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The investigation of the primary combustion products of boron-based fuel-rich propellants is conducive to understanding the mechanism of the two-stage combustion process and provides important information for the prediction and organization of secondary combustion processes. The present investigation methods include numerical simulation and experimental analysis. The former deviates considerably from the actual conditions. The latter is currently a reliable and practical method, although it is difficult to quantify when the components are complex. In this work, SEM, LPSA, XRD, XPS, IC, ICP–MS, AAS, EDS, and chemical analysis methods were employed to qualitatively and quantitatively analyze the 3 solid products (aggregations, condensed-phase products, and flocculent products) at 1, 5, and 10 bar. GC, GC–MS, and IC combined with chemical tests were adopted to investigate the gaseous products (including chlorine-containing gas) for the first time. The solid products primarily included B2O3, B(HO)3, unburned boron, KB5O8(H2O)4, KCl, C, NH4Cl, NH4Mg(H2O)6Cl3, B13C2, Fe3O4, Mo, MgO, Mg(OH)2, MoO3, C3N4, BN, MgCO3(H2O)3 (or MgCO3), MoO2, Fe4(Fe(CN)6)3, and FeB (or BFe4), and the first 7 were dominant. The reaction degree of boron was less than 50 wt% (increasing with increasing pressure), indicating the significance of the secondary combustion process. The reacted boron was likely to react with O and C but not with N. The generated B2O3 vapor easily combined with H2O to form B(HO)3 or reacted with KCl (from the thermal decomposition of KP) and H2O to produce KB5O8(H2O)4 after cooling. There was less B13C2 (its content increased with increasing pressure, resulting in increases in particle size), since it was easily oxidized to generate B2O3. NH4Cl was produced by the oxidation-reduction reaction between the NH3 and HClO4 released from large-sized AP, and some of it crystallized with MgCl2 and H2O to form NH4Mg(H2O)6Cl3. The gaseous products primarily included N2, H2, CO, CH4, CO2, C2H4, C2H2, HCl, C4H6, C3H6, Cl2, C6H6, and other hydrocarbons. N2 and H2 had the highest content. The N2 content increased with increasing pressure, while the content of the combustible gases decreased, indicating that high pressure promotes the propellant reaction. The results also show that high pressure was conducive to the low-temperature decomposition of AP (producing more HCl and Cl2) and the reaction of boron and hydrocarbons (generating more B13C2), while it had an adverse effect on the C reaction (producing less COx and mostly CO). Nevertheless, the increasing pressure had little effect on the combustion product components but did affect the content within the pressure range.

Aluminum hydride (AlH3) is currently receiving considerable attention due to its potential to replace aluminum (Al) as a new metal fuel in solid propellants. In this study, simultaneous thermogravimetric analysis and differential scanning calorimetry and a laser ignition testing system were used to investigate the thermal oxidation and combustion characteristics of AlH3 and Al. The combustion residues were then analyzed using a scanning electron microscope. Results show that the thermal oxidation processes of both AlH3 and Al contain a mass loss stage and two mass gain stages. After dehydrogenation, AlH3 exhibits a stepwise oxidation behavior that is similar to that of a similar sized Al, but has a much higher mass gain. The combustion process of Al can be divided into three stages, which correspond to the combustion of dispersed Al particles, formation of Al agglomerate and combustion of Al agglomerate, respectively. The combustion flame of AlH3 is significantly different from that of Al, but their microscopic combustion behaviors are very similar. A unique flame delamination phenomenon exists in the early stage of AlH3 flame development. AlH3 has shorter ignition delay times and a larger combustion intensity than Al. The release and combustion of hydrogen changes the combustion environment of the remaining AlH3 particles, leading to the appearance of an OH characteristic peak and increasing the production path of AlO. The morphology of combustion residues shows that the dehydrogenation of AlH3 starts from the particle surface and then gradually expands towards its interior.

Introduction It is necessary to clearly understand construction accidents for preventing a rise in Chinese construction accidents and deaths. Better analysis methods are required for Chinese construction sector accidents. Methods Choosing and analyzing a typical construction accident based on four popular contemporary accident causation models: STAMP, AcciMap, HFACS, and the 2-4 Model. Then we evaluated the models' applicability to construction accidents, including their usability, reliability, and validity. Results STAMP addressed how complexity within the accident system influenced the accident development, and its output makes the responsibilities clearer for the accident. AcciMap described the entire system's failure, the entire accident's trajectory, and the relationship between them. AcciMap showed that the accident was a dynamic developing process, and this method has a high usability. The taxonomic nature of HFACS is an important feature that provides it with a high reliability. In the accident reviewed here, we found that poor management was a critical factor rather than the individual factor in the accident. The 2-4 Model provided detailed causes of the accident and established the relationship among the accident causes, the safety management system, and the safety culture. It also avoided capturing all of the complexity in the large sociotechnical system and revealed a dynamic analysis and developing process. We confirmed that it has a high usability and validity. Therefore, the 2-4Model is recommended for future Chinese construction accident analysis efforts. Practical Applications The study provides a useful, reliable, and effective analysis method for Chinese construction accidents.

The maximum and minimum values of dust explosion parameters are indeterminate because the possibility of explosion has not been quantified under conditions that approach the explosion limit. In this review, the influencing factors, laws, and mechanisms of dust explosion characteristics are described by comparing the differences in the methods for measuring the parameters in the ASTM, EN, GB, and ISO standards. The variation law of explosion parameters changes when crossing the limit values of the characteristic diameters and dust concentration. The similarities of certain influence mechanisms are also introduced. In addition, a new method is introduced to quantify explosion probability. The statistical nature of ignition is revealed by discussing the variability of the data overlap region of the “explosion” and the “no explosion” near the MEC and MIE measurements. The influence of sample size and positive factors on the statistical phenomena of ignitions were analyzed. The variability of the results increases when the conditions are not conducive to explosions. The values with 50% probability of explosion are independent of the number of trials and are less affected by the regression models. Reliable statistics can be obtained by sequential analysis methods in a small number of tests. The present work is a more accurate guide for the risk assessment of dust explosions, and introduces a potential statistical method to more accurately analyze the actual explosion characteristics.

The energy release efficiency of boron (B)-containing fuel-rich solid propellants depends largely on the combustion behavior of the B particles contained therein. Here, the combustion process of B particles was first studied using a laser ignition testing system. Then, the combustion and agglomeration characteristics of B particles in a fuel-rich propellant were further investigated. To this end, a microtube burner was specially manufactured to mimic the secondary combustion of boron-containing propellant. Results show that the primary combustion intensity of the boron-containing propellant was relatively weak, and the combustion temperature was 1720±32 K. B particles would participate in the primary combustion reaction of the propellant, resulting in short time detection of the characteristic green flame and BO2 spectrum. Severe self-conglomeration of B particles occurred on the propellant burning surface, forming irregular coral-like B particle conglomerates. The development of the quasi-secondary (the word “quasi” is added to highlight the difference between the experimental conditions and the actual conditions in engines) combustion flame showed that hot B particles were an important ignition source for the gaseous fuel and that the existence of excess gaseous fuel in the primary plume was an important prerequisite for self-sustaining combustion of B particles in air. The emission spectra of propellant quasi-secondary combustion were very similar to those of primary combustion, but the intensity was significantly increased. Microstructures of the combustion residues showed that the oxide layer on the surface of B particles played an important role during the formation of B conglomerates. Irregular B conglomerates detached from the propellant burning surface might further develop into spherical B agglomerates in the high-temperature secondary combustion flame zone.