Ignition Conditions Research Articles

The probability of escape of the products of charged particles resulting from thermonuclear fusion of advanced fuels in the hotspot

Establishing fuel ignition conditions depends on the stability of hotspot ignition. Excessive energy escape from the plasma leads to its cooling and shutdown. In this research, the escape probability of charged particles due to the p/11B fuel fusion has been investigated. First, the contribution of p/11B fuel plasma electrons in the Alpha particle energy loss based on the Krokhin and Rozanov (KR) model and then the auxiliary contribution of plasma ions in the Alpha particle energy loss produced in the hotspot of p/11B fuel, based on Li–Petrasso (LP) calculations were analyzed numerically. By calculating the escape fraction of Alpha particles, it has been shown that the contribution of the stopping power of the electrons of the plasma is dominant at both 70 and 170 keV temperatures, which are the starting temperature of the p/11B fuel reaction and the proper ignition temperature, respectively. By increasing the density of p/11B fuel from 300 to 400 g/cc, it can be seen that at a temperature of 70 keV, the penetration depth of Alpha particles decreases from 1200 to 900 μm based on the KR model. It has also been shown that by reducing the number density ratio of boron to protons from 0.3 to 0.2 and 0.1, due to the reduction of the coupling of electrons and ions, which leads to the reduction of collisions with Alpha particles, the stopping length of Alpha particles increases.

Complex Three-Dimensional Mathematical Model of the Ignition of a Coniferous Tree via a Cloud-to-Ground Lightning Discharge: Electrophysical, Thermophysical and Physico-Chemical Processes

Thunderstorms are the main natural source of forest fires. The ignition mechanism of trees begins with the impact of cloud-to-ground lightning discharge. A common drawback of all predicting systems is that they ignore the physical mechanism of forest fire as a result of thunderstorm activity. The purpose of this article is to develop a physically based mathematical model for the ignition of a coniferous tree via cloud-to-ground lightning discharge, taking into account thermophysical, electrophysical, and physicochemical processes. The novelty of the article is explained by the development of an improved mathematical model for the ignition of coniferous trees via cloud-to-ground lightning discharge, taking into account the processes of soot formation caused by the thermal decomposition phase of dry organic matter. Mathematically, the process of tree ignition is described by a system of non-stationary nonlinear differential equations of heat conduction and diffusion. In this research, a locally one-dimensional method is used to solve three-dimensional partial differential equations. The finite difference method is used to solve one-dimensional heat conduction and diffusion equations. Difference analogues of the equations are solved using the marching method. To resolve nonlinearity, a simple iteration method is used. Temperature distributions in a structurally inhomogeneous trunk of a coniferous tree, as well as distributions of volume fractions of phases and concentrations of gas mixture components, are obtained. The conditions for tree trunk ignition under conditions of thunderstorm activity are determined. As a result, a complex three-dimensional mathematical model is developed, which makes it possible to identify the conditions for the ignition of a coniferous tree trunk via cloud-to-ground lightning discharge.

Characterization of hot electrons generated by laser–plasma interaction at shock ignition intensities

In an experiment carried out at the Prague Asterix Laser System at laser intensities relevant to shock ignition conditions (I > 1016 W/cm2), the heating and transport of hot electrons were studied by using several complementary diagnostics, i.e., Kα time-resolved imaging, hard x-ray filtering (a bremsstrahlung cannon), and electron spectroscopy. Ablators with differing composition from low Z (parylene N) to high Z (nickel) were used in multilayer planar targets to produce plasmas with different coronal temperature and collisionality and modify the conditions of hot-electron generation. The variety of available diagnostics allowed full characterization of the population of hot electrons, retrieving their conversion efficiency, time generation and duration, temperature, and angular divergence. The obtained results are shown to be consistent with those from detailed simulations and similar inertial confinement fusion experiments. Based on the measured data, the advantages, reliability, and complementarity of the experimental diagnostics are discussed.

CRITICISM OF THE “THEORY OF INCANDESCENT SURFACE IGNITION” BY Ya.B. ZELDOVICH. SELF-IGNITION OF INFINITE FUEL BY INCANDESCENT BODIES

The basic principles of thermal explosion were studied and outlined by 1980. It seemed that fundamental research had been completed, but in 1984 A.A. Filippov discovered that the “ignition” condition obtained by Ya.B. Zeldovich did not correlate with the data of the experiment on ignition of a gas mixture by incandescent cylinders. Since Zeldovich’s “Theory of Incandescent surface ignition” has long been the basis of the ignition theory, its analysis was carried out and an error was found. It is proved that the above-mentioned work is not related to ignition, but to the self-ignition of infinite fuel. It is shown that a thermal explosion of fuel from an incandescent body can develop not only as ignition, but also as self-ignition. The conditions of self-ignition of infinite fuel by a flat surface and a cylinder are given, as well as the probable condition of ignition of fuel between parallel walls of different temperatures.

Study of antiprotons as drivers in inertial confinement fusion by fast ignition method

Inertial confinement fusion is a promising approach to achieve controlled nuclear fusion for clean and abundant energy production. One of the key challenges in Inertial confinement fusion is achieving efficient ignition of the fusion fuel. Fast ignition offers a potential solution to this challenge by using an ultra-high intensity laser or a charged particle beam to directly ignite a pre-compressed fusion fuel. In this manuscript, we propose an approach for fast ignition in ICF, utilizing an antiproton beam to drive ignition in a deuteron-tritium fuel with a uranium-238 seed. The use of antiproton beams in Inertial confinement fusion offers unique advantages, including their ability to deposit energy deeply into the fuel, leading to enhanced energy coupling and heating. The addition of uranium-238 as a seed material in the fuel can further improve ignition conditions by enhancing energy deposition and facilitating ignition reactions. We present detailed simulations and analyses to demonstrate the feasibility and potential benefits of this approach. We investigate the effects of antiproton beam parameters, such as energy, intensity, and pulse duration, on ignition conditions, as well as the impact of uranium-238 seed concentration and distribution in the fuel. Our results show that fast ignition driven by an antiproton beam in DT with uranium-238 seed has the potential to significantly improve ignition performance in Inertial confinement fusion, leading to enhanced energy output and higher gain. The use of antiproton beams allows for efficient energy deposition and heating of the fuel, while the inclusion of uranium-238 seed promotes ignition reactions and improves ignition conditions. This concept presents a promising pathway towards achieving practical and efficient ignition in Inertial confinement fusion, and could pave the way for next-generation fusion power plants.

Critical ignition conditions for propane/air premixtures impinging on electric heated surfaces: An experimental study

To contribute to exact predictions of the ignition of flammable mixture by a hot surface and ensure safety against fire, this study aims to clarify the critical ignition condition of a propane/air premixture impinging on a hot surface with external heating. The ignition threshold of heat flux depends on the flow velocity, but that of temperature hardly depends on flow velocity and equivalence ratio. The ignition occurred at the position where the residence time of species was the longest. It corresponds to the position of the lowest heat transfer coefficient. The temperature boundary layer thickness played a significant role in the ignition process as only the heat flux from the top, bottom, and back surfaces of the heater contributed to ignition in the presence of forced convection. The prediction model of the ignition threshold of heat flux and temperature was derived through dimensional analysis. This model effectively explains the ignition characteristics, regardless of the differences in thermal convection mode, dimensions, and geometric arrangements of a heater, as confirmed by comparison with reference data. Additionally, the ignition threshold of temperature can be predicted if the external heating power is known, and it is useful to assess the fire risk.

Prediction of superionic state in LiH2 at conditions enroute to nuclear fusion

Hydrogen and lithium, along with their compounds, are crucial materials for nuclear fusion research. High-pressure studies have revealed intricate structural transitions in all these materials. However, research on lithium hydrides beyond LiH has mostly focused on the low-temperature regime. Here, we use density functional theory and ab initio molecular dynamics simulations to investigate the behavior of LiH2, a hydrogen-rich compound, near its melting point. Our study is particularly relevant to the low-pressure region of the compression pathway of lithium hydrides toward fusion. We discovered a premelting superionic phase transition in LiH2 that has significant implications for its mass transportation, elastic properties, and sound velocity. The theoretical boundary for the superionic transition and melting temperature was then determined. In contrast, we also found that the primary compound of lithium hydrides, LiH, does not exhibit a superionic transition. These findings have important implications for optimizing the compression path to achieve the ignition condition in inertial confinement fusion research, especially when lithium tritium-deuteride (LiTD) is used as the fuel.

Performance and sensitivity analysis of raw biogas combustion under homogenous charge compression ignition conditions

This study aims to investigate the feasibility of using raw biogas as the main fuel in homogeneous charge compression ignition (HCCI) engines and to identify the effects of different trace elements found in biogas on engine performance and emissions. This study was conducted using many models that simulate the operation of HCCI process. These models include the chemical kinetics, combustion model, and the heat transfer models. Raw biogas composition was selected based on the most available traces. The traces that were studied are Hydrogen Sulfide (H2S), Ammonia (NH3), Carbon monoxide (CO), Nitrogen (N2), Oxygen (O2), and Water vapor (H2O). Other parameters like the equivalence ratio, intake temperature, and pressure were also studied. It was found that raw biogas mixtures could produce higher power outputs in the presence of H2S and NH3. However, H2S and NH3 were found to increase the SOx and NOx percentages owing to their higher in-cylinder temperatures. Other traces such as CO and H2O can reduce the power output, but at the same time, they can reduce the engine NOx emissions but not the CO emissions. Engine performance was positively affected by increasing the intake pressure and equivalence ratio within the studied ranges. However, higher intake temperatures and reduce the engine efficiency and increase the engine emissions. On the other hand increasing the compression ratio (CR) increase the system efficiency but up to a certain limit. Considering the exhaust gas residual (EGR), this parameter reduce system performance but at the same time lower the NOx emissions significantly. A sensitivity analysis for the mentioned parameters indicates that EGR is the most effective parameter while CR are the least significant one.

Chemical kinetic analysis on the auto‐ignition characteristics of natural gas–diesel dual‐fuel combustion under engine‐relevant conditions

AbstractWith the advent of energy crisis and stringent emission regulations, natural gas (NG)/diesel dual‐fuel engines have entered the popular view. In this paper, the accurate boundary conditions are determined by the Converge model and the transient parameters were introduced into the chemical kinetics model to make the operating conditions more closely match the actual engine operating conditions. The effects of different methane/n‐heptane ratios, initial temperatures, initial pressures, and equivalence ratios on the IDT of methane/n‐heptane fuel mixtures were analyzed using comprehensive chemical kinetic tools such as mole fraction analysis, ROP analysis, and sensitivity analysis. The IDT of methane/n‐heptane mixture decreased significantly with increasing the equivalence ratio, initial pressure, and the proportion of n‐heptane in the mixture, but the change of temperature was more complicated for the IDT. The NTC behavior of methane/n‐heptane mixture was also affected by the NTC behavior of n‐heptane. With the increase of initial temperature, the consumption of methane and n‐heptane appeared to be significantly accelerated. The peak of radicals and intermediate groups appeared earliest and had the highest radical concentration when the n‐heptane content was 30%. With the increase of initial pressure, the peak moments of the six radicals were further advanced, but the mole concentrations of O, OH, H radicals decreased significantly. This paper not only provides a reference for the design of NG/diesel dual‐fuel engines, but also provides a theoretical basis and data support for improving the ignition and initial condition setting strategies of the engines.

An experimental and modeling study on the ozone-assisted low-temperature ignition of methylcyclohexane in an RCM

Methylcyclohexane (MCH) is a significant component of practical fuels and has garnered significant research interest in recent years. However, due to the low reactivity of MCH, ignition data below the temperature of 680 K are still lacking. To extend the experimental condition of MCH ignition to a lower temperature range, ozone (O3) was used as an active additive to assist in the kinetic investigation of the low-temperature ignition chemistry of MCH. Ignition delay times (IDTs) and the first-stage IDTs for MCH containing varying concentrations of O3 (0, 100, and 500 ppm) were determined in a rapid compression machine (RCM) under the pressure of 20 bar, equivalence ratios of 1.0 and 1.5, and temperatures in the range of 616–864 K. Species profiles during the auto-ignition of MCH at 615 K and 795 K were recorded using a fast sampling system combined with the gas chromatography (GC) technique. A kinetic model for MCH ignition with O3 addition was developed with some key reaction rates updated based on our newly obtained data. The promotional effects of O3 on the ignition of MCH were observed through both the IDTs and species evolutions. The negative temperature coefficient (NTC) behavior of MCH ignition disappeared in the presence of 500 ppm O3. The production of intermediate species displayed varying tendencies, with the formation of acetaldehyde (CH3CHO) being favored over other small alkenes. Moreover, the production of ethane (C2H6) was detected under 615 K with 500 ppm O3 addition, which was not reported in previous works concerning MCH combustion. Model analysis revealed that the oxidation pathways for ROO radicals of MCH altered because the addition of O3 shifted the onset of the oxidation temperature to a lower region, making the generation of RO radicals significantly favored. Therefore, the experimental species profiles can provide additional kinetic information for the low-temperature oxidation chemistry of MCH, and the corresponding data effectively highlighted and constrained the reaction rates of ROO radicals yielding RO radicals. Further model simulation indicated that the intramolecular hydrogen immigration of ROO radicals and the following secondary O2 addition oxidation pathways of MCH can be nearly frozen if the onset oxidation temperature is shifted as low as 550 K with 1000 ppm O3.

Dissociation and ignition of methane hydrate when in contact with typical sources of fire hazard

With their unique properties and structure, gas hydrates are a highly promising energy resource. In this research, we have experimentally studied the methane hydrate dissociation behavior when exposed to various sources of fire hazard. The experiments involved the ignition schemes most commonly identified as causes of fires: an open flame, a massive heated surface, a short circuit, and a local source of fire hazard. We have for the first time explored the impact of the following combination of the key factors on the ignition behavior: gas hydrate dissociation rate, heat flux, coverage of the free surface of the sample, gas diffusion rate, CO2 hydrate dissociation time during the combined methane hydrate and CO2 hydrate dissociation, as well as the background of the thermal field evolution in the gas hydrate. The experimental findings have become a foundation for a model predicting the critical conditions of hydrate ignition.

The contribution of intermediate-temperature heat release to octane sensitivity

Carbon footprint reduction can be achieved by increasing the efficiency of combustion engines. For spark-ignition (SI) internal combustion engines (ICE), fuel octane sensitivity (OS)—defined as the difference between a fuel’s research octane number (RON) and motor octane number (MON)—can help identify ideal fuels for avoiding unwanted knocking conditions. Moreover, it will help to extend the range of operating conditions that can be used in next-generation high-efficiency engines. Presently, there is a controversy over whether high or low OS fuels are beneficial for increased efficiency of next-generation engines depending on application and technology, e.g. high OS fuels are advantageous for down-sized and turbocharged spark-ignition (SI) engines, whereas low OS fuels are superior in novel systems such as pre-chamber engines. This work aims to improve the understanding of the impact of OS on the combustion performance of engines by analyzing the contribution of intermediate-temperature heat release (ITHR) to OS. The heat release of several mixtures of toluene, iso-octane, and n-heptane with similar RON but varying OS were studied under homogeneous charge compression ignition (HCCI) conditions in a cooperative fuel research (CFR) engine. This method gives insight into the heat release of the auto-igniting fuel without the obfuscation of the spark-ignited flame. At RON conditions, ITHR was a marker of OS. Furthermore, a new parameter called ITHR sensitivity was defined as the difference between the ITHR measured at RON conditions and the ITHR measured at MON conditions. ITHR sensitivity decreased with increasing octane sensitivity. This shows that non-sensitive fuels experience a greater change in the amount of ITHR as compared to sensitive fuels when changing from MON conditions to RON conditions. Thus, for engines requiring high OS fuels, it is recommended that components with high ITHR sensitivity are used and vice-versa for engines requiring low OS fuels.

Evaluating the importance of Raman and Brillouin side scattering at ignition conditions

A complete linear model for Raman and Brillouin side scattering is analyzed with infinite and finite boundary conditions. Analytic formulas of absolute thresholds and convective gains for side scatterings are derived when infinite boundary is assumed. Finite beam width introduces finite boundary conditions to the side scattering, which could influence both absolute thresholds and convective gains. Using these formulas and models, we have evaluated the importance of stimulated Raman side scattering and stimulated Brillouin side scattering at ignition conditions, including both direct drive and indirect drive. It shows that side scattering could be a possible way of scattering other than backscattering in several ignition situations.

Global Well-Posedness of Shock Front Solutions to One-Dimensional Piston Problem for Combustion Euler Flows

This paper is devoted to the well-posedness theory of piston problem for compressible combustion Euler flows with physical ignition condition. A significant combustion phenomenon called detonation will occur provided that the reactant is compressed and ignited by a leading shock. Mathematically, the problem can be formulated as an initial-boundary value problem for hyperbolic balance laws with a large shock front as free boundary. In the present paper, we establish the global well-posedness of entropy solutions via wave front tracking scheme within the framework of space. The main difficulties here stem from the discontinuous source term without uniform dissipation structure and from the characteristic boundary associated with degenerate characteristic field. In dealing with the obstacles caused by ignition temperature, we develop a modified Glimm-type functional to control the oscillation growth of combustion waves, even if the exothermic source fails to uniformly decay. As to the characteristic boundary, the degeneracy of contact discontinuity is fully employed to get elegant stability estimates near the piston boundary. Meanwhile, we devise a weighted Lyapunov functional to balance the nonlinear effects arising from large shock, characteristic boundary, and exothermic reaction and then obtain the -stability of combustion wave solutions. Our results reveal that one-dimensional Zeldovich–von Neumann–Döring detonation waves supported by a forward piston are indeed nonlinearly stable under small perturbation in sense. This is the first work on well-posedness of inviscid reacting Euler fluids dominated by ignition temperature.

Low-temperature chemistry in plasma-driven ammonia oxidative pyrolysis

Ammonia is gaining increasing attention as a green alternative fuel for achieving large-scale carbon emission reduction. Despite its potential technical prospects, the harsh ignition conditions and slow flame propagation speed of ammonia pose significant challenges to its application in engines. Non-equilibrium plasma has been identified as a promising method, but current research on plasma-enhanced ammonia combustion is limited and primarily focuses on ignition characteristics revealed by kinetic models. In this study, low-temperature and low-pressure chemistry in plasma-assisted ammonia oxidative pyrolysis is investigated by integrated studies of steady-state GC measurements and mathematical simulation. The detailed kinetic mechanism of NH3 decomposition in plasma-driven Ar/NH3 and Ar/NH3/O2 mixtures has been developed. The numerical model has good agreements with the experimental measurements in NH3/O2 consumption and N2/H2 generation, which demonstrates the rationality of modelling. Based on the modelling results, species density profiles, path flux and sensitivity analysis for the key plasma-produced species such as NH2, NH, H2, OH, H, O, O(1D), O2(a1Δg), O2(b1Σg+), Ar∗, H−, Ar+, NH3+, O2− in the discharge and afterglow are analyzed in detail to illustrate the effectiveness of the active species on NH3 excitation and decomposition at low temperature and relatively higher E/N values. The results revealed that NH2, NH, H as well as H2 are primarily generated through the electron collision reactions e + NH3 → e + NH2 + H, e + NH3 → e + NH + H2 and the excited-argon collision reaction Ar∗ + NH3 + H → Ar + NH2 + 2H, which will then react with highly reactive oxidative species such as O2∗, O∗, O, OH, and O2 to produce stable products of NOx and H2O. NH3 → NH is found a specific pathway for NH3 consumption with plasma assistance, which further highlights the enhanced kinetic effects.

Research on PTFE/Al reactive jet formation of shaped charge

During the PTFE/Al reactive shaped charge jet formation, the burst wave generated by the explosion of the explosive is sufficient to achieve the ignition conditions of the reactive materials (RMS). As a result, the RMS will react, resulting in an energy loss from the reactive jet. In order to research the forming features of PTFE/Al reactive jet and the energy loss during the jet formation, a PTFE/Al reactive liner with a diameter of 74 mm was prepared and an X-ray pulses photography experiment was performed on the jet formation. And the parameters of Johnson-Cook constitutive model of RMS were obtained through quasi-static experiments and Hopkinson pressure bar experiments. The jet formation were simulated, and the AUTODYN secondary development technology was used. The results show that the secondary development technology can intuitively show the reaction of the active material during the jet formation. The PTFE/Al reactive jet is less cohesive, its jet diameter is thicker, and the morphology is divergent. The reactive materials reacted during the PTFE/Al reactive shaped charge jet formation. And during the reactive jet formation, the inner layer of the liner collided. Therefore, most of the chemical reactions occur inside the reactive jet.

Investigation of high rate mechanical flow followed by ignition for high-energy propellant under dynamic extrusion loading

Investigating the ignition response of nitrate ester plasticized polyether (NEPE) propellant under dynamic extrusion loading is of great significant at least for two cases. Firstly, it helps to understand the mechanism and conditions of unwanted ignition inside charged propellant under accident stimulus. Secondly, evaluates the risk of a shell crevice in a solid rocket motor (SRM) under a falling or overturning scene. In the present study, an innovative visual crevice extrusion experiment is designed using a drop-weight apparatus. The dynamic responses of NEPE propellant during extrusion loading, including compaction and compression, rapid shear flow into the crevice, stress concentration, and ignition reaction, have been firstly observed using a high-performance high-speed camera. The ignition reaction is observed in the triangular region of the NEPE propellant sample above the crevice when the drop weight velocity was 1.90 m/s. Based on the user material subroutine interface UMAT provided by finite element software LS-DYNA, a viscoelastic-plastic model and dual ignition criterion related to plastic shear dissipation are developed and applied to the local ignition response analysis under crevice extrusion conditions. The stress concentration occurs in the crevice location of the propellant sample, the shear stress is relatively large, the effective plastic work is relatively large, and the ignition reaction is easy to occur. When the sample thickness decreases from 5 mm to 2.5 mm, the shear stress increases from 22.3 MPa to 28.6 MPa, the critical value of effective plastic work required for ignition is shortened from 1280 μs to 730 μs, and the triangular area is easily triggering an ignition reaction. The propellant sample with a small thickness is more likely to stress concentration, resulting in large shear stress and effective work, triggering an ignition reaction.

Development of thin-walled polymer glass beads reinforced 3D printed solid rocket motors

A polyamide 12(PA12) reinforced with glass beads (GBs) solid rocket motor (SRM) produced by 3D Printing was designed to achieve light weight of the motors. The pressure resistance and heat resistance of four motors with the thicknesses of 6, 2.5, 2 and 1.8 mm were recorded using a high-speed camera and an infrared thermometer. Porosity analysis was performed on four motors before the ignition experiment and the motor with the thicknesses of 2 mm after the ignition experiment. The results of the study show that the shell’s thickness of the motor can be as thin as 2 mm under the ignition condition of 12 Mpa and 2773 K. The overall and local porosity of the motor became larger after ablation. This work provides a new direction and practical references for the application of micropropulsion systems for satellites.

A visualization investigation on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube

Hydrogen is regarded as one of the promising fuels in the next decades. However, the spontaneous ignition of high-pressure hydrogen blocks its safe utilization. In this investigation, a visualization investigation is carried out to study on the characteristic and mechanism of spontaneous ignition condition of high-pressure hydrogen during its sudden release into a tube. A rectangular tube with transparent observation window is employed. The high-speed images of spontaneous ignition conditions and light signals emitted from ignition are obtained. Four spontaneous ignition conditions are observed: (1) no ignition; (2) failed ignition with one ignition spot; (3) failed ignition with two ignition spots and (4) successful ignition. The critical burst pressure for four ignition condition is determined. The jet fire (successful ignition) could be generated in the cases with a shorter distance from ignition spot to leading shock wave and contact surface due to the stronger shock wave affection and sufficient flammable mixture maintaining the chemical reaction and flame propagation. The second ignition is the precondition forming the successful ignition. The scaled ignition delay time and distance could be predicted by using the derived dimensionless shock pressure.

CRITICAL CONDITIONS OF SELF-IGNITION AT BIFURCATION POINTS

The article summarizes the results of the main research in the field of self-ignition theory. For symmetrical vessels, the following are presented: critical conditions of spontaneous ignition in dimensional coordinates, temperature distribution in the vessel, pre-explosive heating of fuel and the critical size of the vessel at the time of explosion. It is noted that the self-ignition conditions for different vessels differ only by a digital multiplier, which indicates that the shape of the vessel does not affect the physic-chemical processes going on in the fuel at the time of the explosion. Each singular point, being a bifurcation point, determines a number that allows finding a single critical condition of self–ignition from the set of solutions to the heat equation. This condition, in the coordinates of its variables, represents a multidimensional surface separating the zone of stationary existence of a combustible system from the zone of “no return”, where a combustible system cannot exist.