Influence of the non-equal aligned nozzles for fuel injection inside the supersonic combustion chamber.

The importance of fuel mixing for the progress of the scramjet engine is indisputable. The present article shows the importance of the non-equal multi-injector system for effective fuel distribution and flame holding inside the combustion segment of a scramjet engine. The supersonic air and fuel jet flow in the non-equal nozzle arrangement is simulated via computational fluid dynamic technique. Two injector types of circular and rectangular nozzle have been analyzed to attain flow characteristics of hydrogen jets at supersonic cross flow. Mach contour is also analyzed for these jet arrangements to show the interface of the jet in the non-equal jet arrangement. Besides, addition of internal air jet is also simulated and evaluated in this research. Our investigation shows that the diffusion height of the fuel jet is higher when a rectangular non-equal nozzle is applied. The circular nozzle is more active in the spreading of the fuel in the combustor and the use of an internal air jet effectively increases fuel in a combustor of the scramjet.

ReaxFF-MPNN machine learning potential: a combination of reactive force field and message passing neural networks.

Reactive force field (ReaxFF) is a powerful computational tool for exploring material properties. In this work, we proposed an enhanced reactive force field model, which uses message passing neural networks (MPNN) to compute the bond order and bond energies. MPNN are a variation of graph neural networks (GNN), which are derived from graph theory. In MPNN or GNN, molecular structures are treated as a graph and atoms and chemical bonds are represented by nodes and edges. The edge states correspond to the bond order in ReaxFF and are updated by message functions according to the message passing algorithms. The results are very encouraging; the investigation of the potential, such as the potential energy surface, reaction energies and equation of state, are greatly improved by this simple improvement. The new potential model, called reactive force field with message passing neural networks (ReaxFF-MPNN), is provided as an interface in an atomic simulation environment (ASE) with which the original ReaxFF and ReaxFF-MPNN potential models can do MD simulations and geometry optimizations within the ASE. Furthermore, machine learning, based on an active learning algorithm and gradient optimizer, is designed to train the model. We found that the active learning machine not only saves the manual work to collect the training data but is also much more effective than the general optimizer.

Applying machine learning to balance performance and stability of high energy density materials.

The long-standing performance-stability contradiction issue of high energy density materials (HEDMs) is of extremely complex and multi-parameter nature. Herein, machine learning was employed to handle 28 feature descriptors and 5 properties of detonation and stability of 153 HEDMs, wherein all 21,648 data used were obtained through high-throughput crystal-level quantum mechanics calculations on supercomputers. Among five models, namely, extreme gradient boosting regression tree (XGBoost), adaptive boosting, random forest, multi-layer perceptron, and kernel ridge regression, were respectively trained and evaluated by stratified sampling and 5-fold cross-validation method. Among them, XGBoost model produced the best scoring metrics in predicting the detonation velocity, detonation pressure, heat of explosion, decomposition temperature, and lattice energy of HEDMs, and XGBoost predictions agreed best with the 1,383 experimental data collected from massive literatures. Feature importance analysis was conducted to obtain data-driven insight into the causality of the performance-stability contradiction and delivered the optimal range of key features for more efficient rational design of advanced HEDMs.

Anisotropic hydrogen bond structures and orientation dependence of shock sensitivity in crystalline 1,3,5-tri-amino-2,4,6-tri-nitrobenzene (TATB).

The orientation dependence of shock sensitivity in high explosive crystals was explored in this study. As a widely used wood explosive, 1,3,5-tri-amino-2,4,6-tri-nitrobenzene (TATB) is insensitive to thermal ignition and mechanical impact. Its typical anisotropic crystal structure suggests anisotropic shock sensitivity. Shockwaves were applied to an incised TATB crystal along three orthogonal directions using the multiscale shock technique (MSST) combined with the ReaxFF method to study the origin of anisotropic shock sensitivity. The physical and chemical responses of the TATB crystal during shock were investigated. The results show that the temperature, stress, volume compressibility, and decomposition rate of TATB are strongly dependent on the shockwave direction. In other words, the sensitivity of TATB to mechanical shock is strongly dependent on the crystal orientation. TATB is relatively sensitive along the directions parallel to the (001) crystal plane (X and Y directions) and is highly insensitive along the [001] direction (Z direction). We calculated the energy of intermolecular hydrogen bonds and the elastic constants of the TATB crystal using ab initio simulations, which also show anisotropy. We found that the unique structure of intermolecular hydrogen bonds and the difference in temperature rise induced by orientation-related compressibility are primarily responsible for the anisotropic shock wave sensitivity.

Comparison study of carbon clusters formation during thermal decomposition of 1,3,5-triamino-2,4,6-trinitrobenzene and benzotrifuroxan: a ReaxFF based sequential molecular dynamics simulation.

Carbon rich clusters are usually found after the detonation of explosives, which greatly hinder their further decomposition into small molecules. A comparison study of thermal decomposition and clusters formation between 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) and benzotrifuroxan (BTF) crystals was conducted to uncover the mechanisms behind their distinct differences in sensitivity and reaction violence, which has not been investigated in detail. The simulations of heating at 3500 K, then expansion and cooling were conducted through reactive molecular dynamics using the ReaxFF-lg force field. As a result, the initial low decay rate indicates that TATB is more stable than BTF under high temperatures, while once ignited it decays faster than BTF. Nevertheless, BTF decomposes more completely with a higher potential energy release, a greater amount of final products, and higher reaction frequencies, and shows higher reaction violence than TATB. More and heavier clusters occur in TATB crystals compared with those in BTF. Large clusters form during the heating process and then partly dissociate during expansion and cooling. A faster cooling rate facilitates larger clusters formation. Graphitic geometries as well as carbon rings and carbon chains are common in the stable clusters. Besides, further simulations show that a lower heating temperature facilitates larger clusters formation both in TATB and BTF. Our results are expected to deepen the insight into the mechanisms of carbon clusters formation and the different performances of TATB and BTF.

Litchi-like Core-Shell HMX@HPW@PDA Microparticles for Polymer-Bonded Energetic Composites with Low Sensitivity and High Mechanical Properties.

The reduction of interfacial interaction and the deterioration of mechanical properties by the introduction of the paraffin wax is a long-standing problem. To address it, a novel litchi-like core-shell 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)@paraffin wax@polydopamine (PDA) structure was constructed with a new high melting point paraffin wax (HPW, 101.9 °C) as the inner shell and the bioinspired strong adhesive PDA as the exterior shell. The evolution of element states on the surface of energetic microcapsules conducted by X-ray photoelectron spectroscopy indicated the successful introduction of paraffin wax and PDA to form the core@double shell structure. Compared with the core@double shell particles based on the conventional low melting point paraffin wax (69.8 °C), the HMX@HPW@PDA particles demonstrated a 117% increase of impact energy EBAM from 6 J to 13 J by the Bundesanstalt für Materialprüfung (BAM) method. Attributed to the stronger interfacial interaction, the litchi-like core-shell HMX@paraffin wax@PDA-based energetic composites also exhibited much superior mechanical properties than that of the corresponding HMX@paraffin wax-based ones and could be equal to or even higher than that of the raw HMX-based ones. In addition, the ß-δ phase transition temperature of HMX in HMX@HPW@PDA crystals was improved by 11.3 °C than that of raw HMX. The simplicity and scalability of the described approach provided a creative opportunity for design and fabrication of energetic composites with high safety performance and mechanical properties.

The solid phase thermal decomposition and nanocrystal effect of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) via ReaxFF large-scale molecular dynamics simulation.

The solid phase thermal decomposition and nanocrystal effect are extremely important to understand the ignition, combustion, reaction growth and buildup to detonation under shock wave action. To explore the basic mechanism at the atomic level and understand the interaction among nanocrystal lattices, molecules, and intermediates during the solid phase decomposition, ReaxFF large-scale molecular dynamics simulation at 1000-3000 K was demonstrated on the solid phase of nanocrystalline RDX with a size in the range of 5-12 nm. Based on the analysis of the RDX decay and chemical species, we found that the whole decomposition process can be divided into the solid-affected stage and the following less-condensed phase stage. From the results of NO2 diffusion and high frequency reaction statistics for the nanocrystal effect on the RDX decay, intermediate diffusion was found to be strongly associated with the chemical pathway. In addition, it was found for the first time that the thermal decomposition of RDX originates from the inside of the nanocrystal instead of its surface. Furthermore, a promising uniform energy distribution mechanism transfer by vibration inside the nanocrystalline RDX was demonstrated. The detailed information derived from this study can aid in the thorough understanding of the size effect on the chemical kinetics of nanoexplosives, especially for thermal decomposition and reaction growth.

Self-assembly of single-wall carbon nanotubes during the cooling process of hot carbon gas.

In this work, self-assembly mechanism of single-wall carbon nanotube (SWCNT) during the annealing process of hot gaseous carbon is presented using reactive force field (ReaxFF)-based reactive molecular simulations. A series of simulations were performed on the evolution of reactive carbon gas. The simulation results show that the reactive carbon gas can be assembled into regular SWCNT without a catalyst. Five distinct stages of SWCNT self-assembly are proposed. For some initial configurations, the CNT was found to spin at an ultra-high rate after the nucleation. Graphical abstract Self-assembly process of single-wall carbon nanotube from the annealing of hot gaseous carbon.

Cluster Evolution at Early Stages of 1,3,5-Triamino-2,4,6-trinitrobenzene under Various Heating Conditions: A Molecular Reactive Force Field Study.

We carried out reactive molecular dynamics simulations by ReaxFF to study the initial events of an insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) against various thermal stimuli including constant-temperature heating, programmed heating, and adiabatic heating to simulate TATB suffering from accidental heating in reality. Cluster evolution at the early stage of the thermal decomposition of condensed TATB was the main focus as cluster formation primarily occurs when TATB is heated. The results show that cluster formation is the balance of the competition of intermolecular collision and molecular decomposition of TATB, that is, an appropriate temperature and certain duration are required for cluster formation and preservation. The temperature in the range of 2000-3000 K was found to be optimum for fast formation and a period of preservation. Besides, the intra- and intermolecular H transfers are always favorable, whereas the C-NO2 partition was favorable at high temperature. The simulation results are helpful to deepen the insight into the thermal properties of condensed TATB.

Cluster evolution during the early stages of heating explosives and its relationship to sensitivity: a comparative study of TATB, ß-HMX and PETN by molecular reactive force field simulations.

Clustering is experimentally and theoretically verified during the complicated processes involved in heating high explosives, and has been thought to influence their detonation properties. However, a detailed description of the clustering that occurs has not been fully elucidated. We used molecular dynamic simulations with an improved reactive force field, ReaxFF_lg, to carry out a comparative study of cluster evolution during the early stages of heating for three representative explosives: 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), ß-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and pentaerythritol tetranitrate (PETN). These representatives vary greatly in their oxygen balance (OB), molecular structure, stability and experimental sensitivity. We found that when heated, TATB, HMX and PETN differ in the size, amount, proportion and lifetime of their clusters. We also found that the clustering tendency of explosives decreases as their OB becomes less negative. We propose that the relationship between OB and clustering can be attributed to the role of clustering in detonation. That is, clusters can form more readily in a high explosive with a more negative OB, which retard its energy release, secondary decomposition, further decomposition to final small molecule products and widen its detonation reaction zone. Moreover, we found that the carbon content of the clusters increases during clustering, in accordance with the observed soot, which is mainly composed of carbon as the final product of detonation or deflagration.

Self-enhanced catalytic activities of functionalized graphene sheets in the combustion of nitromethane: molecular dynamic simulations by molecular reactive force field.

Functionalized graphene sheet (FGS) is a promising additive that enhances fuel/propellant combustion, and the determination of its mechanism has attracted much interest. In the present study, a series of molecular dynamic simulations based on a reactive force field (ReaxFF) are performed to explore the catalytic activity (CA) of FGS in the thermal decay of nitromethane (NM, CH3NO2). FGSs and pristine graphene sheets (GSs) are oxidized in hot NM liquid to increase their functionalities and subsequently show self-enhanced CAs during the decay. The CAs result from the interatomic exchanges between the functional groups on the sheets and the NM liquid, i.e., mainly between H and O atoms. CA is dependent on the density of NM, functionalities of sheets, and temperature. The GSs and FGSs that originally exhibit different functionalities tend to possess similar functionalities and consequently similar CAs as temperature increases. Other carbon materials and their oxides can accelerate combustion of other fuels/propellants similar to NM, provided that they can be dispersed and their key reaction steps in combustion are similar to NM.

Are amino groups advantageous to insensitive high explosives (IHEs)?

There is usually a contradiction between increasing energy densities and reducing sensitivities of explosives. The explosives with both high energy densities and low sensitivities, or the so-called insensitive high explosives (IHEs), are desirable in most cases. It seems from applied explosives that amino groups are advantageous to IHE but the amount of amino groups contained IHEs is very limited. To make this clear, we present systemic examinations of the effects on the two properties stressed in IHEs after introducing amino groups to different molecular skeletons. As a result, the amino groups on resonant sites to nitro groups in conjugated systems can improve distinctly sensitivities and change energy densities in terms of oxygen balance; while the amino groups in unconjugated systems can hardly increase energy densities and usually cause increased sensitivities. It agrees well with a fact that almost all the molecules of applied amino group contained explosives possess conjugated skeletons. We therefore confirm that if amino groups are introduced resonantly to a nitro group in a conjugated system and the introduction improves OB, they are advantageous to IHEs.