Temperature Sensitivity Of Burning Rate Research Articles

Mechanism of HMX combustion in a wide range of pressures

Data obtained in the present work and available publications on combustion of cyclotetramethylene tetranitramine (HMX) at different initial temperatures are analyzed. The temperature sensitivity of the HMX burning rate is demonstrated to increase with increasing initial temperature at pressures of 0.1 to 10 MPa, which is typical for combustion of substances with the leading reaction in the condensed phase (c-phase model). Experimental values of the temperature sensitivity of the burning rate in the pressure interval between 0.1 and 1 MPa are higher than the values predicted by the c-phase model, but this fact indicates the transition of the combustion process to another regime rather than the combustion instability in this area. The flame structure of burning HMX with different additives is studied with the help of thin tungsten-rhenium thermocouples in the pressure range from 0.025 to 1 MPa. The gas-phase flame is found to ignite in an inductive mode, at least up to a pressure of 1 MPa. The surface temperature is obtained as a function of pressure on the basis of experimental data in a wide range of pressures: ln p = −14,092/T + 21.72 (p in atm). Two possible reasons for the oscillatory regime of HMX combustion observed at atmospheric pressure are proposed: the emergence of resonance phenomena during combustion of an inhomogeneous gas mixture in the tube and the lack of correspondence between the chemical reaction rate in the gas phase at the instant of the resonance and its energy capabilities, which do not allow a necessary HMX gasification rate to be ensured. A mechanism of HMX combustion is proposed, which offers an adequate description in a wide range of pressures up to 10 MPa. The mechanism is based on the leading role of HMX decomposition in the melt at the surface temperature.

Thermal stability of the burning of the base subsystem of components of a composite propellant

The problem of the thermal stability of the base subsystem of components of a composite solid propellant with respect to the introduction of heat-absorbing agents, inhibitors, and burning rate catalysts was considered. It was demonstrated that the response of the base subsystem to the introduction in it of additives is equivalent to a change in the initial temperature of the propellant, i.e., is determined by its burning rate temperature sensitivity. The competition of the fuel components for oxidative species and the role of this phenomenon in the formation of the structure of the combustion wave were examined. Extensive experimental data on the effect of heterogeneous fillers of various natures on the burning rate of the composite system were obtained.

Effect of ammonium oxalate/strontium carbonate on the burning rate characteristics of composite propellants

AP/HTPB based composite propellants with additives such as ammonium oxalate (AO), mixture of ammonium oxalate and strontium carbonate (SC) was investigated by burning rate, TG-DTG and FTIR experiments. The results show that the burning rates of these propellants are decreased significantly. TG-DTG experiments indicate that decomposition temperatures of AP with these additives are increased. Furthermore, the activation energy of the decomposition reaction of AP is also increased in the presence of AO or AO/SC. These results show that AO or AO/SC restrains the decomposition of AP. The burning rates of these propellants are decreased. The burning rate temperature sensitivity of AP/HTPB based propellants is reduced significantly by the addition of AO or AO/SC. But the effect of AO is less than that of AO/SC. AO/SC is better effect to reduce temperature sensitivity and at the same time, to reduce pressure exponent. The reduced heat release at the burning surface of AP/HTPB/AO is responsible for the reduced temperature sensitivity. Synergetic action is probably produced between AO and SC within AP/HTPB based propellants in the pressure range tested. This synergetic effect causes the heat release to reduce and the burning surface temperature to increase. Moreover, it makes the net exothermal reaction of condensed phase become little dependent on T0. Thus, the burning rate temperature sensitivity is reduced.

Burning Rate of Solid Propellant Ingredients, Part 2: Determination of Burning Rate Temperature Sensitivity

aep D [± rb=±T0]p Care must be taken in how one evaluates aep when making comparisons of various materials. The collection of burning rate data, number of points measured at a given condition, initial test conditions, and inherent data scatter will have an effect on the value obtained for the burning rate temperature sensitivity. ‡‡;§§ The largestvariationsinburning ratemeasurementsgenerally occur at low pressures, where one approaches the dee agration limit of thematerial,andatregionswhereabreakintheburningratepressure exponentoccurs.Coincidentally,theseareoftentheareasofgreatest interest when determining the burning rate temperature sensitivity. The changes in aep that occur due to data smoothing vs averaging, and with the selection of the initial pressure and temperature conditions, can be signie cant. The effect of initial temperature selection on aep will be demonstrated using the HMX data, and e tting techniques will be illustrated with the ADN data, where considerable data scatter is present. Samples Thedee agrationdataforthesixmonopropellantslisted inTable1 of Ref. 2 will be used to demonstrate the evaluation of burning rate temperature sensitivity. The tabular burning rate data may be found in Refs. 3 and 4. The sample type and preparation was described in Ref.4.Alloftheburningratemeasurementsusedinthispaperforthe calculations were made using the cinephotomicroscopy technique also described in Refs. 3, 4, and elsewhere.

Role of gas- and condensed-phase kinetics in burning rate control of energetic solids

A simplified two-step kinetics model for the combustion of energetic solids has been used to investigate the effect of gas-phase activation energy on flame structure and burning rate and the role of gas- versus condensed-phase kinetics in determining burning rate. The following assumptions are made: a single-step, unimolecular, high activation energy decomposition process which is overall relatively energetically neutral, is followed by a highly exothermic single-step, bimolecular, gas-phase reaction with arbitrary activation energy, E˜ g. The results show that at extremely low (<104 Pa) or high (>1012 Pa) pressures the burning rate is controlled by the condensed-phase reaction kinetics for any E˜g. At intermediate pressures (105-1010 Pa) gas reaction kinetics contribute strongly to the burning rate. In this pressure range the value of E˜g plays an important function in determining the role of gas- and condensed-phase reactions: for high E˜g a gas-phase kinetically controlled regime exists; for low E˜g both condensed and gas-phase kinetics are important. The limiting behaviour of asymptotically large E˜g (gas kinetically controlled burning rate) occurs at about E˜g=20 kcal mol−1 for parameters representative of HMX, while the vanishingly small E˜g behaviour occurs near E˜g. Previous comparison with burning rate and temperature profile data has suggested that the small-E˜g limit is the more accurate of the two extremes. This may imply that the important (burning rate influencing) primary gas reaction zone near the surface has more the character of a chain reaction mechanism than the classical high activation energy thermal decomposition mechanism. To the degree that the low-E˜g chain reaction model is a better approximation than the high-E˜g thermal decomposition model, the possibility exists that the chemistry of either reaction zone, including the molecular structure of the material, might be exploited for favourable tailoring of burning rate. The low-E˜g model also provides a rational mechanistic explanation of observed trends in burning rate temperature sensitivity with pressure and temperature for materials like HMX in terms of a gradual transition from mixed gas- and condensed-phase kinetic control to condensed-phase only kinetic control as the pressure decreases.

Effect of Surface Microstructure on the Temperature sensitivity of Burning Rate of Ammonium Perchlorate

Considering Vielle's law and the new thermodynamic model which the authors have developed recently, the true dependence of temperature sensitivity of burning rate (sigma(p)) of ammonium perchlorate (AP) on pressure is resolved and experimentally verified for pellet burning. The value of sigma(p) decreases with pressure steeply in regime I' (below 20 atm), but gently in regime I (above 20 atm). The sigma(p) value of powder AP has been determined and it is observed that sigma(p(powder)) > sigma(p(pellet)), which clearly suggests that sigma p is influenced by the surface temperature sensitive parameter (partial derivative T-s/partial derivative T-o)(p) and hence by the surface/subsurface microstructure. In powder burning, the buoyant lifting of the particles into the gas phase occurs, which constitutes the so-called 'free board region' (FBR) extending just above the true surface. Consequent to the decomposition of AP particles in FBR, the condensed phase heat release gets curtailed and sigma(p(powder)) becomes larger. A general relationship for sigma(p) in terms of density and surface temperature is suggested, which is applicable to both pellet and powder AP.

Combustion of energetic azide polymers

The combustion-wave structures of azide polymers were examined based on the data obtained by burningrate and temperature-profile measurements. The energetic azide polymer evaluated in this study was bis-azide methyl oxetan (BAMO), which contains two —N3 bonds in the molecular structure. BAMO polymer was copolymerized with tetrahydrofuran and cured with an isocyanate and cross-linked to formulate BAMO copolymer. The results indicate that the burning rate of BAMO copolymer increases with increasing the energy density contained within the unit mass of BAMO copolymer samples. The temperature sensitivity of burning rate is three or four times greater than that of conventional solid propellants. The rate of heat release in the gas phase increases as initial temperature increases at constant pressure. The heat transfer process in the combustion wave is correlated with the observed burning-rate characteristics.

Temperature sensitivity of magnesium-sodium nitrate propellants

Temperature sensitivity of burning rate of magnesium-sodium nitrate propellants has been determined at different mixture ratios in the temperature range between – 40°C and + 50°C. The results indicate that the temperature sensitivity is minimum at high magnesium loadings and increases as the proportion of magnesium in the mixture decreases. Thermal decomposition data indicate that the oxidation of magnesium with sodium nitrate in a propellant is governed by the condenscd phase heat release. This heat which is maximum at high fuel concentration is seen as causing prcssure independent, high burning rate with low temperature sensitivity. The reduction of magnesium content in the mixture increases the metal agglomeration on the surface of the propellant and causes for the decrease of condenscd phasc heat release and the reaction zone temperature. This causes for pressure dependent, low buring rate with high temperature sensitivity. The burning surface temperature data indicate that the change in the initial temperature of the propellant at low fuel concentration might be causing for the change in the degree of mctal agglomcration which lead to the increase of temperature sensitivity of burning rate. The photographs of the sliver surface obtained after static test at different initial temperature of the propellant support the suggested mechanism.

Burning rate temperature sensitivity of HMX propellants

The temperature sensitivity of the burning rate of HMX (cyclotetramethylene tetranitramine) based CMDB (composite modified double-base) propellants were studied in order to elucidate the role of the HMX particles mixed within a double-base propellant matrix. Though the energy contained within the unit mass of propellant increases when the weight fraction of HMX, £, is increased, the burning rate of the propellants tested in this study decreases when % is increased in the range of £ ~ 0.5. In other words, the burning rate becomes a minimum value at £ = 0.5 at a constant pressure. The temperature sensitivity decreases monotonically when £ is increased. The measurements reveal that the fizz zone reaction rate decreases monotonically and the heat of reaction at the burning surface increases monotonically when £ is increased. This combustion mode of the HMX-CMDB propellants demonstrates the observed burning rate and temperature sensitivity characteristics.

Combustion Mechanism of Azide Polymer

AbstractThe combustion wave structure and thermal decomposition process of azide polymer were studied to determine the parameters which control the burning rate. The azide polymer studied was glycidyl azide polymer (GAP) which contains energetic – N3 groups. GAP was cured with hexamethylene diisocyanate (HMDI) and crosslinked with trimethylolpropane (TMP) to formulate GAP propellant. From the experiments, it was found that the burning rate of GAP propellant is significantly high even though the adiabatic flame temperature of GAP propellant is lower than that of conventional solid propellants. The energy released at the burning surface of GAP propellant is caused by the scission of NN2 bond which produces gaseous N2. The heat flux transferred back from the gas phase to the burning surface is very small compared with the heat generated at the burning surface. The activation energy of the decomposition of the burning surface of GAP propellant, Es, is determined to be 87 kJ/mol. The burning rate is represented by r = 9.16 × 103 exp(–Es/RTs) where r (m/s) is burning rate, Ts (K) is the burning surface temperature, and R is the universal gas constant. The observed high temperature sensitivity of burning rate is correlated to the relationship of (∂Ts/∂T0)p = 0.481 at 5 MPa, where T0 is the initial propellant temperature.

Temperature sensitivity of Burning Rate of Ammonium Perchlorate Propellants

AbstractThe temperature sensitivity of burning rate of ammonium perchlorate (AP) composite propellants was studied as a function of AP particle size and a burning rate catalyst. A simplified temperature sensitivity model was presented in order to discriminate the effect of the gas phase and solid phase reactions on the initial propellant temperature (T0). The temperature sensitivity was decreased by the addition of small sized AP particles and/or 2,2‐bis(ethylferrocenyl)propane (BEFP). This is caused by the insensitive burning surface temperature to T0. Thus, the gas phase reaction rate becomes little dependent on T0, and the temperature sensitivity decreases.

The effect of pressure exponent on temperature sensitivity

The paper presents a technique for theoretically determining the motor temperature sensitivity coefficient, π k , from burning rate information generated by a theoretical combustion model, the Petite Ensemble Model, thereby permitting a parametric study of several factors affecting π k that are influenced by the initial propellant temperature: characteristic velocity, burning rate constant and exponent, and burning rate temperature sensitivity. The study is for an AP/HTPB propellant having a unimodal oxidizer distribution with a mode width parameter of 1.5 about a mass mean diameter of 100 microns. The content of AP is varied from 60 to 90%, while the values for the propellant area ratio, K n , range from 200 to 400. The results indicate that at the higher values of oxidizer loading the value of π k appears to be invariant with pressure or K n . At the lower oxidizer loading, the value of π k appears to slightly decrease with increasing pressure or K n . The temperature sensitivity of the burn rate constant and the characteristic velocity both serve to increase the value of π k and both appear to be relatively invariant with pressure and K n . The influence of the pressure exponent on the value of π k appears to be linked with the oxidizer loading. At the higher values of oxidizer loading the n effects term is positive and increases the value of π k , while at the lower oxidizer loading, the n effects term is negative and reduces the value of π k .

Modeling of pressure-coupled response functions of solid propellants

The applicability to various situations of a classic equation formulation commonly used to relate solid propellant pressure-coupled response functions to such steady-state combustion parameters as pressure and temperature sensitivity of burning rate has been examined. It is found that while this equation is valid under certain conditions, allowance for dependence of a surface heat release parameter on propellant conditioning temperature or for oscillations in the composition (oxidizer/fuel ratio) of material ablating from the propellant surface, accompanying surface temperature oscillations, invalidates this approach. In such cases, response functions calculated using this equation differ markedly from those calculated using a complete perturbation analysis. In addition, a potential destabilizing mechanism for heterogeneous propellants which is not present in the case of homogeneous propellants is identified, and an approach to improving combustion stability characteristics of composite propellants is suggested.

Temperature Sensitivity of the Burning Rate of Composite Solid Propellants

Abstract This paper presents the results of an experimental program designed to determine the burning-rate temperature sensitivity of various ammonium perchlorate (A.P.) based propellants in the temperature range from 0 to 140°C The effect of aluminum, copper chromite, and lithium fluoride on the thermal-sensitivity coefficient ( β ≡ ∂ In vb / ∂ To) was also investgated. The results, which are generally well represented by the empirical law vb ≡ vb ∗ exp [β (To-T∗ )] indicate, that the aluminized propellants have a tower value of β and that the presence of additives in the propellants tends to decrease this coefficient still more. Scanning electron microscope observations of the burning surface after the quenching of the combustion, seem to reveal the influence of the difference between the thermal stabilites of the oxidizer (A.P.) and the binder (poly-butadiene, polyumhae).

Dynamic Responses of Solid Rockets during Rapid Pressure Change

Solid rocket performance during rapid pressure excursions differs greatly from predictions based on steady-state burning rate data. Rapid pressurization (150-250 kpsi/sec) following a sudden throat area decrease in a low L* combustor produces pressure overshoots of 10% and indicated burning rate overshoots in excess of 50%. A transient internal ballistics model was developed incorporating nonsteady continuity and energy equations for the chamber, nonsteady energy equation for the propellant condensed phase, and a modified Zeldovich heat feedback function for the propellant (which for the conditions considered is known to burn with a thin quasi-steady reaction zone). Sensitivity analyses using the model indicate that accurate surface temperature and temperature sensitivity data are needed. With reasonable estimates of surface reaction zone temperature and measurements of temperature sensitivity of burning rate, good agreement between the measured and the calculated p vs t was found for a nonmetallized composite propeliant in a low L* combustor. High pressure exponent, high temperature sensitivity of burning rate, high dAJdt, low burning rate, and low L* prominently increase the dynamic effects.

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.

Theory of dynamic extinguishment of solid propellants with special reference to nonsteady heat feedback law

Two methods for predicting instantaneous burning rates are described and compared: 1) theories based on models of the flame and 2) the Zeldovich-Novozhilov method which uses the steady- state burning rate data as functions of pressure p and initial temperature TO to deduce the appropriate nonsteady law for the heat feedback rate from the flame. The latter method offers the important advantage of not requiring detailed knowledge of the flame structure. Theoretical connections between the two methods are demonstrated. Burning rate data r(p,T0) were obtained for pressures from 1.3 to 40.0 atm and for initial temperatures from 130° to 350 °K. The primary limitation encountered in the application of the ZeldovichNovozhilov method is the inadequacy of techniques for obtaining r(p,To) at ambient temperatures below 150 °K. Calculated extinction boundaries agree with measured results for depressurization rates from 4,000 to 15,000 atm/sec and pressures from 20 to 60 atm. An interesting observation is that within a class of similar propellants, the propellant with a high temperature sensitivity of burning rate tends to extinguish more readily, ignite with greater difficulty, and be more prone to instability. A8 = B = c = E = h = H = k = m = n = 0[ ] = p = q = Qs = r = t = T = u = a. = /3,e = X = p =