Flame Structures of Ammonium Perchlorate Composite Propellants with Various Coarse-to-Fine Particle Size Ratios

Transcription

1 Paper # 070HE-0214 Topic: Heterogeneous Combustion 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 Flame Structures of Ammonium Perchlorate Composite Propellants with Various Coarse-to-Fine Particle Size Ratios S. Isert 1, T.D. Hedman 2, K.Y. Cho 3, R.P. Lucht 3, and S.F. Son 3 1 School of Aeronautics and Astronautics, Purdue University, West Lafayette, IN USA 2 Naval Air Warfare Center Weapons Division, China Lake, CA 93555, USA 3 School of Mechanical Engineering, Purdue University, West Lafayette, IN USA The microscopic flame structure of a composite propellant is expected to change significantly as the particle size distribution is varied. However, until recently it has not been possible to observe the flame structure in a burning composite propellant. In this paper we report observations of the diffusion flame structures above ammonium perchlorate (AP) composite propellants with varying ratios of coarse AP and fine AP. The coarse-to-fine (C/F) ratio was varied between 1:16 (mostly fine AP) to 16:1 (mostly coarse AP). Flame structures were imaged at atmospheric and elevated pressures using 5 khz OH planar laser induced fluorescence (PLIF) that allows imaging of these highly transient microscale (100s of microns) flames. Single AP particle ignition delay, burning rate, and lifetime were measured at one atmosphere. While the coarse AP particle lifetime is similar across C/F ratios, the coarse particle ignition delay and the percent of coarse crystal lifetime spent in ignition delay differ between the 1:16 C/F ratio and higher C/F ratio propellants, indicating that nearby particles affect ignition. Jet-like diffusion flames are seen for all C/F ratio propellants at one atmosphere. At elevated pressures, lifted inverted overventilated diffusion flames (IOFs) are seen for all C/F ratio propellants except the lowest, for which jet-like flames are observed. Differences in the flame structures at elevated pressures are postulated to be due to the dissimilar local burning rates of the fine AP/binder matrix (or dirty binder ) as C/F ratios change. Interacting diffusion flames of adjacent coarse AP particles were observed and indicate group combustion. This was observed at all pressures and C/F ratios, except for the 1:16 C/F ratio propellant. Group combustion is more often observed for higher C/F ratios. We present examples of group combustion of particles that are typically not considered in simple models. It is found that at one atmosphere the largest effect of group combustion is on ignition delay. There are also effects on flame height due to group combustion of crystals. At elevated pressures, group combustion is also suspected to distort the bowl-like shape of the lifted IOF. 1. Introduction Composite ammonium perchlorate (AP) propellants typically use a multimodal AP particle size distribution. Having a combination of large and small particle sizes allows for tailorability of propellant burning rates, higher solid loadings, and ensures acceptable propellant rheological properties for filling of propellant molds [1,2]. However, until recently it has not been possible to directly image the flame structure, and the change of flame structure with coarse-to-fine (C/F) ratio could not be directly explored. Recently, high-speed OH PLIF has been successfully applied to solid propellants with a 1:1 C/F ratio [3-6]. Selection of particle size distributions, specifically the ratio of the amount of coarse (hundreds of microns) AP to the amount of fine (tens of microns) AP in the propellant mix, is an essential aspect of composite propellant formulation. Previous studies on the effects of C/F ratio indicate that there are differences in burning rate as the C/F ratio changes [7-9]. Atwood et al. [7] studied propellant ignitability using a laser as the ignition source. The C/F ratios varied between 1.5 and 8.9. It was found that, in general, the time to both first gasification and complete ignition are shorter in propellants with reduced C/F ratios. Additionally, when the mean diameter of the coarse AP particles increased from 200 µm to 400 µm, the time to complete propellant ignition increased as well. Price et al. [8] investigated leading edge flames by relating the burning rates of a range of bimodal composite propellants to the inferred transition of the burning mode from premixed to leading edge burning. Propellant C/F ratios included 7:3 and 4:1. The burning rates of the propellants were very similar at the lowest pressure (~0.4 MPa), but as pressure increased the 7:3 C/F ratio propellant burned faster and had a larger step in burning rate sooner than the 4:1 C/F

2 ratio propellant. The authors attribute these results to the fine AP/binder matrix in the 7:3 C/F propellant being less fuel rich, leading to a shorter premixed flame standoff distance and more heat flux back to the burning surface. Previous work on composite propellant flame structures using 5 khz OH PLIF has examined only propellants with 1:1 C/F ratios [3-6], which is not a typical C/F ratio for solid propellants but was chosen to yield less coarse particle interaction in the initial studies. As the C/F ratio increases beyond this point, or as the coarse particles become more numerous and closer together, it is expected that flame structure, ignition delay, and AP particle regression rate will change. These changes are expected to be due to the effects of group combustion of the large AP crystals. Also, as a result of the lower percentage of fine AP mixed with the binder (or the dirty binder ) the combustion is expected to change; that is, the stoichiometry of the dirty binder will change with C/F ratio. Conversely, as the C/F ratio decreases, flame structure, ignition delay, and AP particle regression rate are expected to change due to the lack of interaction between coarse particles, especially as they become very sparse. The objective of this work is to compare AP crystal ignition delay, lifetime, diffusion flame height, and diffusion flame structure between C/F ratios at 1 atm, and diffusion flame height and structure of lifted inverted overventilated diffusion flames (IOF) at elevated pressures. As many different C/F ratios are used in current propellants, examining the differences in the diffusion flame structure will assist in further understanding of how the C/F ratio impacts propellant performance. Additionally, further understanding of how flame properties and AP crystal response vary with C/F ratio will assist researchers in modeling composite propellants. 2. Methods 2.1 Sample preparation The propellant samples were a bimodal mixture of Table 1. Propellant sample formulation. nominally 400 µm AP and 20 µm AP with a solids loading of 80 wt%. Particle size distributions for the coarse and fine AP Coarse-tofine Ratio µm AP µm AP % 400 % 20 used in this study are given in Hedman et al. [3]. The coarseto-fine ratios chosen were 1:16 (Propellant 1), 7:13 (Propellant 1 1: Propellant 2), 3:1 (Propellant 3), and 16:1 (Propellant 4). Hydroxylterminated polybutadiene (HTPB) binder made up the 3 3: : remaining 20 wt% of the formulation, with small percentages 4 16: of plasticizer, curative, and bonding agent added to the Note: For all C/F ratios, the binder formulation is: prepolymer. Ingredient percentages for each formulation are R-45M: 14.58% shown in Table 1. All propellants except for Propellant 1 Desmodur E744: 2.30% propellant were hand mixes. The large percentage of fines in Isodecyl Pelargonate: 2.92% the Propellant 1 required mixing in a LabRAM Resodyn Tepanol HX-878: 0.20% resonant mixer to ensure a thorough dispersion of the fine AP in the binder. The AP/HTPB was cured in 6.35 mm diameter cylindrical pellets at room temperature. 2.2 Surface Imaging High-speed imaging of the burning propellant surface was performed to observe the propellant surface morphology. An Infinity K2 microscopic lens was attached to a Vision Research Phantom 7.3 high-speed camera, yielding a resolution of approximately 102 pixels/mm. A 1000 W mercury xenon lamp (Newport 66921) was used for illumination of the propellant surface. Video was taken at 1000 fps. 2.3 PLIF system Figure 1. PLIF system setup. For some atmospheric pressure burns a sample pedestal takes place of the pressure vessel. A high speed (5 khz) OH PLIF system, shown in Figure 1, was used to determine the flame structure, ignition delay, and regression rate of individual and grouped coarse AP particles. The high pulse repetition rate of the PLIF system 2

3 allows the very transient microstructure of the diffusion flame between the oxidizer and the binder to be observed in situ. The PLIF system has been described in detail previously [3,4]. Briefly, a Credo dye laser is pumped by an Edgewave solid state Nd:YAG laser. The output is a frequency doubled UV beam with a pulse duration of 7.8 ns. The laser sheet was created by expanding the output UV beam through negative lenses and then directing it through a positive cylindrical lens. The wavelength was tuned to nm to excite the Q 1 (7) OH transition, which causes OH fluorescence at 310 nm. A camera assembly consisting of a Vision Research Phantom 7.3 high-speed camera, Video Scope International UV image intensifier (VS4-1845HS), Semrock interference filter FF01-320/40-25 with a transmission of 74% at 310 nm, bellows, and a UV grade lens (UKA Optics UV1054B 105 mm F/4.0 Quartz Lens) was used to capture images of the fluorescence. The propellant sample was placed on a platform or inside a pressure vessel. The pressure vessel is described in Hedman et al. [5]. A translating stage in the pressure vessel allowed for placement of the sample at the desired height. Feed through wires allowed ignition of the propellant using a loop of nichrome wire on the propellant surface. For elevated pressure burns, the pressures used were between 60 and 75 psig (5-6 atm). Flame heights were calculated as described in [3,6]. As the OH signal tends to taper off instead of stopping abruptly, an arbitrary signal count was chosen to represent where the flame stops. Flame height was measured from the propellant surface up to this signal count, which was kept constant for all experiments. The jet-like diffusion flames lend themselves well to this technique (Fig. 2A). The lifted inverted overventilated diffusion flames (IOF) were somewhat more difficult. For the lifted flames, flame height was measured up to an arbitrary signal count above the top surface. As in some cases the lifted IOF were tilted off of a horizontal line, the point on the surface where the flames were measured from was chosen to be the intersection of the Figure 3. 3D PLIF experimental setup. surface with a line perpendicular to the line connecting the top edges of the lifted IOF at the designated signal count (Fig. 2B). Earlier work reports flame height for a 1:1 C/F ratio propellant [3,6]. Some of the data were collected using a method known as 3D PLIF or volumetric LIF [10,11]. In this setup, which is shown in Figure 3, a rotating galvanometric mirror is placed in the beam path before the beam is expanded into a sheet. As the mirror rotates the laser sheet is scanned across the surface of the propellant; consequently, each image taken by the high-speed camera during a given sweep of the mirror is at a different position on the propellant surface. Due to the high pulse rate of the pump laser, this yields a series of images over a short time period. The images over a given sweep can then be converted into a time-varying pseudo-3d image of the propellant surface and the flame structure above the AP particles. 3. Results and Discussion 3.1 Visible surface imaging Figure 2. Techniques for measuring flame heights: A) for jet-like diffusion flames and B) lifted IOF. Visible surface imaging of the propellant was conducted at 1 atm to collect images of the coarse AP on the propellant surface as the surface burns. Typical results are shown in Figure 4A-C. The Propellant 4 surface image (Fig. 4D) is from a PLIF image, not from the surface imaging. 3

4 Figure 4. Surface of deflagrating AP propellants. A) Propellant 1 (1:16 C/F ratio); B) Propellant 2 (7:13 C/F ratio); C) Propellant 3 (1:3 C/F ratio); D) Propellant 4 (16:1 C/F ratio). In Fig. 4A the surface of Propellant 1 can be seen to have very few coarse particles. These particles are easily distinguished from the surrounding fine AP/binder matrix. Fig. 4B shows the surface of Propellant 2. More particles are present on the surface, but they are not as densely packed as those in Fig. 4C, which shows the surface of Propellant 3. The particles are even more closely packed in Fig. 4D for Propellant 4. None of this is particularly surprising, but it does give a sense of the particle packing in the propellants considered. In Fig. 4A-C, but particularly in Fig. 4B, one can see bright flecks between the coarse particles. Though these could indicate deflagration of the fine AP, review of the high-speed video indicated that these particles are strands of binder that do not appear to be burning to completion, but are instead agglomerating and being ejected from the surface. The brightness may be due to a broad, hot carbon emission. Also of note is that the bright spots are much less evident in the Fig. 4A. This could indicate that, due to the very large amount of fine AP in Propellant 1, the propellant is burning closer to the premixed limit and therefore more of the binder is being fully consumed. 3.2 PLIF results PLIF was performed at 1 atm and at 5-6 atm. Both standard PLIF and 3D PLIF were used for this experiment. With 3D PLIF a much larger section of the surface is visible due to the laser sheet being swept over the surface. This results in many more AP crystals being visible and thus a larger number of measurements were made. Future work will apply a shorter sweep to better resolve single coarse crystals and their diffusion flame structure. Figure 5 shows a typical PLIF sequence of an AP crystal on the surface of Propellant 1 from shortly after crystal protrusion onto the surface (Fig. 5A) through slightly before the crystal is completely Figure 5. A false color PLIF image sequence of an AP crystal burning on the surface of Propellant 1. The crystal is exposed on the propellant surface (A), the diffusion flame begins to form (B,C), the crystal notably regresses (D), and the diffusion flame ends (E). consumed (Fig. 5E). The white dashed line designates the propellant surface location. The images are 0.2 ms apart. Figure 6 shows a typical 3D PLIF scan across the surface of Propellant 2. Each image is 2 ms apart temporally and roughly 200 µm apart spatially. White dashed lines indicate the location of the propellant surface. It can be seen in Fig. 6 that the position and size of the AP crystals varies across the surface, as does the flame structure above the groups of crystals. This diagnostic tool will be particularly useful for investigating the structure of the lifted IOF, which will be reported in the future. Figure 6. A 3D PLIF false color image sequence of a sweep across the surface of Propellant 2 at 1 atm. 4

5 3.2.1 PLIF at 1 atmosphere Ignition delay and crystal lifetime Figure 7 shows ignition delay plotted against AP particle size for the four C/F ratios. Ignition delay was determined by measuring the time between the first exposure of the AP crystal on the propellant surface and when it began deflagrating, as determined by start of consumption of the crystal and the appearance of the diffusion flame above the crystal. The data has considerable scatter, presumably due both to the method of determining ignition delay and the widely varying conditions of the individual AP particles (proximity to other particles, size, preheating, etc.) before ignition. Also, there is simply a distribution of ignition times observed indicating a stochastic behavior that could be expected for this complex system. However, it can be seen that there are two main categories, Propellant 1 and Propellants 2-4. Ignition delay for Propellant 1 tends to be lower than for the other group; the average is 0.17 s with a standard deviation of around 0.05 s. Propellants 2-4 have an average ignition delay of 0.29 s with a standard deviation of about 0.08 s. Below about 375 µm, the two groups appear to have fairly similar ignition delays. As particle size increases, however, ignition delay for the higher C/F ratios increases much more rapidly than that of Propellant 1. A statistical analysis on the ignition delay based on extra sums of squares F-tests gives some interesting results. As expected from the graph, the ignition delay for Propellant 1 has a statistically significant different slope and trend than the other propellants. Propellants 3 and 4 Figure 7. Ignition delay vs. AP particle size as a function of C/F ratio at 1 atm. have the same regression function to a 95% confidence level, and the ignition delay trend for Propellant 2 is different from that of Propellants 3 and 4 only by a constant (that is, the slope is the same as for the other two cases). It would appear from these results that after a certain amount of coarse AP is added to the propellant the ignition delay vs. particle size trend takes on a constant slope. Eventually, after the C/F ratio becomes large enough, the ignition delay changes consistently with particle size regardless of the C/F ratio. Particle lifetime shows a slightly different trend. As can be seen in Figure 8, there is some difference between particle lifetimes for Propellant 1 and Propellants 2-4. The lifetime for Propellant 1 is somewhat lower than the others. A statistical analysis confirmed that Propellants 2-4 have essentially the same lifetime vs. crystal diameter trend to the 95% confidence level, but that the crystals in Propellant 1 do not share that trend. Previous experimental data for a 1:1 C/F ratio propellant shows very similar trends to those found for the Propellants 2-4 for both the lifetime and ignition delay. The reason for the shorter crystal lifetimes for Propellant 1 can be clarified by looking at a chart of the percent of life as ignition delay vs. particle size for the different Figure 8. AP particle lifetime vs. particle size as a function of C/F ratio at 1 atm. propellants (Figure 9). Percent of life as ignition delay was found by dividing the ignition delay for the particle by its lifetime. As can be seen in Fig. 9, the data for Propellant 1 have a different trend than those of Propellants 2-4. A statistical analysis shows that, for all C/F ratio cases and for all crystal diameters within those cases, the percent of life as ignition delay is essentially constant. Further analysis showed that the means for Propellants 3 and 4 are nearly equal. Since, as stated above, the crystal lifetimes of Propellants 3 and 4 are also essentially equal, the hypothesis of a threshold C/F ratio is bolstered as a clear change in behavior is observed. 5

6 Figure 9. Percent of life as ignition delay as a function of crystal diameter for different C/F ratios at 1 atm. The shorter crystal lifetimes of Propellant 1 can be directly related to the amount of the crystal lifetime that is spent in ignition delay. The percentage of life the crystals in the Propellant 1 spent in ignition delay is 54.7% compared to 79.9% for Propellant 2 and 73.7% for Propellants 3 and 4 meaning the coarse AP crystals began to burn 25-32% sooner in Propellant 1 than in the other propellants. The shorter ignition delay leads directly to a shorter lifetime. The reason for the shorter ignition delay can be determined by referring back to Figure 4. As can be seen in Fig. 4A the surface of Propellant 1 consists of few coarse AP crystals surrounded by large amounts of dirty binder (the fine AP/binder mixture between coarse crystals) with a high fine AP percentage. The mixture of fine AP and binder burns in a premixed fashion with temperatures estimated to be around 2150 K for a fine AP/binder ratio of 79/21. This hot, premixed flame closely surrounds the coarse AP crystal. Conversely, as the C/F ratio increases, the coarse AP will burn at the monopropellant flame temperature of 1377 K with some heat feedback from the hotter diffusion flame that sits farther above the surface. The coarse AP crystals in these cases may have the fine AP/binder flame on the edges of the crystals, but as the percentage of fine AP has become much lower the dirty binder s flame temperature will be much lower as well about K depending on the C/F ratio, according to NASA s Chemical Equilibrium Analysis code (CEA). The lower flame temperatures combined with the large thermal mass of the AP crystals and a dirty binder that, comparatively speaking, is not contributing much to the combustion will result in a longer amount of time required for the coarse AP crystals to heat up to the point at which they can burn. Flame Height Figure 10 is a plot of flame height vs. crystal system diameter for the various C/F ratios. The entire data range is shown in Fig. 10A, while flame heights for only individual crystals or crystal groups with group diameters under 650 µm are shown in Fig. 10B. Analysis of the data shows that for crystal sizes below about 750 µm the slope of the regression line is nearly constant, statistically speaking, for all C/F ratios, indicating that flame height differs only by a constant for the different cases. It is possible that below 750 µm the cases are essentially the same and only differ due to the scatter in the data. The scenario changes when the entire data set is considered. The data from Propellants 3 and 4 were found to have equivalent slopes but different intercepts, with Propellant 3 having a larger offset. Both data sets lie above that of Propellant 2, indicating that, for an equivalent crystal system diameter, a propellant environment with large amounts of coarse crystals will produce taller flames than an environment where there are many fine AP particles. Note that Propellant 1 is excluded from this discussion; no crystal group diameters larger than 766 µm were observed, and large variation in flame height between the other C/F ratio cases for a given group diameter does not Figure 10. Flame height vs. A) the entire data set and B) crystals up to 650 µm in diameter at 1 atm. 6

7 begin until crystal group diameters are approximately 1000 µm. The cause for the difference in flame heights between Propellant 2 and Propellants 3 and 4 is thought to be directly related to the fact that coarse AP crystals burn in more of a diffusion mode as opposed to the more premixed mode of fine AP/binder systems. With a large number of coarse AP crystals in a small area surrounded by a dirty binder that has fewer fines, the environment of the coarse particles is more oxidizer rich. There is the same amount of fuel in the higher C/F ratio propellants as in the low C/F ratio propellants; however, in the low C/F ratio propellants, the fuel and fine AP burn together in more of a premixed fashion, leaving less fuel to diffuse into the oxidizer jet produced by the coarse AP crystals. In the larger C/F ratio propellants, more fuel is available to burn with the coarse AP dissociation products, as the equivalence ratio of the dirty binder is higher for these propellants. The dirty binder in Propellant 2 has a lower equivalence ratio than that Propellants 3 and 4; that is, more of the fuel is consumed in the premixed reaction and there is therefore less fuel available to diffuse into the oxidizer jet produced by the coarse particles. The diffusion flames above the coarse AP particles are, therefore, shorter for Propellant 2 than for the larger C/F ratio propellants. Note that Propellant 1 is not being considered here, as there were no large crystal groups observed and therefore conclusions about group combustions cannot be made. It is interesting to note that the flame height only differs above the largest diameter crystal groups; as seen in Fig. 10A, the flame height does not seem to vary much with C/F ratio for a given particle size below about 650 µm. This could be due to the fact that, in these smaller systems, there is a limited amount of oxygen evolved from the dissociating AP crystal and despite the amount of fuel available the diffusion flame can only extend to a given distance above the surface. Figure 11 shows the jet-like flame structures above the surfaces of the different C/F ratio propellants burned at 1 atm. Figures 11A, 11B, and 11C show diffusion flames of grouped crystals over Propellants 4, 3, and 2, respectively. White dashed lines show the approximate position of the propellant surfaces Figure 11. Jet-like diffusion flames over A) 16:1, B) 3:1, C) 7:13, and D) 1:16 C/F ratio propellants at 1 atm. in these figures. Figure 11D shows the jet-like flame over a single crystal in Propellant 1. Due to the way the propellant burned this crystal is set in the middle of a face instead of on a clearly defined edge like the others; hence, there is no line indicating the surface position in Fig. 11D. It was found in previous experiments [3] that AP fluoresces when exposed to UV light; the red areas on the surface are therefore coarse AP crystals. In Fig. 11A it is interesting to note that there are two large crystal groups on the propellant surface and the flames are clearly combining. Later on in the propellant burn these groups combine to form an even larger and taller flame. This is expected because diffusion flame height is a function of diameter PLIF at elevated pressures Recently, lifted inverted overventilated diffusion flames (IOF) have been reported in the literature [5,6]. Hedman et al. [5] describe these flames as occurring when several recessed AP crystals burn faster than the surrounding dirty binder, resulting in an overventilated flame with sufficient mass flux to lift the diffusion flame. That is, for a short period of time, there is excess oxidizer locally from the coarse AP particle. Conversely, flames at one atmosphere are inverted jetlike diffusion flames as described in previous sections. The differences between these flames can be seen schematically in Fig. 12. Figure 12. Differences in causes of inverted, jet-like diffusion flames (A) and inverted, overventilated, lifted diffusion flames (B). Taken from Ref. [5]. 7

8 Figure 13. Flame structures of A) Propellant 4, B) Propellant 3, C) Propellant 2 and D) Propellant 1 at elevated pressures. White dashed lines indicate the propellant surface. The data reported by Hedman et al. [5,6] were from propellants with a 1:1 C/F ratio. As in practice C/F ratios tend more toward 2:1-4:1, it is important to examine the flame structures of propellant formulations closer to those of fielded propellants. Figure 13 shows the flame structures of the different C/F ratio propellants at pressure. Propellants 4, 3, and 2 (Fig. 13 A-C) displayed lifted IOF; however, at no time were lifted IOF seen for Propellant 1. From Fig. 13D it can be seen that, for Propellant 1, a coarse AP crystal is visible on the surface. This is not the case for the other propellants. Instead of protruding above the surface, at higher pressures the coarse AP particles tend to burn away faster than the surrounding surface if the C/F ratio is high enough. The reason for this lies in the burning rate of the fine AP/binder (or the dirty binder) matrix. A series of experiments was performed by Kohga [12] to determine the lower limit of AP content to burn with both coarse AP and fine AP. It was found that, as expected, burning rate increases with increasing AP content and decreasing particle size. Burning rates for propellants with 63% fine AP were in general about one third of those with 75% fine AP. Neither propellant had any coarse AP in it. In this study, Propellant 1 had about 75% fine AP mixed with the binder (see Table 1). The other propellants had 52% fine AP or lower. It can be seen from this that the burning rate of the dirty binder mix will be much lower for the higher C/F ratio propellants due to the smaller percentage of fine AP in the mix, causing the dirty binder to regress slower than the coarse AP particles and consequently form divots on the surface. At 1 atm the coarse AP particles burned at roughly 4 mm/s, and burning rate increases with pressure. Data from Kohga [12] show that the 69% fine AP propellant burned at 2 mm/s at 60 psi, a fine AP/binder matrix burning rate that is already slower than the coarse AP crystal burning rate at 1 atm observed in this study. This result, along with the fact that the fine AP content of the larger C/F ratios is much lower than that of the propellants used in [12], indicates that divots are forming at elevated pressures as Propellants 2-4 burn. Divots have been seen to form on the surface of a 1:1 C/F ratio propellant at elevated pressures by Hedman et al. [5]. The comparison between the work of Kohga and the current work is not exact, as the overall percentage of AP in the propellants of the current study was higher than that of those in [12] even when the amount of fines in the binder was low, probably accounting for the discrepancies in the overall burning rates between studies, but the trend of decreasing dirty binder burning rate with decreasing fine AP content is indicative. Based on these arguments we would expect that at higher pressures there would come a point where even Propellant 1 would exhibit IOF structures and corresponding divots. Flame height analysis for propellants at pressure continues, but results thus far seem to indicate that there is little difference in flame heights between propellants with different C/F ratios. Jet-like flames may be slightly taller than the lifted IOFs, but this may be due to scatter in the data and will be further investigated as a function of C/F ratio in future work. 4. Conclusions Diffusion flame structure, height, ignition delay, and lifetime were investigated as a function of AP crystal size and the coarse-to-fine ratio of the propellant. The C/F ratios investigated were 1:16, 7:13, 3:1, and 16:1 (Propellants 1, 2, 3, and 4 respectively). It was found that, at one atmosphere, diffusion flame structure was jet-like and the flame height varied as a function of crystal system diameter and C/F ratio. For particle sizes below 650 µm the flame height was similar across all C/F ratios, but above crystal sizes of 1000 µm the diffusion flames above Propellant 2 were shorter than those of Propellants 3 and 4. Crystal groups with diameters above about 750 µm were not seen for Propellant 1, and therefore no comparisons can be made in terms of group combustion effects on flame height. It is postulated the differences in flame heights are due to more fuel being consumed in the more premixed type of combustion of the dirty binder for Propellant 2 and therefore not as much fuel is able to diffuse into and combust with the AP dissociation products. Flame structure at elevated pressures differs between Propellant 1 and the other propellants considered. Propellant 1 produces jet-like diffusion flames, while the others produced lifted inverted overventilated diffusion flames. The lifted IOF are produced when the coarse AP particle burns away faster than the dirty binder; for Propellant 1, there is sufficient 8

9 fine AP in the binder to cause the fine AP/binder matrix to burn away faster than the coarse AP particles. At present it appears that the flame heights for both the lifted and jet-like flames have very similar heights across C/F ratios; however, this is still under investigation. Note that the heights measured in this manner are specific to the experimental system. Signal level depends on intensifier gain, lens position, laser energy, and aperture setting, and therefore measurements made on flame heights cannot be directly compared with previous studies. Ignition delay was calculated for one atmosphere cases, as at elevated pressures the coarse AP is (in general) not exposed on the surface to allow for such calculations. Ignition delay trended lower for Propellant 1 than for the other cases. The coarse AP crystals in Propellant 1 are isolated in a sea of fine AP/binder, which burns at a higher temperature than the monopropellant flame above the coarse AP. The combination of a higher temperature close to the surface and a shorter critical radius of heat transfer causes the coarse AP in Propellant 1 to heat up and begin to decompose faster than the AP crystal of the higher C/F cases, which are surrounded by cooler-burning coarse AP and a low AP percentage dirty binder that burns cooler than the dirty binder in Propellant 1. Though there is a hot diffusion flame present above the coarse AP, as group combustion becomes important due to the large amount of coarse AP crystals in a relatively small area the flames become very tall due to groups of coarse particles behaving similarly to a single large particle, decreasing the heat feedback to the surface of the propellant. Finally, crystal lifetime was studied for the 1 atm case and found to be fairly similar across C/F ratios. The lifetime and ignition delay were statistically identical for Propellants 3 and 4, indicating that there may be a limiting C/F ratio in terms of lifetime and ignition delay. As expected, crystal lifetime increased with particle size. Future work will consist of further examination of the elevated pressure data, including burning time (the amount of time the lifted IOF are visible in the gas phase) and flame structure. It is expected that 3D PLIF can be used to further characterize the propellant surfaces, particularly in terms of extend of deflagrating groups and variation of the flame across the crystal system. It is also desired to use the 3D PLIF to gain further insight into the structure of the lifted IOF, and to see if systems of coarse crystals distort the lifted IOF shape. Acknowledgements The authors would like to acknowledge funding provided by the National Science Foundation through funding by NSF GRFP Grant No The authors would also like to acknowledge support from AFOSR MURI contract (#4792- PU-AFOSR-0004) with Mitat Birkan as Program Manager. References [1] R.M. Muthiah, V.N. Krishnamurthy, B.R. Gupta. J. Appl. Polym. Sci. 44 (1992) [2] K. Yang, Z. Tao, G. Wang. Propellants Explos. Pyrotech. 11 (1986) [3] T. D. Hedman, K.Y. Cho, A. Satija, L.J. Groven, R.P. Lucht, S.F. Son. Combust. Flame 159 (2012) [4] T.D. Hedman, D.A. Reese, K.Y. Cho, L.J. Groven, R.P. Lucht, S.F. Son. Combust. Flame 159 (2012) [5] T.D. Hedman, L.J. Groven, K.Y. Cho, R.P. Lucht, S.F. Son. P. Combust. Inst. 34 (2013) [6] T.D. Hedman, L.J. Groven, R.P. Lucht, S.F. Son. The Effect of Polymeric Binder on Composite Propellant Flame Structure Investigated with 5 khz OH PLIF. (2013) Manuscript submitted for publication. [7] A.I. Atwood, K.P. Ford, D.T. Bui, P.O. Curran, T. Lyle. Prog. Propul. Phys. 1 (2009) [8] E.W. Price, S.R. Chakravarthy, J.K. Sambamurthi, R.K. Sigman. Combust. Sci. and Technol. 138 (1998) [9] S.R. Chakravarthy, E.W. Price, R.K. Sigman, and J.M. Seitzman. J. Propul. Power 19 (2003) [10] M. Stohr, J. Schanze, A. Khalili. Exp. Fluids 47 (2009) [11] J.P. Crimaldi. Exp. Fluids 44 (2008) [12] M. Kohga, Propell. Explos. Pyrotech. 36 (2011)