Shorter Ignition Delay Times Research Articles

Understanding the synergistic effect in green propellants 2-azido-N,N-dimethylethanamine and tetramethylethylenediamine: From drop test to gas-phase autoignition

2-Azido-N,N-Dimethylethanamine (DMAZ) and tetramethylethylenediamine (TMEDA) are promising green propellants for hypergolic applications, and blending DMAZ with TMEDA can achieve short ignition delay time (IDT) and comparable specific impulse to hydrazine-based fuels simultaneously. There are reports of a synergistic effect between DMAZ and TMEDA, but its mechanism is not well understood. In this work, drop tests and gas-phase autoignition experiments were combined to provide insights of this synergistic effect from different aspects. The drop tests of DMAZ/TMEDA blends with white-fuming nitric acid were conducted in a confined transparent chamber with controlled O2 concentration. Results show that the hypergolic ignition is much faster for blends with 20–40 wt% DMAZ under all O2 concentration conditions, and higher concentration of O2 in the environment significantly promotes the hypergolic ignition process. To further investigate the chemistry between DMAZ/TMEDA with O2, gas-phase IDTs were measured using a rapid compression machine and a shock tube in a wide temperature range of 500–1100 K and at pressure of 10 bar. TMEDA shows a higher reactivity at lower temperatures (<690 K), while the autoignition of DMAZ is much faster at higher temperatures (>690 K). A synergistic effect between DMAZ and TMEDA was also observed in the gas-phase autoignition, i.e., fuel blend with 30 % DMAZ showed equal or even lower IDTs than pure TMEDA at lower temperatures. The newly measured IDTs of DMAZ/TMEDA were further adopted to validate a previously developed chemical kinetic model for pure TMEDA and DMAZ, which exhibits good predictions for the blending effects nonetheless. Kinetic analyses reveal that the low-temperature reactivity contributed from the TMEDA can be enhanced by the heat release from DMAZ decomposition, causing the synergistic effect in gas-phase autoignition of DMAZ/TMEDA blends.

Measurements of ignition delay time of gas-to-liquid (GTL) fuel blends

The ignition delay time (IDT) is a fundamental combustion parameter of any fuel, which has a direct impact on its combustion and exhaust emissions in diesel engines. In this research, a thorough experimental study was conducted to investigate the ignition delay of Gas-to-Liquid (GTL) fuel and its 50%–50 % blends with diesel under a wide range of operating conditions using a conventional shock tube apparatus. Simultaneously, a comprehensive chemical kinetic model, based on the well-established POLIMI mechanism, was developed to simulate the complex combustion chemistry of diesel, GTL, and their blends. Several loading pressures in the driver section ranging from 6 to 14 bar that results in reflected pressure of 8–16 bar, and equivalence ratios ranging from 0.5 to 1.3 as well as different initial temperatures have been examined. Calibration of the shock tube indicated that a driver gas tailoring technique was necessary to be used, which was conducted by adding a small ratio of either argon or nitrogen to helium to get an extended test time by 32 % with a good rapture pressure. IDT measurements showed that GTL has the shorter ignition delay time than that of diesel fuel for all conditions. The findings show that IDT measurements for GTL fuel has a shorter ignition delay time than that of diesel fuel for all conditions. Under stoichiometric conditions (ɸ = 1.0), IDT of the blended fuel was about 3 ms, very close to that of GTL at 975 K. However, at a temperature above 1000 K, the IDT of the three fuels were almost identical and ranging between 1 and 2 ms. At a temperature of 1100 K, the IDT of the blended fuel was 5.5 ms, almost a middle value between those of the pure diesel, 0.522 ms, and GTL fuel, 0.495 ms. The shortest IDT of the blended fuel was about 0.4 ms with the highest value of the reflected pressure of 14 bar, while the longest IDT was around 10 ms with the rich mixture at ɸ = 1.3. The study can add to the existing literature illustrating the behavior of GTL-diesel blend can affect IDT.

Enhancing Droplet Combustion Dynamics in Trimethyl Borate-Based Blends: Exploring Energetic Phenomena

ABSTRACT Trimethyl borate (TMB) is an excellent alternative to alter the combustion of conventional fuels due to the combination of boron, stable methyl groups, and oxygen, which can improve the combustion behavior in many ways compared to alcohols and etheric hydrocarbon structures. In this research, the combustion and energetic phenomena trends of trimethyl borate blends was investigated on a droplet scale. The camera systems were used at the combustion characteristics look at how the size of the droplets, the structure of the flame, and the flame temperature changed over time. The additions of 20%, 40%, 60%, and 80% trimethyl borate fuel to gasoline were tested for their ability to burn. As the amount of TMB increased, high variations in droplet deformation and high breakups from the hemispheric geometry occurred. At this point, changes were observed in droplet shape change independent of mixing ratio. TMB droplet had the highest flame temperature of 600 K and the lowest extinction time of ~ 1270 ms. As the gasoline content of the droplets increased, the droplet flame temperature tended to decrease. Also, the shortest ignition delay time was observed for pure TMB and fuel droplets containing 40%, 60% and 80% TMB (~0.714 ms).

N-Alkyl-1,2,4-triazole–Borane Complexes as High-Density Hypergolic Materials

AbstractHydrazine and its derivatives have served as a conventional bipropellant fuel for several decades. However, their extremely acute toxicity and carcinogenicity, high volatility, and environmental impact have attracted significant attention. In order to synthesize green bipropellant fuels with high density, high specific impulse, and good thermal stability, three novel N-alkyl-1,2,4-triazole–borane complexes were successfully synthesized by reacting alkylated 1,2,4-triazole coordinated with sodium borohydride in the presence of ammonium sulfate. During the droplet test with white fuming nitric acid, there was a relatively short ignition delay time of 98 ms. Additionally, these hypergolic fuels possessed a high density exceeding 1.10 g cm–3, and the specific impulse is ranging from 187 to 199 s, and the highest decomposition temperature reaches 153.4 °C. These results demonstrate their great potential as hypergolic fuels or hypergolic ionic liquid additives in the field of hypergolic materials.

Investigation of aluminum particle ignition dynamics in various propellant environments

The ignition delay of aluminum particles can be profoundly influenced by the agglomeration and combustion characteristics of solid propellants. In this experimental study, the ignition of individual aluminum particles in nitrate ester plasticized polyether (NEPE) and hydroxyl‑terminated polybutadiene (HTPB) propellant atmospheres was examined using laser ignition tests and high-speed photography. Focus was directed at the effects of the atmospheric composition, pressure, and aluminum particle size (500, 800 and 1000 µm). It was observed that the non-homogeneous reaction heat generation and convective heat exchange rates during particle ignition could be increased by increasing the CO2 content or ambient temperature, resulting in a shorter ignition delay time. This explains why aluminum particles tended to burn more efficiently in nitrate ester plasticized polyether than in hydroxyl‑terminated polybutadiene. In addition, it was found that the ignition delay time decreased with increasing pressure and was inversely proportional to the particle size squared. An ignition model for aluminum particle ignition in a multi-component atmosphere was developed and validated to describe the temperature change and energy transfer during ignition. The calculated ignition delay times were found to be within 5% of the experimental values. The model also showed that increasing the ambient temperature was more effective than increasing the CO2 content at enhancing ignition, and that CO2 was more effective than O2, which was itself more effective than H2O. Overall, this study provides useful insight into the ignition characteristics of aluminum particles in different propellant environments. The results could be used to develop more effective propellant formulations, optimize combustion processes, and improve safety in the handling and use of aluminum-based propellants.

Citric acid enhances the ignition and combustion performance of aluminum: Coating and etching

The combustion efficiency of Al is usually not high due to the influence of alumina film and agglomeration. Therefore, finding ways to enhance the ignition and combustion performance of Al is important. Herein, a simple one-step recrystallization method was used to prepare the Al@ citric acid (CA) sample. The results showed that the one-step recrystallization method could well coat CA on the outer surface of Al particles and could etch Al particles uniformly. After modification by CA, Al was more prone to thermal oxidation. Compared with pure Al, the addition of CA resulted in shorter ignition delay time, longer combustion time, increased combustion intensity, and weakened agglomeration degree. Condensed combustion products (CCPs) showed that Al sintered significantly during combustion, forming a large agglomerate. The CCPs of Al@CA were more dispersed and had many hollow shell particles. Furthermore, the addition of CA led to a decrease in the active Al content and an increase in the α-Al2O3 content in CCPs. The above phenomena indicated that CA could significantly improve the combustion efficiency of Al. At last, the possible mechanism of CA promoting the ignition and combustion of Al was proposed. Overall, the enhancement of Al combustion performance by CA is attributed to the etching and coating effects of CA. Hoping that this paper will provide a possible pathway for exploring ways to improve Al combustion efficiency.

Combustion Characteristics and Application Performance of JP-10/Nano-Sized Aluminum Gel-Fueled Scramjet

Using nano-sized energetic materials as fuel additives to hydrocarbon gel fuel has gained in popularity due to the advantages of higher energy density and shorter ignition delay times. Combustion characteristics and application performance of gel fuel containing 16% wt aluminum nanoparticles were investigated on a direct-connect scramjet platform with an inflow Mach number of 2, an inflow total temperature of 1700 K, and a fuel equivalence ratio ranging from 0.6 to 1.0. The gel fuel could be stably injected and atomized efficiently by a nitrogen-assisted injector. The combustion efficiency of JP-10+Al gel fuel was 4.27% higher than that of JP-10 gel fuel, while the addition of aluminum increased the density specific impulse by 6.34%, indicating that the addition of aluminum nanoparticles indeed presented a positive effect on combustion and engine performance. The wall deposition phenomenon of gel fuel was particularly slight as compared to that of slurry fuel cases. Unlike nanoaluminum-blended slurry fuel, nanoaluminum-blended gel fuel controlled the potential risk of increasing thermal load to the least degree within the scope of existing research, with wall heat flux increasing by 7.43% solely as a result of elevated airflow temperature, indicating its advantageous application prospect.

Alternative Methods for Biodiesel Cetane Number Valuation: A Technical Note.

Biodiesel is an environmentally beneficial and clean energy source that may replace fossil fuels, which are detrimental to the environment and cannot be replenished. Therefore, the physicochemical parameters of biodiesel must be determined in order to verify its quality. The cetane number is a crucial dimensionless fuel property that gauges the fuel ignition quality in power diesel engines. A higher cetane number results in a shorter ignition delay time, and vice versa. Biodiesel's cetane number may fluctuate due to a variety of fatty acid compositions, including variations in carbon chain length and the degree of unsaturation. The cetane number generally increases with increasing saturation and chain length, while it decreases as chain length is reduced and degrees of unsaturation and branching increase. This is the main reason for why alkanes possess a higher cetane number than alkenes and aromatics. The standard protocols for evaluating the cetane number of biodiesel are ASTM D613 and ISO 5165 test techniques using a monocylindrical cetane engine. However, adhering to these conventional procedures is quite challenging and time-consuming, and the cetane number test result may also be affected by the presence of certain gases and fumes. As a result, many researchers are bothered with cetane number valuation, and occasionally they skip it due to a lack of other options. Consequently, the aim of this paper is to present a set of more straightforward and relevant alternative techniques that can be applied to predict the cetane number of biodiesel when engine-based measurement is not practical. The three techniques with their designed pictographic outlooks conferred in this article include color indicator titration, aniline point, and fatty acid composition-based methods. The reported values of these procedures meet the minimum cut point of the biodiesel cetane number required by ASTM D6751 (≥47) and exhibit minimal variation from the typical standard methods. Nevertheless, the above-mentioned techniques are not applicable to other alternative biofuels except biodiesel products because they have a direct implication on the characteristics of the fatty acid profiles of different oil precursors, such as carbon chain length, degree of saturation or unsaturation, and aromaticity, which make up monoalkyl esters.

Novel hydrophobic Ti-PVDF energetic thin films with excellent mechanical and ignition properties and their reaction mechanisms

With the development of traditional aluminum(Al)-based nanothermites, some problems on account of the metal Al powder gradually emerged. Therefore, extensive research is devoted to exploring novel metal fuels to achieve better energy release effects. Titanium (Ti) powder is a potential metal fuel theoretically considering of reducibility and calorific value, but it has rarely been reported in the field of nanothermites. This paper put forward the design strategy by selecting as the metal fuel and polyvinylidene fluoride (PVDF) as oxidant. According to the hydrophilicity of Ti powders, the PVDF film encapsulation can effectively improve hydrophobicity and storage stability. Another advantage of PVDF film state is that it imparts excellent mechanical properties (yield strength of 30.5 MPa and elongation at break of 202.8 %). According to the results of thermal analysis, Ti increased the thermal stability of PVDF while accelerated its thermal decomposition rate. In ignition properties, Ti-PVDF films have the shortest ignition delay time (217 ms) lower than results reported in the literature of other nanothermites under the same conditions. In addition, the energy release process of ignition combustion is analyzed and verified in this paper. Therefore, Ti-PVDF energetic ignition material with diverse properties is expected to be applied in more fields not limited to nanothermites.

Influence of steam dilution on ignition delay time of dimethyl ether/air mixtures at high pressure measured behind reflected shock waves

This work provides the first experimental data on ignition delay times of steam-diluted dimethyl ether/air mixtures at high pressure obtained behind reflected shock waves. Mixtures are prepared with 20 and 40vol% steam dilution and at equivalence ratios of 0.5, 1.0 and 2.0. The measurements comprise temperatures between 731 and 1314K and are recorded at nominal pressures of 15 and 35bar. The obtained experimental data is used for comparison and evaluation of predictions of common chemical-kinetic models. The effect of dilution on ignition delay time is furthermore studied numerically utilizing zero-dimensional reactor simulations with the chemical-kinetic model AramcoMech 3.0. Thermal and chemical effects of steam-dilution are analyzed in comparison to CO2 and N2 dilution. Steam dilution features a smaller heat capacity than CO2 at the investigated temperatures, which leads to a lower thermal effect and therefore shorter ignition delay time. The chemical effect of steam decreases ignition delay time at intermediate and high temperatures below the ignition delay time of a mixture with the same amount of N2 dilution. The chemical effect is mainly due to the high collision efficiency of H2O in the third body reactions H2O2 + H2O ↔ 2OH + H2O, H + O2 + H2O ↔ HO2 + H2O and CH3OCH3 + H2O ↔ CH3 + CH3O + H2O. At high temperatures the direct participation of H2O in reactions and its chemical conversion additionally exerts a further decreasing effect on ignition delay time.

Characteristics of Olive Oil Droplet Combustion with Various Temperatures and Directions of Magnetic Fields in the Combustion Chamber

This study examines the effects of temperatures and directions of the magnetic fields in the combustion chambers on flame characteristics for boiler combustion in power generation systems by burning olive oil droplets. The variations in the temperature of the combustion chamber are 40°C, 50°C, and 60°C. Meanwhile, the directions of the magnetic fields are the repulsive magnetic field (north-north) and the attractive magnetic field (north-south). In the experiment, a droplet of olive oil was placed at a type K thermocouple junction between the two bar magnets. A 250 fps high-speed camera recorded the flame from its ignition to its extinction. The results of this study found that temperature and direction of the magnetic fields in the combustion chamber have an effect on the characteristics of the flame, where the attractive magnetic field (north-south) resulted in increased burning of droplets, round flame, low altitude, increased temperature, and shorter ignition delay time, compared to the repulsive magnetic field (north-north) and without a magnetic field. Furthermore, the combustion chamber temperatures of 40°C, 50°C, and 60°C produced flame temperatures of 799.94°C, 829.25°C, and 879.50°C, and flame heights of 5.97 mm, 5.35 mm, and 4.23 mm, respectively. The strong magnetic fields increased the concentration of oxygen and fuel molecules around the combustion reaction zone, causing shorter droplet combustion and releasing a large amount of energy. These findings are beneficial for designing efficient industrial heat generators with a magnetic field. The results of this study are therefore crucial as a basis for considering the substitution of fossil fuels with environmentally friendly vegetable oils.

The characterization of disruptive combustion of organic gellant-laden ethanol fuel droplets

The future of high-performance rocket propulsion systems depends on superior burn rates, shorter ignition delay times, and eco-friendly nature. Gelled fuel propellants are such candidates which are pivotal to achieving these performance parameters. Gel fuels are multi-component fuels, and their combustion behavior is different from conventional solid and liquid propellants. To develop a fundamental understanding of the vaporization and combustion of gel fuels, single droplet experiments in a pendant mode setup are performed for ethanol-based organic gel fuels by capturing high-speed video data. Primarily, two gel fuel combinations are taken for experiments. The gel fuels are non-metalized ethanol-based fuels containing organic gellants Hydroxypropyl methylcellulose (HPMC) and methylcellulose (MC). Both fuels have shown three distinct combustion stages namely transient heat up (stage I), disruptive burning (stage II), and carbonization (stage III). The demarcation of the stages is done by visual evidence backed by high-fidelity high-speed video data. In stage I, phase separation occurs which leads to the formation of gellant shells. Organic gellants tend to form viscoelastic shells which significantly influence the jetting behavior during the disruptive burning (stage II). The combustion of gel droplets in stage II exhibits characteristic differences in bursting mechanism, for instance, oscillatory and transient bursting in HPMC and transient bursting in MC. Moreover, droplet-bursting behaviors influence jetting attributes and burn rates. Besides, the bursting of gel droplets during combustion is also dependent on the thickness of the gellant shells, the thin and weak shells are prone to bulk droplet motion due to active jetting, whereas the thick and rigid shells act as kinetic energy dampeners and thereby inhibit the bulk motion of droplets. Apart from disruptive burning, carbonization is a significant part of the combustion during which the suppressed jetting behavior is seen due to the traces of fuel trapped in the gellant shells. A significant rise in burn rate is observed due to jetting during the disruptive burning stage in contrast to the other two stages of combustion for both the gel fuels under study. Furthermore, combustion residue analysis is carried out to assess the variation in shell morphology across the stages. Overall, the current study provides detailed insights into the combustion characterization of organic gel fuels, which can help in fabricating gel fuel compositions.

Metal-Free Hybrid Energetic Composites Based on Donor-Acceptor π-Conjugated Organic Energetic Catalysts with Enlightening the Laser Ignition Performance of Multi-Scale Ammonium Perchlorate.

Photosensitive materials, such as energetic complexes, usually have high sensitivity and cause heavy-metal pollution, whereas others, like carbon black and dye, do not contain energy, which affects energy output and mechanical properties. In this work, donor-acceptor π-conjugated energetic catalysts, denoted as D-n, are designed and synthesized. Nonmetallic hybrid energetic composites are prepared by assembling the as-synthesized catalysts into multiscale ammonium perchlorate (AP). Composites containing catalysts and APs can be successfully ignited without the involvement of metals. The new ignition mechanism is further analyzed using experimental and theoretical analyses such as UV-vis-near-infrared (NIR) spectra, electron-spin resonance spectroscopy, and energy-gap analysis. The shortest ignition delay time is 56ms under the experimental condition of a NIR wavelength of 1064nm and a laser power of 10W. At the voltage of 1kV and the electric field of 500V mm-1 , the laser-ignition delay time of D-2/AP hybrid composite decreases from 56 to 35ms because D-2 also exhibits organic semiconductor-like properties. D-2/AP and D-12/AP can also be used to successfully laser ignite other common energetic materials. This study can guide the development of advanced metal-free laser-ignitable energetic composites to address challenges in the field of aerospace engineering.

Influence of different ignition delay times on the pressure rise rate in hybrid mixture explosions in the 20-L sphere

For the development of a standardized method for measuring the explosion safety characteristics of combustible hybrid dust/vapor mixtures, the influence of the ignition delay time needs to be investigated. The ignition delay time, defined as the time between the injection of dust and the activation of the ignition source, is related to the turbulence of the mixture and thus to the pressure rise rate. The ignition source for pure vapors, however, has to be activated in a quiescent atmosphere according to the standards. Nevertheless, when measuring the explosion safety characteristics of hybrid mixtures, it is important that the dust be in suspension around the igniter. Like pure dust/air mixtures, hybrid dust/vapor/air mixtures need to be ignited in a turbulent atmosphere to keep the dust in suspension.This work will therefore investigate the influence of ignition delay times on the severity of hybrid explosions. It was generally found that at shorter ignition delay times, (dp/dt)ex increased due to higher turbulence and decreases as the dust sinks to the bottom of the 20 L-sphere. This effect is more pronounced for hybrid mixtures with higher vapor content compared to dust content.

Effect of particle size on ignition and oxidation of single aluminum: molecular dynamics study

Alumina nanoparticle is one of the attractive nanoparticles synthesized by the plasma method. The oxidation step in this method is challenging to explain experimentally. This work was to perform a molecular dynamics simulation to determine the oxidation mechanism of aluminum nanoparticles with different sizes and oxidation levels in the oxide layer. This work was to perform a molecular dynamics simulation to determine the oxidation mechanism of aluminum nanoparticles with different sizes and oxidation levels in the oxide layer. The simulation method employed the ReaxFF potential. The material used is aluminum nanoparticles in three different sizes (8, 12, and 16 nm) with an oxide layer thickness of 0.5 nm. Aluminum nanoparticles were given a relaxation treatment of 300 K for 1 ps and then heated to a temperature of 3250 K with a heating rate of 5×1013 K/s and cooled to 300 K. The ensemble used is a canonical ensemble with the Nose/Hoover thermostat method. The result shows that the higher the temperature applied to the system, the more oxygen molecules adsorption occurs on the surface of the oxide layer and the diffusion of oxygen to the particle core. The higher temperature applied also causes gaps, or void spaces, between the core and the shell. The reaction barrier for diffusion of oxygen also decreased significantly due to void space, and the surface of the aluminum core dissociates to the surface (alumina shell). Particles with a smaller size have a shorter ignition delay time. In addition, the smaller the particle size, the more oxygen molecules' reacted with aluminum particles in the particle core

Enhancing the combustion and propulsion performance of Al/CuO via introducing AlH3 as fuel substitute

The research on nanothermites mainly focuses on formulation and structural modification, performance optimization and application research. Whereas, the few gaseous products of nanothermites limit the practical applications, especially in microthrusters. Here, various contents of AlH3 was introduced in typical nanothermites (Al/CuO) to replace Al as fuel. The characteristics of Al/AlH3/CuO were examined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ignition and combustion performance such as burning rate, unconstrained combustion, ignition temperature and delay time, pressurization and propulsion properties was investigated. Results revealed that when AlH3 content was 20%, Al/AlH3/CuO had the fastest burning rate (189 m/s), the lowest ignition temperature (441.4 °C), the shortest ignition delay time (5.32 ms), the maximum Pmax (1141.3 kPa), the highest pressurization rate (75.06 kPa/μs) and the maximum microthrust (1.61 N). In short, replacing Al with AlH3 in the Al/CuO provides an innovation for creating nanothermites with completely new metal fuels, and also enhances the combustion and propulsion performance.

Enhancing the reaction efficiency and ignition performance of core-shell Al@HMX composites by precise catalysis of graphene-based carbohydrazide complexes

The incomplete combustion of aluminum in solid propellants leads to a low-level energy release. As a promising strategy, fuel/oxidizer interfacial control is proved to be effective for enhancing their reaction efficiency and less environmental dependence. In this paper, in order to investigate the effect of interfacial control on the reaction efficiency of Al and HMX, spherical Al@HMX composites with polydopamine as interfacial layer were prepared via spray-drying technique. The morphology, structure, heat of reaction, thermal stability, condensed/gaseous products, decomposition kinetics as well as the ignition performance of the composites under the catalytic effects of graphene-based carbohydrazide complexes (GO-CHZ-M, M = Co2+ or Ni2+) were comprehensively investigated. Results showed that the heat of reaction of Al@HMX increased by 220 J g−1 compared to corresponding physical mixture (5655 J g−1), which was further increased to 6210 J g−1 in presence of minor GO-CHZ-Co (1 wt%) as a catalyst. Moreover, the thermal decomposition temperature of HMX was slightly increased in Al@HMX composites. Under the synergy of interfacial control and GO-based catalysts, the enhanced reaction efficiency of Al with HMX was observed and verified by a shorter ignition delay time (reduced from 126 to 71 ms) and the condensed products analysis, with decreasing unreacted Al content and increasing content of submicron-sized particles. Gaseous products investigation illustrated that there were two main decomposition pathways for Al@HMX composites: the CN scission (dominant) and NN scission, where the latter could be enhanced by GO-CHZ-M catalysts.

Spray-wall interaction on n-dodecane spray combustion and soot formation under different wall temperatures

To investigate the effect of the wall on soot formation during spray combustion and to explain the existence of contradictory results, the vapor-phase, ignition, combustion, and soot formation of free- and impingement-spray with different wall temperatures were studied in comparison using optical diagnostic techniques. Results reveal that, compared to free spray, the impingement spray’s vapor-phase spray tip penetration is reduced, and its vapor-phase spray volume is increased by about three times. At conditions where only the vapor-phase impinges on the wall, the impingement spray has a shorter ignition delay time (IDT) for various wall temperatures. Furthermore, the soot formation of the impingement spray combustion is approximately one-third of the free spray, and the increase in air entrainment accounts for this phenomenon. In a comparison of different wall temperature conditions, the higher wall temperature facilitates the spray ignition process with a shorter IDT, and the ignition location is further away from the wall. However, the relationship between wall temperature and soot formation is not linear. The soot formation varies in a pattern of reducing and then increasing as the wall temperature increases due to the high wall temperature accelerating both the generation and oxidation rate of soot.

The impact of ethanol addition on the droplet combustion mechanism of saturated and unsaturated fatty acid/fatty acid methyl ester molecules

The combustion of biodiesel requires complex analysis because it is composed of multicomponent constituents. The use of additives in biodiesel is an option widely used to improve the combustion performance of the fuel blend. The droplet combustion characteristics of single compounds of biodiesel and vegetable oil constituents with ethanol additives were investigated under normal gravity at room temperature and atmospheric pressure. This study reveals the different combustion mechanisms and their interactions with ethanol between laurate and oleate as the base fuel. Ethanol breaks the base fuel into smaller molecular aggregates, reducing the fuel blend density with many larger gaps between the clusters. Thermal expansion and child droplet ejection are key mechanisms that effectively increase the total droplet surface area, causing an increase in the diffusion contact between the fuel vapor and the air. Three mechanisms of child droplet ejection occur during the heating phase until the microexplosive combustion phase. The period of child droplet ejection during the microexplosive combustion phase is determined by the boiling point difference and the solubility level between the base fuel and ethanol. The addition of ethanol increases the burning rate of the fuel mixture with a higher peak temperature in the laurate-ethanol blend compared to the base fuel. Ethanol plays an important role in delaying the start of evaporative cooling with a significant temperature increase rate. This behavior is an effective mechanism to initiate droplet combustion with a short ignition delay time.

A new ternary high-energy composite based on nano titanium powder with low sensitivity and stable combustion

For solving the low combustion efficiency caused by aggregation and further promoting the application of nano titanium powder (n-Ti), a new ternary composite high-substitute nitrochitosan/nano titanium/graphene oxide (NCh/n-Ti/GO) was prepared by a resonant acoustic mixing method. Herein the high-substitute nitrochitosan (NCh) was used as binder and oxidant to modify the surface of n-Ti powder, and GO was introduced as stabilizer and carrier. The energy release rate, laser ignition, combustion performance and sensitivity of NCh/n-Ti/GO were systematically studied in this work. The experiment results confirmed that GO plays a significant role in adjusting combustion performance and reducing the sensitivity of composite. Compared with n-Ti, NCh/n-Ti and NC/n-Ti, the introduction of GO can greatly increase the combustion time and reduce the flame propagation speed. In particular, NCh/n-Ti/GO (3 wt%) presentedan optimal linear flame morphology, the shortest ignition delay time and lowest pressurization rate. The combustion products were also determined and confirmed to be co-existence of TiO, a little TiN and unburned n-Ti powder. The introduction of GO can reduce the sensitivities to impact and electrostatic of composites greatly. The electrostatic sensitivity of NCh/n-Ti/GO (3 wt%) was 5.1 ± 0.1 mJ and decreased by 160% and 230%, compared with that of n-Ti and NCh/n-Ti. So the introduction of NCh and GO would be a good strategy to promote the combustion performances and safety of n-Ti powder.