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.     

Modeling of HMX combustion

A mechanism of HMX combustion was proposed and the corresponding model was developed under the assumption that the combustion wave consists of two zones, with consideration given to the reaction of decomposition and vaporization of the initial energetic material in the condensed phase and the subsequent decomposition of its vapor in the gas phase. An analysis of the results showed that, at low pressures, the burning rate is largely determined by the exothermic decomposition of the material in the condensed phase, but at pressure above ∼20 atm, the processes in the gas phase begin to play an increasingly important role, where the limiting process is the bimolecular activation reaction with the subsequent dissociation of HMX accompanied by the secondary reactions between the products. A comparison of the calculation results with experimental data showed that the model adequately describes a number of characteristics of the combustion wave and ballistic properties, such as the burning rate and its sensitivity to pressure and initial temperature.     

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.     

Comparison of simplified models for nitramine propellant combustion

Although 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are very similar molecularly and their burning rates as a function of pressure are nearly identical, it is well known that they differ significantly in temperature sensitivity, especially at low pressures. To understand these differences better, three simple models were applied to HMX and RDX combustion. Both the Denison–Baum–Williams and Li–Williams–Margolis models have previously been calibrated for use with RDX. However, the RDX calibration of the Ward–Son–Brewster model was developed in the present work. All three models were compared with relevant measured data including: burning rate, flame stand-off/thickness, combustion stability, and temperature sensitivity. It was shown that all models are capable of accurately determining the burning rate of HMX and RDX as a function of pressure at the baseline initial temperature, but only two of the models are capable of capturing the variation in temperature sensitivity for both HMX and RDX, and only one model can replicate all the other measured characteristics within experimental uncertainty. Analysis using this model suggests that the surface reaction of RDX is much less exothermic than HMX and that there is a shifting between the gas phase and surface reaction dominance with pressure for HMX. This explains why the temperature sensitivity for RDX is nearly flat for low pressures while the temperature sensitivity for HMX increases significantly as the pressure decreases. Importantly, these trends are achieved without adding significant model complexity or having parameters change with pressure or initial temperature.     

煤焦燃烧反应动力学的通用规律研究

本文提出了有关煤焦燃烧反应动力学的新思想，即认为煤焦反应动力学参数Ｅ（活化能）与煤质无关，它只是煤焦温度的函数；但其反应频率因子ｋ０，ｃｈ与煤质有密切关系。本文中首次给出了煤焦燃烧反应动力学参数Ｅ／ｋ０，ｃｈ与煤的工业分析基值的通用关系。这样可以根据煤质的工业分析基值，就可获得一确定的Ｅ／ｋ０，ｃｈ值。从而避免了前人关于Ｅ／ｋ０，ｃｈ值与煤质无通用关系的状况。本文还揭示了反应频率因子不仅与煤质有关，而且还与其燃烧状态有关的物理本质，并给出了它们之间的定量关系。     

A semi-analytical,multizone model of droplet combustion with varying properties

A multizone droplet burn model is developed to account for changes in the thermal and transport properties as a function of droplet radius. The formulation is semi-analytical – allowing for accurate and computationally efficient estimates of flame structure and burn rates. Zonal thermal and transport properties are computed using the Cantera software, pre-tabulated for rapid evaluation during run-time. Model predictions are compared to experimental measurements of burning n-heptane, ethanol and methanol droplets. An adaptive zone refinement algorithm is developed that minimizes the number of zones required to provide accurate estimates of burn time without excess zones. A sensitivity study of burn rate and flame stand-off with far-field oxygen concentration is conducted with comparisons to experimental data. Overall agreement to data is encouraging with errors typically less than 20% for predictions of burn rates, stand-off ratio and flame temperature for the fuels considered.     

Transient convective burning of a periodic fuel-droplet array

The transient convective burning of fuel-droplets interacting within 3-D infinite periodic arrays in a hot gas stream is numerically studied for the first time, with considerations of droplet regression, deceleration due to the drag of the droplets, internal liquid motion, variable properties, non-uniform liquid temperature, surface tension, and n-octane one-step oxidation kinetics. Depending upon the initial conditions and other constraints, a flame is established early as either a wake flame or an envelope flame. An initial envelope flame remains an envelope flame, and an initial wake flame has a tendency to develop from a wake flame to an envelope flame. The flame shows no strong tendency to modify significantly the standoff distance during the lifetime of the droplet. For an initial wake flame, the moment of wake-to-envelope transition is advanced as the initial droplet spacing (intermediate) is decreased, but tends to be postponed as the initial droplet spacing is further reduced. The burning rate at smaller initial droplet spacing or smaller initial Reynolds number might be greater for some period during the lifetime because of an earlier wake-to-envelope transition which elevates the average surface temperature. Lower ambient temperature yields a later wake-to-envelope transition time and smaller mass burning rate. At the lower ambient pressure with the same initial relative stream velocity, the average surface temperature is reduced, the wake-to-envelope transition is later, and the mass burning rate is smaller. Validation of our analysis is made by comparing with the results of an isolated droplet Wu and Sirignano [11].     

Pulsating hydrodynamic instability and thermal coupling in an extended Landau/Levich model of liquid-propellant combustion: I. Inviscid analysis

Hydrodynamic (Landau) instability in combustion is typically associated with the onset of wrinkling of a flame surface, corresponding to the formation of steady cellular structures as the stability threshold is crossed. In the context of liquid-propellant combustion, such instability has recently been shown to occur for critical values of the pressure sensitivity of the burning rate and the disturbance wavenumber, significantly generalizing previous classical results for this problem that assumed a constant normal burning rate. Additionally, however, a pulsating form of hydrodynamic instability has been shown to occur as well, corresponding to the onset of temporal oscillations in the location of the liquid/gas interface. In the present work, we consider the realistic influence of a non-zero temperature sensitivity in the local burning rate on both types of stability thresholds. It is found that for sufficiently small values of this parameter, there exists a stable range of pressure sensitivities for steady, planar burning such that the classical cellular form of hydrodynamic instability and the more recent pulsating form of hydrodynamic instability can each occur as the corresponding stability threshold is crossed. For larger thermal sensitivities, however, the pulsating stability boundary evolves into a C-shaped curve in the (disturbance-wavenumber, pressure-sensitivity) plane, indicating loss of stability to pulsating perturbations for all sufficiently large disturbance wavelengths. It is thus concluded, based on characteristic parameter values, that an equally likely form of hydrodynamic instability in liquid-propellant combustion is of a non-steady, long-wave nature, distinct from the steady, cellular form originally predicted by Landau.     

Temperature sensitivity of the burning velocity of nitroglycerin-based propellants

An analysis of the temperature sensitivity of the burning velocity for nitrocellulose and a number of model nitroglycerin-based propellants was performed. The dependences of the temperature sensitivity of the burning velocity on the pressure were obtained: at initial temperatures of below −50°C the pressure dependence of this parameter was virtually absent, whereas at temperatures above −50°C, it decreases sharply with increasing pressure.     

Ammonium Nitrate as an Eco–Friendly Oxidizer for Composite Solid Propellants: Promises and Challenges

Ammonium nitrate (AN) has received attraction globally not only as a nitrogenous fertilizer but also as an oxidizer in gas generators and propellants. Nowadays, great attention is being focused on the development of composite solid propellants with green oxidizers in realizing eco–friendly combustion products. The ammonium perchlorate (AP), which is the work horse oxidizer in composite propellant, needs replacement due to its environmental and human health issues. In this context, AN is regarded as an alternative to AP because of its easy availability and environmentally friendly chlorine free combustion products. However, AN has its own inherent drawbacks such as hygroscopicity, room temperature phase transition, and low burning rate. Recently, several studies have been focused on its phase stabilization and burning rate modification so as to develop solid propellants with improved properties. The knowledge of thermal characteristics of AN is a crucial factor for its applications in propellants and gas generators. This article details the different aspects of polymorphism, phase stabilization, thermal decomposition, hygroscopicity, specific impulse, and burn rate modification of AN and also addresses ways to overcome the inherent weakness of AN as a propellant oxidizer in formulating an effective propellant composition.     

小尺度薄油池火沸腾燃烧特性研究

通过对小尺度薄油池火燃烧特性进行实验研究,分析油池不同燃烧阶段的特点,探讨沸腾燃烧对油池燃烧特性的影响。测量了直径分别为0.10 m、0.14 m、0.20 m和0.30 m正庚烷油池火的燃烧速率以及温度分布随时间变化。分析燃烧过程中燃油液面温度和池壁温度的变化规律,研究池壁沸腾传热对油池沸腾燃烧的影响。结果表明:油池沸腾燃烧阶段的燃烧速率明显大于稳定燃烧阶段;燃油液面温度在油池燃烧初期迅速上升至沸点,随后基本保持不变;池壁温度达到并超过燃料的沸点,从而在油池壁面上发生沸腾现象,是油池发生沸腾燃烧的条件。     

Statistical model for combustion of high-metal magnesium-based hydro-reactive fuel

<正>We investigate experimentally and analytically the combustion behavior of a high-metal magnesium-based hydroreactive fuel under high temperature gaseous atmosphere.The fuel studied in this paper contains 73%magnesium powders.An experimental system is designed and experiments are carried out in both argon and water vapor atmospheres. It is found that the burning surface temperature of the fuel is higher in water vapor than that in argon and both of them are higher than the melting point of magnesium,which indicates the molten state of magnesium particles in the burning surface of the fuel.Based on physical considerations and experimental results,a mathematical one-dimensional model is formulated to describe the combustion behavior of the high-metal magnesium-based hydro-reactive fuel.The model enables the evaluation of the burning surface temperature,the burning rate and the flame standoff distance each as a function of chamber pressure and water vapor concentration.The results predicted by the model show that the burning rate and the surface temperature increase when the chamber pressure and the water vapor concentration increase,which are in agreement with the observed experimental trends.     

Effect of flow circulation on combustion dynamics of fire whirl

In this paper, the effect of flow circulation on the combustion dynamics of fire whirl is systematically investigated by experiments. New correlations for the burning rate, flame height, radial temperature and mass flow rate are established for fire whirl. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, which in turn increases both the heat feedback to the fuel surface and the radial velocity in the ground boundary layer, leading to increase of burning rate. A novel idea for correlation of fire whirl flame height is proposed by assuming that the ratio of the fire whirl flame height to the flame height without circulation solely characterizes the effect of circulation. This idea is fully verified, thereby a new formulation for flame height is established, which successfully decouples the burning rate and the circulation. It is indicated that the fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses fire plume radius and thus decreases its increasing rate with vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the low section of flaming region, the significant decrease of mixture between the combustion products and surrounding air dominates the pure aerodynamic effect of flow circulation on the flame height. Finally, it is clarified that fire whirls maintain higher centerline excess temperature than general pool fires due to the effect of less air entrainment.     

Burning and radiance properties of red phosphorus in Magnesium/PTFE/Viton (MTV)-based compositions

Red phosphorus (RP) a highly efficient smoke-producing agent. In this study different contents of RP are added into the Magnesium/PTFE/Viton (MTV)-based composition, with the aim of investigating the influence of RP on the burning and radiance properties of MTV-based composition by using a high-temperature differential thermobalance method, a Fourier Transform Infrared (FTIR) remote-sensing spectrometer, a FTIR Spectrometer and a far-infrared thermal imager. The results show that RP improves the initial reaction temperature and reduces the mass burning rate by 0.1–0.17 g·s⁻¹ (34–59%). The addition of RP has no obvious effect on the burning temperature and far-infrared radiation brightness, but the radiating area raises substantially (by 141%), and thus improves the radiation intensity (by 155%).     

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.     

合金钢封装光纤Bragg光栅传感器传感特性的研究

提出了一种光纤光栅新型合金钢封装结构。利用等强度悬臂梁和温度控制箱对合金钢封装光纤布拉格光栅的应力和温度传感特性进行了测量。实验表明,采用该种封装的光纤光栅传感器保持了裸光纤光栅的响应灵敏度,其拉应变灵敏度系数为1.17pm/με,压应变灵敏度系数为1.2pm/με,温度灵敏度系数为11.3pm/℃,线性响应度在0.9995以上,可满足实际应用的要求。     

On the structure of the combustion wave of nitroglycerine-based propellants

The parameters of the temperature distribution in the combustion wave of nitroglycerin-based propellants N and NB are analyzed and compared. The aim of the study is to explain the known experimental fact that the size of hotspots and the critical quenching diameter for propellant NB (more rapidly burning) are larger than those for propellant N. It is demonstrated that, at a given burning rate, the burning surface temperature, heat conduction zone thickness, temperature gradient near the burning surface, and the dark zone temperature for propellants N and NB are the same, but the fizz zone thickness for NB is approximately twice as wide as that for N. The dependence of the ratio of the hotspot size to the fizz zone thickness is described by a single power law for both propellants. It is also shown that the hotspot size can be defined as the distance between two consecutive transverse waves, which, in turn, is determined by the delay in the initiation of each following wave.     

着火油罐燃烧特性的理论分析

本文利用自行建立的油罐火灾燃烧特性通用模型,计算得到了燃烧速度、火焰高度,火焰跳动频率和平均温度等油罐燃烧特性的变化规律,并深入探讨了油品燃烧速度在油罐直径、风速、环境温度和油位等因素影响下的变化趋势。为了验证理论计算的准确性,将计算结果与汽油和柴油储罐的燃烧实验数据进行了对比,获得了较为满意的结果。     

Combustion of alane and aluminum with water for hydrogen and thermal energy generation

The combustion of alane and aluminum with water in its frozen state has been studied experimentally and theoretically. Both nano and micron-sized particles are considered over a broad range of pressure. The linear burning rate and chemical efficiency are obtained using a constant-pressure strand burner and constant-volume cell, respectively. The effect of replacing nano-Al particles by micron-sized Al and alane particles are examined systematically with the additive mass fraction up to 25%. The equivalence ratio is fixed at 0.943. The pressure dependence of the burning rate follows the power law, r_b = aPⁿ, with n ranging from 0.41 to 0.51 for all the materials considered. The burning rate decreases with increasing alane concentration, whereas it remains approximately constant with cases containing only Al particles. The chemical efficiency ranged from 32% to 83%, depending on the mixture composition and pressure. Thermo-chemical analyses are conducted to provide insight into underlying causes of the decreased burning rate of the alanized compositions. A theoretical model is also developed to explore the detailed flame structure and burning properties. Reasonably good agreement is achieved with experimental observations.     

Experimental research on combustion dynamics of medium-scale fire whirl

The medium-scale fire whirl was extensively investigated by experimental means, in order to establish correlations of the burning rate, flame height and flame temperature of fire whirl, and to clarify the difference between fire whirls and general pool fires. Experimental observations and data confirmed that a free burning fire whirl is a highly stable burning phenomenon with large quasi-steady periods. Burning rates of fire whirls depend on pool diameter similarly to those of general pool fires; however the transition turbulent burning occurs sooner as the pool diameter increases. The lip height seems to have little effect on the burning rate of fire whirls. The correlation was proposed to couple the height of fire whirl to the fire release rate and ambient circulation. It correlates the data from both this work and the literature. Radial temperature profiles in the continuous region of the fire whirl were confirmed to be hump-type, implying the existence of fuel-rich inner core. The pool diameter and heat release rate do not significantly affect the radial temperature profiles in non-dimensional radial coordinates. It was found that the fire plume of fire whirl involves three distinct zones just like that of pool fire, but with different normalized ranges. Fire whirls maintain a higher ratio of continuous flame height to the overall flame height, and also higher maximum centerline excess temperature in continuous flame region, as compared to general pool fires. It was further demonstrated that the fire whirl plume at its origin behaves like a turbulent jet with moderate swirling, and then tends to become buoyancy dominated downstream, with slight swirling. With an increase in dimensionless height adjusted by the plume origin, the plume centerline excess temperature decays rapidly and approaches the theoretical value of −5/3 for free buoyancy plume.