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BTTN propellant flame studies
RDX/GAP/BTTN Propellant Flame Studies TIM PARR* and DONNA HANSON–PARR
Naval air Warfare Center, Weapons Division, China Lake, CA 93555-6100, USA The non-intrusive techniques of planar laser-induced fluorescence, ultraviolet-visible absorption spectroscopy and spontaneous laser Raman spectroscopy were used to map out species and temperature profiles above the surface of self-deflagrating RDX/GAP/BTTN model propellant with a pseudo pre-mixed flame. HCN, CO, and N2 were found to be the major species near the surface, and CO, N2, and H2O in the burnt gases. A dark zone of about 1200 to 1300 K was observed in which the NO concentration was at its highest. NO2 existed only very close to the surface. No formaldehyde was observed in the gas phase. Analysis of thermocouple measurements showed a surface temperature of 605 K. Thermal diffusivity and specific heat capacity as a function of temperature were also measured. © 2001 by The Combustion Institute
INTRODUCTION In previous years, we have reported on studies of the two-dimensional (2D) diffusion flame structure between AP and various solid binders in sandwich 2D configurations [1]. The multidimensional nature of diffusion flames makes modeling difficult. In addition, these studies have shown that nitramines do not really have strong diffusion flames with either energetic or non-energetic binders. A lot of experimental work has been done on hexahydro-1,3,5-trinitro-s-triazine (RDX) [2–10] and separately, on glycidyl azide polymer (GAP) [11–19]. Therefore, to simplify the comparison with modeling efforts, a study was made of a propellant made with fine RDX and GAP binder to give a homogeneous one-dimensional (1D) flame. The addition of GAP adds the complexity of additional chemistry to the already developed and validated RDX kinetic mechanism [20 –24] without adding the complexity of multidimensionality to the model. Modeling of GAP combustion has begun [25]. Modeling efforts of RDX/GAP propellant are underway and appear promising [26]. Experimental studies were done with RDX/GAP mixtures using a triple quadrupole mass spectrometer [18]. The RDX particle size of these uncured propellants was on the order of 200 m. However, this length scale is of the same order as the RDX self-deflagration flame standoff, so the flame structure would be more like *Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 127:1895–1905 (2001) © 2001 by The Combustion Institute Published by Elsevier Science Inc.
2D diffusion flamelets than a 1D pre-mixed flame. Initial studies of propellants containing only RDX and GAP showed some interesting results [27]. It was concluded that a plasticizer had to be used to increase the oxygen, thereby reducing char formation. 1,2,4 butanetriol trinitrate (BTTN) was chosen as the plasticizer. We reported [27] that cured GAP and uncured GAP have different combustion properties. Cured GAP ignites poorly and can even extinguish the igniting torch. Uncured GAP ignites rapidly, after a heating period, and burns violently. Advanced laser diagnostics, such as saturated planar laser-induced fluorescence (PLIF) and spontaneous Raman spectroscopy, as well as UV/visible absorption spectroscopy and micro thermocouples were used to quantify the micro flame structure of cured RDX/GAP/BTTN propellants via species and temperature profiles. Although there is no strong diffusion flame between RDX and GAP, there are still important effects. For example, it is known that the mixed HMX/GAP propellant burning rate is below that of either HMX or GAP [16]. Effects such as these must be understood on a mechanistic level if modeling is to help formulation of future propellants. Measurements of thermal diffusivity and specific heat capacity were also made for use in modeling efforts. Burning rates as a function of pressure and initial temperature have been measured and will be reported in the near future (Alice Atwood, Pat Curran, Tri Bui, Donna Hanson–Parr, and Tim Parr). 0010-2180/01/$–see front matter PII 0010-2180(01)00296-6
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T. PARR AND D. HANSON–PARR
Fig. 2. Apparatus diagram for ultraviolet/visible absorption spectroscopy.
Fig. 1. Apparatus used for planar laser-induced fluorescence imaging.
EXPERIMENTAL The cured RDX/GAP/BTTN propellant samples were generally about 10 mm in diameter, cut out from a large block. The samples were 71% by weight RDX, 9% GAP polyol, lot L-12369 from 3MTM Corporation, 0.47% HMDI (hexamethyene diisocyanate, aka HDI, aka N-100) curative, 19.51% BTTN plasticizer, and 0.02% T-12 (dibutyl tin dilaurate) catalyst. The military grade RDX was 70% by weight class 5 (mean particle size of about 17 m) and 30% 1.7 m RDX. The propellant was somewhat soft (easy to cut) and was bright yelloworange. The samples were fuel rich. Optical diagnostics applied to the flame included saturated planar laser-induced fluorescence (PLIF) imaging of OH and OH rotational temperature as well as CN and NH, ultravioletvisible (UV-vis) absorption spectroscopy for NO, NO2, CN, NH, OH, and H2CO, and spontaneous Raman spectroscopy for the majority species, HCN, CO, N2, and H2O. Raman can also detect C2H2 and H2CO, but upon analysis of the results, the concentrations of these species were found to be ⬍ 1 mole%. A springloaded sample holder, with a tungsten-rhenium wire stretched taut across the sample, was used in the PLIF and Raman experiments. The PLIF setup used is shown in Fig. 1. Typical laser energies were about 25 mJ/pulse for the nominal 6 ns pulses at 20 Hz. The laser sheet thickness was less than 100 m. The flame structure was imaged in separate experiments in air at 0.093 MPa (0.92 atm), with no added CO2
laser energy, using PLIF just after ignition. Because the flame height was substantially less than the sample diameter, and because the centerline profiles were measured, the oxygen in the surrounding air cannot penetrate and affect the flame. Indeed, no difference was seen in experiments done in nitrogen. For OH, the PLIF laser pumped the R1(3), R1(10), and R1(14) lines of the (1,0) A2⌺⫹-X2⌸ OH band, near 281.5 nm. An averaged OH PLIF image was obtained by summing five separate images taken in sequence from the same sample. Temperature was obtained from Boltzmann population distribution plots for each pixel in the image as discussed in [4]. A temperature profile as a function of height off the surface was obtained from the temperature image by averaging across a narrow (flat) region of the flame in the horizontal direction. PLIF was also used to image NH and CN. The CN radicals were monitored by pumping the (0,0) bandhead of the B-X transition near 388.3 nm while monitoring (0,1) emission around 422 nm. The NH was monitored by pumping the (1,0) Q branch head of the A-X transition near 305.03 nm while monitoring (0,0), (1,1) emission at 336 nm. These images were placed on an absolute scale using results from PLIF measurements done on HMX, and from UV-vis absorption, as discussed below. UV-visible absorption spectroscopy was used to obtain absolute concentrations [5] by mapping out profiles point-by-point. A detailed discussion of this technique and subsequent analysis of the data obtained is given in [3]. Figure 2 shows an apparatus diagram for this technique. A 150W Xe arc lamp source, single pass, was used to obtain concentrations of NO, NO2, CN, NH, OH, and H2CO. The f number of the spectrograph (f4.5) was matched by placing the
RDX/GAP/BTTN PROPELLANT FLAMES cylindrical lens the proper distance from the spectrograph slits. Because the focal point of a lens is wavelength dependent, the lens upstream of the sample had to be moved to refocus the lamp over the sample in the flame. Video imaging (VCR camera) was done for each sample piece of the material burned to observe the flame structure and to measure height off the surface and path length. The spectral resolution was about 1.2Å at 230 nm, and the spatial resolution was determined to be 24 m, but was also dependent upon the time frame of the acquisition. Except where noted, all UV-visible absorption experiments with the cured propellants were done in a bomb with nitrogen at 0.10 MPa pressure. A CO2 laser at about 400 Watts/cm2 was used to ignite the samples in air at 0.093 MPa, and upon ignition, the CO2 laser was turned off and nitrogen was allowed to flow into the bomb. The bomb was essentially a cube with three 63.5 mm (2.5 inch) diameter sapphire windows for optical access and a 12.7 mm (0.5 inch) diameter ZnSe window for CO2 laser access. The bomb volume was about one liter. It took approximately 1 s to completely displace all the air in the bomb (0.10 MPa N2 inside the bomb), so only data after that time is presented. The CO2 laser timing was controlled by a digital delay generator, which triggered the CO2 laser at time zero, also the start of spectra acquisition. In some cases, species were imaged in an HMX flame, for which the concentrations have been measured [5]. The signal levels from HMX combustion and the RDX/GAP/BTTN flame results, with the same detector conditions, were compared. Absolute concentrations could be measured this way only if the quenching processes were the same. However, the transitions are all saturated, which largely removes the effect of quenching, so this method of absolute concentration measurement is not subject to errors as large as one would first imagine. Another technique used to obtain absolute concentration profiles, spontaneous Raman spectroscopy, was used previously to measure species profiles in XM39 (an RDX-containing propellant) flames [6,28]. This technique was fully discussed in [28]. Numerous species can be detected in one spectrum. For each species, the Raman signal appears shifted from the exciting
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Fig. 3. Apparatus experiments.
layout
for
spontaneous
Raman
laser beam frequency (here, 532 nm) by the vibrational frequency in the case of vibrational Raman. The apparatus diagram for this technique is shown in Fig. 3. Temperature measurements were also done with embedded thermocouples in self-deflagrating samples. To embed the 75-mm diameter bead platinum-platinum 10% rhodium thermocouple, a cylinder of propellant was cut into two pieces using an angled razor blade. The bottom piece had a ridge across its centerline, where the thermocouple junction was placed. The thermocouple leads fell away from the bead at a 10° angle. The top half of the propellant cylinder was epoxied onto the lower half around the edges, without getting epoxy on the thermocouple, except outside the cylinder, where epoxy was dabbed on the leads to prevent premature burn-through. Voltage-time traces were measured and converted to temperature-time traces post measurement. Thermal diffusivity and specific heat capacity measurements were made as described in the literature [29]. By comparing pre-test to posttest thicknesses (L), it was seen that the samples readily flattened. Vapors were observed at temperatures above about 140°C and the samples turned brown around the edges and disintegrated. At 148°C the sample had disappeared completely, and no measurements could be made; the sample appeared to have cooked off and a scent was observed. Numerous different samples, each of different thickness were used at room and elevated temperature to get an idea of reproducibility and to judge the uniformity of the particle distribution in the propellant. The thickness could be measured to within about 20 m,
1898 temperature rise to about 0.01°C, and t1/2 to about 10 ms. The stiffer the sample (i.e., the less it compressed when clamped down upon) the more accurately the thickness could be measured post-test. Based on the uncertainties in L and t1/2, a ⫾ 2% variation for the thermal diffusivity would be typical; however, it is believed that the largest source of variation is because of the sample uniformity, including contact between crystals and the rest of the matrix. Several references were used for the specific heat capacity (cp) measurements, to make sure that the heat input (H) into the sample was well-characterized. H ⫽ cpL⌬Tmax, where is the density, and ⌬Tmax is the maximum temperature rise of the sample. The references used were: Teflon™, HDPE, and Al. Although the thermal diffusivity of Al is too large to measure with the apparatus used here, the temperature rise of the sample is representative of the amount of heat input into the sample. Teflon™ cannot be used as a reference at room temperature because it undergoes a crystal-crystal transition at about 25°C. HDPE can only be used below about 80°C. At room temperature, the heat input obtained for Al and HDPE agreed to within ⫾ 15%. The thermal diffusivities measured for these references matched literature values (except for Al). The used to obtain cp for the RDX/GAP/BTTN propellant was given as 1.688 g/cm3, obtained from the NAWC thermochemical code, PEP. Original sample thicknesses were from 500 to 800 m and postexperiment thicknesses ranged from about 200 to 500 m. ⌬Tmax was generally about 0.5°C to 1°C, depending on the sample thickness. The variation in measured cp would be about ⫾ 10%, since the literature values of cp for the references also contain some uncertainty. The thermal conductivity (k) could then be calculated:
T. PARR AND D. HANSON–PARR
Fig. 4. Thermal diffusivity of RDX/GAP/BTTN propellant.
cp,mixture ⫽ ⌺iMicp,i, where Mi is the mass fraction of species i, and cp,i is the specific heat capacity of species i. From previous results of measured cp for RDX [29], and a GAP/BTTN binder (not the mixture ratio used here) [29], an estimate was calculated and plotted on Fig. 5 (normalized at 20°C). These values of cp appear to be at the lower boundaries of what was currently measured, but essentially within the data scatter, except for the highest temperature. The law of mixtures cannot be used to estimate thermal diffusivity or conductivity because these properties depend largely on the degree of contact between constituents. Nevertheless, thermal diffusivity values that were measured were similar to those for pure RDX, especially at the higher temperatures. At low temperature, the thermal diffusivity was about 20% larger than that for pure RDX. Values of thermal conductivity calculated from the ␣ and cp data are shown in Fig. 6. Fits
k ⫽ alpha*cp* (J cm-1 s-1 deg-1)
RESULTS AND DISCUSSION Thermal diffusivities are shown in Fig. 4 and the specific heat capacity results are shown in Fig. 5. The law of mixtures can be used to estimate cp:
Fig. 5. Specific propellant.
heat
capacity
of
RDX/GAP/BTTN
RDX/GAP/BTTN PROPELLANT FLAMES
Fig. 6. Thermal conductivity of RDX/GAP/BTTN propellant obtained from data of Figs. 4 and 5.
of ␣, cp, and k versus temperature (°C) are shown on the respective Figures. Combustion Experiments At 0.65 MPa (94 psia), the samples ignited readily (in less than 0.2s) in N2 with a CN (seen in emission as a bluish flame sheet) standoff of about 2 mm. After the laser shut off and the sample deflagration reached steady state, the CN standoff was small, less than 0.5 mm. This also happened at 0.21 MPa (30 psia), with standoffs of 9 mm and less than 0.5 mm for laser-supported and steady-state deflagration, respectively. It took longer than 0.2 s, but less than 0.5 s to ignite the sample at 0.21 MPa. At 0.12 MPa (17 psia) N2, the CO2 laser drilled a crater into the sample before ignition could be established; because the sample height was short, the sample extinguished after the crater formed. The samples could be readily ignited in air, however, (CN standoff about 9 mm), and then burned steady-state in an N2 atmosphere (CN standoff about 2–2.5 mm), as described in the experimental section. Therefore, this procedure was followed to measure species profiles at atmospheric pressure (of N2) for which the flame standoff is sufficient to obtain meaningful data. Very little char was seen on the surface at 0.65 MPa, with more char seen as the pressure decreased. No char “snakes” were formed, unlike results from 50%/50% RDX/GAP cured propellants that had been examined earlier [27]. Unlike cured 70%/30% RDX/GAP propellant [27], it didn’t appear that any flamelets were
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Fig. 7. PLIF [CN], [NH], and [OH] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The absorption measurements done to scale these profiles were done at 0.1 MPa in N2.
holding on the char on the surface. The char on the surface did not burn when lit with a torch. A reproducible char ring was seen to form about 1 s after ignition. Little or no smoke was seen except for a minor amount when the sample extinguished near burnout. This was in stark contrast to the 50%/50% propellants which produced copious amounts of smoke so that no data could be obtained shortly after ignition [27]. Species and temperature profiles from PLIF, UV-vis, and Raman measurements are shown in Figs. 7-14. Measurements of [H2CO] were done with both UV-vis absorption and Raman. Whether laser-supported or self-deflagration, the [H2CO] present was below the detection limits of either apparatus, less than 1 mole%. The [CN], [NH], and [OH] profiles from PLIF were put on an absolute scale using previous results from HMX self-deflagration by scaling the intensity, as described in the experimental section. Peak concentrations were 170 parts per million (ppm) for CN, 150 ppm for NH, and 1.4 mole % for OH. The [OH] from the PLIF results agreed with that from UV-vis absorption. Somewhat lesser concentrations compared with the PLIF results were obtained from the UV-vis absorption results for CN and NH. As viewed in 3D, CN and NH are formed in a shell around the sample, so if the lamp beam is sampling a region below the peak of the [CN] or [NH] shell, then it is going through two edges. The cross-section of each edge is essentially a (skewed) Gaussian profile in concentration (see Fig. 7). Integration over the PLIF
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Fig. 8. [NO] and [NO2] profiles from UV-vis absorption for self-deflagration of RDX/GAP/BTTN propellant at 0.10 MPa in N2.
Gaussian profile was compared with the integrated results from UV-vis absorption to obtain peak [CN] and [NH] of 120 and 110 ppm, respectively. The CN and NH profiles are nearly identical in shape and concentration and peak about halfway up the rising edge of the [OH] profile. [NO] absorption results are seen in Fig. 8 together with [NO2] results. The NO2 concentration profile is actually from a laser-assisted experiment, which was then scaled to the flame height of the self-deflagration experiments. The [NO2], therefore, is possibly three times what it would be for no CO2 laser-support [4]. For self-deflagration, the [NO2] was too close to the surface to measure with absorption spectroscopy; this conclusion was made after many attempts to measure [NO2] under these conditions. Thermodynamic equilibrium calculations show that [NO] should be about 0.12 mole% in the burnt gas region. Many UV-vis absorption experiments were done to obtain the [NO] profile, especially near the surface. The points from 0 to 0.9 mm are averages of multiple determinations. Starting from 0 mm, and going in steps of 0.05 mm, up to 0.9 mm, data points were averaged ⫾ 0.05 mm, standard deviations were obtained, and are shown in Fig. 8. The NO concentration appears to be increasing from 0 to 0.8 mm, but within the error bars could also be flat. The magnitude of the error bars arises mostly from uncertainty in the path length, temperature, especially at positions above the surface where it is steeply
T. PARR AND D. HANSON–PARR
Fig. 9. Temperature profile for self-deflagration of RDX/ GAP/BTTN propellant. The solid line is the temperature obtained from PLIF OH rotational temperature, the dots are from fitting the NO UV-vis absorption spectra, and the dashed line is from thermocouple measurements, corrected for thermal effects, all at 0.093 MPa in air.
rising, and baseline noise [5]. As discussed in [5] typical uncertainties in mole fractions would be 15% near the surface to 25% nearer to the secondary flame front. The temperature profile (Fig. 9) is a combination of the OH rotational temperature profile from PLIF results, NO vibrational temperature obtained from fitting the UV-vis data, and results from thermocouple-measured temperature, corrected for lead heat loss and thermal lag. The equations of [30] were used to obtain the fit to the thermocouple trace. The gas phase temperature used was that from the NO absorption results. The thermocouple errors (thermal lag and heat loss) must be removed to recover an accurate condensed phase temperature profile. It is much easier to do this in a forward instead of a backwards analysis. A temperature profile is assumed throughout the domain. Then the thermal lag and heat loss simulation is performed on this temperature profile to recover what the thermocouple would have measured. This is compared with what the thermocouple actually measured and the initial temperature profile guess modified to minimize the difference. The ‘guess’ temperature profile is constructed from the actual measured gas phase temperature above the surface (from NO absorption and OH PLIF) coupled to a condensed phase profile simulation based on an assumed surface temperature parameter, the measured thermal dif-
RDX/GAP/BTTN PROPELLANT FLAMES
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Fig. 10. Spontaneous Raman [HCN] and [CO] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The mark on the right axis shows the adiabatic thermoequilibrium value for [CO].
Fig. 11. Spontaneous Raman [N2] and [H2O] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The marks on the right axis show the adiabatic thermoequilibrium values.
fusivity of the propellant as a function of temperature, and a constant diffusivity fit parameter for regions outside the measured temperature range. The simulation assumed a non-reactive condensed phase. The two fitting parameters, that is, the assumed surface temperature and the diffusivity outside the measured temperature range, were varied to get the best fit to the actual measured thermocouple temperature. The gas phase part of the actual temperature was never altered, and the gas phase heat transfer coefficient was calculated based on the relevant Reynolds and Nusselt numbers. The heat transfer coefficient in the condensed phase came from prior measurements [31]. The best fit was for a true surface temperature of about 605 K, compared with the raw measured value of 575 K. The best fit to the subsurface temperature trace was obtained for a thermal diffusivity of 8 ⫻ 10⫺4 cm2/s for temperatures above 120°C, using measured values (Fig. 4) below 120°C. The [OH] profile follows the temperature profile quite closely. Similar relationships were seen for RDX self-deflagration at 0.093 MPa in air [4]. Spontaneous Raman results for RDX/GAP/ BTTN are shown in Figs. 10-14. Besides [CN], measured with PLIF and UV-vis absorption spectroscopy, the other carbon-containing species profiles measured in the flame were [HCN] and [CO] (Fig. 10). [CO2] can be measured with Raman, but the C2* emission in the flame was rather high in intensity and could not be ade-
quately removed from the Raman signal by subtracting off the signal obtained when the laser was y-polarized [28]. [HCN] is highest at the surface and decays exponentially. [CO] is present at the surface at about 15 to 20% and grows to its thermodynamic equilibrium value of 29.4%. The nitrogen-containing species in the flame included [CN] and [NH] (Fig. 7), and NO2 and NO (Fig. 8), measured with PLIF and UV-vis absorption spectroscopy, as well as HCN (Fig. 10), N2O, and N2. The [N2] profile measured with Raman is shown in Fig. 11. The [N2] profile matches the [CO] profile rather closely and grows toward its equilibrium value of 27.7%. The N2O signal appears at about the same
Fig. 12. Spontaneous Raman [H2] profile for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The data points have large uncertainties associated with them. The mark on the right axis shows the adiabatic thermoequilibrium value.
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Fig. 13. [N2O] and [CO2] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa derived from atom balance. The mark on the right axis shows the adiabatic thermoequilibrium value for [CO2].
wavelength region as CO2, and for reasons discussed above for CO2, could not successfully be measured. It is estimated that the N2O concentration is at most a few percent, based on the atom balance results. The hydrogen-containing species in the flame are HCN (Fig. 10), NH and OH (Fig. 7), H2O, H2, and H-atoms. The [H2O] profile obtained with Raman is shown in Fig. 11 and is seen to grow to its equilibrium value of 18.8%. [H2] can be measured with Raman, but in these experiments, the detector used is about half as sensitive in the wavelength region where the H2 signal appears than where N2 appears, but the cross section for H2 is about twice that for N2. However, at elevated temperatures, the nitrogen Q-branch Raman is still a relatively narrow peak, while the hydrogen Q-branch Raman signal spreads out significantly, reducing the
Fig. 14. Atom balance at 0.093 MPa as a function of height off the surface, including [H2], [CO2], and [N2O].
T. PARR AND D. HANSON–PARR peak value and making it hard to distinguish from the noise. It is also in this region where the noise from the power supply is worse. (See [28] for discussion of sources of noise.) Our direct measurements were unable to quantify only two major and one minor species: H2 and CO2 are the major species and N2O the minor. The reason is the hostile nature of this flame. The high fuel level leads to soot and soot precursors in the flame. LIF from C2 (Swan band) interferes with the CO2 and N2O Raman measurements. Also, char buildup on the surface limits the time over which Raman can be integrated. This increased the noise level of all species, but seriously impacted the ability to detect H2, as the detector has poor sensitivity in the near IR where the H2 Raman appears. Atom balance calculations were undertaken in an attempt to verify that results obtained were consistent with reality. All atoms that were in the starting material (propellant) will be in the flame species and therefore, have to be accounted for (neglecting the small mass left in the char). The sum of the mole fractions at a given position in the flame should sum to one. The assumption made was that the flame species present at a given position were: HCN, CO, CO2, N2, N2O, H2O, H2, and NO. Any H-atoms (equilibrium thermochemical calculations show about 3 mole% of H-atoms) would therefore, be lumped together with H2. 16.1% of the atoms are C, 30.4% H, 25.1% N, and 28.4% O. If there are unmeasured large un-dissociated fragment molecules in the flame, especially near the surface, this atom balance approach will not work, but may be close if the fragment molecules constitute only a few mole%. Large molecules do exist near the surface, like the nitrosamine seen in [3]. RDX vapor may also be present; Fig. 15 of [3], attributed to RDX absorption, may actually be because of the nitrosamine (“unknown”). If Fig. 15 of [3] is actually RDX, then for a surface temperature of 605 K, [RDX] is only about 0.6 mole % at the surface, or at least as close to the surface as the measurement spatial resolution allowed. An estimate of the nitrosamine concentration near the surface is possibly 1%, but more work needs to be done for that species. The atom balance will also be in error if species diffusion is significant. Light species such as H2 and H may
RDX/GAP/BTTN PROPELLANT FLAMES rapidly diffuse axially or outwards radially. Since for each position measured, the mole fractions add up to one within a percent, we don’t think that the latter is too large of a problem. Still the failure of atom balance because of differential diffusion of light molecules could affect the accuracy of the species concentrations calculated from atom balance (N2O and CO2). The following equations were used in a least squares fit to recover the H2, CO2, and N2O mole fractions based on atom balance criteria: 1. mole fractions Propellant 3 c1HCN ⫹ c2CO ⫹ c3N2 ⫹ c4H2O ⫹ c5NO ⫹ c6OH⫹ c7H2 ⫹c8CO2 ⫹ c9N2O ⌺ici ⫽ 1.0 2. 3. 4. 5.
Carbon atoms: (c1⫹c2⫹c8)/n ⫽ 0.161 H atoms: (c1 ⫹ 2c4⫹ c6 ⫹2c7)/n ⫽ 0.304 N atom: (c1 ⫹ 2c3 ⫹ c5 ⫹ 2c9)/n ⫽ 0.251 O atoms (c2 ⫹ c4 ⫹ c5 ⫹ c6 ⫹ 2c8 ⫹ c9)/n ⫽ 0.284
n ⬀ sum of atoms ⫽ (c1⫹c2⫹c8)⫹( c1 ⫹ 2c4⫹ c6 ⫹2c7)⫹( c1 ⫹ 2c3 ⫹ c5 ⫹ 2c9)⫹( c2 ⫹ c4 ⫹ c5 ⫹ c6 ⫹ 2c8 ⫹ c9) ⫽ 2(⌺ici)⫹(c1 ⫹c4 ⫹c8 ⫹c9) ⫽ 2 ⫹ (c1 ⫹c4 ⫹c8 ⫹c9) The Minerr (least squares) function of MathCAD™ was used to solve for the mole fractions of H2, CO2, and N2O, using the fits to the data shown in Figs. 8, 10 to 13, and the OH concentration profile in Fig. 7. Figures 12 and 13 show the results. Figure 12 also includes a comparison with the measured H2 mole fractions. As stated above, the atom balance H2 result in Fig. 12 actually includes any H atoms as well, which adds a few percent in the hotter parts of the flame. Still, the measured H2, although noisy, is slightly below the atom balance results in the mid regions of the flame. Further work may be undertaken to minimize noise sources for H2 Raman and the H2 mole fraction may be measured again more accurately. Figure 14 shows the resulting atom balance plots. The consistency of the data with atom balance is quite good except very near the surface where measurements are difficult. It should be noted that although H2 and CO2 were calculated from atom balance, as well as
1903 N2O which is small, this does not guarantee perfect atom balance given all the species that are measured. H2 has a very high diffusion coefficient and the diffusion of H2 either radially or axially could change the [H2] in the flame. If the amount of diffusion is large, it would significantly affect the atom balance results. The coefficient c7, and n of Eq. 1 through 5 would decrease. Therefore, the atom balances presented do not necessarily have to be valid, and the species calculated from atom balance, such as N2O and CO2 could be in error. Nevertheless, the atom balance results give us some degree of confidence in the validity of our measurements. Figure 13 contains the atombalance-derived concentration profiles of N2O and CO2 based on the atom balance fits to the data. CO2 in the burnt gas region should be 6.3%, in agreement with the atom balance results. N2O is never larger than 4%. RDX versus RDX/GAP/BTTN Propellant The idea of using small particle size RDX was to get an approximately premixed flame of binder and RDX as opposed to a diffusion flame. The differences in flame chemistry between the propellant and pure RDX should therefore, be because of the species produced by GAP/BTTN decomposition. Because of GAP decomposition, char forms on the surface of the propellant. Whereas the surface of RDX is liquid and bubbly, the propellant surface is much less so. GAP decomposition is known to produce substantial amounts (around 35 mole%) of N2 [10 –18]. The extra gasification rate from the GAP acts to push the flame further away from the surface, causing higher flame standoffs for the propellant than for pure RDX. For pure RDX, the CN flame sheet is about 0.5 mm above the surface, compared with about 2 mm for the propellant. High mole fractions of CO (20%) and N2 (18%) were seen at the surface of the propellant, compared with about 8% and 5%, respectively for pure RDX [8] at atmospheric pressure. Both [CO2] and [H2] were high at the propellant surface at about 11% and 15%, respectively, compared with only about 0 through 3% and 3%, respectively, for pure RDX. The [OH], [NH], and [CN] for the propellant
1904 were about half that for pure RDX. At the surface [H2O] and [HCN] were about the same for either material, but the [NO], [NO2], and [N2O] were about three times larger for pure RDX than for the propellant. At most, only a few mole% of H2CO were seen for either material [5,8]. The surface temperature (605 K) appeared to be the same for both RDX and RDX/GAP/BTTN propellant. Based on the different gas mole percentages at the surface of the propellant compared with pure RDX, one would expect to see differences in the flame chemistry for the two materials, especially in the nitrogen chemistry. SUMMARY An experimental propellant consisting of fine RDX, GAP, and BTTN was formulated to produce a pseudo one-dimensional homogeneous flame that would be a “simple” propellant flame to model. The goal was to use fine RDX with length scales much lower than the flame structure, making a pseudo-premixed propellant. This propellant would include the chemical complexity of interaction between energetic material and binder without the added complexity of multi-dimensionality brought on by heterogeneity and diffusion flames. Thus, the kinetics of interaction could be modeled with simpler 1D pre-mixed models. To maximize the solids loading, a bimodal distribution of RDX was necessary, but even the larger particle-size fraction was kept below the flame structure length scales to try to maintain a pseudo homogeneous pre-mixed flame. PLIF, UV-visible absorption spectroscopy, spontaneous laser Raman spectroscopy, and micro-thermocouples were used to map out species and temperature profiles above the surface of this propellant as inputs for the modelers. HCN, CO, and N2 were found to be the major species near the surface, and CO, N2, and H2O in the burnt gases. The surface temperature was determined to be 605 K, and a dark zone of about 1200 to 1300 K was observed in which the NO concentration was at its highest. NO2 existed only very close to the surface. No formaldehyde was observed in the gas phase at measurable distances off the surface.
T. PARR AND D. HANSON–PARR The results were consistent with atom balance when species that were not quantified (CO2, N2O, and H2) were taken into account. Results in the burnt gases largely matched thermochemical calculations so the profiles tended to the right boundary conditions. These species and temperature profiles and the values for thermal conductivity and specific heat capacity presented here will be helpful to modelers to validate detailed kinetic 1D models of the deflagration of this model RDX/GAP/BTTN propellant. This is a step in the direction of developing models that can a priori predict ballistic properties of real composite solid propellants. We gratefully acknowledge the support of Dr. Judah Goldwasser of ONR, Dr. May Chan for the propellants and patience, and Alice Atwood and Pat Curran for making ballistics measurements on this propellant.
REFERENCES 1.
2.
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4.
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Parr, T. P., and Hanson–Parr, D. M., Proceedings of the 33rd JANNAF Combustion Subcommittee Meeting, Vol. I. I., CPIA Pub. 653. Chem. Propulsion Information Agency, Columbia, MD, 1996, p.35. Parr, T. P., and Hanson–Parr, D. M., Proceedings of the 26th JANNAF Combustion Committee Meeting, Vol. I, CPIA publication 529. Chem. Propulsion Information Agency, Columbia, MD, 1989, p. 27. Hanson–Parr, D. M., and Parr, T. P., Proceedings of the 31st JANNAF Combustion Subcommittee Meeting, Vol. II, CPIA publication 620. Chem. Propulsion Information Agency, Columbia, MD, 1994, p. 407. Parr, T. P., and Hanson–Parr, D. M., Proceedings of the 32nd JANNAF Combustion Subcommittee Meeting, Vol. I, CPIA publication 631. Chem. Propulsion Information Agency, Columbia, MD, 1995, p. 429. Parr, T. P., and Hanson–Parr, D. M., in Solid Propellant Chemistry, Combustion, and Motor Interior Ballistics, Progress in Astronautics and Aeronautics, Vol. 185, (V. Yang, T. Brill, and W.-Z. Ren, Eds.), Am. Institute of Aeronautics and Astronautics, Inc., Reston, Virginia, 2000, Chapter 2.5, p.381. Parr, T. P., Hanson–Parr, D. M., Smooke, M. D., and Yetter, R. A., Paper AIAA 2000 –2683, 21st AIAA Aerodynamic Measurement Technology and Ground Test Conference, 19 –22 June 2000, Denver, CO. Am. Institute of Aeronautics and Astronautics, Inc., Reston, Virginia. Brill, T. B, and Brush, P. J., Phil. Trans. R. Soc. Lond. A 339:377 (1992). Gongwer, P. E., and Brill, T. B., Combust. Flame 115:417 (1998).
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Lee, Y. J., Tang, C. J., and Litzinger, T. A., Combust. Flame 117:600 (1999). Homan, B. E., Miller, M. S., Vanderhoff, J. A., Combust. Flame 120:301 (2000). Chen, J. K, and Brill, T. B., Combust. Flame 87:157 (1991). Arisawa, H., and Brill, T. B., Combust. Flame 112:533 (1998). Frankel, M. B., Grant, L. R., and Flanagan, J. E., J. Propulsion and Power 8:560 (1992). Nazare, A. N., Asthana, S. N., Singh, H., J. Energetic Materials 10:43 (1992). Haas, Y., and Ben Eliahu, Y., Combust. Flame 96: 212 (1994). Kubota, N., and Sonobe, T., 23rd Symposium (International) on Combustion, The Combustion Institute, 1990, p. 1331. Schedlbauer, F., Propellants, Explosives, and Pyrotechnics 17:164 (1992). Tang, C. -J., Lee, Y. J., and Litzinger, T. A., Proceedings of the 34th JANNAF Combustion Subcommittee Meeting MURI Technology, Vol. II., CPIA Pub. 662. Chem. Propulsion Information Agency, Columbia, MD, 1997, p. 491. Tang, C. -J., Lee, Y. J., and Litzinger, T. A., Combust. Flame 117:244 (1999). Prasad, K., Yetter, R. A., and Smooke, M. D., Combust. Sci. and Tech. 124:35 (1997). Erikson, W. W., and Beckstead, M. W., 35th AIAA/ ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, 20 –23 June 1999, Los Angeles, California, AIAA 99 –2498. Davidson, J. E., and Beckstead, M. W., J. Propulsion and Power. 13:375 (1997).
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Liau, Y. C., and Yang, V., J. Propulsion and Power. 11:729 (1995). Liau, Y. C. (1997). PhD. Thesis, Dept. of Mechanical Engineering, Pennsylvania State University. Davidson, J. E., and Beckstead, M. W., Proceedings of the 33rd JANNAF Combustion Subcommittee Meeting, Vol. II, CPIA Publication 653. Chem. Propulsion Information Agency, Columbia, MD, 1996, p.91. Liau, Y. -C., Thynell, S. T., and Yang, V., Proceedings of the 34th JANNAF Combustion Subcommittee Meeting MURI Technology, Vol. II., CPIA Pub. 662. Chem. Propulsion Information Agency, Columbia, MD, 1997, p. 61. Parr, T. P., and Hanson–Parr, D. M., Proceedings of the 37th JANNAF Combustion Subcommittee Meeting, Vol. I, CPIA publication 701. Chem. Propulsion Information Agency, Columbia, MD, 2000, p.391. Parr, T. P., and Hanson–Parr, D. M., Proceedings of the JANNAF Joint 35th Combustion Subcommittee Meeting and 7th Propulsion Systems Hazards Subcommittee Meeting, CPIA publication 685. Chem. Propulsion Information Agency, Columbia, MD, 1998, p. 87. Hanson–Parr, D. M., and Parr, T. P., J. Energetic Materials 17:1 (1999). Suh, N. P., and Tsai, C. L., J. Heat Transfer, 93:77 (1971). Hanson–Parr, D. M., and Parr, T. P., Proceedings of the 20th JANNAF Combustion Subcommittee Meeting, Vol. I, CPIA publication 38. Chem. Propulsion Information Agency, Columbia, MD, 1983, p. 281.
Received 21 February 2001; revised 28 June 2001; accepted 29 June 2001
Naval air Warfare Center, Weapons Division, China Lake, CA 93555-6100, USA The non-intrusive techniques of planar laser-induced fluorescence, ultraviolet-visible absorption spectroscopy and spontaneous laser Raman spectroscopy were used to map out species and temperature profiles above the surface of self-deflagrating RDX/GAP/BTTN model propellant with a pseudo pre-mixed flame. HCN, CO, and N2 were found to be the major species near the surface, and CO, N2, and H2O in the burnt gases. A dark zone of about 1200 to 1300 K was observed in which the NO concentration was at its highest. NO2 existed only very close to the surface. No formaldehyde was observed in the gas phase. Analysis of thermocouple measurements showed a surface temperature of 605 K. Thermal diffusivity and specific heat capacity as a function of temperature were also measured. © 2001 by The Combustion Institute
INTRODUCTION In previous years, we have reported on studies of the two-dimensional (2D) diffusion flame structure between AP and various solid binders in sandwich 2D configurations [1]. The multidimensional nature of diffusion flames makes modeling difficult. In addition, these studies have shown that nitramines do not really have strong diffusion flames with either energetic or non-energetic binders. A lot of experimental work has been done on hexahydro-1,3,5-trinitro-s-triazine (RDX) [2–10] and separately, on glycidyl azide polymer (GAP) [11–19]. Therefore, to simplify the comparison with modeling efforts, a study was made of a propellant made with fine RDX and GAP binder to give a homogeneous one-dimensional (1D) flame. The addition of GAP adds the complexity of additional chemistry to the already developed and validated RDX kinetic mechanism [20 –24] without adding the complexity of multidimensionality to the model. Modeling of GAP combustion has begun [25]. Modeling efforts of RDX/GAP propellant are underway and appear promising [26]. Experimental studies were done with RDX/GAP mixtures using a triple quadrupole mass spectrometer [18]. The RDX particle size of these uncured propellants was on the order of 200 m. However, this length scale is of the same order as the RDX self-deflagration flame standoff, so the flame structure would be more like *Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 127:1895–1905 (2001) © 2001 by The Combustion Institute Published by Elsevier Science Inc.
2D diffusion flamelets than a 1D pre-mixed flame. Initial studies of propellants containing only RDX and GAP showed some interesting results [27]. It was concluded that a plasticizer had to be used to increase the oxygen, thereby reducing char formation. 1,2,4 butanetriol trinitrate (BTTN) was chosen as the plasticizer. We reported [27] that cured GAP and uncured GAP have different combustion properties. Cured GAP ignites poorly and can even extinguish the igniting torch. Uncured GAP ignites rapidly, after a heating period, and burns violently. Advanced laser diagnostics, such as saturated planar laser-induced fluorescence (PLIF) and spontaneous Raman spectroscopy, as well as UV/visible absorption spectroscopy and micro thermocouples were used to quantify the micro flame structure of cured RDX/GAP/BTTN propellants via species and temperature profiles. Although there is no strong diffusion flame between RDX and GAP, there are still important effects. For example, it is known that the mixed HMX/GAP propellant burning rate is below that of either HMX or GAP [16]. Effects such as these must be understood on a mechanistic level if modeling is to help formulation of future propellants. Measurements of thermal diffusivity and specific heat capacity were also made for use in modeling efforts. Burning rates as a function of pressure and initial temperature have been measured and will be reported in the near future (Alice Atwood, Pat Curran, Tri Bui, Donna Hanson–Parr, and Tim Parr). 0010-2180/01/$–see front matter PII 0010-2180(01)00296-6
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T. PARR AND D. HANSON–PARR
Fig. 2. Apparatus diagram for ultraviolet/visible absorption spectroscopy.
Fig. 1. Apparatus used for planar laser-induced fluorescence imaging.
EXPERIMENTAL The cured RDX/GAP/BTTN propellant samples were generally about 10 mm in diameter, cut out from a large block. The samples were 71% by weight RDX, 9% GAP polyol, lot L-12369 from 3MTM Corporation, 0.47% HMDI (hexamethyene diisocyanate, aka HDI, aka N-100) curative, 19.51% BTTN plasticizer, and 0.02% T-12 (dibutyl tin dilaurate) catalyst. The military grade RDX was 70% by weight class 5 (mean particle size of about 17 m) and 30% 1.7 m RDX. The propellant was somewhat soft (easy to cut) and was bright yelloworange. The samples were fuel rich. Optical diagnostics applied to the flame included saturated planar laser-induced fluorescence (PLIF) imaging of OH and OH rotational temperature as well as CN and NH, ultravioletvisible (UV-vis) absorption spectroscopy for NO, NO2, CN, NH, OH, and H2CO, and spontaneous Raman spectroscopy for the majority species, HCN, CO, N2, and H2O. Raman can also detect C2H2 and H2CO, but upon analysis of the results, the concentrations of these species were found to be ⬍ 1 mole%. A springloaded sample holder, with a tungsten-rhenium wire stretched taut across the sample, was used in the PLIF and Raman experiments. The PLIF setup used is shown in Fig. 1. Typical laser energies were about 25 mJ/pulse for the nominal 6 ns pulses at 20 Hz. The laser sheet thickness was less than 100 m. The flame structure was imaged in separate experiments in air at 0.093 MPa (0.92 atm), with no added CO2
laser energy, using PLIF just after ignition. Because the flame height was substantially less than the sample diameter, and because the centerline profiles were measured, the oxygen in the surrounding air cannot penetrate and affect the flame. Indeed, no difference was seen in experiments done in nitrogen. For OH, the PLIF laser pumped the R1(3), R1(10), and R1(14) lines of the (1,0) A2⌺⫹-X2⌸ OH band, near 281.5 nm. An averaged OH PLIF image was obtained by summing five separate images taken in sequence from the same sample. Temperature was obtained from Boltzmann population distribution plots for each pixel in the image as discussed in [4]. A temperature profile as a function of height off the surface was obtained from the temperature image by averaging across a narrow (flat) region of the flame in the horizontal direction. PLIF was also used to image NH and CN. The CN radicals were monitored by pumping the (0,0) bandhead of the B-X transition near 388.3 nm while monitoring (0,1) emission around 422 nm. The NH was monitored by pumping the (1,0) Q branch head of the A-X transition near 305.03 nm while monitoring (0,0), (1,1) emission at 336 nm. These images were placed on an absolute scale using results from PLIF measurements done on HMX, and from UV-vis absorption, as discussed below. UV-visible absorption spectroscopy was used to obtain absolute concentrations [5] by mapping out profiles point-by-point. A detailed discussion of this technique and subsequent analysis of the data obtained is given in [3]. Figure 2 shows an apparatus diagram for this technique. A 150W Xe arc lamp source, single pass, was used to obtain concentrations of NO, NO2, CN, NH, OH, and H2CO. The f number of the spectrograph (f4.5) was matched by placing the
RDX/GAP/BTTN PROPELLANT FLAMES cylindrical lens the proper distance from the spectrograph slits. Because the focal point of a lens is wavelength dependent, the lens upstream of the sample had to be moved to refocus the lamp over the sample in the flame. Video imaging (VCR camera) was done for each sample piece of the material burned to observe the flame structure and to measure height off the surface and path length. The spectral resolution was about 1.2Å at 230 nm, and the spatial resolution was determined to be 24 m, but was also dependent upon the time frame of the acquisition. Except where noted, all UV-visible absorption experiments with the cured propellants were done in a bomb with nitrogen at 0.10 MPa pressure. A CO2 laser at about 400 Watts/cm2 was used to ignite the samples in air at 0.093 MPa, and upon ignition, the CO2 laser was turned off and nitrogen was allowed to flow into the bomb. The bomb was essentially a cube with three 63.5 mm (2.5 inch) diameter sapphire windows for optical access and a 12.7 mm (0.5 inch) diameter ZnSe window for CO2 laser access. The bomb volume was about one liter. It took approximately 1 s to completely displace all the air in the bomb (0.10 MPa N2 inside the bomb), so only data after that time is presented. The CO2 laser timing was controlled by a digital delay generator, which triggered the CO2 laser at time zero, also the start of spectra acquisition. In some cases, species were imaged in an HMX flame, for which the concentrations have been measured [5]. The signal levels from HMX combustion and the RDX/GAP/BTTN flame results, with the same detector conditions, were compared. Absolute concentrations could be measured this way only if the quenching processes were the same. However, the transitions are all saturated, which largely removes the effect of quenching, so this method of absolute concentration measurement is not subject to errors as large as one would first imagine. Another technique used to obtain absolute concentration profiles, spontaneous Raman spectroscopy, was used previously to measure species profiles in XM39 (an RDX-containing propellant) flames [6,28]. This technique was fully discussed in [28]. Numerous species can be detected in one spectrum. For each species, the Raman signal appears shifted from the exciting
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Fig. 3. Apparatus experiments.
layout
for
spontaneous
Raman
laser beam frequency (here, 532 nm) by the vibrational frequency in the case of vibrational Raman. The apparatus diagram for this technique is shown in Fig. 3. Temperature measurements were also done with embedded thermocouples in self-deflagrating samples. To embed the 75-mm diameter bead platinum-platinum 10% rhodium thermocouple, a cylinder of propellant was cut into two pieces using an angled razor blade. The bottom piece had a ridge across its centerline, where the thermocouple junction was placed. The thermocouple leads fell away from the bead at a 10° angle. The top half of the propellant cylinder was epoxied onto the lower half around the edges, without getting epoxy on the thermocouple, except outside the cylinder, where epoxy was dabbed on the leads to prevent premature burn-through. Voltage-time traces were measured and converted to temperature-time traces post measurement. Thermal diffusivity and specific heat capacity measurements were made as described in the literature [29]. By comparing pre-test to posttest thicknesses (L), it was seen that the samples readily flattened. Vapors were observed at temperatures above about 140°C and the samples turned brown around the edges and disintegrated. At 148°C the sample had disappeared completely, and no measurements could be made; the sample appeared to have cooked off and a scent was observed. Numerous different samples, each of different thickness were used at room and elevated temperature to get an idea of reproducibility and to judge the uniformity of the particle distribution in the propellant. The thickness could be measured to within about 20 m,
1898 temperature rise to about 0.01°C, and t1/2 to about 10 ms. The stiffer the sample (i.e., the less it compressed when clamped down upon) the more accurately the thickness could be measured post-test. Based on the uncertainties in L and t1/2, a ⫾ 2% variation for the thermal diffusivity would be typical; however, it is believed that the largest source of variation is because of the sample uniformity, including contact between crystals and the rest of the matrix. Several references were used for the specific heat capacity (cp) measurements, to make sure that the heat input (H) into the sample was well-characterized. H ⫽ cpL⌬Tmax, where is the density, and ⌬Tmax is the maximum temperature rise of the sample. The references used were: Teflon™, HDPE, and Al. Although the thermal diffusivity of Al is too large to measure with the apparatus used here, the temperature rise of the sample is representative of the amount of heat input into the sample. Teflon™ cannot be used as a reference at room temperature because it undergoes a crystal-crystal transition at about 25°C. HDPE can only be used below about 80°C. At room temperature, the heat input obtained for Al and HDPE agreed to within ⫾ 15%. The thermal diffusivities measured for these references matched literature values (except for Al). The used to obtain cp for the RDX/GAP/BTTN propellant was given as 1.688 g/cm3, obtained from the NAWC thermochemical code, PEP. Original sample thicknesses were from 500 to 800 m and postexperiment thicknesses ranged from about 200 to 500 m. ⌬Tmax was generally about 0.5°C to 1°C, depending on the sample thickness. The variation in measured cp would be about ⫾ 10%, since the literature values of cp for the references also contain some uncertainty. The thermal conductivity (k) could then be calculated:
T. PARR AND D. HANSON–PARR
Fig. 4. Thermal diffusivity of RDX/GAP/BTTN propellant.
cp,mixture ⫽ ⌺iMicp,i, where Mi is the mass fraction of species i, and cp,i is the specific heat capacity of species i. From previous results of measured cp for RDX [29], and a GAP/BTTN binder (not the mixture ratio used here) [29], an estimate was calculated and plotted on Fig. 5 (normalized at 20°C). These values of cp appear to be at the lower boundaries of what was currently measured, but essentially within the data scatter, except for the highest temperature. The law of mixtures cannot be used to estimate thermal diffusivity or conductivity because these properties depend largely on the degree of contact between constituents. Nevertheless, thermal diffusivity values that were measured were similar to those for pure RDX, especially at the higher temperatures. At low temperature, the thermal diffusivity was about 20% larger than that for pure RDX. Values of thermal conductivity calculated from the ␣ and cp data are shown in Fig. 6. Fits
k ⫽ alpha*cp* (J cm-1 s-1 deg-1)
RESULTS AND DISCUSSION Thermal diffusivities are shown in Fig. 4 and the specific heat capacity results are shown in Fig. 5. The law of mixtures can be used to estimate cp:
Fig. 5. Specific propellant.
heat
capacity
of
RDX/GAP/BTTN
RDX/GAP/BTTN PROPELLANT FLAMES
Fig. 6. Thermal conductivity of RDX/GAP/BTTN propellant obtained from data of Figs. 4 and 5.
of ␣, cp, and k versus temperature (°C) are shown on the respective Figures. Combustion Experiments At 0.65 MPa (94 psia), the samples ignited readily (in less than 0.2s) in N2 with a CN (seen in emission as a bluish flame sheet) standoff of about 2 mm. After the laser shut off and the sample deflagration reached steady state, the CN standoff was small, less than 0.5 mm. This also happened at 0.21 MPa (30 psia), with standoffs of 9 mm and less than 0.5 mm for laser-supported and steady-state deflagration, respectively. It took longer than 0.2 s, but less than 0.5 s to ignite the sample at 0.21 MPa. At 0.12 MPa (17 psia) N2, the CO2 laser drilled a crater into the sample before ignition could be established; because the sample height was short, the sample extinguished after the crater formed. The samples could be readily ignited in air, however, (CN standoff about 9 mm), and then burned steady-state in an N2 atmosphere (CN standoff about 2–2.5 mm), as described in the experimental section. Therefore, this procedure was followed to measure species profiles at atmospheric pressure (of N2) for which the flame standoff is sufficient to obtain meaningful data. Very little char was seen on the surface at 0.65 MPa, with more char seen as the pressure decreased. No char “snakes” were formed, unlike results from 50%/50% RDX/GAP cured propellants that had been examined earlier [27]. Unlike cured 70%/30% RDX/GAP propellant [27], it didn’t appear that any flamelets were
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Fig. 7. PLIF [CN], [NH], and [OH] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The absorption measurements done to scale these profiles were done at 0.1 MPa in N2.
holding on the char on the surface. The char on the surface did not burn when lit with a torch. A reproducible char ring was seen to form about 1 s after ignition. Little or no smoke was seen except for a minor amount when the sample extinguished near burnout. This was in stark contrast to the 50%/50% propellants which produced copious amounts of smoke so that no data could be obtained shortly after ignition [27]. Species and temperature profiles from PLIF, UV-vis, and Raman measurements are shown in Figs. 7-14. Measurements of [H2CO] were done with both UV-vis absorption and Raman. Whether laser-supported or self-deflagration, the [H2CO] present was below the detection limits of either apparatus, less than 1 mole%. The [CN], [NH], and [OH] profiles from PLIF were put on an absolute scale using previous results from HMX self-deflagration by scaling the intensity, as described in the experimental section. Peak concentrations were 170 parts per million (ppm) for CN, 150 ppm for NH, and 1.4 mole % for OH. The [OH] from the PLIF results agreed with that from UV-vis absorption. Somewhat lesser concentrations compared with the PLIF results were obtained from the UV-vis absorption results for CN and NH. As viewed in 3D, CN and NH are formed in a shell around the sample, so if the lamp beam is sampling a region below the peak of the [CN] or [NH] shell, then it is going through two edges. The cross-section of each edge is essentially a (skewed) Gaussian profile in concentration (see Fig. 7). Integration over the PLIF
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Fig. 8. [NO] and [NO2] profiles from UV-vis absorption for self-deflagration of RDX/GAP/BTTN propellant at 0.10 MPa in N2.
Gaussian profile was compared with the integrated results from UV-vis absorption to obtain peak [CN] and [NH] of 120 and 110 ppm, respectively. The CN and NH profiles are nearly identical in shape and concentration and peak about halfway up the rising edge of the [OH] profile. [NO] absorption results are seen in Fig. 8 together with [NO2] results. The NO2 concentration profile is actually from a laser-assisted experiment, which was then scaled to the flame height of the self-deflagration experiments. The [NO2], therefore, is possibly three times what it would be for no CO2 laser-support [4]. For self-deflagration, the [NO2] was too close to the surface to measure with absorption spectroscopy; this conclusion was made after many attempts to measure [NO2] under these conditions. Thermodynamic equilibrium calculations show that [NO] should be about 0.12 mole% in the burnt gas region. Many UV-vis absorption experiments were done to obtain the [NO] profile, especially near the surface. The points from 0 to 0.9 mm are averages of multiple determinations. Starting from 0 mm, and going in steps of 0.05 mm, up to 0.9 mm, data points were averaged ⫾ 0.05 mm, standard deviations were obtained, and are shown in Fig. 8. The NO concentration appears to be increasing from 0 to 0.8 mm, but within the error bars could also be flat. The magnitude of the error bars arises mostly from uncertainty in the path length, temperature, especially at positions above the surface where it is steeply
T. PARR AND D. HANSON–PARR
Fig. 9. Temperature profile for self-deflagration of RDX/ GAP/BTTN propellant. The solid line is the temperature obtained from PLIF OH rotational temperature, the dots are from fitting the NO UV-vis absorption spectra, and the dashed line is from thermocouple measurements, corrected for thermal effects, all at 0.093 MPa in air.
rising, and baseline noise [5]. As discussed in [5] typical uncertainties in mole fractions would be 15% near the surface to 25% nearer to the secondary flame front. The temperature profile (Fig. 9) is a combination of the OH rotational temperature profile from PLIF results, NO vibrational temperature obtained from fitting the UV-vis data, and results from thermocouple-measured temperature, corrected for lead heat loss and thermal lag. The equations of [30] were used to obtain the fit to the thermocouple trace. The gas phase temperature used was that from the NO absorption results. The thermocouple errors (thermal lag and heat loss) must be removed to recover an accurate condensed phase temperature profile. It is much easier to do this in a forward instead of a backwards analysis. A temperature profile is assumed throughout the domain. Then the thermal lag and heat loss simulation is performed on this temperature profile to recover what the thermocouple would have measured. This is compared with what the thermocouple actually measured and the initial temperature profile guess modified to minimize the difference. The ‘guess’ temperature profile is constructed from the actual measured gas phase temperature above the surface (from NO absorption and OH PLIF) coupled to a condensed phase profile simulation based on an assumed surface temperature parameter, the measured thermal dif-
RDX/GAP/BTTN PROPELLANT FLAMES
1901
Fig. 10. Spontaneous Raman [HCN] and [CO] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The mark on the right axis shows the adiabatic thermoequilibrium value for [CO].
Fig. 11. Spontaneous Raman [N2] and [H2O] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The marks on the right axis show the adiabatic thermoequilibrium values.
fusivity of the propellant as a function of temperature, and a constant diffusivity fit parameter for regions outside the measured temperature range. The simulation assumed a non-reactive condensed phase. The two fitting parameters, that is, the assumed surface temperature and the diffusivity outside the measured temperature range, were varied to get the best fit to the actual measured thermocouple temperature. The gas phase part of the actual temperature was never altered, and the gas phase heat transfer coefficient was calculated based on the relevant Reynolds and Nusselt numbers. The heat transfer coefficient in the condensed phase came from prior measurements [31]. The best fit was for a true surface temperature of about 605 K, compared with the raw measured value of 575 K. The best fit to the subsurface temperature trace was obtained for a thermal diffusivity of 8 ⫻ 10⫺4 cm2/s for temperatures above 120°C, using measured values (Fig. 4) below 120°C. The [OH] profile follows the temperature profile quite closely. Similar relationships were seen for RDX self-deflagration at 0.093 MPa in air [4]. Spontaneous Raman results for RDX/GAP/ BTTN are shown in Figs. 10-14. Besides [CN], measured with PLIF and UV-vis absorption spectroscopy, the other carbon-containing species profiles measured in the flame were [HCN] and [CO] (Fig. 10). [CO2] can be measured with Raman, but the C2* emission in the flame was rather high in intensity and could not be ade-
quately removed from the Raman signal by subtracting off the signal obtained when the laser was y-polarized [28]. [HCN] is highest at the surface and decays exponentially. [CO] is present at the surface at about 15 to 20% and grows to its thermodynamic equilibrium value of 29.4%. The nitrogen-containing species in the flame included [CN] and [NH] (Fig. 7), and NO2 and NO (Fig. 8), measured with PLIF and UV-vis absorption spectroscopy, as well as HCN (Fig. 10), N2O, and N2. The [N2] profile measured with Raman is shown in Fig. 11. The [N2] profile matches the [CO] profile rather closely and grows toward its equilibrium value of 27.7%. The N2O signal appears at about the same
Fig. 12. Spontaneous Raman [H2] profile for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa in air. The data points have large uncertainties associated with them. The mark on the right axis shows the adiabatic thermoequilibrium value.
1902
Fig. 13. [N2O] and [CO2] profiles for self-deflagration of RDX/GAP/BTTN propellant at 0.093 MPa derived from atom balance. The mark on the right axis shows the adiabatic thermoequilibrium value for [CO2].
wavelength region as CO2, and for reasons discussed above for CO2, could not successfully be measured. It is estimated that the N2O concentration is at most a few percent, based on the atom balance results. The hydrogen-containing species in the flame are HCN (Fig. 10), NH and OH (Fig. 7), H2O, H2, and H-atoms. The [H2O] profile obtained with Raman is shown in Fig. 11 and is seen to grow to its equilibrium value of 18.8%. [H2] can be measured with Raman, but in these experiments, the detector used is about half as sensitive in the wavelength region where the H2 signal appears than where N2 appears, but the cross section for H2 is about twice that for N2. However, at elevated temperatures, the nitrogen Q-branch Raman is still a relatively narrow peak, while the hydrogen Q-branch Raman signal spreads out significantly, reducing the
Fig. 14. Atom balance at 0.093 MPa as a function of height off the surface, including [H2], [CO2], and [N2O].
T. PARR AND D. HANSON–PARR peak value and making it hard to distinguish from the noise. It is also in this region where the noise from the power supply is worse. (See [28] for discussion of sources of noise.) Our direct measurements were unable to quantify only two major and one minor species: H2 and CO2 are the major species and N2O the minor. The reason is the hostile nature of this flame. The high fuel level leads to soot and soot precursors in the flame. LIF from C2 (Swan band) interferes with the CO2 and N2O Raman measurements. Also, char buildup on the surface limits the time over which Raman can be integrated. This increased the noise level of all species, but seriously impacted the ability to detect H2, as the detector has poor sensitivity in the near IR where the H2 Raman appears. Atom balance calculations were undertaken in an attempt to verify that results obtained were consistent with reality. All atoms that were in the starting material (propellant) will be in the flame species and therefore, have to be accounted for (neglecting the small mass left in the char). The sum of the mole fractions at a given position in the flame should sum to one. The assumption made was that the flame species present at a given position were: HCN, CO, CO2, N2, N2O, H2O, H2, and NO. Any H-atoms (equilibrium thermochemical calculations show about 3 mole% of H-atoms) would therefore, be lumped together with H2. 16.1% of the atoms are C, 30.4% H, 25.1% N, and 28.4% O. If there are unmeasured large un-dissociated fragment molecules in the flame, especially near the surface, this atom balance approach will not work, but may be close if the fragment molecules constitute only a few mole%. Large molecules do exist near the surface, like the nitrosamine seen in [3]. RDX vapor may also be present; Fig. 15 of [3], attributed to RDX absorption, may actually be because of the nitrosamine (“unknown”). If Fig. 15 of [3] is actually RDX, then for a surface temperature of 605 K, [RDX] is only about 0.6 mole % at the surface, or at least as close to the surface as the measurement spatial resolution allowed. An estimate of the nitrosamine concentration near the surface is possibly 1%, but more work needs to be done for that species. The atom balance will also be in error if species diffusion is significant. Light species such as H2 and H may
RDX/GAP/BTTN PROPELLANT FLAMES rapidly diffuse axially or outwards radially. Since for each position measured, the mole fractions add up to one within a percent, we don’t think that the latter is too large of a problem. Still the failure of atom balance because of differential diffusion of light molecules could affect the accuracy of the species concentrations calculated from atom balance (N2O and CO2). The following equations were used in a least squares fit to recover the H2, CO2, and N2O mole fractions based on atom balance criteria: 1. mole fractions Propellant 3 c1HCN ⫹ c2CO ⫹ c3N2 ⫹ c4H2O ⫹ c5NO ⫹ c6OH⫹ c7H2 ⫹c8CO2 ⫹ c9N2O ⌺ici ⫽ 1.0 2. 3. 4. 5.
Carbon atoms: (c1⫹c2⫹c8)/n ⫽ 0.161 H atoms: (c1 ⫹ 2c4⫹ c6 ⫹2c7)/n ⫽ 0.304 N atom: (c1 ⫹ 2c3 ⫹ c5 ⫹ 2c9)/n ⫽ 0.251 O atoms (c2 ⫹ c4 ⫹ c5 ⫹ c6 ⫹ 2c8 ⫹ c9)/n ⫽ 0.284
n ⬀ sum of atoms ⫽ (c1⫹c2⫹c8)⫹( c1 ⫹ 2c4⫹ c6 ⫹2c7)⫹( c1 ⫹ 2c3 ⫹ c5 ⫹ 2c9)⫹( c2 ⫹ c4 ⫹ c5 ⫹ c6 ⫹ 2c8 ⫹ c9) ⫽ 2(⌺ici)⫹(c1 ⫹c4 ⫹c8 ⫹c9) ⫽ 2 ⫹ (c1 ⫹c4 ⫹c8 ⫹c9) The Minerr (least squares) function of MathCAD™ was used to solve for the mole fractions of H2, CO2, and N2O, using the fits to the data shown in Figs. 8, 10 to 13, and the OH concentration profile in Fig. 7. Figures 12 and 13 show the results. Figure 12 also includes a comparison with the measured H2 mole fractions. As stated above, the atom balance H2 result in Fig. 12 actually includes any H atoms as well, which adds a few percent in the hotter parts of the flame. Still, the measured H2, although noisy, is slightly below the atom balance results in the mid regions of the flame. Further work may be undertaken to minimize noise sources for H2 Raman and the H2 mole fraction may be measured again more accurately. Figure 14 shows the resulting atom balance plots. The consistency of the data with atom balance is quite good except very near the surface where measurements are difficult. It should be noted that although H2 and CO2 were calculated from atom balance, as well as
1903 N2O which is small, this does not guarantee perfect atom balance given all the species that are measured. H2 has a very high diffusion coefficient and the diffusion of H2 either radially or axially could change the [H2] in the flame. If the amount of diffusion is large, it would significantly affect the atom balance results. The coefficient c7, and n of Eq. 1 through 5 would decrease. Therefore, the atom balances presented do not necessarily have to be valid, and the species calculated from atom balance, such as N2O and CO2 could be in error. Nevertheless, the atom balance results give us some degree of confidence in the validity of our measurements. Figure 13 contains the atombalance-derived concentration profiles of N2O and CO2 based on the atom balance fits to the data. CO2 in the burnt gas region should be 6.3%, in agreement with the atom balance results. N2O is never larger than 4%. RDX versus RDX/GAP/BTTN Propellant The idea of using small particle size RDX was to get an approximately premixed flame of binder and RDX as opposed to a diffusion flame. The differences in flame chemistry between the propellant and pure RDX should therefore, be because of the species produced by GAP/BTTN decomposition. Because of GAP decomposition, char forms on the surface of the propellant. Whereas the surface of RDX is liquid and bubbly, the propellant surface is much less so. GAP decomposition is known to produce substantial amounts (around 35 mole%) of N2 [10 –18]. The extra gasification rate from the GAP acts to push the flame further away from the surface, causing higher flame standoffs for the propellant than for pure RDX. For pure RDX, the CN flame sheet is about 0.5 mm above the surface, compared with about 2 mm for the propellant. High mole fractions of CO (20%) and N2 (18%) were seen at the surface of the propellant, compared with about 8% and 5%, respectively for pure RDX [8] at atmospheric pressure. Both [CO2] and [H2] were high at the propellant surface at about 11% and 15%, respectively, compared with only about 0 through 3% and 3%, respectively, for pure RDX. The [OH], [NH], and [CN] for the propellant
1904 were about half that for pure RDX. At the surface [H2O] and [HCN] were about the same for either material, but the [NO], [NO2], and [N2O] were about three times larger for pure RDX than for the propellant. At most, only a few mole% of H2CO were seen for either material [5,8]. The surface temperature (605 K) appeared to be the same for both RDX and RDX/GAP/BTTN propellant. Based on the different gas mole percentages at the surface of the propellant compared with pure RDX, one would expect to see differences in the flame chemistry for the two materials, especially in the nitrogen chemistry. SUMMARY An experimental propellant consisting of fine RDX, GAP, and BTTN was formulated to produce a pseudo one-dimensional homogeneous flame that would be a “simple” propellant flame to model. The goal was to use fine RDX with length scales much lower than the flame structure, making a pseudo-premixed propellant. This propellant would include the chemical complexity of interaction between energetic material and binder without the added complexity of multi-dimensionality brought on by heterogeneity and diffusion flames. Thus, the kinetics of interaction could be modeled with simpler 1D pre-mixed models. To maximize the solids loading, a bimodal distribution of RDX was necessary, but even the larger particle-size fraction was kept below the flame structure length scales to try to maintain a pseudo homogeneous pre-mixed flame. PLIF, UV-visible absorption spectroscopy, spontaneous laser Raman spectroscopy, and micro-thermocouples were used to map out species and temperature profiles above the surface of this propellant as inputs for the modelers. HCN, CO, and N2 were found to be the major species near the surface, and CO, N2, and H2O in the burnt gases. The surface temperature was determined to be 605 K, and a dark zone of about 1200 to 1300 K was observed in which the NO concentration was at its highest. NO2 existed only very close to the surface. No formaldehyde was observed in the gas phase at measurable distances off the surface.
T. PARR AND D. HANSON–PARR The results were consistent with atom balance when species that were not quantified (CO2, N2O, and H2) were taken into account. Results in the burnt gases largely matched thermochemical calculations so the profiles tended to the right boundary conditions. These species and temperature profiles and the values for thermal conductivity and specific heat capacity presented here will be helpful to modelers to validate detailed kinetic 1D models of the deflagration of this model RDX/GAP/BTTN propellant. This is a step in the direction of developing models that can a priori predict ballistic properties of real composite solid propellants. We gratefully acknowledge the support of Dr. Judah Goldwasser of ONR, Dr. May Chan for the propellants and patience, and Alice Atwood and Pat Curran for making ballistics measurements on this propellant.
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Received 21 February 2001; revised 28 June 2001; accepted 29 June 2001