Boron Ignition Research Articles

Ignition and combustion characteristics of boron-based nanofluid fuel

With its high gravimetric and volumetric heat value, boron-based nanofluid fuel is potentially well suited for future high-energy propulsion systems. In this experimental study, we investigate the ignition and combustion of boron-based nanofluid fuel, with a focus on the effects of oleic acid (OA) concentration (0 to 10 wt.%), boron particle concentration (2.5 to 20 wt.%), and boron particle size (300 nm to 13 µm). Thermogravimetry−differential scanning calorimetry and laser ignition tests reveal that adding boron nanoparticles to a fuel can improve its combustion intensity, but without much affecting the oxidation and ignition characteristics, primarily because of agglomeration and thicker oxide layer properties relative to boron microparticles. Pure kerosene droplets are found to undergo pre-ignition heating and classic combustion, whereas OA or boron addition promotes puffing and micro-explosions, accelerating the classic combustion process. The co-existence of OA and boron is found to lead to more frequent and stronger disintegration of the nanofluid fuel droplet. The ignition delay reaches 156 ms with only 2.5 wt.% OA inclusion, compared to just 73 ms for pure kerosene. Both the ignition delay and combustion rate increase, while the solid residual decreases owing to enhanced micro-explosions as the OA concentration increases. Unlike OA, boron addition promotes ignition, with 2.5 wt.% boron yielding an ignition delay reduction of 24 ms, but this effect weakens with increasing boron concentration. The combustion rate is found to be relatively insensitive to boron addition. Although the particle size affects the ignition and combustion performance of boron powder, its effect on the combustion of the nanofluid fuel droplet is, under the present test conditions, relatively weak. Finally, a phenomenological model is presented to capture the ignition and combustion of a boron-based nanofluid fuel droplet. The predicted ignition delay and combustion rate are found to be within 7.15% of their experimental values. This study contributes to an improved understanding of boron ignition and combustion in high-energy liquid fuel systems.

Zero-Dimensional Ignition Model of Boron Agglomerates.

Boron primarily exists in the form of agglomerates in ramjet combustion chambers. However, the model used to predict the ignition time of boron agglomerates is usually based on the single-particle assumption, resulting in inaccurate predictions. This study aims to develop a numerical model that can accurately describe the ignition of boron agglomerates. The model is based on the ignition model of a single particle boron proposed by the group of Kuo. Thiele modulus and effectiveness factor are introduced to represent the diffusion resistance of reaction gases in the pores of boron agglomerates. The model includes the necessary physical processes to accurately predict the ignition time. The rates of evaporation and heterogeneous reactions involved in the oxide layer removal process are corrected based on the fact that the diffusion rate of (BO)n in the liquid oxide layer equals to its consumption rate at the oxide-air interface. To evaluate the accuracy of the model, the obtained results for ignition time are compared with experimental data, showing reasonable consistency between them. The model is then applied to investigate the ignition characteristics of boron agglomerates. Parameters, such as initial average pore diameter, oxide layer thicknesses, initial particle diameter, O2 concentration, H2O concentration, and environmental pressure, are studied for their effects on the ignition time. In summary, the boron ignition model established in this study is a powerful tool to investigate the ignition mechanisms and characteristics of boron agglomerates. It can be further coupled with flow analysis for the detailed simulation of turbulent combustion in ramjet combustors.

Effect of alkali metal perchlorate and iodate type on boron ignition: The role of oxidizer phase change

Boron often demonstrates low reactivity despite having high gravimetric and volumetric energy density. Here we explore a family of oxidizers with very similar chemistries to understand controlling mechanisms in boron ignition. Alkali metal perchlorates as well as iodates (LiClO4, NaClO4, KClO4, LiIO3, NaIO3 and KIO3) were investigated under rapid heating conditions (∼105 K/s) by temperature-jump (T-Jump) ignition and time-of-flight mass spectrometry (TOFMS). T-Jump ignition tests in atmospheric pressure argon show that B/LiClO4, B/NaClO4, B/LiIO3, and B/NaIO3 ignite while B/KClO4 and B/KIO3 do not, despite the near identical chemistries. T-Jump TOFMS results demonstrate a dramatic increase of Cl and I containing species for Li and Na systems relative to K when B is added. Thermogravimetry/differential scanning calorimetry (TGA/DSC) analysis reveals that there is a temperature gap between melting and decomposition onset of LiClO4, NaClO4, LiIO3, and NaIO3, respectively, whereas KClO4 and KIO3 melt and decompose concurrently. We find that the larger interval between melting and decomposition renders more time for the oxidizer to melt and surround/wet the nanoscale B before decomposing, providing instant access to oxygen for B nanoparticles that results in vigorous ignition. This study of a set of equivalent oxidizers illustrates the important role of transport phenomena and physical properties have on ignition.

Effect of Fluoroalkylsilane Surface Functionalization on Boron Combustion.

Boron has been regarded as a promising high-energy fuel due to its high volumetric and gravimetric heating values. However, it remains challenging for boron to attain its theoretical heat of combustion because of the existence of its native boron oxide layer and its high melting and boiling temperatures that delay ignition and inhibit complete combustion. Boron combustion is known to be enhanced by physically adding fluorine-containing chemicals, such as fluoropolymer or metal fluorides, to remove surface boron oxides. Herein, we chemically functionalize the surface of boron particles with three different fluoroalkylsilanes: FPTS-B (F3-B), FOTS-B (F13-B), and FDTS-B (F17-B). We evaluated the ignition and combustion properties of those three functionalized boron particles as well as pristine ones. The boron particles functionalized with the longest fluorocarbon chain (F17) exhibit the most powerful energetic performance, the highest heat of combustion, and the strongest BO2 emission among all samples. These results suggest that the surface functionalization with fluoroalkylsilanes is an efficient strategy to enhance boron ignition and combustion.

Promoting the combustion properties of boron powder through in‐situ coating

AbstractBoron powder has been considered as a prime candidate as an additive in solid fuels or propellants. Unfortunately, boron ignition and combustion is hindered by the pre‐existing B2O3 surface layer which slows the oxidizer attack on the underlying boron particles. Therefore, removing the pre‐existing B2O3 surface layer is critical to improve the ignition and combustion performance of boron. Herein, an in‐situ coating strategy is reported to be very effective in removing the pre‐existing B2O3 surface layer of boron powder and improving its combustion performance. Through the in‐situ coating process in ethanol, flammable 3‐aminophenol/formaldehyde resin (AF) protection layer will be formed on the surface of the boron powder, replacing the pre‐existing B2O3 surface layer, and preventing the surface reoxidation of the boron powder. Comparing with the untreated boron powder, such coated boron powder exhibits higher burning rate and can be ignited at lower temperature. This in‐situ coating strategy can not only contribute to the extensive utilization of boron powder in high enthalpy fuels and propellants, but also shed light on the surface modification of functional materials in different fields.

IGNITION AND COMBUSTION OF SINGLE SOLID PARTICLES AS NON-ISOTHERMAL METHODS OF CHEMICAL KINETICS

The ignition and combustion of single particles of crystalline boron continue to produce major scientific interest due to the particularities of the process and diversity of potential applications of boron compounds. The full valorization of boron energetic potential is a very current scientific challenge. The objective of the paper is to systematize the methodology for evaluating the kinetic parameters of boron ignition and combustion reactions in various oxidizing gaseous environments. Experimental dependencies between the ignition temperature and the particle size, as well as the combustion time as a function of oxidizing temperature are used for the calculation of the kinetic constants. As a main result, the kinetic parameters of the ignition and combustion reactions of boron in oxygen and water vapor are calculated.

Augmentation of Aluminum and Boron Ignition

Effect of K2CO3 and KCl additives on the process of high-temperature oxidation and ignition of conglomerates of powdered boron and aluminum particles and on the critical conditions for the ignition of particle–gas mixtures of these metals is studied experimentally. It is revealed that the mixtures of boron and aluminum particles covered (encapsulated) with salts ignite at temperatures lower than the ignition temperatures of mechanical mixtures of metals with these salts in appropriate proportions and significantly lower ignition temperatures of pure boron and aluminum. This indicates the active role of salts in the process of heterogeneous ignition. It is established that, for boron conglomerates, the accelerating effect of salt additives on the oxidation of conglomerates is noticeable at all investigated concentrations and compositions starting from 1% (by weight). For aluminum, an increase in the reaction rate is noticeable only for KCl additives at concentrations of 3% and above.

Dynamic ignition and combustion characteristics of agglomerated boron-magnesium particles in hot gas flow

Boron-based fuel is the most attractive energy source for ramjets and scramjets because of its high calorific value. Due to magnesium's excellent combustion performance, adding magnesium as a promoter for boron ignition and combustion was proposed. The combustion mechanisms of boron-based fuel become more complicated after adding magnesium. At the same time, because of the high viscosity B2O3 on boron's surface, fuels always come into combustion in agglomerated state. Therefore, the dynamic ignition and combustion characteristics of agglomerated boron-magnesium particles were investigated experimentally in this paper, which could provide the basic and key parameters for combustor design. The particles investigated here were prepared by drying a mixture of micron sized boron and magnesium slurry. The experiment results indicated that the combustion process of agglomerated boron-magnesium particles can be divided into agglomerated combustion and micro-explosion combustion. The micro-explosion phenomenon is related to the core-shell structure formed during the particle's heating process. With magnesium content being increased, the combustion type will transit from agglomerated combustion to micro-explosion combustion. The transition region of combustion type is the condition that when magnesium content reaches 10-20%. When magnesium content reaches 30%, it has no obvious effect on the combustion of agglomerated boron-magnesium particles. The critical ignition temperature for magnesium containing samples is ranging from 1279 K to 1346 K, and for magnesium free sample is 1670 K. The ignition delay time for samples with magnesium is shortened from 132-164 ms at 1279 K to 26-38 ms at 1673 K, with a reduction of 80%.

Aluminum and Boron Ignition Intensification

Aluminum and Boron Ignition Intensification

Prospects of Using Boron Powders As Fuel. II. Influence of Aluminum and Magnesium Additives and Their Compounds on the Thermal Behavior of Boron Oxide

Physicochemical changes occurring in an oxide shell of boron particles during heating are extremely important for the oxidation and combustion of boron, including those in solid propellant compositions. The Al2O3, MgO, MgF2, Al, and Mg impurities were experimentally discovered in an oxide layer on boron particles obtained by various methods. The goal of this work is to determine the effect of these impurities on the thermal behavior of boron oxide, with particular attention paid to evaporation of B2O3. The temperature and thermal effects of reactions between the components are thermoanalytically determined, and the processes of dehydration, melting, and evaporation of boron oxide are analyzed in detail. The enthalpy of evaporation of boron oxide, starting at a temperature above 1300°C is experimentally determined, and is equal to 347 ± 3 kJ/mol. An interaction is observed between magnesium fluoride and boron oxide at a temperature of about 1000°C with a mass loss corresponding to the content of magnesium fluoride and the formation of gas-phase boron fluoride. It is established that boron dissolved in oxide has virtually no effect on the evaporation of the boron oxide melt, while the addition of Al2O3or MgO significantly increases its thermal stability. Based on the analysis of the results obtained, an assumption is made about the effect of impurities on boron activity in the processes of boron ignition and combustion.

Characteristics of the combustion chamber of a boron-based solid propellant ducted rocket with a chin-type inlet

The scope of this study is to improve combustion performance of a boron-based solid propellant ducted rocket (SPDR). A numerical approach for the internal reaction flow in SPDR was built, and an improved ignition and combustion model of boron particles was used. Many integrated physical processes of boron ignition and combustion, such as the balance of (BO)n on particle surface at ignition stage, the evaporation and boiling processes of boron at combustion stage, were considered. A new configuration of SPDR with swirling flow was proposed. Moreover, numerical and experimental investigations were conducted, and the effects of equivalence ratio and swirling flow on combustion performance were analysed. Validation of the developed numerical approach showed good agreement between the approach's results and available experimental data in open literature and also those in this study. Experimental results indicated that combustion efficiencies increase from 88% to 95% with swirling flow.

Boron ignition and combustion with doped δ-Bi2O3: Bond energy/oxygen vacancy relationships

The purpose of this paper is to extract a clearer relationship between atomic properties of the oxidizer, and ignition temperature and combustion kinetics. Pure Bi2O3 and a series of Y3+ and W6+ doped Bi2O3 nanoparticles with the same crystal structure and morphology were synthesized via aerosol spray pyrolysis and used as oxidizers in boron-based thermites. This enabled us to vary bond energy and oxygen vacancy concentration systematically. The ignition temperatures and the reactivities of different B/Bi2O3 thermites were measured by rapid heating (> 105 K/s) temperature-jump/time-of-flight mass spectroscopy and a confined pressure cell, respectively. With pure Bi2O3, the boron could be ignited at a temperature as low as 520 °C. In-situ high heating rate TEM was used to observe the reaction before/after heating. We find very clear relationships that higher oxygen vacancy concentration and smaller metal–oxygen bond energy lead to lower ignition temperature and higher combustion reactivity.

Effect of plasma on combustion characteristics of boron**Project supported by the National Natural Science Foundation of China (Grant No. 11372356).

As it is very difficult to release boron energy completely, kinetic mechanism of boron is not clear, which leads to the lack of theoretical guidance for studying how to accelerate boron combustion. A new semi-empirical boron combustion model is built on the King combustion model, which contains a chemical reaction path; two new methods of plasma-assisted boron combustion based on kinetic and thermal effects respectively are built on the ZDPLASKIN zero-dimensional plasma model. A plasma-supporting system is constructed based on the planar flame, discharge characteristics and the spectral characteristics of plasma and boron combustion are analyzed. The results show that discharge power does not change the sorts of excited-particles, but which can change the concentration of excited-particles. Under this experimental condition, plasma kinetic effect will become the strongest at the discharge power of 40 W; when the discharge power is less than 40 W, plasma mainly has kinetic effect, otherwise plasma has thermal effect. Numerical simulation result based on plasma kinetic effect is consistent with the experimental result at the discharge power of 40 W, and boron ignition delay time is shortened by 53.8% at the discharge power of 40 W, which indicates that plasma accelerates boron combustion has reaction kinetic paths, while the ability to accelerate boron combustion based on thermal effect is limited.

Ignition and combustion model of a single boron particle

Boron is one of the most attractive additives of fuel used for ramjet engines, due to its high volumetric and gravimetric heating value. Studies focusing on the ignition and combustion model of a single boron particle have been taken place for a long time, the most outstanding model is proposed by the group of Kuo, which is simple and can be well used in CFD application. However, some drawbacks were found in this model, and a new model was proposed. In the new extended model, some physical processes of boron ignition and combustion were considered. At the ignition stage, the balance of (BO)n on the liquid oxide layer surface was considered. It was diffused from the boron-boron oxide interface, and then consumed by both the heterogeneous reactions and the evaporation of diboron dioxide on the surface. The evaporation process of boron oxide and reaction between water vapor and boron oxide were considered. At the combustion stage, the evaporation and boiling processes of boron were considered due to the high combustion temperature, some chemical reaction rates were adjusted. The influence of forced convection on the particle was considered in both ignition and combustion stage. A careful validation shows that the extended model is suitable for a single boron particle, especially at the combustion stage. Effects of gas temperature, gas pressure, oxygen mole fraction on boron ignition and combustion in ramjet engine were studied and could be useful for ramjet engine design.

Ignition kinetics of boron in primary combustion products of propellant based on its unique characteristics

Study on the boron-based primary combustion products can bridge the gap between primary combustion and secondary combustion in solid rocket ramjets. To clarify the initial state and ignition characteristics of boron particles in the after-burning chamber of solid rocket ramjets, the elemental, composition and morphology of the primary combustion products collected under gas generator chamber pressure of 0.2MPa and 6MPa were investigated by energy dispersive (EDS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy with energy dispersive (SEM-EDS) individually. The ignition times of boron particles among the primary combustion products were determined using a high temperature tube furnace system. The BD model was adopted for numerical verification. The numerical solution procedure of boron ignition model in a real afterburner chamber was modified. The results show that the sum of B, C, O elements in the primary combustion products reaches approximately 90%. The primary combustion products are mainly consisted of B, C, and B2O3. Images of the primary combustion products present highly agglomeration, indicating an oxidation of boron surface. Numerous spherical carbon particles with a diameter around 100nm are observed in the products. Three features of the boron in the primary combustion products are obtained, compared to virgin boron. First most of the boron lumps are covered by carbon particles on the surface. Second the mean particle size is five times larger than that of virgin boron. Third the overall initial oxide layer covered on boron surface increases its thickness by above 0.1µm. The ignition time of boron in the primary combustion products reaches 20–30ms under 1673–1873K, which is quite different from virgin boron of ~4ms. Numerical calculation results show the key reason leading to such a long ignition time is the variation of the initial oxide layer thickness. In conclusion, the physicochemical properties of boron particles are found to differ with virgin boron after primary combustion process. The accurate evaluation of the initial oxide layer thickness and initial particle radius is a crucial procedure before the numerical calculation of boron ignition kinetics. Results of our study are expected to provide better insight in the simulation of solid rocket ramjets working process.

Ignition and Combustion Enhancement of Boron with Polytetrafluoroethylene

The ignition and combustion properties of fuel-rich mixtures of boron and polytetrafluorethylene in air and argon were studied as a function of pressure at a heating rate of approximately to simulate heating rates that individual ingredients may be subjected to in propellant burning. Mixtures ranging from 20 to 100 wt% boron (balance polytetrafluorethylene) and pressures up to 7 MPa were considered in this study. Ignition of the samples was achieved by joule heating of a platinum filament within a pressure vessel of selected atmospheres. Ignition of the mixture was characterized by monitoring broadband light emission, whereas boron ignition specifically was verified by identification of the molecule using emission spectroscopy. At atmospheric pressure, none of the mixtures ignited within the duration of the experiment. Mixtures containing more than 80% boron did not consistently ignite under any conditions within the duration of the experiment. It was found that the ignition temperature for all of the mixtures was a function of pressure with a reduction in ignition temperature of about 300 K when increasing the pressure from 2 to 7 MPa. At pressures greater than approximately 3.5 MPa, the ignition temperature was insensitive to mixture composition for all mixtures that ignited. The results suggest that the enhancement of boron ignition is the result of sufficient polytetrafluoroethylene decomposition products to remove the oxide layer of boron.

Role of Oxalic Acid in Promoting Ignition and Combustion of Boron: an Experimental and Theoretical Study

AbstractBoron is an attractive fuel for propellants and explosives because of its high energy density. However, its combustion is inhibited by the oxide layer that covers the particles. The use of oxalic acid as an additive was shown to promote boron oxidation. In this study, the thermodynamic model FactSage 6.2 and a laser ignition facility were used to investigate the effect of oxalic acid on the burning characteristics of boron particles. The results of the thermodynamic analyses show that oxalic acid can reduce B2O3(l) production during boron combustion. This enables removal of the the oxide film and promotes the burning of boron. However, only at high temperatures (>1500 K) H2O(g) (produced from H2C2O4) can react with B2O3 and remove the oxide film. The evolution of boron combustion flame takes place in three stages: ignition, stable combustion, and extinction; the bright yellow color in the flame indicates boron ignition, the bright white color indicates boron combustion, and the bright green color is interpreted as BO2 emission. Addition of oxalic acid into boron powders can significantly promote boron ignition and combustion. The ignition delay time of the resulting mixture is reduced by 42.4 %, the combustion intensity is raised by 16.7 %, and the combustion efficiency of boron is increased by 21.5 percentage points. The mechanism of action of oxalic acid on enhancing the combustion of boron was studied by scanning electron microscopy.

Metal Oxides as Catalysts for Boron Oxidation

Boron is a beneficial fuel for propellants and explosives because of its high energy density. However, efficient combustion of boron particles is difficult to obtain because of an inhibiting oxide layer that covers the particles. Various metal oxides are active catalysts in graphite/carbon oxidation, but no study has been carried out to investigate metal oxides as catalysts for boron oxidation. In this paper, the effects of metal oxides on boron oxidation are introduced. The instruments used in the experiments include a thermobalance, FactSage 6.2 software, and a laser ignition facility. The results reveal that is the most active catalyst: it can reduce the ignition temperature by 15.2%. and are the second and third most active catalysts, respectively. The other four metal oxides used in the experiments exhibit little activity on boron thermal oxidation. The catalytic action of metal oxides possibly involves the cyclic reduction of the metal oxides and the reoxidation of the resulting metals. The catalysts help transfer oxygen from the surroundings to the interface. All metal oxides used in the experiments help decrease boron ignition delay time. Two reasons are proposed to interpret the effect of metal oxides on the boron ignition delay time.

Effect of magnesium on the burning characteristics of boron particles

Boron is an attractive fuel for propellants and explosives because of its high energy density. However, boron particles are difficult to combust because of inhibiting oxide layers that cover the particles. The use of magnesium as additives has been shown to promote boron oxidation. In this study, laser ignition facility and thermobalance were used to investigate the effect of magnesium on the burning characteristics of boron particles. The influences of magnesium addition on sample combustion flame, boron ignition delay time, boron combustion efficiency and initial temperature of boron oxidation. Results show that all Mg/B samples exhibit the same type of flame structure, i.e., a bright plume surrounded by green radiation which is interpreted as BO2 emission. The combustion flame intensity of a sample increases with the increasing magnesium content of boron particles. An increase in magnesium content results in a decrease and a subsequent increase in boron ignition delay time. (Mg/B)0.2 has a minimum ignition delay time of ~48ms. Boron combustion efficiency increases with increasing magnesium addition. (Mg/B)0.5 shows a maximum boron combustion efficiency of ~64.2%. Magnesium addition decreases the initial temperature of boron oxidation.

Effect of Initial Oxide Layer on Ignition and Combustion of Boron Powder

AbstractOnly few data exist for experimental studies on ignition and combustion of boron particles with initial oxide thickness. The oxidation, ignition and combustion characteristics including the onset temperatures, weight gain, apparent activation energy, emission spectra during combustion, and ignition delay time of crystalline boron powders with different initial oxide thickness (x0) were studied by a laser ignition and thermogravimetric (TG) analyses. Simulations of the kinetics of oxide layer during boron ignition were conducted using a common model. The results indicated that the onset temperature was approximately 775 °C, independent of x0. The total weight gain decreased with increasing x0, whereas the weight gain at 775 °C did not change. The apparent activation energy was found to be insensitive to x0 and had a constant value of about 210 kJ mol−1. The intensity of the emission spectra gradually decreased while the ignition delay time increased with increasing x0. Numerical simulation showed that the removal rate of oxide layer enhanced with increasing x0. The experimental results revealed that the oxidation of boron powder was no diffusion‐controlled process at low temperatures. But the diffusion of oxygen could become important to the oxidation reaction at high temperatures