High Ignition Research Articles

Micro-optics for ultra-intense lasers

Table-top, femtosecond lasers provide the highest light intensities capable of extreme excitation of matter. A key challenge, however, is the efficient coupling of light to matter, a goal addressed by target structuring and laser pulse-shaping. Nanostructured surfaces enhance coupling but require “high contrast” (e.g., for modern ultrahigh intensity lasers, the peak to picosecond pedestal intensity ratio >1012) pulses to preserve target integrity. Here, we demonstrate a foam target that can efficiently absorb a common, low contrast 105 (in picosecond) laser at an intensity of 5 × 1018 W/cm2, giving ∼20 times enhanced relativistic hot electron flux. In addition, such foam target induced “micro-optic” function is analogous to the miniature plasma-parabolic mirror. The simplicity of the target—basically a structure with voids having a diameter of the order of a light wavelength—and the efficacy of these micro-sized voids under low contrast illumination can boost the scope of high intensity lasers for basic science and for table-top sources of high energy particles and ignition of laser fusion targets.

Combustion characteristics of n-heptane spray combustion in a low temperature reform gas/air environment

This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case.

Study of lignocellulosic biomass ignition properties estimation from thermogravimetric analysis

In the last decade, the use of renewable resources has increased significantly in order to reduce the energetic dependence on fossil fuels, as they have an important contribution to the global warning and greenhouse gasses effect. Because of that, research on biofuels has been increased in the last years as its characteristics of use match those of the conventional fuel's: solid biomass can be used instead of coals, and biodiesel could replace diesel. Research on solid biomass ignition properties has been considerably developed because of the amount of industrial accidents related to the treatment and use of solid biomass (self-ignition, dust explosions, etc.). On the other hand, thermogravimetric analysis (TGA) is becoming and important characterization technique as it can be used to determine a wide spectrum of properties, such as kinetics, composition, proximate analysis, etc. This research aims to combine thermal analysis and ignition properties, by using the TGA to obtain the elemental composition of lignocellulosic biomass and compare those results to Minimum Ignition Energy (MIE) values test output, so a relation between composition and MIE can be found.To achieve this aim, biomass samples from different origins have been used: oil palm wastes (empty fruit bunches, mesocarp fiber and palm kernel shell), agricultural wastes (straw chops) and forestry wastes (wood chips and wood powder). Also, raw materials and torrefied biomass were compared. The hemicellulose/cellulose ratio was calculated and compared to different flammability properties, finding out that the greater the ratio and the lower the onset temperature (temperature at which the pyrolysis reaction accelerates), the lower was the minimum ignition energy. From this basis it was possible to define “tendency areas” that grouped the samples whose MIE values were similar. Three tendency areas were found: high minimum ignition energy, medium minimum ignition energy, and low ignition energy.

Effects of high ignition energy on lean combustion characteristics of natural gas using an optical engine with a high compression ratio

Natural gas engines often suffer from slow-burning rate and large cyclic variations, which significantly affects engine performance and thermal efficiency. In this study, using an optical spark-ignition engine with a high compression ratio, the role of high ignition energy in lean combustion characteristics of natural gas was investigated. Synchronization measurement of in-cylinder pressure and visualization images were performed to analyze the correlations of engine performance and combustion characteristics. The results show that elevating ignition energy can improve combustion instability and thermal efficiency, manifesting more concentrated heat release and greater influences on the combustion duration from 10% to 50% in burned mass fraction than that of 50%–90% range. The combustion visualizations show that ignition energy mainly affects initial flame kernel formation and early flame development, and improved flame speed and combustion instability can be observed even when excess air coefficient reaches 1.4 at sufficiently high ignition energy. Furthermore, an empirical criterion based on burned mass fraction and combustion visualizations was adopted to quantify early flame development. The shortage of ignition delay time and the acceleration of initial flame propagation are observed with the increases of ignition energy, which leads to lower cyclic variations and a faster burning rate at the early combustion stage.

Ammonia Combustion Catalysts

Abstract Recent developments for ammonia (NH3) combustion catalysts are covered in this highlight review. NH3 has been proposed as a renewable and carbon-free energy source. However, use of NH3 fuel poses the problems of high ignition temperature and nitrogen oxide (N2O/NOx) production. In order to overcome these issues, a novel catalytic combustion system was probed, and high performance catalysts were developed. This review introduces their research with including related studies.

Modelling ignition probability with pairwise mixing-reaction model for flame particle tracking

Reduced order models for ignition analysis can offer insights into ignition processes and facilitate the combustor optimization. In this study, a Pairwise Mixing-Reaction (PMR) model is formulated to model the interaction between the flame particle and the surrounding cell mixture during Lagrangian flame particle tracking. Specifically, the model accounts for the two-way coupling of mass and energy between the flame particle and the surrounding shell layer by modelling the corresponding turbulent mixing, chemical reaction and evaporation process if present. The state of a flame particle, e.g., burnt, hot gas or extinguished, is determined based on particle temperature. This model can properly describe the ignition process with a spark kernel being initiated in a nonflammable region, which is of practical importance in certain turbine engines and has not been rigorously accounted for by the existing models based on the estimation of local Karlovitz number. The model is integrated into an ignition probability analysis platform and is demonstrated for a methane/air bluff-body flame with the flow and fuel/air mixing characteristics being extracted from a non-reacting simulation. The results show that for the spark location being at the extreme fuel-lean outer shear layer of the recirculation zone, PMR can yield ignition events with a significant number of active flame particles. The mechanisms for the survival of the initial flame particles and the entrainment of the survived flame particles into the recirculation zone are analyzed. The results also show that the ignition probability map from PMR agrees well with the experimental observation: a high ignition probability in the shear layer of the recirculation zone near the mean stoichiometric surface, and low ignition probabilities inside the recirculation zone and the top stagnation region of the recirculation zone. The parametric study shows that the predicted shape of the ignition progress factor and ignition probability is in general insensitive to the model parameters and the model is adequate for quantifying the regions with high ignition probabilities.

Numerical Characterization of Corona Spark Plugs and Its Effects on Radicals Production

A mono-dimensional code for the simulation of the effects of High Frequency Ignition systems (HFI) on the production of chemical radicals was developed and here presented. The simulations were carried out by considering the typical environmental thermodynamic conditions of a nowadays engine at full load. An electron transport model is linked with a Boltzmann solver coupled with a chemistry solver, affecting the Electron Energy Distribution Function (EEDF) in order to obtain the physical conditions leading to the production of radical components for a given fuel mixture. The transport equations for the electrons, the positive and the negative ions, and the Gauss’ law in a steady-state plasma region. Then the Boltzmann equation for the electrons, in a spatially homogeneous steady-state case, is solved in order to obtain the EEDF. Finally the chemical kinetics model is employed assuming a fuel-air mixture neglecting the fuel carbon atoms due to the assumption that electron-impact dissociation reactions, which initiate the combustion, exhibit a greater reaction rate compared to those based on hydrocarbon thermal dissociation and therefore can be neglected in this work. Results show the production of the hydrogen (H), nitrogen (N), and oxygen (O) radicals and the radius of the initial discharge under different simulated engine operating conditions characterizing the role of a plasma corona effect for the induced chemical ignition in gasoline-powered engines.

Inhibition Mechanism of CH4 Addition on the Pressurized Hydrogen Spontaneous Ignition

It is well known that the high spontaneous ignition risk of pressurized hydrogen poses a serious threat to hydrogen storage safety. Aiming to reveal the inhibition mechanism of the fuel-blending st...

Effects of pre-injection strategy on combustion performance of methanol/diesel dual fuel engine at low load

In this paper, the effect of pre-injection timing on the combustion and emission performance of methanol/diesel high premixed charge compression ignition (HPCCI) combustion mode at low load was studied by three-dimensional numerical simulation. The simulation is carried out under the conditions of 1900 r/min and 25% load, and the pre-injection diesel mass remains constant and the pre-injection timing is changed under the conditions of 50% and 60% methanol ratio respectively. The results show that with the delay of pre-injection timing, the evaporation of pre-injected fuel in cylinder becomes faster, the low temperature reaction is enhanced, and the peak value of heat release rate is higher. In addition, the distribution of pre-injected diesel in cylinder and the position of impact wall are the important factors affecting the combustion and emission performance in cylinder, and when the injection timing is -35 °CA ATDC, the combustion and emission characteristics of the engine are optimal. Finally, by comparing with the traditional RCCI combustion mode, the enhancement ability of HPCCI combustion mode on methanol combustion efficiency under low load is verified and analyzed.

Design and experimental validation of a high flow rate pyrogen igniter for cryogenic engine testing

Design and experimental validation of a high flow rate pyrogen igniter for cryogenic engine testing

ИССЛЕДОВАНИЕ ФАКТОРОВ, СПОСОБСТВУЮЩИХ НЕCАНКЦИОНИРОВАННОМУ ИНИЦИИРОВАНИЮ ЭНЕРГОНАСЫЩЕННЫХ МАТЕРИАЛОВ

The results of experimental studies on the initiation of compositions containing energy-saturated materials by high-current electron beams and electrodischarge plasma are presented. Experimental studies simulate the effects on energy-saturated materials of electromagnetic fields of natural and man-made origin. The threshold voltage of the electrical breakdown of the compositions during a high-voltage electric discharge was determined and the possibility of initiation of the compositions by high-current electron beams of nanosecond duration using a pulsed high-current electron accelerator GKVI-300 was studied. Four compositions were selected as objects of study: a composition based on potassium picrate, a composition containing ammonium perchlorate, a composition based on lead minium with zirconium and colloxylin additives, and ballistite composition SBK-3. It is shown in the work that the initiation of compositions by a high-current electron beam is potentially possible only for compositions with low ignition temperatures, ignition of compositions with high ignition temperatures is possible due to the introduction of a powdery nanoscale additive, particles of which have specific heat less than particles of an energy-saturated material, and ignition of ammonium perchlorate is possible only if the particles of the additive have lower thermal conductivity than the particles of perchlorate ammonium.

Development of plasma electrolytic oxidation coatings on Ti-42Al-2V-2Nb alloy

Ti-Al intermetallics based on their low density (3.9–4.7 g/cm3), high melting point (>1450 °C), and high ignition resistance were selected as the optimal materials for the aerospace applications replacing Ni-based superalloys. The current study aims at enhancing the performance of Ti-42Al-2V-2Nb alloy in the corrosive marine environment by developing plasma electrolytic oxidation (PEO) coatings. Three different electrolytes (5 g/l Na2SiO3 + 2.5 g/l KOH, 5 g/l Na3PO4 + 2.5 g/l KOH and 2.5 g/l Na2SiO3 + 2.5 g/l Na3PO4 + 2.5 g/l KOH) were selected for developing the coatings. PEO coatings were developed at a current density of 170 mA/cm2 over a processing time of 12 min. The phase composition and wettability of the obtained PEO coatings were studied by the X-ray diffraction (XRD) and contact angle goniometric studies. XRD results indicated the formation of the ceramic Al2TiO5 phase. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopic (EIS) studies were carried out in simulated marine condition using 3.5 wt% NaCl solution to analyze the performance of the samples in the corrosive medium. PDP and EIS results revealed that the PEO coated samples exhibited significant enhancement in corrosion properties compared to the uncoated substrate on the account of ceramic oxide layer formation. PDP studies unveiled the successful shielding of the Ti-42Al-2V-2Nb sample from pitting was obtained by developing the PEO coatings. The sample PEO coated using phosphate electrolyte exhibited superior corrosion resistance (icorr = 1.75 × 10−6 mA/cm2) compared to the other PEO coated samples attributing to the higher thickness (12.7 ± 0.5 µm) and higher contact angle (89°).

Effect of oxygen and steam on gasification and power generation in industrial tests of underground coal gasification

To optimize the gasification process and improve the underground coal gasification (UCG) technology for power generation, a pilot project of UCG integrated with the power generation was built in the abandoned mine “Ankou”. The UCG industrial tests with different oxygen concentrations of oxidant along with the pulsed injection method, and power generation experiments using UCG syngas were conducted. The results indicate that a decrease in the rate of air blowing for a short time exhibited a limited influence on syngas production, but a change in the amount of oxygen injection presented an obvious effect. With the increase in the oxygen concentration in the agent, the H2 content, combustible components and calorific value of syngas, flow ratio, and the consumption of coal, steam and oxygen increased, along with a rise in the gasification efficiency after an initial drop. The oxygen-enriched gasification trials showed higher air consumption, lower consumption of coal, steam and oxygen, and a low combustible components and calorific value of syngas compared to the gasification tests of the oxygen-enriched steam. The evaporation of ground water and coal moisture in the UCG panel provided steam for the oxygen-enriched gasification process, and its utilization rate was about 6–32 wt%. From the characteristics of UCG syngas, the technical transformations such as exhaust gas booster combustion control, high energy ignition, low pressure intake, expansion of the air, and gas inlet pipe diameter were carried out on the existing gas generator sets. The effectiveness of the measures was verified by the power generation experiments.

Pt/CeO2 catalyst synthesized by combustion method for dehydrogenation of perhydro-dibenzyltoluene as liquid organic hydrogen carrier: Effect of pore size and metal dispersion

Liquid organic hydrogen carrier (LOHC) is a chemical hydrogen storage method that stores hydrogen in the form of liquid organics. Dibenzyltoluene (DBT) is a promising LOHC material due to its high storage density, low ignitability, and low cost. In this study, Pt/Al2O3 and Pt/CeO2 catalysts are synthesized using a combustion nanocatalyst synthesis method called the glycine nitrate process (GNP) to obtain high catalytic activity for the dehydrogenation of perhydro-dibenzyltoluene (H18-DBT). Pt/CeO2 exhibits much faster dehydrogenation than Pt/Al2O3, 80.5%/2.5 h versus 3.5%/2.5 h. To investigate the causes of the difference in the dehydrogenation rates, microstructural characterization by N2 physisorption, CO chemisorption and transmission electron microscopy analysis are conducted, and the catalytic activities are evaluated at various liquid hourly space velocities (LHSVs). The differences in dehydrogenation can be attributed to the mass transport of liquid H18-DBT into the catalyst pores being slow due to the small pores in Pt/Al2O3, which is a rarely addressed issue for other LOHC materials. This is because many LOHC materials are dehydrogenated at the gas phase, which has higher diffusivity than that of the liquid phase. Pt/CeO2 synthesized by the GNP is also compared with a commercial Pt/Al2O3 catalyst. The commercial Pt/Al2O3 catalyst shows a dehydrogenation of 17.8%/2.5 h, which is much slower than that of Pt/CeO2 synthesized by the GNP, at 80.5%/2.5 h.

A versatile set-up to study plasma/microwave sources for liquid fuel ignition

Motivated by the high demand for an alternative, more reliable, high energy ignition source to facilitate the re-ignition of lean-burn combustion chambers which are necessary to reduce pollutant emissions, a new set-up has been designed to study plasma/microwave sources. The use of a waveguide-based resonant cavity leads to very low power plasma ignition. An example in this paper shows that a plasma at atmospheric pressure can be maintained with less than 2 W input power. Such a performance is possible using the large variety of possible adjustments (resonance frequency, different kind of initiators, etc.) that this versatile set-up offers. To illustrate the wide range of possible studies, another example is given and discussed : minimum ignition energy for an ethanol droplet stream with aluminum and stainless steel initiators. The results show that the initiator material and its surface quality have an influence on the minimum ignition energy, especially for large gaps. Depending on the gap size we can get down to under 10 W entering the cavity to ignite the droplet stream.

Numerical simulation of a mixed-mode reaction front in a PPC engine

The ignition process, mode of combustion and reaction front propagation in a partially premixed combustion (PPC) engine running with a primary reference fuel (87% iso-octane, 13% n-heptane by volume) is studied numerically in a large eddy simulation. Different combustion modes, ignition front propagation, premixed flame and non-premixed flame, are observed simultaneously. Displacement speed of CO iso-surface propagation describes the transition of premixed auto-ignition to non-premixed flame. High temporal resolution optical data of CH2O and chemiluminescence are compared with simulated results. A high speed ignition front is seen to expand through fuel-rich mixture and stabilize around stoichiometry in a non-premixed flame while lean premixed combustion occurs in the spray wake at a much slower pace. A good qualitative agreement of the distribution of chemiluminescence and CH2O formation and destruction shows that the simulation approach sufficiently captures the driving physics of mixed-mode combustion in PPC engines. The study shows that the transition from auto-ignition to flame occurs over a period of several crank angles and the reaction front propagation can be captured using the described model.

A perspective on the use of ammonia as a clean fuel: Challenges and solutions

A perspective on the use of ammonia as a clean fuel: Challenges and solutions

A numerical study of accelerated moderate or intense low-oxygen dilution (MILD) combustion stability for methane in a lab-scale furnace by off-stoichiometric combustion technology

Moderate or intense low-oxygen dilution (MILD) combustion has become a promising low-NO X emission technology, while the delayed mixing of reactants and slower oxidation rate could potentially cause ignition instability in some scenarios. This paper proposes a new idea for enhancing the ignition stability for methane MILD combustion by combining with off-stoichiometric combustion (OSC), and its performances have been numerically assessed through a comparison against the original MILD combustion burner. The results reveal although non-premixed pattern has the lowest NO emission, it suffers from a larger liftoff distance, thus less ignition stability. Contrarily, both partially-premixed and fully premixed patterns exhibit excellent ignition stability. Among the considered OSC conditions, the pattern of Inner ultra-rich and Outer lean produces the lowest NO emission while maintains a high ignition stability. Furthermore, the enhancement of the combustion stability by implementing OSC to the original MILD combustion burner is shown by comparing the operational range of furnace wall temperature ( T f ), CO and NO emissions, as well as the evolution of chemical flame. The comparison reveals that OSC can extend the lowest operational T f from 900 K to 800 K. More importantly, OSC can significantly improve the ignition stability in the whole range of T f as compared to the original MILD combustion burner.

Features of modeling the combustion processes of flammable liquids with high ignition temperature in FDS software

The article describes the features of modeling the dynamics of fire during the combustion of flammable liquids, linked with dependence of the velocity of spreading along the liquid surface from a large number of factors. The method of computer simulation of combustion processes of flammable liquids with high ignition temperature is viewed via the Fire Dynamics Simulator software on the example of turbine oil. The influence of the linear velocity of flame spreading over the surface of turbine oil on the dynamics of temperature changes of the gaseous medium in the room is explored. The results of dynamic modeling of turbine oil combustion for cases of a simultaneously ignited area of the spilled flammable liquid and spreading of the flame over its surface from the ignition point with a calculated linear speed. The conclusion is made that it is expedient to use the described method for estimating the linear velocity of flame movement over the liquid surface when modeling combustion over large areas of flammable liquids with a high ignition temperature.

Understanding cool flames and warm flames

Low-temperature flames such as cool flames, warm flames, double flames, and auto-ignition assisted flames play a critical role in the performance of advanced engines and fuel design. In this paper, an overview of the recent progresses in understanding low-temperature flames and dynamics as well as their impacts on combustion, advanced engines, and fuel development will be presented. Specifically, at first, a brief review of the history of cool flames is made. Then, the recent experimental studies and computational modeling of the flame structures, dynamics, and burning limits of non-premixed and premixed cool flames, warm flames, and double flames are presented. The flammability limit diagram and the temperature-dependent chain-branching reaction pathways, respectively, for hot, warm, and cool flames at elevated temperature and pressure will be discussed and analyzed. After that, the effect of low temperature auto-ignition of auto-igniting mixtures at high ignition Damköhler numbers at engine conditions on the propagation of cool flames, warm flames, and double flames as well as turbulent flames will be discussed. Finally, a new platform using low temperature flames for the development and validation of chemical kinetic models of alternative fuels will be presented. Discussions of future research of the dynamics and control of low temperature flames under engine conditions will be made.