Theoretical Research on the Combustion Characteristics of Ammonium Dinitramide-Based Non-Toxic Aerospace Propellant

Abstract

Propellants play a crucial role in the propulsion systems of aerospace vehicles, and their combustion characteristics are susceptible to external environmental conditions. This study systematically investigated the impact of various initial conditions on the combustion process of ADN-based propellant, including combustion products, equilibrium pressure, adiabatic temperature, and ignition delay time. The results indicate that the primary combustion products of ADN-based propellant include N₂O, N₂, CO₂, OH, and others. ADN-based propellant exhibits a distinct two-stage combustion process under low pressure and temperature conditions (P₀ = 2 atm, T₀ = 586 K). Conversely, under high pressure and temperature conditions (P₀ = 10 atm, T₀ = 2930 K), the two stages of combustion occur almost simultaneously, making them difficult to distinguish. Furthermore, as the initial temperature increases, the ignition delay time decreases significantly, and the combustion rate accelerates. When the initial temperature rises from 400 K to 2800 K at a pressure of P₀ =10 atm, the ignition delay time decreases from 3.5 ms to 0.6 μs. Interestingly, changes in initial pressure have a relatively minor impact on the ignition delay time compared to changes in temperature. Therefore, temperature has a more crucial influence on the combustion characteristics of ADN-based propellant than pressure. This study holds promise for providing new combustion optimization strategies for the aerospace industry and promoting the development of aircraft designs towards higher performance and sustainability.

1. Introduction

With the rapid development of aerospace technology, small satellites, microsatellites, and planetary probes, among other space exploration technologies, are thriving. Propellants, serving as the core power source for various carrier rockets and spacecraft, directly impact the performance of thrusters and are crucial for advancing new-generation carrier rockets and spacecraft. Ammonium dinitramide (ADN)-based propellants [1,2,3,4,5], with their significant advantages of high energy, high density, low toxicity, positive oxygen balance, and high specific impulse, have gradually emerged as a strong candidate for the new generation of hydrazine-based propellants. ADN, with the chemical formula NH₄N(NO₂)₂, is a white crystal composed of the anion N(NO₂)⁻ and the cation NH₄⁺ [6,7,8]. Since its first synthesis by the Zelinsky Institute of the former Soviet Union in 1971, the successful on-orbit application of ADN-based propellants has attracted widespread attention in the industry [9,10]. The combination of ADN as an oxidizer with methanol (CH₃OH) dissolved in water has been recognized as a formulation strategy for ADN-based propellants, which are regarded as a promising substitute for conventional hydrazine-based monopropellants [11,12,13,14]. Yao et al. [15] from the Beijing Institute of Control Engineering used Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology to accurately measure the CO concentration and gas temperature generated during the decomposition and combustion processes of ADN-based propellants. Zeng et al. [16] combined TDLAS with monochromatic radiation temperature measurement technology to deeply investigate the combustion characteristics of ADN-based thrusters under different operating modes and successfully measured intermediate products and gas temperatures. Wang et al. [17] from the Institute of Mechanics, Chinese Academy of Sciences systematically studied the influence of different laser energies and delay times on the plasma characteristics of ADN-based propellants using laser-induced emission spectroscopy technology, revealing important findings such as an initial electron temperature of 68,000 K and an electron density of 1.6 × 10⁻¹⁹. Yu et al. [18,19,20] from Beijing Jiaotong University focused on the influence laws of different microwave powers, gas flow rates, and propellant flow rates on microwave plasma ignition and the combustion of ADN-based liquid propellants. Experimental results showed that increasing microwave power significantly promotes the combustion of ADN-based liquid propellants. Within our research group, Jiang et al. [21] investigated the performance of ADN-based liquid propellant in a pulse laser ablation micro-propulsion system based on laser-induced breakdown spectroscopy. The results demonstrated that when the incident laser energy was added to 100 mJ and 21 wt. % AMIMDCA was used as fuel, a strong and stable plasma signal was observed. Moreover, Du and colleagues [22] documented the application of high-energy pulsed laser ablation to ADN-based propellant droplets positioned within micron-scale pits, yielding an impulse of 9.8 µN·s and a specific impulse of 234.9 s, thereby proving the feasibility of generating micro-thrust via the pulsed laser ablation of microdroplets.

Simultaneously, significant progress and fresh breakthroughs have been achieved in related theoretical modeling endeavors. Gustavo F. Velardez et al. [23] performed molecular dynamics calculations on the melting point and liquid properties of ADN. Zhang et al. [24] conducted comprehensive simulations to analyze the atomization characteristics, decomposition and combustion, and thruster structural parameters of ADN-based propellants, revealing the temperature and pressure distribution patterns inside the thruster. Jing et al. [25,26] at Tsinghua University proposed a detailed reaction mechanism for the combustion process of ADN-based propellants, encompassing 48 species and 242 chemical reactions. After that, the chemical mechanism of ADN-based propellants consisting of 18 species and 39 reactions was successfully simplified to simulate the reaction process of gaseous ADN and methanol, revealing the asynchrony between ADN decomposition and methanol oxidation within the reaction chamber. Liu et.al [27] at the Beijing Institute of Control Engineering employed three-dimensional numerical simulations to investigate the flow and phase change characteristics of the propellant at the microscale of capillaries within the ADN-based micro-thruster.

In summary, previous research on ADN has primarily focused on its physicochemical properties and dynamic behaviors. However, the thermal decomposition and detailed combustion mechanisms of ADN-based propellants remain unclear. To address these gaps, we investigated the combustion characteristics of ADN-based propellants under varied initial conditions. Our research systematically analyzed the influence of different temperatures and pressures on core combustion parameters, including combustion products (N₂O, N₂, CO₂, NO, and OH), the equilibrium pressure (P_E), adiabatic combustion temperature (T_ad), and ignition delay time (τ). Our findings are expected to provide guidance for enhancing thruster propulsion performance and promoting the development of aerospace towards high performance and sustainability.

2. Theoretical Methods

OpenFOAM V6 is a well-established software program for computational fluid dynamics (CFD) calculations. Being an open-source platform, it effectively handles the computation of combustion reactions [28,29,30]. In this study, we utilize the reacting Foam solver within OpenFOAM to perform numerical simulations of ADN-based propellant. The solver introduces adaptive mesh refinement technology to enhance computational speed. The pertinent equations governing mass conservation, momentum conservation, species conservation, and energy conservation are presented in Equation (1), as detailed in [31]. The solution to the control equation is obtained through the integrated finite volume method solver, while the pressure implicit operator splitting (PISO) algorithm, sourced from [32], is employed to adjust the velocity field based on the pressure equation’s output.

$$\begin{array}{l}{\displaystyle \frac{\partial \rho }{\partial t}}+{\displaystyle \frac{\partial (\rho u_i)}{\partial x_i}}=0\\ {\displaystyle \frac{\partial (\rho u_j)}{\partial t}}+{\displaystyle \frac{\partial }{\partial x_i}}(\rho u_i{u}_j)=-{\displaystyle \frac{\partial p}{\partial x_j}}+{\displaystyle \frac{\partial {\tau }_{ij}}{\partial x_i}}\\ {\displaystyle \frac{\partial (\rho Y_k)}{\partial t}}+{\displaystyle \frac{\partial (\rho u_i{Y}_k)}{\partial x_i}}+{\displaystyle \frac{\partial (\rho V_{k,i}{Y}_k)}{\partial x_i}}={\dot{\omega }}_k\begin{array}{cc}& k=1,\dots ,N\end{array}\\ {\displaystyle \frac{\partial (\rho E)}{\partial t}}+{\displaystyle \frac{\partial }{\partial x_i}}(\rho u_{i}E)={\dot{\omega }}_T-{\displaystyle \frac{\partial q_i}{\partial x_i}}+{\displaystyle \frac{\partial }{\partial x_j}}({\sigma }_{ij}{u}_i)\end{array}$$

(1)

where ${\tau }_{ij}$ is the viscous stress tensor; $V_{k,i}$ is the i-component of the diffusion velocity for the kth component; $Y_k$ is the mass fraction of the kth component; ${\dot{\omega }}_k$ is the reaction rate of the kth component; ${\dot{\omega }}_T$ is the heat release rate of the combustion process; $q_i$ is the heat flux, including the thermal diffusion term caused by the temperature gradient and the thermal diffusion term caused by the diffusion of components with different specific enthalpies; and ${\sigma }_{ij}$ is the deviatoric stress tensor [31].

The ADN-based propellant employed in the present investigation is designed as a mixture consisting of three components: 63% ADN, 11% methanol (CH₃OH), and 26% water (H₂O). The percentage mentioned here refers to the mass fraction. The ADN-based propellant composition (63% ADN, 11% methanol, and 26% water) was determined based on the experimental measurements by Jing et al. from Tsinghua University [26]. A simplified mechanism of ADN-based propellant with 18 species and 39 elementary reactions was used in our investigation [4]. The solution to the chemical reaction process employs the seulex ode solver, which utilizes the explicit Euler method for time advancement. Furthermore, to ensure the reliability of numerical simulation results obtained using the simplified mechanism, a comparative analysis was conducted for typical scenarios. As illustrated in Figure 1, we calculated the zero-dimensional homogeneous ignition process of ADN-based propellant at various temperatures (370 K, 400 K, 450 K, and 500 K) under atmospheric pressure conditions and compared the time-varying curves of mixture temperature obtained from both the simplified and detailed chemical reaction mechanisms. The results indicate that the simplified mechanism can accurately predict the ignition process of the mixture under different initial temperature conditions. Moreover, the diffusion problems (Figure S1) and shock tube problems (Figure S2) serve as validation cases. The research findings indicate a high degree of concordance between the data produced by our solver and the predictive results of the ASURF program, confirming the trustworthiness of our model. Therefore, all numerical simulation results in this work were obtained using the simplified mechanism. Moreover, the simulated operating conditions with different initial pressures and temperatures are shown in

Table 1. The simulated operating conditions with different initial pressures and temperatures.

Conditions	Pressure (atm)	Temperature (K)	Conditions	Pressure (atm)	Temperature (K)
1	1	400	13	7	2000
2	1	1200	14	7	2800
3	1	2000	15	10	400
4	1	2800	16	10	1200
5	4	400	17	10	2000
6	4	1200	18	10	2800
7	4	2000	17	15	400
8	4	2800	18	15	1200
9	7	400	19	15	2000
10	7	1200	20	15	2800

.

3. Results and Discussion

3.1. Time-Evolution Temperature and Pressure Analysis of ADN-Based Propellant

As we all know, temperature and pressure are two key factors that affect the combustion reaction rate and product distribution. Figure 2 illustrates the evolution of temperature and pressure over time during the combustion of ADN-based propellants under different initial conditions. Under the initial conditions of pressure P₀ = 2 atm and temperature T₀ = 586 K (hereinafter referred to as the “low-temperature, low-pressure scenario”), the trends in temperature and pressure caused by the combustion of ADN-based propellants exhibit overall consistency. As shown in Figure 2a, the temperature and pressure generated by the combustion of ADN-based propellants display a clear two-stage increase at t = 0.3 ms and t = 0.75 ms. For instance, at t = 0.3 ms, the temperature rapidly rises from the initial 586 K to approximately 1200 K. Here, we define the period t < 0.3 ms as the first-stage ignition process and t > 0.3 ms as the second-stage ignition process, using 0.3 ms as the dividing line. In contrast to the low-temperature, low-pressure scenario, under the initial conditions of pressure P₀ = 10 atm and temperature T₀ = 2930 K (hereinafter referred to as the “high-temperature, high-pressure scenario”), as shown in Figure 2b, no two-stage characteristics are observed in the temperature and pressure of ADN-based propellant combustion. This is primarily due to the higher initial temperature, which accelerates the combustion reaction process, with the second-stage combustion occurring simultaneously with the first-stage combustion. Additionally, it is noteworthy that during the initial ignition stage (at t = 0.1 μs), there is a decline in temperature (approximately 40 K), which differs from the continuous temperature increase observed in the low-temperature, low-pressure scenario. This phenomenon may be attributed to the different initial conditions affecting the chemical combustion reaction of ADN-based propellants. By comparing the simulation results of the low-temperature, low-pressure scenario and the high-temperature, high-pressure scenario, it is found that the ignition time of ADN-based propellants is shortened from 0.8 ms in the low-temperature, low-pressure scenario to 4 μs in the high-temperature, high-pressure scenario, indicating that the high-temperature, high-pressure initial conditions effectively accelerate the combustion reaction rate of ADN-based propellants.

3.2. Combustion Products Analysis of ADN-Based Propellant

The evolution of primary combustion components and its mass fractions of ADN-based propellant were performed and are shown in Figure 3. The main combustion components involved in the combustion process of ADN-based propellant include N₂O, N₂, CO₂, OH, CH₃OH, and NH₄N(NO₂)₂. As shown in Figure 3a, under low-temperature, low-pressure conditions, when t = 0.3 ms, the mass fraction of the reactant NH₄N(NO₂)₂ rapidly decreases to zero, while the mass fraction of N₂O increases from zero to 0.20, indicating that NH₄N(NO₂)₂ is completely decomposed within the first stage, producing the intermediate product N₂O. Concurrently, the mass fraction of CH₃OH decreases from 0.16 to 0.10 during this stage, suggesting that CH₃OH has begun to decompose. In the second-stage ignition process (t > 0.3 ms), as time progresses, CH₃OH is completely consumed and converted into the product CO₂, while the mass fraction of the intermediate product N₂O decreases and results in the formation of the final product N₂; complete ignition occurs in the mixture, and the temperature reaches the adiabatic combustion temperature. For high-temperature, high-pressure conditions (Figure 3b), due to the higher initial temperature, intermediate species such as N₂O generated from the combustion reaction of NH₄N(NO₂)₂ in the first stage are quickly consumed, leading to simultaneous production and consumption of N₂O that cannot be clearly distinguished. Therefore, the second-stage ignition occurs concurrently with the first stage, which is consistent with previous analysis of temperature and pressure changes. Additionally, the consumption rate of CH₃OH accelerates under high-temperature, high-pressure conditions and its mass fraction has already decreased to zero when NH₄N(NO₂)₂ begins to be consumed. This contrasts with the pattern observed under low-temperature, low-pressure conditions, where NH₄N(NO₂)₂ reacts with CH₃OH first, potentially due to the rapid decomposition of CH₃OH at high temperatures and pressures leading to a temperature drop during the initial ignition stage. Different initial conditions result in different combustion reaction mechanisms of ADN. Therefore, varying initial conditions can effectively regulate the combustion reaction of ADN-based propellants.

3.3. Equilibrium Pressure and Adiabatic Temperature Analysis of ADN-Based Propellant

The influence of initial temperature and pressure on the equilibrium pressure (P_E) and adiabatic combustion temperature (T_ad) of ADN-based propellants was further investigated. As shown in Figure 4a, when the initial pressure P₀ remains constant, gradually increasing the initial temperature T₀ results in an overall decreasing trend in P_E after the combustion of ADN-based propellants. Additionally, there is a positive correlation between the initial pressure P₀ and P_E; as P₀ increases, P_E also increases. Unlike the trend observed for equilibrium pressure, when the initial pressure P₀ is constant, the adiabatic combustion temperature T_ad of ADN-based propellants after combustion increases with an increase in initial temperature, as illustrated in Figure 4b. Notably, the increase in T_ad is much smaller than the change in T₀. Taking the condition with P₀ = 15 atm as an example, when the initial temperature varies from 400 K to 2800 K, the adiabatic combustion temperature only rises from 2962 K to 3789 K. This is because the heat release during the combustion process raises the temperature, inducing a shift in the reaction equilibrium towards the reverse direction. Simultaneously, the specific heat capacity of the substances increases with temperature, leading to the adiabatic combustion temperature increasing by a smaller amount than the initial temperature. In summary, the choice of initial temperature has a crucial impact on the combustion reaction. There exists an appropriate range for selecting the initial temperature; it is not necessarily better to choose a higher one.

Subsequently, the impact of different initial pressures on the P_E and T_ad of ADN-based propellants was studied. As shown in Figure 5a, research indicates that, with a constant initial temperature, the P_E of ADN-based propellants after ignition increases with an increase in initial pressure P₀, and a clear positive correlation is observed between P_E and P₀. The slope of the curve representing the change in P_E with P₀ decreases as the initial temperature of the mixture rises. Figure 5b displays the variation in the adiabatic combustion temperature Tad of the mixture with initial pressure. The results show that the adiabatic combustion temperature increases with an increase in initial pressure and the effect of initial temperature on the adiabatic combustion temperature of the mixture is more pronounced at higher temperatures (T₀ = 2800 K) compared to lower temperatures (T₀ = 400 K).

3.4. Ignition Delay Time Analysis of ADN-Based Propellant

Figure 6 displays the curves illustrating the impact of various initial temperatures and pressures on the ignition delay time (τ) of ADN-based propellants. As shown in Figure 6a, under constant initial pressure conditions, the ignition delay time decreases as the initial temperature increases. This is because a higher initial temperature facilitates the achievement of ignition conditions for ADN propellants and accelerates the rate of combustion chemical reactions. For different initial pressures, the ignition delay times of ADN-based propellants at the same temperature are similar, indicating that the sensitivity of the ignition delay time of ADN fuels to initial temperature is significantly higher than to initial pressure. Figure 6b depicts the effect of different initial pressures on the ignition delay time. The calculation results show that the ignition delay time decreases slightly with an increase in initial pressure. However, for the same initial pressure, the ignition delay time of the mixture decreases significantly as the temperature increases. Taking P₀ = 10 atm as an example, when the initial temperature of the mixture increases from 400 K to 2800 K, the ignition delay time decreases from 3.5 ms to 0.6 μs. These results indicate that the sensitivity of the ignition delay time of ADN-based propellants to initial temperature is higher than to initial pressure.

4. Conclusions

In summary, this paper investigates the impact of various initial conditions on the combustion characteristics of ADN-based propellants, encompassing combustion products, equilibrium pressure, adiabatic temperature, and ignition delay time. Our findings contribute to enhancing the performance of aircraft propulsion systems and promotes the development of future aircraft designs towards higher performance and greater sustainability. The main conclusions are as follows:

• ADN-based propellants exhibit distinct two-stage combustion characteristics under low-temperature and low-pressure conditions. However, under high-temperature and high-pressure conditions, these two combustion stages occur concurrently and cannot be clearly distinguished.
• An increase in temperature effectively shortened the ignition delay time and accelerated the combustion reactions. When the initial temperature rises from 400 K to 2800 K, the ignition delay time decreases from 3.5 ms to 0.6 μs at a pressure of P₀ = 10 atm.
• Compared to changes in initial temperature, an increase in initial pressure has a relatively minor effect on the ignition delay time. These results indicate that temperature has a more significant impact on the combustion characteristics of ADN-based propellants than pressure.