Rapid Ignition Research Articles

Ignition study of hypergolic solid fuels through counterflow experiment using hydrogen peroxide spray

Hypergolic solid fuels provide rapid and reliable ignition for hybrid rockets, offering significant advantages over conventional methods. However, a practical experimental approach is necessary to study ignition and combustion behavior under realistic oxidizer flow conditions. This study introduced a novel counterflow spray experiment using 90 % rocket-grade hydrogen peroxide sprays applied to low-density-polyethylene-based fuel pellets embedded with sodium borohydride and manganese acetate tetrahydrate. The results showed that sodium borohydride caused vigorous reactions, damaging the pellets, while the addition of manganese acetate tetrahydrate moderated this effect. Hypergolic ignition and sustained combustion were achieved at oxidizer mass flow rates of 0.38–0.57 g/s, with ignition delay times of 15 to 30 ms, consistent with droplet test results. In addition, reignition was successful under the same flow conditions, although the char layer formed on pellet surface after the first ignition limited the surface additive reactivity, leading to longer ignition delay times. 3D surface analysis provided quantitative data on surface roughness across various fuel compositions and flow rates. This novel counterflow spray experiment is expected to be both effective and simple, with the ability to offer valuable insights into hypergolic ignition and combustion process.

Investigation of combustion mode conversion driven by fuel flow variation in a cavity-based scramjet

This work aimed to investigate combustion mode conversions by rapid variation of the fuel flow rate in a cavity-based scramjet combustor. The experiments were carried out on a direct-connected facility with an inflow condition of Mach number 2.52, a total pressure of 1.35 MPa, and a total temperature of 1650 K. The fuel injector consisted of two injection ports: fuel was continuously injected from one port while the other controlled the fuel flow for mode conversions by switching it on or off. Simultaneous schlieren and CH* imaging techniques were used to characterize the dynamics of combustion mode conversions. It was recognized that the combustion modes characterized by different flow field structures and heat release distributions can be classified into three types: the shear-layer mode, the transition mode, and the jet-wake mode. During the combustion mode conversion, the mixing region of the transverse jet and air became thicker with the increase in fuel flow rate, and the gradient of the flow field density and the flame area increased, making the flame more likely to propagate upstream. The combustion suppression induced by rapid fuel addition was observed at low equivalence ratios. It was speculated that the weak heat supply was insufficient to provide adequate heat for the rapid ignition of the added fuel. Furthermore, it was found that the flame-flow matching process with frequent flame propagation upstream occurred during the combustion mode conversion. This process was attributed to the mismatch between the increasing heat release and the original flow field structure.

Role of volatile secondary char on the combustion behavior of cellulose-based hydrochars

Hydrothermal carbonization (HTC) can transform a wide range of biomass into renewable biofuels. Hydrochar, the solid fuel resulting from HTC, has a similar energy density to low-rank coals. Despite an extensive body of literature detailing the thermogravimetric analysis (TGA) of hydrochar under slow pyrolysis and oxidation, there remains a limited understanding of hydrochar's behavior in realistic combustion settings. In this work, we integrate combustion experiments and TGA to understand the combustion characteristics of a model hydrochar fuel. Cellulose-based hydrochars produced at different temperatures along with solvent-extracted chars are tested in a Hencken burner across varying temperatures and oxygen concentrations in the oxidizer gas. Simultaneous CH* chemiluminescence, particle image velocimetry, and two-color pyrometry are used to measure ignition delay time and identify homogeneous/heterogeneous ignition mechanisms and combustion phases. Incorporating TGA with combustion results shows that the ignition mode and combustion processes are strong functions of surrounding gas temperature and oxygen mole fraction. The level of carbonization of the hydrochar dictates the ignition delay time and combustion modes. Further, the presence of a tar-like secondary char on the as-carbonized hydrochars leads to more rapid ignition due to high volatility.

Innovative carbonization techniques for food waste: A comparative study of hydrothermal carbonization (HTC) and vapor-thermal carbonization (VTC)

This study addresses the pressing issue of global food waste (FW) by conducting a comprehensive comparison between two conversion methods: hydrothermal carbonization (HTC) and vapor-thermal carbonization (VTC) for the production of hydrochar. The analysis encompasses physicochemical characterizations, Fuel characteristics, and surface properties. Under identical reaction conditions (180–250 °C, 30 minutes), both processes significantly elevate the carbon content of hydrochar. VTC-hydrochar with a carbon content ranging from 54 % to 62 %, while HTC-hydrochar with a slightly higher range of 55–63 %. Notably, hydrochar derived from both methods results in higher heating values (HHV) of 24.04–27.40 MJ/kg for HTC and 22.85–26.60 MJ/kg for VTC, indicating enhanced fuel quality. But with less volatile matter, VTC-hydrochar shows lower HHV. Moreover, HTC-hydrochar demonstrates superior attributes for rapid ignition and energy release, while VTC-hydrochar proves more suitable for applications requiring higher combustion temperatures and stability. In terms of surface properties, VTC-hydrochar demonstrates a higher NH4+ adsorption capacity attributed to its superior pore structure and abundant key functional groups, making it an ideal choice for adsorption applications. Most notably, through comparative analysis, finding that VTC-hydrochar with a more uniform structure from the complex and diverse FW feedstock. This study establishes a robust scientific foundation for selecting appropriate hydrochar production technologies.

“Fireworks” ignition and combustion of metal nanoparticles: Based on the rapid conversion and transmission of microwave energy by energetic ionic liquids

The metal nanoparticles (MNPs) are widely used to improve the output performance of various explosives and propellants, which ignition is an important link for the energy release of fuel. The microwaves have been recognized as a safer and smart ignition method. However, it is difficult to achieve rapid ignition of MNPs under microwave radiation due to the low heating rate. The ADN energetic ionic liquids could efficiently absorb microwaves owing to excellent polarity. This work proposes a method of fast “fireworks” ignition and combustion of MNPs under microwave radiation, by employing ADN as the “central hub” for microwave energy conversion. The dispersion of Al and Ti NPs in ADN endowed the solution with nanofluid properties and significantly improved the thermal conductivity. The solution boiled violently under microwave radiation, causing the temperature of MNPs to rise continuously. Meanwhile, the acidic components produced by ADN pyrolysis destroyed the oxide shell of MNPs, further accelerating the ignition process. The ignition phenomenon of Al and Ti NPs were different due to distinct microwave absorption properties and ignition temperature. When the expanding liquid film broken, the shock wave blew the Al NPs into the air to realize the “fireworks” ignition and combustion. The Ti NPs was ignited in the solution, which further ignited the ADN solution, allowing the system for controllable ignition and combustion. The “fireworks” ignition and combustion performance of MNPs could be modulated by the input microwave power. In summary, the new ignition and combustion method of MNPs will have a profound impact on the design of ignition sequence, the improvement of microwave energy utilization and the expansion of the application of ADN-based liquid propellants.

Analysis of the combustion characteristics of ammonia/air ignited by turbulent jet ignition with assisted hydrogen injection in pre-chamber

Turbulent jet ignition (TJI) is an advanced ignition strategy that can improve the ignition and combustion characteristics of low-reactivity mixtures. The utilization of TJI system may be reliable to achieve the application of ammonia (NH3) internal combustion engines. Hydrogen (H2) is a potential auxiliary fuel for the pre-chamber, and the injection of a small amount of H2 in the pre-chamber is beneficial for promoting the ignition and combustion of NH3/air in the main chamber. In this study, the ignition and combustion characteristics of NH3/air adopting the active TJI with assisted H2 injection in pre-chamber were investigated, and the relevant experiments were conducted in the constant volume combustion bomb system. The results show that the H2 pre-chamber can improve the flammability of NH3/air, and properly increasing H2 injection is conducive to the rapid ignition of NH3/air in the main chamber. The turbulence introduced into the main chamber by the hot jet enhances the combustion process, and the generation of turbulence weakens the sensitivity of the combustion rate to the reactivity of the unburned mixture. The turbulence intensity can be increased by decreasing the pre-chamber orifice diameter, which increases the ignition delay but significantly shortens the combustion duration.

Study of the copper structure samples externed to extreme influences

This work is devoted to the study of changes in the crystal structure, chemical and phase composition of copper samples subjected to extreme effects of temperature, pressure and electromagnetic fields. With the help of X-ray diffraction, as well as microanalysis, it was revealed that the plastic deformation of copper wires in the car power supply system leads to the formation of a superconducting Cu2O phase. This is the reason for the rapid ignition of the car, as it leads to a sharp increase in the magnitude of the electric current and temperature in the plastic deformation zone of copper wires. During explosion welding of copper samples, the Cu2O phase appears on their surface, which has superconducting properties. This significantly changes the electrophysical properties of copper samples. In metallurgical processes during the smelting of copper products, there is a possibility of the appearance of a superconducting Cu2O phase. When modifying a copper melt with hardening additives, the superconducting Cu2O phase makes it possible to obtain fracture-resistant copper products with high electrical conductivity. Plastic deformation of a copper foil 30 mkm thick by a magnetic field generated by a current of 180 kA leads to the formation of a texture and rupture of the foil. This has been detected using X-ray diffraction, as well as optical and scanning electron microscopy.

Superior Al90Mg10-Based Composite Fuel Grain for a Hybrid Rocket Engine

A paraffin-based fuel coupled with a nested helical matrix structure is a low-cost, high-performing alternative for hybrid rocket engine applications. The mechanical and combustion properties of the composite fuel grain can be enhanced by replacing the conventional polymer matrix with a metal skeleton. Three-dimensional printing was used to design an Al90Mg10 skeleton embedded with the paraffin-based fuel. The combustion characteristics of the composite fuel grain, including the ignition behavior, pressure oscillations, regression rate, and combustion efficiency, were comprehensively investigated. The properties of grains with and without a secondary perforated structure were compared. The flame structure and metal burning behavior of the hybrid rocket engine were monitored by endoscopic radioluminescence imaging, and the emission spectral characteristics of the plume were analyzed simultaneously. Good flammability makes the Al90Mg10 helical skeleton a promising candidate for enhancing the combustion performance of a paraffin-based fuel grain. The combustion process with rapid ignition was relatively stable, and no additional pressure oscillation frequency was observed. The metal-based composite fuel grains had a superior regression rate to that of a paraffin-based fuel grain (up to 100% higher using the perforated skeleton). Introducing a secondary structure into the fuel grain promoted the reaction and thereby enhanced the combustion performance.

EFFECT OF REFUSE-DERIVED FUEL FEEDING RATIO ON PERFORMANCE AND STABILITY OF COMBUSTION TEMPERATURE CONTROL

In this study, the effect of Refuse-Derived Fuel type 3 (RDF-3) feeding ratio on a performance and a stability of combustion temperature control was exmined. A new direct-combustion furnace was designed and constructed for co-combustion of RDF-3 and woodchip (WC) feedstock. In this work, four main ratios on wet basis of WC and RDF-3; 100 : 0, 90 : 10, 80 : 20, and 70 : 30 wt.% were tested at the combustion temperature of 970°C. The results showed that the complete combustion was achieved from all of feeding ratios. From an actual fuel feed rate, an increase in RDF mixing ratio could reduce the using of pure woodchip by 5.3 wt.%, 14.3 wt.% and 34.0 wt.% when mixing the RDF at 10 wt.%, 20 wt.% and 30 wt.%, respectively. At increasing of RDF mixing ratio, non-smoothy burning was also found to increase, however, a more rapid ignition behavior of was achieved

Enhancement of flame spread and droplet dynamics behavior of biodiesel constituents with ethanol and MWCNT-OH additions

The interactive droplet combustion between three adjacent droplets of biodiesel constituents was investigated to obtain flame spread and droplet dynamics behavior enhancement with ethanol and hydroxyl-functionalized multi-walled carbon nanotubes (MWCNT-OH) additives. The experiment was conducted at atmospheric pressure and under normal gravity using spatial and temporal tracking. This study found that ethanol effectively increases flame spread velocity and shortens droplet heating time at a close droplet distance. Meanwhile, MWCNT-OH shortens flame spread induction time at a large droplet distance when thermal conduction time mainly controls the flame spread. Both additives improve the volatility and thermal conductivity of fuel blends, extending the flame spread limit distance with strong reciprocal interaction between droplets and accelerating combustion with a higher burning rate. Different flame spread mechanisms were observed and determined by the fuel additives. Child droplet ejection and microburst flame encourage rapid ignition and flame spread to the next unburned droplet. The puffing and microexplosion mechanisms of the fuel blends were determined by the nucleation type preceding the child droplet ejection. Combining ethanol and MWCNT-OH as fuel additives is the most effective way to improve combustion and flame spread performance because ethanol induces nanoparticle burns at the earlier combustion stage.

Viscous shear flow and heating of impact-extruded composite energetic materials

Investigating the macroscopic deformation and localized heating behaviors of high-ductility composite energetic material (CEM) under impact extrusion is crucial to understand the accidental ignition phenomenon of CEM charged weapon systems with structural defects and design the advanced safe munitions. In this study, the rapid viscous shear flow and ignition responses of impact-extruded CEM are first experimentally studied and then simulated using a physically based thermomechanical model. The experimental results show that CEM sample impacted by a 6 kg and ∼2 m/s falling drop-weight could be extruded into the narrow crack with a high flow speed (∼20 m/s) and ignited. A thermomechanical model is developed to describe the continuum viscoelastic-plastic deformation and localized heating due to viscoplastic deformation in solid material, viscous flow in melted liquid and chemical reaction. By simulation, a semicircular extrusion region with a shear concentration is formed in CEM sample above the crack, and high shear stress and strain rate at this region contributes to the localized viscous shear heating and ignition of CEM. The effects of the diameter ratio between crack and sample (0.1∼0.4) on dynamic responses of CEM are further investigated. The simulated main features of CEM sample, including flow history into the crack, ignition time and location, are consistent with the experimental observations.

Research on on-site measurement factors and performance of coal calorific value based on laser ignition

The laboratory method used for coal calorific value measurement has a significant hysteresis. In this paper, an on-site measurement method of coal calorific value based on high power laser rapid ignition is proposed. The effects of oxygen transverse passage velocity, thermal insulation crucible bracket and coal samples on the measurement time, unburned rate and calorific value measurement error are gradually investigated. The experimental results shows that the measurement error of calorific value is within 2% and unburned rate is within 2% at the oxygen transverse passage velocity of 120 mm/s of Huainan bituminous coal. Then the utilization of porous thermal insulation crucible bracket has shown a good performance in heat dissipation to shorten the measurement time without increasing the unburned rate. Then, the effects of coal sample thicknesses, species and particle sizes on calorific value measurement are investigated. Results show that under the coal thickness from 4.2 to 5.4 mm, the calorific value measurement error is within 3%, and the unburned rate is within 2% of Huainan antiminous coal. Finally, the results shows that the method has a good performance on different coal species and particle sizes which the calorific value measurement error is within 5%. This research quantifies the effects of the experimental condition parameters on the measurement of coal calorific value based on laser ignition, which lays the foundation for the subsequent development of online coal calorific value meter.

Accreting white dwarfs: effect of WD composition on helium ignition during slow accretion

Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon–oxygen (CO) white dwarf (WD) from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on various parameters during the accretion of helium-rich matter at a slow rate. We have used the computer simulation code Modules for Experiments in Stellar Astrophysics (MESA) to understand the variations in the properties, such as specific heat ( $$C_P$$ ) and degeneracy parameter ( $$\eta $$ ). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the difference in carbon abundance in a CO-WD. We found that WDs of the lower abundance of carbon, accrete slightly longer before the onset of helium ignition.

Experimental study on the combustion process of a kerosene-fueled scramjet with strut injection

The ground combustion test was used to study the effects of kerosene fuel on the start-up ignition and flame stabilization of a scramjet with reverse strut injection. The test analyzed the changes of flow field wave system and flame propagation law under various working conditions after pilot hydrogen was injected into the combustor. The results showed that when hydrogen was injected into the combustor, it started to combust in only 0.0128 s. The heat released by combustion led to the generation of reverse pressure gradient, which made the wave system move forward and concentrate near the backward step. In the case of hydrogen self-ignition, the entire flow field remained stable combustion. At t = 0.07 s, kerosene was gradually injected into scramjet and ignited by pilot flame. The pressure at the monitoring point increased rapidly from 0.064 MPa to 0.135 MPa, and severe combustion occurred in the cavity. The flame began to propagate from the shear layer to the low-speed recirculation zone. Subsequently, the pressure of the monitoring point was reduced to 0.121 MPa, and the combustion flame continuously passed from the cavity downstream. When the hydrogen was removed, the pressure was reduced to 0.09 MPa, and the flame in the cavity was relatively weakened. However, the rapid ignition condition of kerosene could still be achieved, and the combustion flame continued to generate. After fuel began to be gradually removed, kerosene could not maintain continuous flame combustion due to the reduction of injection volume. The combustor changed between ignition and flameout, showing periodic oscillation with a period of 4.8 ms.

Influence of Adding Carbonaceous Fuels to Ionic Liquids on Propellant Properties.

To make ionic liquids (ILs) accessible and economical, ethylene glycol was mixed in 1-ethyl-3-methylimidazolium-dicyanamide ([EMIm]DCA) to obtain droplets that could experimentally collide white fuming nitric acid. To investigate the ignition delay (ID) time theoretically in terms of hydrodynamics, alcohol fuels and kerosene were used as combustibles, while the intermiscibility between them and nitric acid (HNO3) was calculated using the ternary phase-field method alongside finite element analysis. The specific impulses of blend fuels were calculated by a thermodynamic method and compared to ILs. When the droplet was ethylene glycol/[EMIm]DCA with a 2.1 mm diameter and a 1.69 m/s colliding velocity, the ID time was the shortest. Kerosene was not an applicable additive for [EMIm]DCA owing to its lower intermiscibility with ILs and HNO3 than alcohol fuels; alcohol fuels, however, were appropriate. The concentration of ethylene glycol in the oxidizer pool increased faster than the concentration of propylene glycol, triggering more rapid hypergolic ignition in the first 50 ms. The protocols regarding the hypergolic ignition conditions were verified, i.e., the size of the droplet had to be minute when the colliding velocity was as fast as possible; this was carefully calculated using ethylene glycol. According to thermodynamic calculations, the addition of alcohol fuels can improve the specific impulse of fuels, with ethylene glycol performing the best. The feasibility of adding alcohol fuels to ILs was confirmed via experiments and thermodynamic computations, with the simulation results providing some guidance on selecting the experimental or engineering conditions or both.

Pre-vaporized ignition behavior of ethyl- and propyl-terminated oxymethylene ethers

Oxymethylene ethers (OMEs) have been studied in recent years for use as compression ignition fuel blendstocks, but the methyl-terminated OMEs commonly studied exhibit properties that are poorly optimized for engine use and distribution. Recent work has shown that OMEs with larger (ethyl, propyl, or butyl) end groups may have superior properties for fuel usage/storage. In this work, we consider ignition of four OMEs - diethoxymethane (E-1-E), dipropoxymethane (P-1-P), ethoxy-(methoxy)2-ethane (E-2-E), and diisopropoxymethane (iP-1-iP) - as representatives of the possible effects of changes to OME structures. To our knowledge, ignition behaviors of the latter three fuels have not been studied prior to this work. We find that all of the tested linear OMEs (E-1-E, P-1-P, and E-2-E) show two-stage ignition at low temperatures and nonlinear ignition behavior, consistent with literature on methyl-terminated OMEs and E-1-E. The nonlinear, branched OME (iP-1-iP) required higher pressure and temperature to ignite than the linear OMEs; further, this fuel experienced only single stage ignition and a linear ignition delay curve. By analogy to existing kinetic mechanisms for ethers and higher alcohols, the chemical basis for the observed trends are hypothesized. Faster ignition of E-2-E results from the additional oxymethylene group providing additional sites for ROO formation and more possible QOOH structures. Slower low temperature ignition of P-1-P is driven by lower H abstraction rates in comparison to E-1-E, however at high temperatures P-1-P ignites faster, driven by increasing abstraction from the additional H site on the propyl group that opens up additional QOOH formation pathways. iP-1-iP ignition is slowed significantly by preferential H abstraction from the central carbon of the isopropyl group, which is crowded and unlikely to bond with O2, however at high temperatures, abstraction from H sites on the methyl groups allows for the ROO cascade initiation and subsequent rapid ignition.

Facile electro-thermal igniters based on freestanding CNTs films for the ignition of energetic materials

Facile electro-thermal igniters based on freestanding carbon nanotubes (CNTs) films are investigated for exploring the use of igniting energetic materials. Increases in electro-thermal temperature, dynamic pictures of ignition process, and temporal changes of voltage, current and resistance as well as the ignition ability of freestanding CNTs film–based igniters with different film thickness are compared with their counterparts with paper-like substrate. The results demonstrated that freestanding CNTs film–based igniters had faster rise in electro-thermal temperature and better ignition ability than their counterparts. The ignition delay of loaded energetic materials employing freestanding CNTs film based–igniters was only 40%–67% of their counterparts depending on the film thickness. Freestanding CNTs film–based igniters with larger thicknesses were more suitable for the rapid ignition of energetic materials. The shortest ignition delay for igniters of 8 × 8 mm bridge area under 20 V was less than 80 ms. The initial temperature rise rate of igniters was crucial for deciding the ignition delays of loaded energetic materials. The presence of substrates in CNTs film–based igniters impeded their temperature rise rate and thus increased the ignition delay of loaded energetic materials. Moreover, due to the property of negative resistance-temperature coefficient, freestanding CNTs film–based igniters are favourable for the realization of low voltage ignition of energetic materials.

Analysis of Dispersibility Effect of Carbon Additives on Ignitability of Ammonium-Dinitramide-Based Ionic Liquid Propellants Using Continuous Wave Laser Heating

ABSTRACT Ammonium dinitramide-based ionic liquids (ADN-EILPs) are a promising alternative to hydrazine monopropellants. Continuous wave (CW) laser heating using carbon wools is an effective approach to attain the rapid ignition of ADN-EILPs. This study aims to verify the influence of the dispersibility of carbon additives in ADN-EILPs on their ignition. The investigation was performed by performing fluorescence microscopy of samples imitating the mixture of ADN-EILPs with carbon additives and CW laser ignition tests of ADN-EILPs with several yarn-lengths of carbon wools. Based on these results, the dispersity mechanism of carbon additives in ADN-EILPs is proposed, which indicates that the use of high-power laser is not an effective approach to ignite ADN-EILPs consisting of carbon additives with high dispersibility. During sample preparation for the ignition tests, it was verified that the difference in the length of carbon yarns affects the bulk density and morphology of the prepared samples, and dispersibility of carbons. The results of the ignition tests indicate that samples whose morphology altered into a liquid-like morphology cannot be ignited and the ones who retained their original one can be ignited. The physical distribution of the residue of samples with a liquid-like morphology, observed after the ignition tests, agrees with the discussion regarding the dispersibility mechanism of carbon additives, obtained through fluorescence microscopy. Moreover, for the samples exhibiting an ignition capacity, the bulk density of additives would be crucial to be considered to achieve the effective ignition.

Fuel trait effects on flammability of native and invasive alien shrubs in coastal fynbos and thicket (Cape Floristic Region).

In June 2017, extreme fires along the southern Cape coast of South Africa burnt native fynbos and thicket vegetation and caused extensive damage to plantations and residential properties. Invasive alien plants (IAPs) occur commonly in the area and were thought to have changed the behaviour of these fires through their modification of fuel properties relative to that of native vegetation. This study experimentally compared various measures of flammability across groups of native and alien invasive shrub species in relation to their fuel traits. Live plant shoots of 30 species (10 species each of native fynbos, native thicket, and IAPs) were sampled to measure live fuel moisture, dry biomass, fuel bed porosity and the proportions of fine-, coarse- and dead fuels. These shoots were burnt experimentally, and flammability measured in terms of maximum temperature (combustibility), completeness of burn (consumability), and time-to-ignition (ignitability). Multiple regression models were used to assess the relationships between flammability responses and fuel traits, while the Kruskal-Wallis H test was used to establish if differences existed in flammability measures and fuel traits among the vegetation groups. Dry biomass significantly enhanced, while live fuel moisture significantly reduced, maximum temperature, whereas the proportion of fine fuels significantly increased completeness of burn. Unlike other similar studies, the proportion of dead fuels and fuel bed porosity were not retained by any of the models to account for variation in flammability. Species of fynbos and IAPs generally exhibited greater flammability in the form of higher completeness of burn and more rapid ignition than species of thicket. Little distinction in flammability and fuel traits could be made between species of fynbos and IAPs, except that fynbos species had a greater proportion of fine fuels. Thicket species had higher proportions of coarse fuels and greater dry biomass (~fuel loading) than species of fynbos and IAPs. Live fuel moisture did not differ among the vegetation groups, contrary to the literature often ascribing variation in flammability to fuel moisture differences. The fuel traits investigated only explained 21–53% of the variation in flammability and large variation was evident among species within vegetation groups suggesting that species-specific and in situ community-level investigations are warranted, particularly in regard fuel moisture and chemical contents.

Evolution of solid-liquid coupling combustion characteristics of boron suspension fuel in O2/Ar atmosphere

Boron suspension fuel is considered a good candidate to meet the energy requirements of hypersonic aircraft. As a solid-liquid mixed fuel, its combustion is a complex multi-stage process coupling of solid boron and liquid JP-10 combustion affected by various factors. In this study, A CO2 laser ignition test system was used to obtain the combustion characteristics of boron suspension fuel with solid loading of 10 wt.% in O2/Ar atmosphere. Focus is placed on the key factor of oxygen supply on the evolution of its solid-liquid coupling combustion characteristics. Theoretical analysis of the underlying mechanism of oxygen transportation and consumption caused by boron and JP-10 and their coupling characteristics was carried out for different stages. The results indicate that the combustion process could be divided into ignition, evaporation combustion, agglomerate combustion and extinguishment under full oxygen supply. In ignition stage, boron particles first ignite and promote the rapid ignition of JP-10. Oxygen supply can transfer boron combustion from heterogeneous reaction into homogeneous combustion. In evaporation combustion stage, a clear competition for oxygen between JP-10 and boron is observed. JP-10 is the main oxygen consumer whose flame forms an oxygen-lean zone around the droplet and suppresses boron combustion within the area. Micro-explosion could help carry boron particles into the outer oxygen-rich zone and achieve single-particle combustion. Increasing oxygen content to 60% can effectively compress the oxygen-lean zone area and realize simultaneous combustion of boron and JP-10. Agglomerate combustion stages could only occur when oxygen content is higher than 80%, which is the main energy releasing stage of boron. In extinguishment stage, black smoke is observed due to incomplete combustion of JP-10 when oxygen content is less than 60% while white smoke is observed due to boron oxide cooling deposition. This study provides a reference for the full energy exploitation of boron suspension fuel.