Numerical Investigation of Droplet Impact Dynamics of Ammonium Dinitramide (ADN)- Based Liquid Propellant on Solid Surfaces

The mechanism for sublimation of NH(4)N(NO(2))(2) (ADN) has been investigated quantum-mechanically with generalized gradient approximation plane-wave density functional theory calculations; the solid surface is represented by a slab model and the periodic boundary conditions are applied. The calculated lattice constants for the bulk ADN, which were found to consist of NH(4)(+)[ON(O)NNO(2)](-) units, instead of NH(4)(+)[N(NO(2))(2)](-), agree quite well with experimental values. Results show that three steps are involved in the sublimation/decomposition of ADN. The first step is the relaxation of the surface layer with 1.6 kcal/mol energy per NH(4)ON(O)NNO(2) unit; the second step is the sublimation of the surface layer to form a molecular [NH(3)]-[HON(O)NNO(2)] complex with a 29.4 kcal/mol sublimation energy, consistent with the experimental observation of Korobeinichev et al. (10) The last step is the dissociation of the [H(3)N]-[HON(O)NNO(2)] complex to give NH(3) and HON(O)NNO(2) with the dissociation energy of 13.9 kcal/mol. Direct formation of NO(2) (g) from solid ADN costs a much higher energy, 58.3 kcal/mol. Our calculated total sublimation enthalpy for ADN(s) → NH(3)(g) + HON(O)NNO(2)) (g), 44.9 kcal/mol via three steps, is in good agreement with the value, 42.1 kcal/mol predicted for the one-step sublimation process in this work and the value 44.0 kcal/mol computed by Politzer et al. (11) using experimental thermochemical data. The sublimation rate constant for the rate-controlling step 2 can be represented as k(sub) = 2.18 × 10(12) exp (-30.5 kcal/mol/RT) s(-1), which agrees well with available experimental data within the temperature range studied. The high pressure limit decomposition rate constant for the molecular complex H(3)N···HON(O)NNO(2) can be expressed by k(dec) = 3.18 × 10(13) exp (-15.09 kcal/mol/RT) s(-1). In addition, water molecules were found to increase the sublimation enthalpy of ADN, contrary to that found in the ammonium perchlorate system, in which water molecules were shown to reduce pronouncedly the enthalpy of sublimation.

The vigorous development of micro–nano satellites urgently requires satellite-borne propulsion systems as support. Pulsed laser ablation micro-propulsion can meet these high demands. Ammonium dinitramide (ADN), as a green monopropellant, can serve as the working substance for laser ablation. This work investigated the micro-propulsion performance of liquid propellants composed of ADN and water with different ADN mass fractions, aiming to clarify the enhancement effect of chemical energy. Through the single-pulse impulse measurement, the results show that the 70 wt.% ADN had a maximum specific impulse of 167.55 s, a 19% increase compared to H2O. The established semi-empirical model of the micro-propulsion performance fits well with the experimental data and can effectively explain the variations in the patterns of the propulsion’s parameters. The chemical energy’s actual rate of contribution to the increase in the kinetic energy was positively correlated with the ADN’s mass fraction and negatively correlated with the laser energy, with an actual contribution rate of 36% for 70 wt.% ADN at a laser energy of 60 mJ. Furthermore, based on the relationship between the ablation efficiency, chemical-specific energy, and laser specific energy, it was found that the ablation efficiency can be improved by increasing the chemical specific energy and reducing the laser specific energy while ensuring the breakdown. This work provides a scientific approach to quantitatively analyze the enhancement in the propulsion’s performance by chemical energy in laser micro-ablation, which is expected to be extended to other energetic liquid propellants.

Ammonium dinitramide (ADN) is a promising high energy oxidizer for rocket propellants because it offers a good oxygen balance and has a significant energy content. As a result, ADN-based energetic ionic liquid propellants (EILPs) have been studied, based on ADN combined with urea and monomethyl ammonium nitrate (MMAN). The thermal decomposition of ADN in the condensed phase affects the combustion of both pure ADN and ADN-based EILPs; thus, it is important to understand the reactions of EILPs in the condensed phase. The present study assessed the reactivity of ADN mixtures in the condensed phase, focussing on hydrogen abstraction reactions with NO2· formed from the thermal decomposition of ADN. The potential energy surfaces of these reactions were obtained using ab initio calculations. The effects of functional groups and of carbon chain length on hydrogen abstraction by NO2· were examined. Mixtures of ADN with urea and acetamide (AA) as amide compounds, and with MMAN and monoethanol amine nitrate (MEAN) as nitrate salts, were examined. Thermal analysis was conducted to investigate the properties of these mixtures, using differential scanning calorimetry (DSC). The calculation results shows that AA and MEAN are more reactive with ADN than urea and MMAN, which is supported by the DSC data. Hydrogen abstraction by NO2· is evidently an important condensed phase reaction in ADN mixtures, and substances having alkyl groups and longer carbon chains are more highly reactive.

In this work, the evaporation and combustion processes of liquid propellant ammonium dinitramide–methanol aqueous solution were investigated by a thermogravimetric analyzer and a Curie-point pyrolysis unit, respectively, and the gas-phase products were measured by a Fourier transform infrared spectroscopy. The results showed that methanol and water evaporated first, followed by the decomposition of ammonium dinitramide at heating rates of 5–. When a catalyst was added, ammonium dinitramide decomposed much faster and released heat that let methanol evaporate immediately as the propellant solution was in contact with the catalyst particles. To explore how the decomposition of ammonium dinitramide is coupled with the oxidation of methanol, the propellant solution was heated by a Curie-point pyrolysis unit at a rate of to temperatures ranging from 160 to 670°C. Large amounts of nitrous oxide and ammonia were detected at all temperatures, suggesting that most of the decomposition of ammonium dinitramide occurred at the condensed phase. Finally, a detailed gas-phase ammonium dinitramide–methanol reaction model was built to analyze the coupling between the decomposition of ammonium dinitramide and the oxidation of methanol.

ABSTRACTThe thermal decomposition of ammonium dinitramide (ADN)‐based liquid propellant (ALP), which is composed of 63.4% ADN, 25.4% H2O, and 11.2% CH3OH by mass fraction, is significantly influenced by nitrogenous gases. To get a better insight into the effect of nitrogenous gases (N2, NH3, and NO2) on the thermal decomposition of ALP, the reactive force field ReaxFF‐lg was employed to describe the bonding and debonding of ALP. Three models (ALP/N2, ALP/NH3, and ALP/NO2) have been constructed to study the reaction pathways, main products, as well as the influences on mechanisms in the thermal high‐temperature decomposition of ALP at 1000–2500 K. The N─N bond in N2 was hardly broken to participate in the thermal high‐temperature decomposition of ALP. NH3 inhibited the decomposition of •NH4 in ADN, while NO2 enhanced the dehydrogenation of NH3 and then accelerated the thermal decomposition of ADN. NH3 facilitated the generation of H2O and reduced the number of •H and •OH, whereas NO2 inhibited the generation of H2O. NO2 promoted the decomposition of CH3OH to produce CO2 at high temperatures. This study provided an atomic mechanism for the decomposition process of ALP/gases, which would be helpful for further studies on the reaction mechanism of energetic fuels.

The search for a smokeless propellant has encouraged scientists and engineers to look for a chlorine-free oxidizer as a substitute for AP (ammonium perchlorate). Endeavors seemed to come to an end when ADN (ammonium dinitramide) appeared in the West in the early 1990s. Although some drawbacks soon became apparent by that time, the foremost obstacle for its use in rocket-motors came from the patent originally applied for in the United States in 1990. Furthermore, environmental concerns have also increased during these two decades. Ammonium perchlorate is believed to cause thyroid cancer by contaminating soil and water. In addition, AP produces hydrogen chloride during burning which can cause acid rain and ozone layer depletion. Unlike AP, ADN stands for both smokeless and green propellant. Since then, much progress has been made in its development in synthesis, re-shaping, microencapsulation and solid propellant. The high solubility of ADN in water has also allowed its application as liquid monopropellant. Tests have revealed Isp (specific impulse) superior to that normally observed with hydrazine, one of the most harmful and hazardous liquid propellants. With constraints of use, along with the patent near to expiry, scientists and engineers are rushing to complete developments and patents until then.

The transmissive mode laser micro-ablation performance of near-infrared (NIR) dye-optimized ammonium dinitramide (ADN)-based liquid propellant was investigated in laser plasma propulsion using a pulse YAG laser with 5 ns pulse width and 1064 nm wavelength. Miniature fiber optic near-infrared spectrometer, differential scanning calorimeter (DSC) and high-speed camera were used to study laser energy deposition, thermal analysis of ADN-based liquid propellants and the flow field evolution process, respectively. Experimental results indicate that two important factors, laser energy deposition efficiency and heat release from energetic liquid propellants, obviously affect the ablation performance. The results showed that the best ablation effect of 0.4 mL ADN solution dissolved in 0.6 mL dye solution (40%-AAD) liquid propellant was obtained with the ADN liquid propellant content increasing in the combustion chamber. Furthermore, adding 2% ammonium perchlorate (AP) solid powder gave rise to variations in the ablation volume and energetic properties of propellants, which enhanced the propellant enthalpy variable and burn rate. Based on the AP optimized laser ablation, the optimal single-pulse impulse (I)~9.8 μN·s, specific impulse (Isp)~234.9 s, impulse coupling coefficient (Cm)~62.43 dyne/W and energy factor (η)~71.2% were obtained in 200 µm scale combustion chamber. This work would enable further improvements in the small volume and high integration of liquid propellant laser micro-thruster.

The thermal decomposition characteristics and fuel oxidation mechanism of ammonium dinitramide (ADN)‐based liquid propellant are of great practical significance, but it is difficult to detect the whole intermediates and products using traditional experimental methods. In this paper, the thermal decomposition of ADN‐based liquid propellant at different temperatures (1500, 2000, 2500, and 3000 K) was simulated using the reactive molecular dynamics method. The effect of temperature on the thermal decomposition of propellant and the distribution of the main products were investigated. The results showed that the temperature played a role in promoting the decomposition of propellant. The main products of ADN‐based liquid propellant were N2, H2O, NH3, HNO3, ⋅NHO, CH2O, CO2, and NO2, while their evolution and generation pathways were also described in detail. The initial decomposition of ADN was the breakage of N−H bonds in ⋅NH4 or N−N/N−O bonds in ⋅N(NO2)2, and methanol generated free radicals by dehydrogenation. There was no direct chemical reaction between ADN and methanol, and the interaction was accomplished through the generation and consumption of free radicals. H2O provided a large amount of ⋅H and ⋅HO leading to a lower activation energy of ADN in propellant than that from reference. The overall reaction mechanism of the liquid propellant was deduced. The results would help to further investigate the reaction mechanism of ADN‐based liquid propellants.

In this paper, the flame structure of a liquid propellant ammonium dinitramide–methanol aqueous solution in a small model thruster is investigated. The thruster is composed of an injector and a combustion chamber (including a catalyst bed) with optical windows, through which the combustion process in the atmosphere is inspected, and the temperature distribution along the flame is measured by the tunable diode laser absorption spectroscopy method. Obvious fluctuations in temperature and flickering yellowish flame downstream of the combustion chamber are observed, indicating that the combustion is not stable. The mean temperature distribution has a significant peak where a yellowish flame appears. To explain the experimental observations, a detailed gas-phase ammonium dinitramide–methanol reaction model is used to establish the flame structure of an ideal liquid propellant, which can be divided into several different regions, including the liquid phase, aerosol zone, and two flame zones. The decomposition of ammonium dinitramide and the evaporation of methanol take place in the liquid phase and aerosol zone, whereas the gas phase reactions of methanol and the decomposition products of ammonium dinitramide take place in the two flame zones. Based on the proposed theory of flame structure, it is seen that, in our experiment, the physical and chemical processes in the liquid phase and aerosol zone are completed upstream of the catalyst bed, and the unstable yellowish flame appearing in the combustion chamber is caused by competitive influence between the two flame zones.

Effects of ignition voltage and electrode structure on electric ignition and combustion characteristics of Ammonium Dinitramide (ADN)-based liquid propellants in electric ignition mode in inert gas environment

Although the solid propellant, ammonium dinitramide (ADN, NH4N(NO2)2) is safe and thermally stable, it requires high purity for practical commercial applications. Even a small amount of impurities in ADN can create negative effects, including catalyst poisoning and thruster nozzle cloggings when it is used as a liquid propellant. Thus, we explored several purification processes for the precipitated ADN particles, such as repetition extraction, adsorption by activated carbons, and low-temperature extraction. These purifying methods help to improve the chemical purity as evaluated by FTIR, UV-vis, DSC, and IC analyses. Among the purification processes, adsorption was found to be the best method, showing a final purity of 99.768% based on relative quantification by ion chromatography.

This paper focuses on the thermal behavior of mixtures of ammonium dinitramide (ADN) and amine nitrates. Because some mixtures of ADN and amine nitrate exhibit low melting points and high-energy content, they represent potential liquid propellants for spacecraft. This study focused on the melting behavior and thermal-decomposition mechanisms in the condensed phase of ADN/amine nitrate mixtures during heating. We measured the melting point and exothermal behavior during constant-rate heating using differential scanning calorimetry and performed thermogravimetry–differential thermal analysis–mass spectrometry (TG–DTA–MS) to analyze the thermal behavior and evolved gases of ADN/amine nitrate mixtures during simultaneous heating to investigate their reaction mechanisms. Results showed that the melting point of ADN was significantly lowered upon the addition of amine nitrate with relatively low molecular volume and low melting point. TG–DTA–MS results showed that the onset temperature of the thermal decomposition of ADN/amine nitrates was similar to that of pure ADN. Furthermore, during thermal decomposition in the condensed phase, ADN produced highly acidic products that promoted exothermic reactions, and we observed the nitration and nitrosation of amines from the dissociation of amine nitrates.

Ammonium dinitramide (ADN) was characterized during recrystallization from the melt. The surface tension of molten ADN at 97 °C was measured to be 89 mN/m. The wetting angles between molten ADN and different solid surfaces (polytetrafluoroethylene, glass, steel, and aluminum) were determined. The wettability depends on the surface tension of molten ADN, the free surface energy of the solid surfaces and the interfacial tension between the solid and liquid. Observations of the recrystallization behavior of molten ADN showed that nucleation does not occur, even at super cooling rates of 70 K. Crystallization can be initiated by the application of seed crystals.

Experimental and numerical studies of ammonium dinitramide based liquid propellant combustion in space thruster

Controlling impact dynamics of droplets on solid surfaces is a significant problem in a variety of applications, such as inkjet printing, spray cooling and coating and so on. Most of fluids used in industries always contain various kinds of additives such as surfactants, polymers and particles. Therefore, these fluids exhibit non-Newtonian behaviors, for instance, yield-stress, viscoelastic, shear-thickening and shear-thinning. The impact dynamics of Newtonian droplets on solid surfaces has been extensively investigated. However, the number of researches about fluids with non-Newtonian properties is comparatively very small. In this work, we employ the finite element scheme coupled with level set method to simulate the impact process of droplets on solid surfaces. The numerical simulation models the presence of shear-thinning viscosity by using the truncated power-law rheological model. We first conduct a mesh convergence study and verify the numerical model. The simulation results are found to be in good agreement with experimental data in the literature. By performing extensive numerical simulations and varying the rheological parameters and surface wettabilities, the influences of these parameters on the impact dynamics are evaluated, and the dominant effects that govern the spreading and receding process are determined. The simulation results show that for the case of droplet impacting on surface with contact angle <i>θ</i> = 55°, the spreading is stronger with power-law index decreasing as evidenced by larger shape deformation and faster interface moving speed. As power-law index decreases, we expect the maximum dimensionless diameter to increase and the minimum dimensionless height to decrease during inertial spreading. For the case of droplet with lower power-law index (<i>m</i> = 0.85 and 0.80), which indicates lower viscous dissipation during impact, the dimensionless parameters have significant differences. After first receding, the impacting droplet is not balanced any more and it starts to spread again until its kinetic energy is completely damped by fluid viscous dissipation. For the case of droplet (<i>m</i> = 0.80) impacting on surface, the center breakage can be observed during droplet spreading, which results from the effect of strong shear-thinning property. When a shear-thinning droplet impacts on a surface with contact angle <i>θ</i> = 100°, the oscillation behavior can be observed and the oscillation amplitude increases as power law index decreases. Bouncing phenomenon can be observed when a droplet impacts on surface with contact angle <i>θ</i> = 160°, regardless of rheological property. Finally, we propose an empirical model to predict the maximum dimensionless diameter of shear-thinning droplet impacting on the surface with contact angle <i>θ</i> = 55° as a function of non-Newtonian Reynolds number <i>Re</i><sub>n</sub>.