Low-pressure Deflagration Limit Research Articles

Combustion of Ammonium Perchlorate monopropellant: Role of heat loss

The combustion characteristics of Ammonium Perchlorate (AP) monopropellant have been revisited. The use of silica grease to prevent heat loss from the sides of the pellet and the ignition methodology opened up new area; one of them being the low pressure deflagration limit (LPDL) of AP itself and the other being the burn rate of AP at different pressures. The new burn rate law for pure AP was obtained for conditions close to adiabatic. Using slow depressurization method, wherein the LPDL was independent of ignition transients, the LPDL of pure AP was found to be 14 bar The burn rate of pure AP at 70 bar was found to be 10.66 mm/s, which was much higher than the ones reported in literature. The pressure index for this case was found to be 0.71. This new result completely changes the understanding of AP combustion and called for a new set of parameters to be developed to predict the results obtained with silica grease on the sides. Using the same set of parameters, along with a heat loss model the predictions for the case when there was no silica grease applied was made. These results were in reasonable agreement with the experimental data reported in literature without the silica grease on the sides of the AP pellet. This brings out the critical role of heat loss in AP combustion.

Experimental studies on low pressure deflagration limit of ammonium perchlorate with additives

This paper deals with the low pressure deflagration limit (LPDL), which is the minimum pressure below which burning of a propellant ceases without the assistance of an external energy source. The LPDL of AP monopropellant reported in literature is around 20 bar and it is reported to shift to different higher pressures with the addition of small fraction of impurities or additives. Most of these additives (such as iron oxide and copper chromite) are known to catalyze AP combustion, but yet their reported LPDL is significantly higher than the LPDL of AP itself. In present study, reasons for this behavior have been explored experimentally. Three additives (each of 1% by mass) namely iron oxide (IO), copper chromite (CC) and activated charcoal (AC) were used. The LPDL of AP with these additives was determined using three methods. Two of these methods (method I and method II) include the effect of ignition dynamics on LPDL while in method III (slow depressurization) ignition dynamics does not play a role. In method I, hot nichrome wire was used for ignition, while in method II, nichrome wire ignition in conjunction with an in-situ propellant was used to ignite the pellets. The effect of convective heat loss on LPDL was studied with the use of silica grease whose role through carefully thought out experimentation has been identified as an insulation on sides of pellet. The LPDL of AP with additives was found to be higher for method I compared to method II. Lastly, method III (slow depressurization) was found to result in lowest LPDL for these additives. The LPDL of pure AP, which has been reported as around 20 bar, was revisited and method III along with the use of silica grease has lowered the LPDL of pure AP to 14 bar.

A novel approach to composite propellant combustion modeling with a new Heterogeneous Quasi One-dimensional (HeQu1-D) framework

This paper is concerned with a new and novel approach to modeling composite propellant burn rate behavior. It is founded on the fact that composite propellant combustion is largely boxed between the premixed limits – of pure AP and fine AP-binder (HTPB, here) whose burn behaviors are taken as known. The current strategy accounts for particle size distribution using the burn time averaging approach. The diffusional effects are accounted for through a calibrated heterogeneous quasi-one-dimensional model (HeQu1-D for short) that allows for the flame temperature dependence on the local AP size-binder thickness geometry. Fine AP-binder homogenization is adopted as in recent models with refinement on the particle size as a function of pressure. The specialty of the present approach is that it invokes local extinction for fuel rich conditions for specific particle sizes when the heat balance causes the surface temperature to drop below the low pressure deflagration limit of AP; this feature allows for the prediction of extinction of propellant combustion. Combining these ideas into a MATLAB® calculation framework that uses a single dataset on properties of AP and binder consistent with burn rate vs. pressure of pure AP and fine AP-binder system allows for making the predictions of propellants with multiple particle sizes and different fractions. Comparisons of burn rate data over nearly thirty compositions from different sources appear excellent to good. It is found that it is important to treat the full particle size distribution to achieve better predictions. Low burn rate index (∼0.25) observed with addition of SrCO3 is captured by extending the model to include the effect of binder melt; the gas phase effect is accounted for by calibration against catalytic effect on the fine AP-binder propellant. An interesting deduction from the model is that the temperature sensitivity of propellants should not exceed that of AP. The robustness of the current model and speed of determining the burn rate behavior allow for the possibility of determining the particle size distribution required to meet the burn rate specifications of a specific propellant for practical applications before actually embarking on making the propellant.

Near-surface flame structure characterization of simplified ammonium perchlorate/hydroxyl-terminated polybutadiene compositions

Simplified model propellant configurations, such as monomodal propellants, can be valuable in the development and validation of predictive numerical tools. These idealized experiments also yield insight into the effect of diffusion length scales on combustion, but comprehensive data covering a large range of diffusional length scales do not currently exist. Here, monomodal propellants with ammonium perchlorate (AP) particle sizes under 800 µm and AP pellets ported and filled with hydroxyl-terminated polybutadiene (HTPB) were used to systematically study the effect of diffusion length scales, or AP equivalent particle sizes) of up to 4.1 mm on flame structure. In general, burning rates increased with pressure and decreasing particle size, as expected. Burning rates for samples with particle sizes greater than 400 µm converged with AP monopropellant burning rate data above approximately 2 MPa, the AP low-pressure deflagration limit (LPDL). For a given pressure above the LPDL, burning rates eventually became constant for both increasing and decreasing particle sizes. Conversely, for a given pressure below the LPDL burning rate was shown to be a function of particle diameter. Flame structures above the composite propellants were observed using 5 kHz OH planar laser-induced fluorescence (PLIF). The transient flames were underventilated (jet-like) over the AP particles at 1 atm while lifted, inverted, and overventilated at 5 atm. Distinct diffusion flame structures were observed visually above the ported samples at 1 atm. Very luminous flames were observed at the interface between the AP and binder. The effect of strain rate on sample combustion was examined using an opposed flow burner; at 1 atm, sample burning rate was not affected by strain rate. At the largest strain rate, the sample self-extinguished after igniter shutoff, indicating that secondary diffusion flames are important in the opposed flow configuration.

Enhancing Composite Solid Propellant Burning Rates with Potassium Doped Ammonium Perchlorate-Part I

The objective of this paper is to understand the observed changes in behavior of ammonium perchlorate deflagration characteristics reported in literature, when it is doped with potassium. Successive recrystallization is employed to dope potassium into ammonium perchlorate. This paper, through various recrystallization techniques, conclusively puts an end to doubts regarding the actual process of acquisition of potassium by ammonium perchlorate. It is concluded that the increase in the potassium content in ammonium perchlorate is due to the filter used during recrystallization process. From the experiments conducted to obtain the deflagration rates of ammonium perchlorate, it is observed that potassium doped ammonium perchlorate has higher burning rates and also a higher low-pressure deflagration limit. As potassium doped is within the ammonium perchlorate crystal, thermal conductivity and specific heat values for ammonium perchlorate with various percentages of potassium are obtained. These values are incorporated into the computational studies on ammonium perchlorate. The computational studies performed in this paper are not intended to reproduce the experimental results but to illustrate the various features of the potassium doped ammonium perchlorate deflagration. With these computational studies, the observed burning rate increase and low-pressure deflagration limit increase of ammonium perchlorate with doped potassium are explained.

Revisiting the Modeling of Ammonium Perchlorate Combustion: Development of an Unsteady Model

This paper examines afresh features of ammonium-perchlorate (AP) combustion. Here an AP model, which predicts most experimental observations, is proposed. The one-dimensional aero-thermo-chemical field is captured through the solution of mass, energy, and species conservation equations. The unsteady one-dimensional phase heat transfer accounting for regression is solved for in the condensed phase. The uncertain parameters for AP pyrolysis, gas-phase reaction, and extent of surface exothermicity are chosen so that the most certain of the measured parameters of AP combustion, namely, pressure index of stable combustion (0.77 between 2 and 7 MPa), initial temperature sensitivity of burn rate, σp (about 0.0021 to 0.0015 K −1 ), range of surface temperatures measured and suggested in several studies (∼850 to 875 K), and low-pressure deflagration limit (LPDL) at 300 K (∼2 MPa), are correctly predicted. The results obtained show the pressure index of AP combustion to be 0.77 and σp to be 0.0024‐0.0023 K−1. The LPDL is caused by a combination of loss of liquid layer and transient conduction into the condensed phase and not by heat loss.

Sandwich propellant combustion: Modeling and experimental comparison

This paper reports reacting fluid dynamics calculations for an ammonium percholrate binder sandwichand extracts experimentally observed features including surface profiles and maximum regression rates as a function of pressure and binder thickness. These studies have been carried out by solving the twodimensional unsteady Navier-Stokes equations with energy and species conservation equations and a kinetic model of three reaction steps (ammonium perchlorate decomposition flame, primary diffusion flame, and final diffusion flame) in the gas phase. The unsteady two-dimensional conduction equation is solved in the condensed phase. The regressing surface is unsteady and two dimensional. Computations have been carried out for a binder thickness range of 25–125 μm and a pressure range of 1.4 to 6.9 MPa. Good comparisons at several levels of detail are used to demonstrate the need for condensed-phase two-dimensional unsteady conduction and three-step gas-phase reactions. The choice of kinetic and thermodynamic parameters is crucial to good comparison with experiments. The choice of activation energy parameters for ammonium percholrate combustion has been made with stability of combustion in addition to experimentally determined values reported in literature. The choice of gas-phase parameters for the diffusion flames are made considering that (a) primary diffusion flame affects the low-pressure behavior and (b) final diffusion flame affects high-pressure behavior. The predictions include the low-pressure deflagration limit of the sandwich apart from others noted above. Finally, this study demonstrates the possibility of making meaningful comparisons with experimental observations on sandwich propellant combustion.

A thermodynamic proposal for the exposition of criticality and other subtle features in self-deflagrating ammonium perchlorate systems

A first comprehensive investigation on the deflagration of ammonium perchlorate (AP) in the subcritical régime, below the low pressure deflagration limit (LPL, 2.03 MPa) christened as régime I', is discussed by using an elegant thermodynamic approach. In this régime, deflagration was effected by augmenting the initial temperature ( T 0 ) of the AP strand and by adding fuels like aliphatic dicarboxylic acids or polymers like carboxy terminated polybutadiene (CTPB). From this thermodynamic model, considering the dependence of burning rate ( ṙ ) on pressure ( P ) and T 0 , the true condensed ( E s,c ) and gas phase ( E s,g ) activation energies, just below and above the surface respectively, have been obtained and the data clearly distinguishes the deflagration mechanisms in régime I' and I (2.03–6.08 MPa). Substantial reduction in the E s,c of régime I', compared to that of régime I, is attributed to HClO 4 catalysed decomposition of AP. HClO 4 formation, which occurs only in régime I', promotes dent formation on the surface as revealed by the reflectance photomicrographs, in contrast to the smooth surface in régime I. The HClO 4 vapours, in régime I', also catalyse the gas phase reactions and thus bring down the E s,g too. The excess heat transferred on to the surface from the gas phase is used to melt AP and hence E s,c , in régime I, corresponds to the melt AP decomposition. It is consistent with the similar variation observed for both the melt layer thickness and ṙ as a function of P . Thermochemical calculations of the surface heat release support the thermodynamic model and reveal that the AP sublimation reduces the required critical exothermicity of 1108.8 kJ kg -1 at the surface. It accounts for the AP not sustaining combustion in the subcritical régime I'. Further support for the model comes from the temperature-time profiles of the combustion train of AP. The gas and condensed phase enthalpies, derived from the profile, give excellent agreement with those computed thermochemically. The σ p expressions derived from this model establish the mechanistic distinction of régime I' and I and thus lend support to the thermodynamic model. On comparing the deflagration of strand against powder AP, the proposed thermodynamic model correctly predicts that the total enthalpy of the condensed and gas phases remains unaltered. However, 16% of AP particles undergo buoyant lifting into the gas phase in the ‘free board region’ (FBR) and this renders the demarcation of the true surface difficult. It is found that T s lies in the FBR and due to this, in régime I', the E s,c of powder AP matches with the E s,g of the pellet. The model was extended to AP/dicarboxylic acids and AP/CTPB mixture. The condensed (∆ H 1 ) and gas phase (∆ H 2 ) enthalpies were obtained from the temperature profile analyses which fit well with those computed thermochemically. The ∆ H 1 of the AP/succinic acid mixture was found just at the threshold of sustaining combustion. Indeed the lower homologue malonic acid, as predicted, does not sustain combustion. In vaporizable fuels like sebacic acid the E s,c régime I', understandably, conforms to the AP decomposition. However, the E s,c in AP/CTPB system corresponds to the softening of the polymer which covers AP particles to promote extensive condensed phase reactions. The proposed thermodynamic model also satisfactorily explains certain unique features like intermittent, plateau and flameless combustion in AP/polymeric fuel systems.

Combustion characteristics of AP/HTPB propellants with burning rate modifiers

Fe2O3, copper chromite catalyst (CC), LiF, and CaCO3 are the four burning rate modifying additives considered in this study. A wide variety of subatmospheric flames are observed for the propellants with additives. Under subatmospheric pressures a dark zone is present; this zone becomes thicker at lower pressure and with finer ammonium perchlorate (AP) particles. A smoldering surface without a gas-phase flame is noted for the additive Fe2O3. From the smoke deposits, obtained from the subatmospheric burning of propellants with burning rate enhancers, it is shown that the breakdown of heavy fuel molecules is better with finer AP particles. The low-pressure deflagration limit is reduced by the addition of Fe2O3 or CC and increased by the addition of LiF or CaCO3. In the presence of LiF or CaCO3, above certain additive concentration levels, reduction in AP particle size is found to depress the burning rate further. LiF, below certain concentration levels, acts as a burning rate enhancer up to a certain pressure and as a depressant thereafter. The type of additive influences the pressure at which the shape of burning AP surface changes from convex to concave and also the pressure at which the AP surface recesses relative to the mean surface.

Propellant Gas Phase Chemical Kinetics

An adiabatic constant pressure chemical kinetic study of the AP gas phase is conducted. The objective is to examine the importance of chemical kinetic studies for propellant deflagration modelling in general and for AP in particular. A reaction mechanism consisting of ninety five elementary reactions involving twenty one chemical species is developed to represent the AP gas phase. An important parameter that can be obtained from the kinetics is the time rate of change of temperature of the gas phase at the condensed phase-gas phase boundary (dT/dt at t = 0). This is directly related to the heat feedback from the gas phase to the condensed phase. Comparison with earlier work indicates that the use of a one-step overall reaction could lead to underestimates of heat feedback from the gas phase by a factor of about four. In addition it is found that the rate of decrease of dT/dt at t = 0, with pressure is much larger at lower pressures, suggesting a possible gas phase role in the low pressure deflagration limit. Further, the kinetic study does not support the conclusion of a constant fraction of AP reacting in the condensed phase in the pressure range 20–100 atmospheres. These results are at variance with what is obtained by use of a one-step overall reaction. Thus, the one-step overall reaction cannot represent even qualitatively several gas phase characteristics important for deflagration studies, and its use leads to erroneous results. It is concluded that a full reaction mechanism representation of gas phase chemical kinetics is essential to AP deflagration modelling. The same conclusion is probably valid for propellant deflagration modelling in general.

Radiative properties of burning aluminum droplets

The spectral intensity (790 nm) emitted during the combustion of pressed pellets composed of ammonium perchlorate (AP) and aluminum has been measured. Aluminum mass fraction content was varied from 0.5% to 6.0%. The combustion bomb pressure ranged from the low pressure deflagration limit (near 650 psia or 4.5 MPa) to 1100 psia or was well above the local gas temperature and in excess of the adiabatic flame temperature assuming complete Al 7.6 MPa. Based on the intensity measurements, the effective blackbody emission temperature of the burning droplets combustion. The higher intensities were attributed to continuous emission by molten Al 2O 3 in the flame envelope surrounding the molten Al droplets as well as emission by the Al droplets themselves. A two part model of the radiant transport in the burning droplet system and between droplets was developed for comparing the theoretical and measured spectral intensities to estimate the magnitude of the radiative parameters of the burning Al droplets. From this model, the optical depth of the Al 2O 3 flame envelope surrounding the droplet was estimated to be of the order of 10 −2. It was concluded that scattering and absorption by the envelope were negligible whereas emission was not. The effective radiative properties of the burning droplet system were predicted and the importance of radiative transfer in vapor phase combustion of liquid metal droplets was discussed.

IGNITION OF AMMONIUM PERCHLORATE

A simplified theory for the ignition of ammonium perchlorate is proposed, which is derived from a unified theory that also explains the low-pressure deflagration limit as well as the steady deflagration. The theory provides an approximate method of calculating the ignition delay and the minimum external he»t flux for a successful ignition, as functions of pressure and initial solid temperature. The ignition calculations show that there exists a pressure limit due to the weakness of the igniter strength, in addition to the low-pressure deflagration limit which is an inherent property of the solid independent of the igniter strength. The theory can be extended to other monopropellants for which exothermic reaction occurs only in the gas phase.

A Unified Theory of Ammonium Perehlorate Deflagration and the Low Pressure Deflagration Limit

Abstract A unified theory is proposed to explain using a single framework the hitherto separately treated aspects of the combustion of ammonium perehlorate: burning rate, low-pressure deflagration limit and ignition. The model on the basis of which the theory is formulated incorporates the experimentally observed microstructure of the burning surface of ammonium perehlorate, which provides an intermediate heat sink in an overall adiabatic system. The theory explains the existence of the low-pressure deflagration limit of ammonium perehlorate, which has eluded previous attempts at theoretical explanation. Furthermore, the theory predicts accurately the variation of the limiting pressure with initial solid temperature and also the nearly constant burning rate at the low-pressure deflagration limit regardless of initial solid temperature. The comparison of the theory and experiment for the low-pressure deflagration limit and burning rate of ammonium perehlorate indicates that the same reaction mechanisms pre...

Rocket Propellant Combustion Studies in a Constant Volume Bomb

Abstract A constant volume window bomb has been used to measure the characteristic velocity (c*) of rocket propellants. Analysis of the combustion process inside the bomb including heat losses has been made. The experiments on double base and composite propellants have revealed some (i) basic heat transfer aspects inside the bomb and (ii) combustion characteristics of Ammonium Perchlorate-Polyester propellants. It has been found that combustion continues even beyond the peak pressure and temperature points. Lithium Fluoride mixed propellants do not seem to indicate significant differences in c*) though the low pressure deflagration limit is increased with percentage of Lithium Fluoride.

An Experimental Study of Ammonium Perchlorate-Binder Sandwich Combustion in Standard and High Acceleration Environments

Abstract An experimental investigation of ammonium perchlorate (AP)-binder (PBAA) sandwich combustion was made in standard and high acceleration environments. An optically equipped combustion bomb mounted on a centrifuge and a standard combustion bomb were used in the investigation. High speed color motion pictures and schlieren were taken during the combustion process. Principal results were: binder thickness as little effect upon regression rate; sandwich combustion is acceleration sensitive and results in part from binder melt/AP deflagration interaction; below the low pressure deflagration limit (P dt ) of ammonium perchlorate the sandwich combustion process is laminar with combustion continuing for large distances above the binder/AP interface; above the P dt of AP the sandwich combustion process appears to be turbulent and consists of two distinct flame regions.

Mects of Low-Temperature Ammonium Perchlorate Decomposition on the Ballistic Properties of a CTPB Propellant

The objective of the subject work was to characterize the effect of the low-temperature condensed-phase decomposition (LTD) of ammonium perchlorate (AP) on the ballistic properties of a CTPB propellant. Propellants were prepared from three chemically altered AP oxidizers, differing widely in initial LTD rate, but having essentially identical particle size distributions. Other physiochemical properties of the propellants were maintained constant during processing. The three oxidizers were as follows, in order of increasing initial LTD rate: 1) AP cocrystallized with (NH4)2 HPO4, 2) recrystallized control, and 3) AP cocrystallized with KC1O3. Experimental evidence is cited for the probable absence of phosphate-induced catalysis of the high-temperature decomposition of AP. Despite a demonstrated tenfold spread in isothermal LTD weight loss rates of the special oxidizers used in the three propellants, these differences had no measurable effects on strand burning rate, critical P extinction characteristics, low-pressure deflagration limit, burning-rate temperature sensitivity, or ignition delay time.

The Deflagration of Solid Propellant Oxidizers

Abstract Experimental results are reported of the self-deflagration of the solid oxidizers hydroxylammonium perchlorate and hydrazine nitroform. The burning rate-pressure relationships were measured and combustion temperatures were determined. Quenching diameters were also measured for these two oxidizers plus ammonium perchlorate. Two findings of significance have emerged: first, hydroxylammonium perchlorate exhibits a low-pressure deflagration limit of 146 atm, analogous to the 20 atm limit of ammonium perchlorate. This high value obtains in spite of aflame temperature for HAP that is 100° higher than that of AP; and in spite of the fact that at pressures at which HAP does burn, it burns two to three times more rapidly than AP. Comparison of results for several oxidizers suggests that AP should be regarded as exhibiting an unusually low low-pressure limit. The second item of importance is an hypothesis that ammonium perchlorate deflagration occurs by a cellular mechanism. A simplified analysis of the AP...

Strand size and low-pressure deflagration limit in a composite propellant

The low-pressure deflagration limit, LPDL, has been determined for a composite ammonium perchlorate propellant over a range of pressures from 35 to 235 torr. The independent variable was strand cross section area. A linear relation was found between the LPDL and the inverse of the hydraulic radius of the strand. It was thus possible to extrapolate and determine the low-pressure limit for a strand of infinite extent. It can be argued that such an infinite strand is adiabatic. A possible mechanism to explain a finite extinguishing pressure for an adiabatic strand is the lack of sufficient oxidant in the gas phase due to differences in the rate of change of fuel and oxidizer gasification rates with decreasing pressure. I. Introduction S OME time ago we attempted to examine the structure of the combustion zone above a strand of composite solid propellant by means of interferometry. In order to increase spatial resolution we expanded the zone by burning propellant strands at subatmospheri c pressure in a chamber which could be continuously pumped and which was provided with windows for observation. For reasons that we probably should have anticipated we fell somewhat short of our primary objective. In the course of our experiments, however, we did obtain some data on the low-pressure deflagration limit, LPDL, for two composite propellant compositions. In particular, we determined the dependence of LPDL on strand size. It seems in order to record these results because they may provide some grist for the theorist's mills and shed