Microwave-assisted ignition is a new ignition method, which has the advantages of reliable ignition and high ignition energy and requires no preheating. In this study, experimental methods were used to study the microwave-assisted ignition and combustion characteristics of ADN-based liquid propellant, and the effects of microwave power and propellant flow rate on combustion flame structure, spectral emission characteristics, and flame temperature were investigated. In the experiment, a microwave-assisted ignition experimental device was established first. The ADN-based liquid propellant was injected into the microwave high-energy region from the bottom of the resonator through a hollow straight tube with an inner diameter of 1 mm, and the gas was introduced in a coaxial manner. The research results demonstrated that when the microwave power increased from 1000 to 2000 W, the flame height increased from 11.12 to 17.32 mm; the free radical intensity of C₂*, NH₂, and HNO* increased significantly; the flame temperature increased by about 28.9%. The increase in microwave power is helpful to the generation of intermediate products and the combustion performance of propellant is better. When the propellant flow rate was increased from 25 to 45 mL/min, it was found that the spray combustion effect was the best when the propellant flow rate was 30 mL/min, the flame height increased by 25.2%, and the flame temperature increased by about 11.3%.

Microwave ignition technology has the advantages of high ignition energy, stable ignition, and spatial multi-point ignition. These advantages make this technology promising for future application in green single-component propellants. In this paper, the ignition characteristics of ammonium dinitramide (ADN)-based liquid propellant droplets under the influence of microwaves at room temperature are investigated using experimental methods. The effects of microwave power on puffing, micro-explosion, and combustion behavior of ADN-based liquid propellant droplets were studied. The droplet and flame diameters were statistically related to time, and the microwave-assisted droplet ignition mechanism was analyzed. A new rectangular waveguide resonant cavity was designed in which the droplet is placed at the maximum electric field strength of the device. The droplet morphology and flame profile inside the resonant cavity were photographed with a high-speed camera. The experimental results showed that the microwave positively influenced the puffing, micro-explosion, and combustion behavior of droplets. When the microwave power was increased from 200 to 280 W, the total droplet evaporation time and ignition delay time were reduced by 56.5% and 35.2%, respectively. The positive effects of microwaves on combustion have been summarized as the thermal effect of microwaves on polar molecules and the promotion of fuel oxidation reactions by microwave-induced plasma. The plasma was found to control the development of the initial flame propagation front and to influence the temperature during the combustion reaction process. In this paper, we propose the mode of droplet combustion under microwave induction as a plasma discharge and several stages of the droplet combustion process. This research provides novel insight into the study of the microwave ignition mechanism of liquid fuels.

In this paper, numerical simulations were used to study the decomposition and combustion processes inside the 0.2 N-class ADN-based thruster, and the effects of two geometrical parameters (length and diameter) of the combustion chamber on the combustion performance were evaluated. The decomposition and combustion processes of the thruster were simulated using a reduced chemical reaction mechanism with 22 components and 20 reactions steps. According to the distribution of the basic physical fields, the variation patterns of the pressure field, velocity field, temperature field, and key component parameters caused by different combustion chamber geometrical parameters were observed and analyzed. The results show that the specific impulse and thrust of the thruster increased and then decreased with the increase of the combustion chamber diameter. When the combustion chamber diameter is 7.9 mm, the specific impulse reaches the maximum value of 206.6 s. Additionally, the specific impulse increased from 186 s to 206 s when the combustion chamber length was changed from 7 mm to 11 mm; the specific impulse increased gradually but not significantly, and the growth trend started to flatten out. The results from the paper can serve as a reference for the design and vacuum testing of an ADN-based thruster.

The objective of this study was to investigate the effects of alumina (Al₂O₃) nanoparticles on the evaporation, ignition, and combustion of ammonium dinitramide (ADN)-based liquid propellants under microwave irradiation. The alumina nanoparticles were uniformly dispersed in the ADN-based propellant at mass ratios of 25, 50, and 100 ppm, resulting in the formation of nano-fuel. Experimental methods were employed to examine the influence of alumina nanoparticles on the micro-explosion intensity, ignition delay time, combustion duration, critical ignition power, and emission spectra of the nano-fuel droplets. The experimental findings revealed a significant increase in the intensity and frequency of micro-explosion events with the inclusion of alumina nanoparticles. Compared to pure propellant, the nano-fuel with a concentration of 25 ppm exhibited a reduction of 33.5% in ignition delay time and a decrease of 9% in critical ignition power. Furthermore, the emission spectra characteristics of the pure propellant and nano-fuel were analyzed, indicating that alumina nanoparticles notably enhanced the thermal decomposition of ADN and the exothermic reaction of methanol combustion. This study provides an effective approach to improve the evaporation and combustion performance of ADN-based liquid propellants, offering potential possibilities for future applications.

As a new type of energy-containing material, Ammonium dinitramide based liquid propellant has the advantages of being green, having low toxicity, good stability, and high safety performance. Traditional catalytic combustion methods require preheating of the catalytic bed and deactivation of the catalytic particles at high temperatures, while microwave ignition methods can effectively solve these problems. To study the combustion characteristics of ADN-based liquid propellants during microwave ignition, the influence of microwave power and gas flow rates on the combustion process are analyzed using experimental methods. A high-speed camera was used to observe the enhanced effects of microwave power and gas flow on plasma and flame. Combined with temperature measurement, the combustion process of ADN-based liquid propellants under the action of plasma was analyzed. The combustion process in the presence of microwaves was observed by comparing parameters such as flame length, flame temperature, and radical intensity. Those results show that, with the increase in microwave power, the luminous burning area of the flame grows significantly. The microwave power is increased by 250 W each, and the flame jet length is increased by nearly 20%. The increase in microwave power also leads to an increase in propellant combustion temperature, however, this increase gradually slows down. At a gas flow rate of 20 L/min, the ADN-based liquid propellant showed the best combustion performance with a maximum jet length of 14.51 cm and an average jet length increase of approximately 85.9% compared to 14 L/min. Too much gas flow rate will hinder the development of the jet, while the high-velocity airflow will have a cooling effect on the flame temperature. The results provide a basis for the specific parameter design of microwave ignition and promote the application of ADN-based liquid propellants in the aerospace field.

Microwave-assisted ignition is an emerging high-performance ignition method with promising future applications in aerospace. In this work, based on a rectangular waveguide resonant cavity test bed, the effects of two parameters (material and diameter) of the microwave antenna on the ignition and combustion characteristics of ADN-based liquid propellant droplets were investigated using experimental methods. A high-speed camera was used to record the droplet combustion process in the combustion chamber, the effect of the microwave antenna on the propellant combustion response was analyzed based on the emission spectroscopy method, and finally, the loss of the microwave antenna was evaluated using a scanning electron microscope. The experimental results show that the droplet has the lowest critical ignition power (179 W) when the material of the microwave antenna is tungsten, but the ignition delay time is higher than that of copper. A finer diameter of microwave antenna is more favorable for plasma generation. At a microwave power of 260 W, the ignition delay time of the droplet with a microwave antenna diameter of 0.3 mm is 100 ms lower than that of 0.8 mm, which is about 37.5%. In addition, this study points out the mechanism of microwave discharge in the droplet combustion process. The metallic microwave antenna not only collects the electrons escaping from the gas discharge, but also generates a large amount of metallic vapor, which provides charged particles to the plasma. This study provides the possibility for the application of microwave-assisted liquid fuel ignition.