concentration would exhibit a more uniform profile simialr to that shown in Figs. 3 and 7. However, because the flamelet attachment is not a stable phenomenon, it is impossible to identify appropriate path lengths to be used in the data analysis; thus, no attempt was made to determine the "true" path length for any distance above the burning surface. The propellant diameter (a constant value) was used as the light path length in the data analysis for all distances above the burning surface (as in the simulation study). According to Eq. 3, the concentration of the absorbing species is nearly inversely proportional to the path length for a measured transmittance. Therefore, the line-of-sight averaged NO concentration decreases as the vertical distance from the burning surface increases because the pathlength used in data analysis is greater than the true value. Since the NO concentration is low in the luminousflame region, the deduced temperature is dominated by the properties of the NO molecules in the dark zone. Therefore, the temperature can still exhibit a uniform profile within the dark zone region even when flamelet attachment occurs. Based on the above explanation, there is actually no inconsistency between the deduced temperature and line-of-sight averaged NO concentration profiles. Video records revealed that the dark-zone size of M43 propellant (6.5-7.2 mm at 1.27 MPa, 2.9-4.3 mm at 1.6 MPa, and 1-1.5 mm at 2.17 MPa) in the covered pressure range is smaller than that of XM39 propellant (6-7 mm at 1.69 MPa, 1.7-2.2 mm at 2.17 MPa, and 0.7-1.1 mm at 3.55 Mpa), and that it decreases more rapidly with an increase of pressure than XM39. At 1.69 MPa, the luminous flame of XM39 was hardly seen in the gas phase, and a
TABLE 1 D e d u c e d T e m p e r a t u r e s and N O Concentrations for Simulated A b s o r p t i o n Spectra Ld.z. = 0.9D
Ld,z. = 0.5D
Ld.z. = 0.1D
Deduced T e m p e r a t u r e (K)
1227.6 = 1.02Td.z.
1255.1 = 1.04Td.z.
1501.1 = 1.25T0.z.
Deduced NO N u m b e r Density × 10-19 ( m o l e c u l e s / c m 3)
2.225 = 0-85Nr, a.z.
1.2898 = 0.49Nr, ,J.z.
0.2728 = 0. lONr, d.z.
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Y. C. LU ET AL.
great amount of smoke was generated from the burning surface ("fizz burning" mode [17]) of the self-deflagrating propellant sample. However, a luminous flame always appeared above the uniform dark-zone region (with intermittent fiamelet attachment) of a steadily regressing M43 propellant sample ("flame-burning" mode [17]) even at a pressure as low as 1.27 MPa. For tests at high pressures, flamelet attachment became almost inevitable. Because of this, it was impossible to obtain the plateautype profiles, and only line-of-sight-averaged temperatures and NO concentrations were deduced. These results show trends similar to those in Fig. 9, except that the dark-zone size is smaller at high pressures. Figure 12 shows the deduced temperatures and NO concentrations in the dark zone of XM39 propellant at 3.55 MPa with flamelet attachment. The darkzone size is about 1.1 mm as estimated from the sharp rise of temperature in Fig. 12. According to the descriptions given in the previ-
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ous paragraphs, the deduced temperatures should still be close to the "true" dark-zone temperatures at this pressure. It was found that the dark-zone temperature has a very weak dependency on pressure for both XM39 and M43 propellants; its variation was less than 100 K in the pressure range covered in this study (1.27-3.55 MPa), which is within the measurement uncertainty.
CONCLUSION UV/Visible absorption spectroscopy has been applied to determine gas-phase temperature and NO species concentration profiles of RDX-based solid propellants at elevated pressures. XM39 and M43 propellant flames showed a nearly uniform visible dark zone in some of the tests. For these tests, deduced temperature and NO concentration exhibit quite uniform profiles within the dark-zone region. The deduced dark-zone temperature
1
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1800
X M 3 9 O 3.55 M P a (Flamelet a t t a c h m e n t • Temperature 1500
• XNo
0.09 o m.
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•
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600
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,
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Z o
D A R K Z O N E S IN S O L I D - P R O P E L L A N T F L A M E S and N O mole fraction plateaus are 1180 K and 15% for XM39 and 1050 K and 20% for M43. However, in many of the tests, especially at higher pressures, luminous flamelets were attached to the burning surface at numerous carbonaceous patches. The flamelet attachment resulted in shorter intercepted lengths in the dark zone by the U V light b e a m as the vertical distance from the burning surface increases. Therefore, when a constant path length is used in the data analysis, the spatially averaged, line-of-sight m e a s u r e m e n t yields a relatively uniform temperature profile, but a decreasing N O concentration profile in the darkzone region. It was found that the deduced temperatures could still represent the true dark-zone temperatures, even though the measurements were taken when the luminous flame was attached to the burning surface. This aspect was simulated numerically using flame properties similar to those of the real propellant flame. According to video records, M43 has a smaller dark-zone size than XM39 at the same pressure, and its dark-zone size decreases m o r e rapidly with an increase of pressure than XM39 in the pressure range studied. The flame luminosity of XM39 is weaker than that of M43, except at high pressures (e.g., 3.5 MPa) where they are similar. There was almost no luminous flame for XM39 at pressure below 1.69 MPa; only a significant amount of smoke was produced at the regressing burning surface. M43, however, could maintain a stable flame burning at a pressure as low as 1.27 MPa (lowest pressure tested in this work). There were numerous narrow illuminating streaks (burning carbonaceous residues) in M43 flame at low pressures (e.g., 1.6 MPa). However, at higher pressures such as 2.17 MPa, there were nearly no illuminating streaks in the flame region, and the flame luminosity became weaker. The change of visual flame characteristics occurs in the same pressure range where the M43 burning rate vs pressure relationship shows a slope variation. Because flame attachment is almost inevitable for most of the nitramine solid-propellant applications, spatially averaged quasi-l-D solid-propellant combustion models should be able to simulate this aspect of the flame struc-
351
ture. Thus, the data reported in this p a p e r should be very useful for theoretical model validations and improvements. This work was performed under the sponsorship o f the A r m y Research Office, Contract No. DAALO3-92-G-0118. The support and encouragement of Dr. Robert W. Shaw of A R O are highly appreciated. The authors would like to acknowledge Dr, J. A. Vanderhoff and Dr. A. J. Kotlar of A R L for useful discussions and for providing important molecular constants. The authors would like to thank Mr. Abdullah Ulas o f PSU for making some N O absorption measurements for X M 3 9 propellants.
REFERENCES 1. Edwards, T., Weaver, D. P., and Campbell, D. H., Appl. Opt. 26:3496 (1987). 2. Hanson-Parr,D., and Parr, T., in Nonsteady Burning and Combustion Stability of Solid Propellants (L. DeLuca, E. W. Price, and M. Summerfield, Eds.), Vol. 143 of Progress in Astronautics and Aeronautics, AIAA, 1992, ch. 8, pp. 261-324. 3. Hanson-Parr, D., and Parr, T., Proceedingsof the 25th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 1635-1643. 4. Stuffiebeam,J. H., and Eckbreth, A. C., Combust. Sci. Technol. 66:163-179 (1989). 5. Vanderhoff, J. A., and Kotlar, A. J., Proceedingsof the 23rd Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 13391344. 6. Vanderhoff, J. A, Teague, M. W., and Kotlar, A. J., Proceedings of the 24th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1915-1922. 7. Lu, Y. C., Freyman, T. M., and Kuo, K. K., Combust. Sci. Technol. 104:193-205 (1995). 8. Korobeinichev, O. P., Fizika Goreniya i Vzryva 23(5):64-76 (1987). 9. Zenin, A., J. Propulsion and Power 11(4):752-758 (1995). 10. Kotlar,A. J., Private communication, U.S. Army Research Laboratory, 1994. 11. Henry, A., Le Moal, M. F., Cardinet, Ph., and Valentin, A. J. Molecular Spectroscopy 70:18-26 (1978). 12. Engleman, R., Jr., and Rouse, P. E., J. Molecular Spectroscopy 37:240-251 (1971). 13. McDerrnid, I. S., and Laudenslaer, J. B., J. Quantitative Spectroscopyand Radiative Transfer 27(5):483-492 (1982). 14. Huber, K. P., and Herzberg, G., Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979.
352 15. Press, W. P., Flannery, B. P., Teukolsky, S. A., and Vetterling, W. T., NumericalRecipes, Cambridge University Press, New York, 1986, ch. 14. 16. Teague, M. W, Singh, G., and Vanderhoff, J. A., Spectral Studies of Solid Propellant Combustion IV: Absorption and Burn Rate Results for M43, XM39, and M10 Propellants, Army Research Laboratory Report ARL-TR-180, Aug. 1993.
Y . C . LU ET AL. 17. Hsieh, W. H., Li, W. Y., and Yim, Y. J., Combustion Behavior and Thermochemical Properties of RDXBased Solid Propellants, AIAA Paper 92-3628, 1992. 18. Huang, T. H., Thynell, S. T., and Kuo, K. K., J. Propulsion and Power 11(4):781-790 (1995). Received 15 September 1995
INTRODUCTION D e t e r m i n a t i o n o f t e m p e r a t u r e a n d speciesconcentration profiles of solid-propellant flames u n d e r v a r i o u s o p e r a t i n g c o n d i t i o n s is t h e first step in u n d e r s t a n d i n g the d e t a i l e d c o m b u s t i o n - w a v e s t r u c t u r e o f solid p r o p e l l a n t s a n d assessing t h e effects o f individual ingredients on the p r o p e l l a n t c o m b u s t i o n process. T h e s e d a t a a r e also i m p o r t a n t in establishing and validating solid-propellant combustion m o d e l s which i n c o r p o r a t e d e t a i l e d c h e m i c a l r e a c t i o n m e c h a n i s m s . H o w e v e r , d u e to the hostile c o m b u s t i o n e n v i r o n m e n t , m o s t o f the p r e v i o u s e x p e r i m e n t a l studies for s o l i d - p r o p e l lant c o m b u s t i o n f o c u s e d o n b u r n i n g - r a t e m e a s u r e m e n t s a n d qualitative o b s e r v a t i o n s o f flame-zone and near-surface phenomena. Only a l i m i t e d n u m b e r o f t h e s e studies q u a n t i t a tively m e a s u r e d the profiles o f g a s - p h a s e t e m p e r a t u r e s o r species c o n c e n t r a t i o n s o f solidp r o p e l l a n t flames using e i t h e r intrusive p r o b ing m e t h o d s (such as t h e r m o c o u p l e s a n d mi-
* Corresponding author. 0010-2180/97/$17.00 PII S0010-2180(96)00095-8
croprobe mass spectrometry) or nonintrusive d i a g n o s t i c t e c h n i q u e s (such as l a s e r - i n d u c e d f l u o r e s c e n c e a n d a b s o r p t i o n s p e c t r o s c o p y ) (e.g., see Refs. 1-9). A b s o r p t i o n s p e c t r o s c o p y is a well-established m e t h o d for s i m u l t a n e o u s l y d e t e r m i n i n g temperature and absolute concentration of c h e m i c a l species f r o m o n e m e a s u r e d s p e c t r u m . R e c e n t l y , this t e c h n i q u e was successfully u s e d for s o l i d - p r o p e l l a n t flame diagnostics at p r e s sure u p to 6.99 M P a [3, 5-7]. Since a b s o r p t i o n s p e c t r o s c o p y is a n o n i n t r u s i v e d i a g n o s t i c m e t h o d , it can b e u s e d to d e t e r m i n e t e m p e r a tures in a r a n g e b e y o n d the capability o f conv e n t i o n a l t h e r m o c o u p l e s . This w o r k e x t e n d e d t h e t e c h n i q u e p r e s e n t e d in Ref. 7 to d e t e r m i n e profiles o f g a s - p h a s e t e m p e r a t u r e a n d N O conc e n t r a t i o n in the d a r k - z o n e r e g i o n o f two n i t r a m i n e c o m p o s i t e s o l i d - p r o p e l l a n t flames u n d e r e l e v a t e d p r e s s u r e s by m e a n s o f U V / Visible a b s o r p t i o n spectroscopy. T h e m a j o r ing r e d i e n t s for t h e solid p r o p e l l a n t s s t u d i e d in this w o r k are: . XM39: cyclotrimethylene trinitramine ( R D X , 76% by weight), cellulose a c e t a t e COMBUSTIONAND FLAME 109:342-352 (1997) © 1997 by The Combustion Institute Published by Elsevier Science Inc.
DARK ZONES IN SOLID-PROPELLANT FLAMES butyrate (CAB, 12%), nitrocellulose (NC (nitration level 12.6%), 4.0%), acetyl triethyl citrate (ATEC,7.6%), and ethyl centralite (EC, 0.4%); 2. M43: similar to XM39, except that ATEC is replaced by an energetic plasticizer. EXPERIMENTAL SETUP The experimental setup used in this work is very similar to that reported in Ref. 7 for OH measurements. Therefore, to avoid duplication, only a brief description of the test facility is given below, with emphasis on the modifications and improvements made to the orevious setup in order to allow NO measurements. Absorption transitions of NO molecules in solid-propellant flames were accessed by passing a high-intensity Xenon arc-lamp (ORIEL 6262, 450-W UV-enhanced lamp) light beam across the flame zone. This lamp is different from the one used in Ref. 7 since the previous 1000-W arc lamp did not produce enough light intensity at UV wavelengths pertinent to NO absorption. The present light source produces a spectral irradiance about four to ten times greater than the 1000-W light in the 230-250 nm range. A portion of the light energy is absorbed by NO molecules in the propellant flame, and the remaining light is focused onto a 100-/xm pinhole and then onto the entrance slit of the spectrometer by means of fused-silica lenses. The pinhole minimizes collection of emission signals from the flame and dictates the spatial resolution of measurements. The focal length ( f ) of the fused-silica lenses is 25.40 cm for the light at the wavelength of 589.3 nm. It varies significantly when the wavelength decreases to the low UV range. Based on the specifications in the manufacturer's catalog, the focal length at any other wavelength ;t (in unit of nm) can be determined by the following equation; (n589. 3 -- 1) fA ---~A89.3
(n~ - 1)
(1)
where na is the index of refraction at any particular wavelength A. For example, n589.3 1.4584 and n240 = 1.5133 for fused silica, and f5893 = 25.4 cm in this case; thus, the focal =
343
length at 240 nm is reduced to 22.68 cm. The distance on the focusing side of the lenses was thus adjusted based on Eq. 1. The slit opening was set at 80 /xm for NO measurement, yielding a spectral resolution of around 0.9 A. The spectrometer has a focal length of 75 cm, equipped with a 1200groove/mm grating (Instruments S.A., Inc., 12001-250 M). The grating is blazed at 250 nm, with about 65% efficiency at 230 nm and about 73% at 250 nm. The dispersed light was detected by a thermoelectrically cooled CCD camera and stored in a microcomputer for further processing. The CCD camera covered a 28-nm band of light when the grating was operated at first order. Propellant samples were cylindrical strands 0.64 cm in diameter and 3-5 cm in length. Propellant samples were ignited by a COe laser, and burned inside a windowed high-pressure strand burner under well-controlled operating conditions. During the propellant burning, the propellant sample was fed by a linear actuator system at a prespecified speed so that the propellant surface could be kept at a fixed position or slowly moved downward. Thus, it was possible to conduct either multiple samplings at a fixed position or scanning through a certain range in the flame zone in a single test. Continuous nitrogen purge flow was supplied during tests to carry combustion product gases out of the test chamber. The sampling time was 0.04 s, with 20-30 samplings per test. The chamber was also equipped with an in situ propellant-strand heating unit for investigating the effect of propellant initial temperature on the combustion process and for stretching the propellant flame zone at high pressures. DATA ANALYSIS The data-reduction program is exactly the same as that used in Ref. 7. This program, based on Beer's law, was used to extract temperature and concentration from the measured absorption spectrum. Since the spectrometer's bandwidth is larger than the linewidth of a typical transition line, convolution between molecular absorption and instrument function is required and considered. Due to scattering and absorption by soot particulates and large molecules,
344
Y . C . LU ET AL.
strong attenuation of the transmitted light was observed; the attenuation effect was displayed by shifting the baseline away from unity in either a linear or nonlinear fashion. By assuming the Boltzmann distribution, the observed transmittance r at any frequency v0 cm-1 can be described by the following equation [5-7]:
7"(Vo) = (A o + AlVo + A2Vo2)
is given below [7]:
r(v0) = (Ao + A~oo + A~,o ~) 1
x/1 /
x 1-exp
Es%,-o) J
q(T~Bjgje-e#kr
[ hvb N~ × -~S(v' vo)eXp c q(T)
f
× EBgjej-EJkVP(vj)],
dv
(3)
(2)
where h is the Planck constant, b the path length, c the speed of light, k the Boltzmann constant, T the absolute temperature, Bj the Einstein coefficient for the jth transition in energy density units [m3/j s2], gj the degeneracy of the jth energy level, Ej the energy of the jth level, Pj(v) the transition line shape, Nr the total number density, q(T) the partition function, and S the normalized instrument function. The summation j is performed over all of the energy transitions which contribute to the light absorption at frequency v 0. The second-order polynomial A o + Air o + A2vo2 (sometimes a different order polynomial or an exponential function) was used to account for the baseline shift. Each transitional line was treated as a Delta function to improve the efficiency of data analysis. As demonstrated in Ref. 7, line broadening at pressures as high as 6.99 MPa does not change the characteristics of absorption signals nor deteriorate the quality of the curve fit; thus, the Delta-function treatment for transitional line shape is acceptable in the pressure range considered in this paper (up to 3.55 MPa). Since the calibration between wavelength and CCD pixel using an Hg lamp was only an approximation, adjustment of wavelengths in measured spectra was considered in the data analysis. To further simplify data reduction, an average instrument bandwidth was used in each test for the wavelength range considered. The final equation for curve fitting the observed transmittance at any frequency v0
Measured absorption spectra were used as input files of the data analysis program (which was based on Eq. 3). The program analyzed the measured spectrum and adjusted a set of parameters (including temperature, number density, etc.) to best fit the data by the least-squares method. The Einstein coefficients, degeneracies, and energy levels for NO were obtained from Kotlar [10], who obtained the ground state constants from Henry et al. [11], the upper state constants from Engleman and Rouse [12], and the radiative lifetimes from McDermid and Laudenslager [13]. 2125 transitions in the A2E-XZlI(O, 1) and (0,2)vibrational band systems were considered in the data analysis. Other molecule-specific parameters were obtained from Huber and Herzberg [14]. Levenberg-Marquardt's algorithm [15] was used to perform the least-squares analysis for the measured absorption spectra. Absolute number density as converted to mole fraction and molar concentration using the perfect gas law. It should be noted that the propellant diameter was used as the path length in all data analyses in this study. RESULTS AND DISCUSSION Figure 1 presents an image of the M43 propellant flame at 1.6 MPa (217 psig). A uniform visible dark zone was seen to separate the burning surface and the luminous-flame region. Because the lower edge of the luminous flame was not stationary, the size of the dark zone was not constant, and ranged from 2.9 to 4.3 mm at this pressure. Numerous narrow, bright streaks were always observed in the luminous-flame region. According to video im-
DARK ZONES IN SOLID-PROPELLANT FLAMES
the measured data and the solid line the fitted result from data analysis. A series of analyses similar to that shown in Fig. 2 was performed to determine temperature and NO concentration profiles in the dark zone of M43 propellant flame at 1.6 MPa; the results are shown in Fig. 3. It is evident that temperature and NO concentration are quite uniform (i.e., plateau) in the dark-zone region, which is very similar to the characteristics of double-base propellants. NO reduction near the luminous-flame zone is accompanied by energy release; therefore, temperature tends to increase gradually, while NO concentration tends to decrease as the vertical distance approaches the luminousflame region. The deduced averaged dark-zone temperature is about 1050 K, and the NO mole fraction is about 20%. Vanderhoff et al. [16] reported a dark-zone temperature of 1200K and an NO mole fraction of 22% for M43 propellant (in a slightly lower pressure range), which is not far from that reported in the current study. However, it should be noted that even though Ref. 16 reported the above results, many of its absorption measurements did not show the plateau feature of the dark zone, and it did not explain why the NO concentra-
M43 propellant
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ages, these streaks were carbonaceous residues ejected from the burning surface. The residues burned in the flame region and were strongly luminous. Figure 2 shows a typical NO absorption spectrum or deducing one pair of temperature and NO number density. The dots represent
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Fig. 3. Deduced gas-phase temperature and N O propellant in the dark-zone region at 1.6 MPa.
mole fraction profiles of M43
346
Y . C . LU ET AL.
tion showed a decreasing trend in the dark-zone region. These issues will be explored in detail later. The spatially uniform dark zone shown in Fig. 1 did not occur in all of the tests. In fact, only a limited number of tests exhibited the feature of a quasi-steady luminous flame above a dark zone; in many tests, the luminous flame was attached to the burning surface through several flamelets (see Fig. 4). The size of the dark zone decreased drastically, and the flamelet attachment became more pronounced with an increase of pressure. At a higher pressure of 2.17 MPa (300 psig), the size of the visible dark zone (when it indeed existed during the test) was only about 1-1.5 mm. The uniform dark zone only appeared for very short periods of time during the propellant burn; the luminous flame almost always attached to the burning surface through many conical shape flamelets, as shown in Fig. 5. Note that as pressure increases, both the number of illuminating streaks in the flame region and the flame luminosity decrease, in addition to the very pronounced flamelet attachment. It is interesting to note that the change of visual flame characteristics mentioned above occurs
I
I
in the same pressure range where the burning-rate curve of M43 propellant shows the slope variation reported in Ref. 17. NO absorption measurements were also conducted for XM39 propellant at different pressures. Compared to M43 propellant flame at 2.17 MPa, the luminous flame of XM39 propellant is much smaller and nearly indistinguishable from the ambient environment (see Fig. 6 in which a dark zone exists between the burning surface and luminous flame). The size of the visual dark zone is about 1.7-2.2 mm for tests at 2.17 MPa, which is slightly larger than that of M43 propellant at the same pressure. The results of NO absorption measurements conducted in the dark-zone region to deduce temperature and NO concentration profiles for XM39 propellant flame at 2.17 MPa are shown in Fig. 7. Similar to Fig. 3, the temperature and NO concentration are quite uniform in the dark-zone region. The deduced averaged temperature is about 1180 K, and the NO mole fraction is 15% in the dark-zone region of XM39 propellant. Vanderhoff et al. [16] reported a dark-zone temperature of 1275 K and an NO mole fraction of 15% for XM39 propellant. Note that XM39 and M43 have similar
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Fig. 1. An image of M43 propellant flame with a uniform dark zone at 1.6 MPa.
Fig. 5. An image of M43 propellant flame showing Aamelet attachment at 2.17 MPa.
Fig. 4. An image of M43 propellant flame showing tlamelet attachment at 1.6 MPa.
Fig. 6. An image of XM3Y propellant flame with a uniform dark zone at 2.17 MPa.
Fig. 8. An image of XM3Y propellant flamelet attachment at 2.17 MPa.
flame showing
DARK ZONES IN SOLID-PROPELLANT FLAMES
correlates well with the video records for the boundary between the dark zone and the luminous flame. Similar to M43 propellant, the luminous flame of XM39 has the tendency to attach to the burning surface, especially at high pressures. Figure 8 shows an image of XM39 propellant flame with flamelet attachment at 2.17 MPa. A series of NO absorption measurements were conducted at different vertical distance above the burning surface to deduce temperatures and line-of-sight averaged NO concentrations in the dark zone of XM39 with flamelet attachment; the results are shown in Fig. 9. Based on this series of measurements, the following two observations were made:
dark-zone temperatures; however, M43 has about 5% more NO mole fraction than XM39 in the dark zone. This difference is due mainly to the 7.6% energetic plasticizer used in M43, which is known to produce a significant amount of NO 2 upon decomposition [18]. NO 2 then reacts with other chemical species such as CH20 in the primary reaction zone to form NO and other products. Therefore, NO is more abundant in the dark zone of M43 than in XM39. At a distance about 1.6 mm above the burning surface (see Fig. 7), the NO concentration begins to decrease. NO reduction is generally considered to be the major exothermic chemical reactions responsible for heat release to produce the high-temperature environment in the final flame region. Thus, the NO mole faction is expected to be very low in the luminous-flame region, and higher temperatures are expected and were detected at larger vertical distances. The temperature will continue to increase until it reaches the final flame temperature. Note that the position with rapid NO reduction (between 1.6 and 2.0 mm) indeed
I
!
347
1. The temperature profile is quite uniform, while the line-of-sight averaged NO concentration profile exhibits a decreasing trend in the dark-zone region. 2. The deduced temperatures are still very close to the "true" dark-zone temperatures (see Fig. 7) even when the flamelet attachment occurs.
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348
Y . C . LU ET AL.
A similar decreasing trend of NO concentration in the dark zone was also exhibited in many of the NO concentration profiles reported by Vanderhoff et al. in Ref. 16. At first glance, there seems to be two unusual scenarios in the deduced results associated with the situation of flamelet attachment (e.g., Fig. 9): 1. The uniform temperature profile conflicts with the decreasing NO concentration profile since both of them occur in the same spatial range. 2. The decreasing NO concentration profile in the dark zone violates the energy conservation law since it shows that NO reduction occurs throughout the dark zone (in which the temperature profile is uniform) and is completed at the end of the dark zone. However, these two scenarios can be explained by thorough examination of the flame structure and the nature of the line-of-sight average technique. The concurrence of a uniform temperature profile and a decreasing NO concentration profile can also be constructed through numerical simulation for the observed flame structure with flamelet attachment. The details are given below. Considering a very idealized flame at 2.17 MPa as shown in Fig. 10, there are two distinct regions intercepted by the UV light beam: 1) Region 1 simulates the luminous-flame zone which has a temperature of 2400 K and a NO mole fraction of 1% (or NO number density 6.547 × 1017 molecules/cm3), and 2) Region 2
simulates the dark zone which has a temperature of 1200 K and a NO mole fraction 20% (or NO n u m b e r density 2.619 × 1019 molecules/cm3). These assumed flame properties have certain similarities with those obtained from either UV/Visible absorption measurements or chemical equilibrium calculations for both XM39 and M43 propellants. Consider three particular vertical positions where the UV light beam intercepts the flame zone" 1. Position I, the dark-zone region intercepted by the light beam (Lo.z.) has a length of 0.9D (D is the propellant diameter); 2. Position II, Ld.z.= 0.5D; and 3. Position III, Ld.z.= 0.1D. Figure 11 shows the simulated absorption spectra for Position I. The short dash represents the absorption spectrum caused by NO molecules in the luminous-flame zone, the long dash the absorption spectrum caused by NO molecules in the dark zone, and the circles the "overall" absorption spectrum caused by NO molecules in both the luminous-flame and dark zones (combined effect). As can be seen, the characteristics of the overall absorption spectrum are dominated by absorption due to NO molecules in the dark zone. Similar numerical simulation was also conducted for Positions II and III. It was found that the effect of absorption due to NO molecules in the luminous
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I
232
236
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240
244
248
Wavelength (nm) Fig. 11. Simulated absorption spectra for Position I shown in Fig. 10.
DARK ZONES IN SOLID-PROPELLANT FLAMES flame is nearly negligible on the overall absorption spectrum, except in the Position III case. The simulated overall absorption spectra (e.g., circles in Fig. 11) for these three positions were then used as input files for the data analysis program to deduce line-of-sight averaged temperatures and NO concentrations in the dark-zone region as if they were truly "measured" absorption spectra. The propellant diameter was used as the path length (i.e., the parameter "b" in Eqs. 2 and 3) in the data analysis. The calculated results are summarized in Table 1. It can be seen that the deduced temperatures are very close to the "exact" dark-zone temperature, except for the case where the majority of the UV light beam is in the luminous-flame zone (Position III). The deduced number density decreases as the intercepted length in the dark-zone region decreases. It should be noted that if the assumed NO concentration in the luminous flame is lower, its effect on the simulated overall absorption spectra and deduced temperatures is even smaller. Based upon this set of simulations, the following explanations can be made for the two observations mentioned earlier. When the UV excitation light beam travels any particular distance above the surface, it passes through both the dark-zone and luminous-flame regions when flamelet attachment occurs. In the luminous-flame region, the temperature is high, but NO concentration is very low. In the dark-zone region, the temperature is lower, but the NO concentration is very high. As can be seen from Figs. 4, 5, and 8, the dark-zone region intersected by the light beam becomes smaller as the distance from the surface increases. If the true light path length is known and used in
349
the data analysis, the NO concentration would exhibit a more uniform profile simialr to that shown in Figs. 3 and 7. However, because the flamelet attachment is not a stable phenomenon, it is impossible to identify appropriate path lengths to be used in the data analysis; thus, no attempt was made to determine the "true" path length for any distance above the burning surface. The propellant diameter (a constant value) was used as the light path length in the data analysis for all distances above the burning surface (as in the simulation study). According to Eq. 3, the concentration of the absorbing species is nearly inversely proportional to the path length for a measured transmittance. Therefore, the line-of-sight averaged NO concentration decreases as the vertical distance from the burning surface increases because the pathlength used in data analysis is greater than the true value. Since the NO concentration is low in the luminousflame region, the deduced temperature is dominated by the properties of the NO molecules in the dark zone. Therefore, the temperature can still exhibit a uniform profile within the dark zone region even when flamelet attachment occurs. Based on the above explanation, there is actually no inconsistency between the deduced temperature and line-of-sight averaged NO concentration profiles. Video records revealed that the dark-zone size of M43 propellant (6.5-7.2 mm at 1.27 MPa, 2.9-4.3 mm at 1.6 MPa, and 1-1.5 mm at 2.17 MPa) in the covered pressure range is smaller than that of XM39 propellant (6-7 mm at 1.69 MPa, 1.7-2.2 mm at 2.17 MPa, and 0.7-1.1 mm at 3.55 Mpa), and that it decreases more rapidly with an increase of pressure than XM39. At 1.69 MPa, the luminous flame of XM39 was hardly seen in the gas phase, and a
TABLE 1 D e d u c e d T e m p e r a t u r e s and N O Concentrations for Simulated A b s o r p t i o n Spectra Ld.z. = 0.9D
Ld,z. = 0.5D
Ld.z. = 0.1D
Deduced T e m p e r a t u r e (K)
1227.6 = 1.02Td.z.
1255.1 = 1.04Td.z.
1501.1 = 1.25T0.z.
Deduced NO N u m b e r Density × 10-19 ( m o l e c u l e s / c m 3)
2.225 = 0-85Nr, a.z.
1.2898 = 0.49Nr, ,J.z.
0.2728 = 0. lONr, d.z.
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Y. C. LU ET AL.
great amount of smoke was generated from the burning surface ("fizz burning" mode [17]) of the self-deflagrating propellant sample. However, a luminous flame always appeared above the uniform dark-zone region (with intermittent fiamelet attachment) of a steadily regressing M43 propellant sample ("flame-burning" mode [17]) even at a pressure as low as 1.27 MPa. For tests at high pressures, flamelet attachment became almost inevitable. Because of this, it was impossible to obtain the plateautype profiles, and only line-of-sight-averaged temperatures and NO concentrations were deduced. These results show trends similar to those in Fig. 9, except that the dark-zone size is smaller at high pressures. Figure 12 shows the deduced temperatures and NO concentrations in the dark zone of XM39 propellant at 3.55 MPa with flamelet attachment. The darkzone size is about 1.1 mm as estimated from the sharp rise of temperature in Fig. 12. According to the descriptions given in the previ-
I
I
ous paragraphs, the deduced temperatures should still be close to the "true" dark-zone temperatures at this pressure. It was found that the dark-zone temperature has a very weak dependency on pressure for both XM39 and M43 propellants; its variation was less than 100 K in the pressure range covered in this study (1.27-3.55 MPa), which is within the measurement uncertainty.
CONCLUSION UV/Visible absorption spectroscopy has been applied to determine gas-phase temperature and NO species concentration profiles of RDX-based solid propellants at elevated pressures. XM39 and M43 propellant flames showed a nearly uniform visible dark zone in some of the tests. For these tests, deduced temperature and NO concentration exhibit quite uniform profiles within the dark-zone region. The deduced dark-zone temperature
1
I
I"
0.12
I
1800
X M 3 9 O 3.55 M P a (Flamelet a t t a c h m e n t • Temperature 1500
• XNo
0.09 o m.
v
•
• •
•
1200
• • 0.06
k
E~
o
900 0.03
600
300
, 0.0
I 0.2
,
I
=
0.4
0.6
i
i
0.8
1.0
,
i
'
0.00
1.2
Distance above surface (ram) Fig. 12. Deduced gas-phase tempcrature and N O molc fraction profiles of X M 3 9 propellant in the dark-zonc region with flamelet attachment at 3.55 MPa.
Z o
D A R K Z O N E S IN S O L I D - P R O P E L L A N T F L A M E S and N O mole fraction plateaus are 1180 K and 15% for XM39 and 1050 K and 20% for M43. However, in many of the tests, especially at higher pressures, luminous flamelets were attached to the burning surface at numerous carbonaceous patches. The flamelet attachment resulted in shorter intercepted lengths in the dark zone by the U V light b e a m as the vertical distance from the burning surface increases. Therefore, when a constant path length is used in the data analysis, the spatially averaged, line-of-sight m e a s u r e m e n t yields a relatively uniform temperature profile, but a decreasing N O concentration profile in the darkzone region. It was found that the deduced temperatures could still represent the true dark-zone temperatures, even though the measurements were taken when the luminous flame was attached to the burning surface. This aspect was simulated numerically using flame properties similar to those of the real propellant flame. According to video records, M43 has a smaller dark-zone size than XM39 at the same pressure, and its dark-zone size decreases m o r e rapidly with an increase of pressure than XM39 in the pressure range studied. The flame luminosity of XM39 is weaker than that of M43, except at high pressures (e.g., 3.5 MPa) where they are similar. There was almost no luminous flame for XM39 at pressure below 1.69 MPa; only a significant amount of smoke was produced at the regressing burning surface. M43, however, could maintain a stable flame burning at a pressure as low as 1.27 MPa (lowest pressure tested in this work). There were numerous narrow illuminating streaks (burning carbonaceous residues) in M43 flame at low pressures (e.g., 1.6 MPa). However, at higher pressures such as 2.17 MPa, there were nearly no illuminating streaks in the flame region, and the flame luminosity became weaker. The change of visual flame characteristics occurs in the same pressure range where the M43 burning rate vs pressure relationship shows a slope variation. Because flame attachment is almost inevitable for most of the nitramine solid-propellant applications, spatially averaged quasi-l-D solid-propellant combustion models should be able to simulate this aspect of the flame struc-
351
ture. Thus, the data reported in this p a p e r should be very useful for theoretical model validations and improvements. This work was performed under the sponsorship o f the A r m y Research Office, Contract No. DAALO3-92-G-0118. The support and encouragement of Dr. Robert W. Shaw of A R O are highly appreciated. The authors would like to acknowledge Dr, J. A. Vanderhoff and Dr. A. J. Kotlar of A R L for useful discussions and for providing important molecular constants. The authors would like to thank Mr. Abdullah Ulas o f PSU for making some N O absorption measurements for X M 3 9 propellants.
REFERENCES 1. Edwards, T., Weaver, D. P., and Campbell, D. H., Appl. Opt. 26:3496 (1987). 2. Hanson-Parr,D., and Parr, T., in Nonsteady Burning and Combustion Stability of Solid Propellants (L. DeLuca, E. W. Price, and M. Summerfield, Eds.), Vol. 143 of Progress in Astronautics and Aeronautics, AIAA, 1992, ch. 8, pp. 261-324. 3. Hanson-Parr, D., and Parr, T., Proceedingsof the 25th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1994, pp. 1635-1643. 4. Stuffiebeam,J. H., and Eckbreth, A. C., Combust. Sci. Technol. 66:163-179 (1989). 5. Vanderhoff, J. A., and Kotlar, A. J., Proceedingsof the 23rd Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 13391344. 6. Vanderhoff, J. A, Teague, M. W., and Kotlar, A. J., Proceedings of the 24th Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, pp. 1915-1922. 7. Lu, Y. C., Freyman, T. M., and Kuo, K. K., Combust. Sci. Technol. 104:193-205 (1995). 8. Korobeinichev, O. P., Fizika Goreniya i Vzryva 23(5):64-76 (1987). 9. Zenin, A., J. Propulsion and Power 11(4):752-758 (1995). 10. Kotlar,A. J., Private communication, U.S. Army Research Laboratory, 1994. 11. Henry, A., Le Moal, M. F., Cardinet, Ph., and Valentin, A. J. Molecular Spectroscopy 70:18-26 (1978). 12. Engleman, R., Jr., and Rouse, P. E., J. Molecular Spectroscopy 37:240-251 (1971). 13. McDerrnid, I. S., and Laudenslaer, J. B., J. Quantitative Spectroscopyand Radiative Transfer 27(5):483-492 (1982). 14. Huber, K. P., and Herzberg, G., Molecular Spectra and Molecular Structure IV. Constants of Diatomic Molecules, Van Nostrand Reinhold, New York, 1979.
352 15. Press, W. P., Flannery, B. P., Teukolsky, S. A., and Vetterling, W. T., NumericalRecipes, Cambridge University Press, New York, 1986, ch. 14. 16. Teague, M. W, Singh, G., and Vanderhoff, J. A., Spectral Studies of Solid Propellant Combustion IV: Absorption and Burn Rate Results for M43, XM39, and M10 Propellants, Army Research Laboratory Report ARL-TR-180, Aug. 1993.
Y . C . LU ET AL. 17. Hsieh, W. H., Li, W. Y., and Yim, Y. J., Combustion Behavior and Thermochemical Properties of RDXBased Solid Propellants, AIAA Paper 92-3628, 1992. 18. Huang, T. H., Thynell, S. T., and Kuo, K. K., J. Propulsion and Power 11(4):781-790 (1995). Received 15 September 1995