Al-Mg alloys have been identified as having significant potential for application in energetic materials and propulsion systems. The thermophysical properties of these alloys directly influence a number of processes, including propellant preparation, storage safety, and combustion efficiency. In this study, a high-precision NNP for Al-Mg alloys was constructed based on first-principles data covering the phase diagram composition. This potential function demonstrates excellent performance in predicting a number of key properties, including energy, atomic forces, crystal structure, mechanical properties, vibrational characteristics, and dynamic evolution over a wide temperature range. This renders it a dependable instrument for atomic-scale thermal property research. Employing the innovative NNP potential, molecular dynamics simulations were employed to systematically investigate the effects of Mg content on the melting point, thermal expansion coefficient, specific heat capacity, thermal conductivity, radial distribution function, and Warren-Cowley parameters of Al-Mg alloys. The findings suggest that even a minimal amount of Mg doping can substantially reduce the alloy's melting point and thermal conductivity, while concurrently increasing the thermal expansion coefficient and specific heat capacity. As the Mg content rises, the thermal expansion coefficient and thermal conductivity demonstrate non-monotonic trends, with inflection points at 40 wt.% and 30 wt.%, respectively. This phenomenon is intimately linked to the stability of the Al-Mg alloys crystal structure. These findings reveal, at the atomic scale, the regulatory mechanisms of Mg content on the thermal properties of Al-Mg alloys, providing an atomic-scale basis for composition design and performance control in propellants.

For graph G , the first Zagreb index M_1(G) and second Zagreb index M_2(G) are defined as: M_1(G)=∑_u∈ V(G)d_G^2(u) and M_2(G)=∑_u∈ E(G)d_G(u)d_G() , where d_G(v) denotes the degree of vertex v in G . The Hyper-Zagreb index HM(G) is defined as: HM(G)=∑_uν∈ E(G)[d_G(u)+d_G(ν)]^2 . In this paper, we introduce a novel class of F-multiplicity corona graphs and derive explicit formulas for their M_1(G) , M_2(G) and HM(G) . Additionally, we demonstrate the chemical relevance of these graphs through applications in molecular design, highlighting their potential for modeling complex chemical structures.

In the current landscape of underwater vehicle development, high-tech advances are increasingly being utilised, especially in the application of high-speed underwater equipment. Based on the concept of water ram engine operation, the formation of a ventilated supercavity by converting seawater into high-temperature water vapour has the potential to reduce the consumption of ventilating gases. However, the kinetic process that promotes the formation of a supercavity by ventilating it with high-temperature water vapour remains unexplored. This study attempts to bridge this knowledge gap by carefully investigating the cavitation flow around an axisymmetric object under various high-temperature water vapour ventilation conditions. Experiments conducted in a cavitation tunnel equipped with a high-temperature water vapour generator showed that the formation of a transparently ventilated supercavity requires a significant increase in the ventilation rate compared to the ventilation rate facilitated by room-temperature non-condensable gases. The injection of high-temperature water vapour produces perturbations along the cavity boundaries, thereby triggering interfacial instability, a phenomenon that is not present in super-cavities generated by non-condensable gases. Further observations revealed a correlation between the elevated ventilation temperature and elevated cavity boundary instability. In addition, the stability of a high-temperature water vapour ventilated supercavity (HWVVS) exhibits high sensitivity to external environmental conditions. The irrational pairing of the ambient cavitation number and ventilation volume poses a potential risk of supercavity collapse.

In this work, the induced coalescence mechanism and dynamic contact behavior of alumina (Al 2 O 3 ) droplets at different impact velocities were investigated for the first time from a microscopic point of view by molecular dynamics (MD) methods through the analysis of axial speed, shrinkage, neck radius ratio, contact force, temperature, kinetic energy, surface energy, and the amount of change in the internal energy of the droplets. The results show that the minimum speed at which collisional coalescence of Al 2 O 3 droplets occurs is 30 m/s. When the speed is lower than 30 m/s, the droplets undergo bounce phenomena due to the Coulomb force. Under the high-speed impact, the inertia force of Al 2 O 3 droplets acts less than the surface tension and viscous resistance. The droplets don’t get squashed in the whole collision process. For the different initial velocities, the magnitude of the contact force on a unilateral droplet during the collision process does not always increase with speed. When the collision speed is not higher than 400[Formula: see text]m/s, the contact force on the droplets eventually stabilizes at about 0.28[Formula: see text]Kcal/(mol⋅Å), whereas this value is about 0.36[Formula: see text]Kcal/(mol⋅Å) and about 0.5[Formula: see text]Kcal/(mol⋅Å) for the intervals from 500[Formula: see text]m/s to 700[Formula: see text]m/s and from 800[Formula: see text]m/s to 1000[Formula: see text]m/s, respectively. The increase of the droplet’s initial speed has a limited contribution to the temperature of the system after the collision, and the amount of loss of the total energy (the sum of kinetic energy, surface energy, and internal energy changes) becomes more pronounced, even up to about 20% when the speed reaches 900[Formula: see text]m/s. At the same time, we predicted the Al 2 O 3 melting point and compared it with the standard melting point with an error of 2%, proving the accuracy of the model. This work can strengthen our understanding of the industrial processes with applications in high-energy nanomaterials, rocket propellants, rocket structure design and performance optimization.

Symmetrization of the coefficient matrix of the wave equation is an effective method to integrate different types of wave equations and reduce the difficulty of wave propagation simulation. It has been successfully applied to acoustic wave equations and elastic wave equations in isotropic and anisotropic media. In this paper, the symmetric format of the coefficient matrix of the two-term medium wave equation will be derived. Subsequently, a multi-axis perfectly matched layer is introduced, and the upwind scheme SBP-SAT difference method is used to discretize the wave equation, and the stability is evaluated by the energy method. Numerical simulations show that the proposed discrete framework has the characteristics of high integration, good stability and strong expansibility. In addition, the method in this paper can stably simulate wave propagation in curved domains and reduce its implementation cost, indicating that the symmetrization method of the coefficient matrix of the wave equation and its discrete framework have broad application prospects in the field of wave propagation simulation

When a supercavitating vehicle performs a flight, the water ramjet of the vehicle exploits the water in the surrounding environment as an oxidiser and is inevitably affected by the vehicle motions. In this study, the characteristics of the performance change in a water ramjet were investigated when a supercavitating vehicle performs a flight to gain more insights into the design of underwater power systems. Given the principle of independent cross-section expansion of cavities, a longitudinal ventilated supercavity model was built to predict the development of a ventilated cavity and its interaction with the body. A thermal calculation model was developed based on the minimum free energy method to calculate the performance parameters of water ramjets. A novel coupling algorithm was proposed based on the aforementioned model to integrate the motion of a supercavitating vehicle with the operation of a water ramjet. The integration described above was achieved by linking the water intake and body motion and calculating the performance change characteristics of the water ramjet during the flight of the supercavitating vehicle. The operating characteristics of the water ramjet with changes in the designed water-fuel ratios were investigated to provide a basis for exploring the unperturbed flight of the vehicle, depth regulation, and underwater environmental changes. The thrust of the water ramjet is less affected by external factors, and its performance feedback tends to exacerbate external disturbances at a designed water-fuel ratio lower than the optimal water-fuel ratio. A water ramjet with a designed water-fuel ratio higher than the optimal water-fuel ratio has a rapid response time, adaptive thrust, and stable combustion chamber pressure. External disturbances slightly affected the thrust and combustion chamber pressure of the water ramjet when the designed water-fuel ratio was equal to the optimal water-fuel ratio. The results of this study provide technical support for optimising the design of water ramjet.

Al nanoparticles (ANPs) have high reactivity and can improve the system's combustion performance. However, ANPs are susceptible to inactivation by external oxidants. Here, we use ethanol and ether molecules to coat ANPs and then compare and discuss the combustion process between coated ANPs and bare ANPs. Our results show that the ethanol/ether coating can adsorb more H2O molecules and increase the active Al atom number and the Al core area in the ignition stage. The combustion phase can be divided into four stages according to the rate of the combustion temperature. The ethanol/ether coating can enable ANPs to deliver a better combustion performance, reducing the ignition delay time of particles, greatly increasing the combustion temperature, and making the whole system enter the gas phase combustion stage. These will enable the ethanol/ether-ANPs systems to release more energy and improve the combustion efficiency of the system.

Supercavitation is a beneficial method to reduce skin friction of underwater objects. To better predict and control the features of a supercavity under the influence of the tail jet, an experimental study of the supercavity under the tail jet condition is conducted in this paper. In this study, some underwater experiments were conducted in the test channel to observe the cavity appearance and internal flow in an environment with free-surface boundaries in which a body is moving and the fluid is resting. In the present experiments, the periodic gas leakage and ring vortices were observed and investigated the Strohal number of periodic gas leakage, which has rarely been mentioned in previous studies. In addition, the effect of tail-jet ventilation conditions on the natural supercavity was investigated. The findings demonstrate that when the ventilation volume is limited, the water vapor can still maintain its dominance and that gas and water vapor interaction takes place in the cavity's inner layer. After increasing the ventilation amount, the interaction surfaces of gas and water vapor in the cavity become more obvious and dominated by gas. The expansion trend of the cavity also alters as ventilation is increased.

Sintering is one of the key factors limiting the application of aluminum nano particles (ANPs) as additives in solid propellant. Because of its high specific surface area and the surface is covered by atoms with low coordination number, ANPs are prone to aggregate or sinter even during the production process which blocks the energy release of ANP during combustion. We perform reactive molecular dynamics simulations to study sintering behaviors for bare and ethanol coated ANPs. Several parameters including shrinkage ratio, gyration radius and moment of inertia are responsible for judging the progress of particle sintering process. A three-stage sintering process has been observed. ANPs follow different sintering laws below and above their melting temperatures. At the same temperature, the smaller the ANP size, the faster the sintering process ends. Before melting, the effect of raising temperature on the sintering degree is not obvious. The coated ANPs show an excellent anti-sinter behavior even when temperature is above their melting point. Different heating rates (1012 K/s–1014K/s) are used to simulate different ignition environments and results show a slow heating rate can make ANPs enter sintering process earlier. For the organic coating layer, carbon and oxygen atoms hinder the formation of the neck region which is an important step in the sintering process. Hydrogen atoms tend to diffuse into the interior of ANPs rather than stay on the surface.

Al nano-particle (ANP) is vulnerable to external oxidation environment due to its high reactivity. To overcome such drawback, we coat ANPs with organic materials: ethanol and diethyl ether. Comparative discussions are raised from the coating to combustion process between bare ANP and other ANPs with different organic coating layers. Our results show the structure of organic coating layer is different for ethanol and diethyl ether molecules. ANPs prefer adsorbing ethanol molecules for its relatively simple structure and molecular polarity. In the heating stage, the presence of organic coating layers is beneficial for ANPs to attract external oxygen molecules, but the content of active Al atoms is reduced. The combustion stage can be divided into four stages according to the rate of temperature change. ANPs coated with organic compounds show a better combustion performance: they sacrifice ignition time but dramatically increase the combustion temperature which leads the whole ANP into a gas-phase combustion. Such improvement will enable ANPs to release more heat, obtain higher burning rate and effectively prevent nozzle blockage.

Jinyun Wang1, Mengjun Wang1, 2, Zailin Yang2, Wenshui Lin3, Huijie Zhou4 1 Hebei Key Laboratory of dual medium power technology, Handan, Hebei; 2 College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin, Heilongjiang; 3 School of informatics, Xiamen University, Xiamen, Fujian; 4 College of Science and Technology, Ningbo University, Ningbo, Zhejiang 【Abstract】Metal fuel has a bright future because it can replace the traditional non renewable energy as a new fuel. In this paper, the research progress of metal fuel technology at home and abroad was summarized. The energy performance of several typical metal fuels were compared and analyzed, and the combustion mechanism of micro-nano metal aluminium based fuel was expounded. In addition, the latest research progress of water-reactive metal fuel engine technology was summarized at home and abroad, and the research status of oxygen-reactive nano iron powder metal fuel engine technology was reviewed. It can provide reference for the future application of metal fuel engine technology and engineering. 【Keywords】Metal fuel technology; Metal fuel engine; Nano metal fuel; Combustion mechanism; Research progress

Molecular dynamics simulations are performed to study thermal properties of bulk iron material and Fe nanoparticles (FNP) by using a ReaxFF reactive force field. Thermodynamic and energy properties such as radial distribution function, Lindemann index and potential energy plots are adopted to study the melting behaviors of FNPs from 300 K to 2500 K. A step-heating method is introduced to obtain equilibrium melting points. Our results show ReaxFF force field is able to detect size effect in FNP melting no matter in energy or structure evolution aspect. Extra storage energy of FNPs caused by defects (0%-10%) is firstly studied in this paper: defects will not affect the melting point of FNPs directly but increase the system energy especially when temperature reaches the melting points.

The shape of an aluminum particle is assumed to be spherical or an equivalent sphere during the combustion process.Such an assumption lacks objectivity and leads to unreasonable approximations of burning efficiency and performance.To investigate the influence of non-spherical particles on burning behavior,this study focused on a theoretical and experimental investigation of the combustion of nanoscale aluminum ellipsoidal particles.Models for prolate and oblate spheroids in aluminum combustion were established to explore combustion properties such as mass release rate,linear burning rate,burning rate,and burnout time.To validate the theoretical results,combustion experiments were conducted on three samples.Reasonable agreement between the results of numerical simulation and experimental findings was obtained in terms of the particle burning characteristics.It was found that particle morphology (such as prolate or oblate spheroid shape) and size play a significant role in the combustion performance of nanosized aluminum particles.

Computational fluid dynamics (CFD) simulation is used to test two body design methods which use negative pressure gradient to suppress laminar flow separation and drag reduction. The steady-state model of the Transition SST model is used to calculate the pressure distribution, wall shear stress, and drag coefficient under zero angle of attack at different velocities. Four bodies designed by two different methods are considered. Our results show the first method is superior to the body of Hansen in drag reduction and the body designed by the first method is more likely to obtain the characteristics of suppressing or eliminating separation, which can effectively improve laminar flow coverage to achieve drag reduction under higher Reynolds number conditions. The results show that the negative pressure gradient method can suppress separation and drag reduction better than the second method. This successful design method is expected to open a promising prospect for its application in the design of small drag, small noise subsonic hydrodynamic hull and underwater weapons.

Combustion mechanism of nano Al particles remains controversies for years. In this paper, we use ReaxFF molecular dynamics simulations to study thermodynamic and structure properties of Al particles from ignition to combustion. Two typical core–shell nano Al models with different oxide shell thickness are studied. Effect of radiative heat transfer is also considered in the combustion stage. Our results show that no shell cracks are observed during both heating and burning periods. The oxide shell hinders the thermal diffusion of core Al atoms and causes the abnormal melting rule. The stress variations at the core–shell interface are the most dramatic during the heating period. The combustion processes are controlled by diffusion behaviors of components in particles and external oxygen molecules. Detailed structural evolution processes are studied by displacement magnitude analysis. Particle with thinner shell has a shorter ignition delay and a higher combustion temperature which directly affect the radiative heat transfer rate during the combustion period.

In this study, novel carbon nanotube (CNT) and transition-metal oxides (TMOs) nanoparticle catalysts were prepared and their effects on the thermal decomposition of potassium perchlorate (KP) were investigated. Nanocatalysts have been widely studied because of their excellent properties, such as the high surface energies of nanoparticles and their ease of agglomeration adversely affect their catalytic performances. Good dispersion is the key to good nanocatalytic activity. The composite catalysts were prepared by loading a nanocatalyst on CNTs; this improves nanoparticle dispersion, promotes electron transfer during the reaction, and increases the catalytic effect. Different types of CNT-based composite catalysts and their corresponding single nanocatalysts were each added to KP. The effects of the catalysts on the thermal decomposition of KP were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The activity of each catalyst was evaluated on the basis of the changes in the thermal decomposition peaks and the apparent decomposition heats. The results show that the activity of a composite catalyst in KP thermal decomposition is much higher than that of its corresponding single nanocatalyst.

In this paper, the CoO/graphene, CuO/graphene, and Fe2O3/graphene composite particles were successfully prepared by liquid deposition method. The crystal structure, morphology and specific surface area of the composite particles were analyzed by XRD, SEM and BET. Micromorphology analysis showed that CoO, CuO, and Fe2O3 were uniformly loaded on graphene slices. The catalytic effect of each catalyst on the thermal decomposition of potassium perchlorate (KP) was investigated by DSC. The results show that CoO/graphene has the best catalytic performance.

The mechanism of coating effects between ether molecules and iron (Fe) nanoparticles was generally estimated using first-principle calculations and molecular dynamic (MD) simulations coupling with Fe (110) crystal layers and sphere models. In the present work, the optimized adsorption site and its energy were confirmed. The single sphere model in MD simulations was studied for typical adsorption behaviors, and the double sphere model was built to be more focused on the gap impact between two particles. In those obtained results, it is demonstrated that ether molecules were prone to be adsorbed on the long bridge site of the Fe (110) crystal while comparing with other potential sites. Although the coating was not completely uniform at early stages, the formation of ether layer ended up being equilibrated finally. Accompanied with charge transfer, those coated ether molecules exerted much binding force on the shell Fe atoms. Additionally, when free ether molecules were close to the gap between two nanoparticles, they were found to come under double adsorption effects. Although this effect might not be sufficient to keep them adsorbed, the movement of these ether molecules were hindered to some extent.

The adsorption process of ethanol molecules on Al slabs was investigated by molecular dynamic simulations with a ReaxFF force field. The force field used in this paper has been validated by comparing adsorption energy results with quantum mechanical (QM) calculations. All simulations were performed under the canonical (NVT) ensemble. The single-molecule adsorption simulation shows that the hydroxyl group plays a more important role in the whole progress than the ethyl group. Besides, decomposition of hydroxyl groups was also observed during multimolecule adsorption processes. Simulations of adsorption processes of Al slab by ethanol molecules at different temperatures and pressures (controlled by the number of ethanol molecules) was also performed. System energy and radial distribution function (RDF) plots were invoked to describe adsorption processes and centro-symmetry parameter (CSP) analysis was adopted to study the surface properties with coating layers. Our results indicate that the whole adsorption process can be divided into two periods and the greater the pressure, the more ethanol molecules diffuse into the Al slab. How raising the temperature helps the adsorption processes is related to the initial number of molecules. The crystal structure of the Al surface will become amorphous under the constant impact of ethanol molecules.

The adsorption of ethanol molecules on annealed aluminum nanoparticles was investigated by ReaxFF molecular dynamics simulations. The force field used in this paper has been validated by comparing the obtained adsorption process results with the results of quantum mechanical (QM) calculations. First, the case of single molecule adsorption was investigated and was shown to be a kind of chemisorption. We also simulated the processes of coating the aluminum nanoparticles with ethanol molecules at different temperatures and pressures. The results indicate that two zones were formed around the surface of aluminum nanoparticles by different interactions during the adsorption processes. The radial distribution function plots and isothermal adsorption curves were used to describe the behaviors. The results show that hydrogen-bonding formed by the ethanol molecules in the solution is a constituent of the coating layer and that the adsorption rate is proportional to the pressure. The cycle-coating method is adopted to produce a fully coated particle, and the oxidation resistance test shows that at 300 K, the organic layer can adsorb the oxygen atom rather than decompose by oxidation. Our simulation results are in good agreement with the results of experimental observations.