Subsequent Ignition Research Articles

Investigation of Al-Li particle ignition dynamics with different Li content

Promoting the ignition of aluminum powder is considered an effective method to inhibit the agglomeration of aluminum powder in propellants and enhance combustion efficiency. This study utilizes laser ignition technology and high-speed photography to investigate the ignition and combustion processes of single aluminum particles and aluminum-lithium alloy particles. The focus is on comparing the effects of the diameter of micron-sized metal particles and the lithium content in aluminum-lithium alloy particles on the ignition and combustion processes of metal particles. The results show that the ignition delay time is directly proportional to the diameter of the metal particles and inversely proportional to the lithium content. For the aluminum-lithium alloy particle with a lithium content of 3.5 %, even if the diameter is close to 300 μm, the ignition delay time is only 125.5 ms, which is much smaller than that of the pure aluminum particle with a diameter of 208 μm. Compared to aluminum particles and aluminum-lithium alloy particles, there is basically no difference between the two during the combustion stage. However, in the ignition stage, aluminum-lithium alloy particles sequentially exhibit a red gas-phase flame corresponding to lithium and a yellow gas-phase flame corresponding to aluminum. This indicates that during the ignition process of aluminum-lithium alloy particles, lithium first reacts with the oxidative atmosphere and releases heat, providing a heat source for the subsequent ignition of aluminum particles. This also explains why the ignition delay time of metal particles is inversely proportional to the lithium content. An ignition model for aluminum particles in a multi-component atmosphere is established, which further considers the chemical reactions between lithium and oxidative gases, making the model applicable to aluminum-lithium alloy particles. This ignition model effectively describes the impact of particle diameter and lithium content on the ignition process of metal particles. The model is further verified, and the results show that the calculated ignition delay is in good agreement with the experimental data. Overall, this study provides deeper experimental and theoretical insights into the ignition and combustion processes of aluminum-lithium alloys, and the findings can guide the application of aluminum-lithium alloys in propellants.

Experiments and modeling of the thermal spray-chasing phenomenon during split injection processes

Spray chasing is a specific manifestation of the interaction between split injections, where the mass and heat transfer between the pilot and main sprays significantly influence fuel mixing and ignition processes. In this study, a spray-chasing model is developed and validated based on optical diagnostic results, aiming to theoretically support the development of injection strategies that are more environmentally adaptive and offer improved performance. A series of optical diagnoses of diesel split injection is performed in a constant volume combustion chamber using diffused back-illumination imaging and high-speed schlieren imaging. The extent of the spray-chasing phenomenon is modulated by regulating the injection dwell time (DT) and ambient temperature. For inert sprays, when the background temperature exceeds 790 K, the liquid spray-chasing phenomenon can be disregarded owing to the premature evaporation of the pilot spray. Although shortening the DT can promote the development and mixing of the main spray, the lack of evaporation in the pilot fuel pocket leads to a higher local fuel concentration in the interaction zone after spray chasing, which will hinder the subsequent ignition of the main spray. For reactive sprays, the variations in spray chasing patterns under different operating parameters are determined and their impact on the ignition process is evaluated by combining optical test results with theoretical models. Under cold start conditions, the ignition delay and ignition location of the main spray exhibit a non-monotonic trend with increasing injection dwell time. Optical diagnosis observes that compared to DT of 200 us and 1500 μs, the main spray exhibits both valley in ignition delay time and ignition location at DT = 900 μs. The spray-chasing model identifies approximately 1020 μs as the optimal theoretical DT, expected to best facilitate ignition of the main spray. However, under hot-start conditions, even though the spray-chasing mode varies with different injection dwell times, the ignition characteristics remain essentially unchanged due to the limited promotion effect of split injection on ignition.

Reduced chemical kinetics mechanism for modelling of n-Heptane/syngas combustion with NOx formation in a micro-pilot ignited dual fuel engine

A reduced chemical kinetics model was developed to simulate the combined combustion of n-Heptane with syngas, as well as the standalone syngas combustion and NOx formation in a pilot-ignited dual fuel engine. This simplified model comprises 75 different species and 276 reactions. Its accuracy was confirmed by comparing it with experimental data from existing literature and numerical results from established models. The comparison encompassed key factors such as ignition delay, the speed of combustion in a laminar flame, and the concentration profiles of NOx. The reduced model closely aligned with both experimental and simulated outcomes, particularly under conditions relevant to engine combustion.This reduced mechanism was then employed in a comprehensive computational fluid dynamics computations to predict syngas combustion in a dual-fuel engine that employs the injection of reduced amount of liquid n-Heptane fuel with subsequent micro-pilot ignition. The model accurately captured the necessary conditions for n-Heptane injection and the co-oxidation of syngas with n-Heptane. Moreover, it exhibited strong agreement with experimental measurements of the heat release rate (ROHR) and cylinder pressures. The study's results highlight that the reduced mechanism can precisely simulate combustion of syngas-based fuels in engine cylinder containing up to 57% hydrogen (H2) content. As a result, this simplified model is highly suitable for simulating engine conditions involving fuels derived from syngas.

The synergistic effect mechanism of H2 generation during coal/ammonia co-pyrolysis

Coal/ammonia (NH3) co-combustion faces the problem of unstable ignition and combustion due to the high ignition temperature and low flame propagation speed of NH3. The pre-pyrolysis of coal and NH3 generates combustible gases such as hydrogen (H2) that aid in enhancing the stability of the subsequent ignition and combustion process. In this paper, the synergistic effect mechanism of H2 generation during coal/NH3 co-pyrolysis are investigated by constant temperature thermogravimetric experiments and reactive molecular dynamics (ReaxFF MD) simulations. The experimental results show that NH3 promotes coal pyrolysis, leading to an increase in the release of coal pyrolysis volatiles during coal/NH3 co-pyrolysis. Furthermore, both experimental and ReaxFF MD simulations results affirm a synergistic effect on H2 generation during coal/NH3 co-pyrolysis. The atomic labeling method is employed to reveal the mechanism of the synergistic effect on H2 generation during coal/NH3 co-pyrolysis. It is found that H2 generated solely from either coal or NH3 during co-pyrolysis is lower than that from the individual pyrolysis of coal or NH3, respectively. The synergistic effect on H2 generation during coal/NH3 co-pyrolysis is mainly attributed by the interactions between coal and NH3, resulting in the generation of coupling H2. The main reaction pathways for H2 generation during coal/NH3 co-pyrolysis are determined by analyzing the frequencies of H2 generation reactions. The mechanisms of the coupling H2 generation are revealed by tracing to the source of the important reactants in the main reactions for H2 generation. The coupling H2 are mainly formed through the directly pyrolysis of the coupling NHi, and the dehydrogenation reactions of the coupling hydrocarbons.

Upward flame spread behaviour of cladding materials on a medium-scale ventilated façade experimental setup with a single combustible wall

A parametric experimental study was performed to characterise the fire spread dynamics in a simplified ventilated façade using a medium-scale testing rig comprised of a non-combustible and a combustible cladding wall (1800 × 600 mm). Three different cavity widths and four different cladding materials were tested. Measurements of the flame height, the incident heat flux on the non-combustible cavity wall and oxygen consumption calorimetry were performed. A strong relationship between flame height and heat release rate was found for the growth phase of the fire. It has been shown that the time for encapsulation failure and subsequent cladding material core ignition decreased as the cavity width was reduced since the heat transfer to the walls was enhanced. The increase in the heat transfer to the opposite wall with all the materials could lead to external heat fluxes above the critical heat flux for ignition of a number of combustible cladding materials. This highlights the importance of considering the interaction of the products used in the façade and its geometry for the design of façade assemblies when accounting for the fire performance of the system. The results also show the need to understand the impact of the interaction between the design variables and the system performance, since the material performance observed at bench-scale may fail to capture the performance in heat transfer and flame spread scenarios observed at a system scale.

Influence of the air gap between two cells of the storage battery on the thermal conditions of its operation: Numerical analysis and reliability assessment

To control the operating conditions of battery energy storage systems (BESS), the cells are combined into assemblies and modules located mostly in a closed space limited by the battery case. There are air gaps between the cells of the battery assembly. Energy dissipation in cells leads to an intense heat removal in the closed region of the air gap. As a result, the temperature of the battery assembly increases with possible further uncontrolled thermal runaway and subsequent battery ignition. Despite the increasing number of fires and explosions of battery ESSs every year, there is still no theory providing the ability to predict the conditions of fire and explosion safety of energy-intensive energy storage systems based on storage batteries. Particularly, there are no reliable data on the temperature fields of the cells of battery assemblies during intensive charge/discharge processes.The aim of the work was to analyze the thermal conditions of operation of a storage battery consisting of two typical prismatic cells, taking into account heat transfer in the air gap between the cells. Electrothermal modeling was performed for a fairly typical battery by solving a non-stationary heat conduction equation in a two-dimensional formulation by the finite difference method. The most realistic ranges of variation in the values of influencing factors were considered. In addition, the paper presents assessment of the thermal conditions of operation of a single cell and a comparative analysis of the obtained representative temperatures with the representative temperatures of a battery assembly of two battery cells. It has been established that the temperature of the case of cells of the battery assembly is 3–7 °C (depending on current loads) higher than the temperature of the case of a single cell at equal conditions. The obtained results show the need to take into account the heat transfer between the cells of the battery assembly when analyzing the temperatures of such assemblies.

Analytical model for the temperature field of a plasma heated by fast and monoenergetic ion beams

An analytical model for the determination of the spatial temperature field of a deuterium–tritium plasma heated by a fast and monoenergetic fully stripped ion beam is presented. This analytical model provides relevant hot spot properties in good agreement respect those generated by a numerical model for fast monoenergetic ion beams. The model is useful as it can serve as starting point of detailed numerical radiative-hydrodynamics simulation of the subsequent ignition and burning process of the plasma, avoiding the difficulties of the different spatial and temporal scales involve in the problem. Furthermore, the analytical model provides relationships between the main variables of the hot spots generated that help to a better understanding of the interaction. The results presented in this work were obtained for a beam of fully stripped ions of carbon in a wide range of the beam parameters.

Thermal-hydraulic optimization of a proposed EU-DEMO hydrogen passive removal system

As the R&D of magnetic fusion power demonstrating plants are approaching important steps toward concept designs, analysts are working parallelly on the safety assessment of such concepts to identify any potential risk. One of the safety concerns involves the confinement of radioactive substances during normal operation and accidental conditions. Several accident sequences inside the tokamak vacuum vessel or pressure suppression systems are characterized by the risk of hydrogen buildup and subsequent ignition that could threaten the structural confinement integrity. In the Safety and Environment work package of the EUROfusion consortium, possible approaches to mitigate the hydrogen explosion risk are under investigation. One of the exploratory solutions is based on limiting the hydrogen concentration that could reach flammable gas mixture conditions and using Passive Autocatalytic Recombiners (PARs) installed into the atmosphere of the pressure suppression systems tanks to recombine hydrogen.This paper examines the theoretical effectiveness of the PARs intervention during an in-vessel loss of coolant accident without the intervention of the decay heat removal system for the Water-Cooled Lithium Lead (WCLL) concept of EU-DEMO, using an optimization methodology. The involved systems have been modelled in MELCOR to estimate the PARs recombination capability as a function of the thermal-hydraulic parameters of the suppression tanks. Furthermore, the optimizer entity of the RAVEN tool is applied to perform optimization studies on the hydrogen recombination system design parameters. The goal is to explore the geometrical and thermal-hydraulic parameters that maximize the capability of the hydrogen removal system for the WCLL concept.

Passive Hydrogen Recombination during a Beyond Design Basis Accident in a Fusion DEMO Plant

One of the most important environmental and safety concerns in nuclear fusion plants is the confinement of radioactive substances into the reactor buildings during both normal operations and accidental conditions. For this reason, hydrogen build-up and subsequent ignition must be avoided, since the pressure and energy generated may threaten the integrity of the confinement structures, causing the dispersion of radioactive and toxic products toward the public environment. Potentially dangerous sources of hydrogen are related to the exothermal oxidation reactions between steam and plasma-facing components or hot dust, which could occur during accidents such as the in-vessel loss of coolant or a wet bypass. The research of technical solutions to avoid the risk of a hydrogen explosion in large fusion power plants is still in progress. In the safety and environment work package of the EUROfusion consortium, activities are ongoing to study solutions to mitigate the hydrogen explosion risk. The main objective is to preclude the occurrence of flammable gas mixtures. One identified solution could deal with the installation of passive autocatalytic recombiners into the atmosphere of the vacuum vessel pressure suppression system tanks. A model to control the PARs recombination capacity as a function of thermal-hydraulic parameters of suppression tanks has been modeled in MELCOR. This paper aims to test the theoretical effectiveness of the PAR intervention during an in-vessel loss of coolant accident without the intervention of the decay heat removal system for the Water-Cooled LithiumLead concept of EU-DEMO.

Покращення показників автомобільних дизельних двигунів при додаванні водневої каталітичної добавки

The aim of the study is to present a new proposed method for improving the efficiency of transportation diesel engines. Considering the rising cost of transportation, where 80% of the expenses are attributed to fuel costs, there is a necessity to develop methods for reducing fuel consumption. Among the main approaches are the use of alternative fuels or fuel additives. One of the most effective and promising options is the utilization of hydrogen, both as an alternative fuel and a fuel additive. Among the crucial factors significantly influencing the efficiency of hydrogen additives is the method of their delivery to the internal combustion engine. Injecting hydrogen during the engine's intake stroke, although a simple method, faces challenges in achieving precise engine control and poses risks due to the potential formation of an explosive mixture in the intake tract and subsequent ignition. A proposed solution involves introducing small hydrogen additives into the high-pressure fuel line, between the fuel pump and the injector. After the completion of the injection process in the high-pressure line, a "rarefaction wave" is generated. Utilizing this effect allows introducing a small amount of hydrogen into the diesel fuel. Hydrogen delivery is ensured by a special device equipped with a check valve that reacts to changes in pressure in the fuel line. Hydrogen, when introduced into the fuel, promotes improved combustion and increased engine efficiency. This results in a reduction in fuel consumption by 0.4 to 3.5% compared to nominal values, with particularly high fuel efficiency observed at partial load conditions, as well as during acceleration and maneuvers. It is worth noting the positive environmental impact of this technology. When adding hydrogen in a proportion of 0.1% of the fuel mass, a decrease in hydrocarbon emissions by 40–50% and carbon monoxide by 15–25% is observed. However, an increase in nitrogen oxide emissions by 3–7% has been identified, which is associated with a certain elevation of the maximum cycle temperature. Nevertheless, NOx emissions increase can be mitigated by implementing appropriate adjustments to the engine's operating parameters.

A Systematic Review and Bibliometric Analysis of Wildland Fire Behavior Modeling

Wildland fires have become a major research subject among the national and international research community. Different simulation models have been developed to prevent this phenomenon. Nevertheless, fire propagation models are, until now, challenging due to the complexity of physics and chemistry, high computational requirements to solve physical models, and the difficulty defining the input parameters. Nevertheless, researchers have made immense progress in understanding wildland fire spread. This work reviews the state-of-the-art and lessons learned from the relevant literature to drive further advancement and provide the scientific community with a comprehensive summary of the main developments. The major findings or general research-based trends were related to the advancement of technology and computational resources, as well as advances in the physical interpretation of the acceleration of wildfires. Although wildfires result from the interaction between fundamental processes that govern the combustion at the solid- and gas-phase, the subsequent heat transfer and ignition of adjacent fuels are still not fully resolved at a large scale. However, there are some research gaps and emerging trends within this issue that should be given more attention in future investigations. Hence, in view of further improvements in wildfire modeling, increases in computational resources will allow upscaling of physical models, and technological advancements are being developed to provide near real-time predictive fire behavior modeling. Thus, the development of two-way coupled models with weather prediction and fire propagation models is the main direction of future work.

Computational Fluid Dynamics Modeling of Low Temperature Ignition Processes From a Nanosecond Pulsed Discharge at Quiescent Conditions

Abstract Recent interest in nonequilibrium plasma discharges as sources of ignition for the automotive industry has not yet been accompanied by the availability of dedicated models to perform this task in computational fluid dynamics (CFD) engine simulations. The need for a low-temperature plasma (LTP) ignition model has motivated much work in simulating these discharges from first principles. Most ignition models assume that an equilibrium plasma comprises the bulk of discharge kernels. LTP discharges, however, exhibit highly nonequilibrium behavior. In this work, a method to determine a consistent initialization of LTP discharge kernels for use in engine CFD codes like converge is proposed. The method utilizes first principles discharge simulations. Such an LTP kernel is introduced in a flammable mixture of air and fuel, and the subsequent plasma expansion and ignition simulation is carried out using a reacting flow solver with detailed chemistry. The proposed numerical approach is shown to produce results that agree with experimental observations regarding the ignitability of methane-air and ethylene-air mixtures by LTP discharges.

Cooperative spot ignition by idealized firebrands: Impact of thermal interaction in the fuel

Firebrand spotting is a major cause for structure losses in wildland-urban interface (WUI) fires. When firebrands land nearby and accumulate into groups or piles, they can act as a more competent ignition source compared to single firebrands. While experimental studies have demonstrated this, and measured the enhanced ability to ignite fuels, it is not understood under what critical conditions the firebrands act as an accumulation rather than individual isolated firebrands. This work is an experimental and numerical study of how the fuel bed thermal interactions influence the subsequent flaming ignition propensity of wood in contact with two electric heaters acting as idealized firebrands. The experiments use electrical cartridge heaters of two sizes (7.5 and 15 mm in width) which produce an incident heat flux to the fuel (up to 60 kW/m2) to create controllable heating conditions comparable to those from firebrands. It is observed that decreasing the heater size requires higher heat fluxes to the fuel for ignition to occur. When a second heater is introduced within a certain critical distance it is found to hasten ignition or make ignition possible with a lower incident heat flux to the fuel. Numerical modelling is used to elucidate the importance of thermal interactions in the fuel on the propensity of flaming ignition. The numerical modelling is also used to examine a broader range of idealized firebrand sizes (5 − 50 mm) and the separation distance between idealized firebrands in finer resolution. The model 2D captures the qualitative ignition behaviour well and even shows quantitative agreement in most cases.

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

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

Effects of ‘step-like’ amplitude-modulation on a pulsed capacitively coupled RF discharge: an experimental investigation

We present measurements of the time evolution of plasma and electrical parameters in a pulsed capacitively coupled argon discharge operated at a radio frequency of 12.5 MHz, whose amplitude is ‘step-up’ and ‘step-down’ modulated. The ‘step-up (-down)’ amplitude-modulated waveform consists of three segments, i.e., a low (high)-voltage, a high (low)-voltage, and a zero-voltage stage. Here, we focus on the effect of the ratio (ζ = V L/V H ⩽ 1) of the low-(V L) to high-voltage (V H) amplitude (measured at the end of the respective segment) on the time evolution of discharge parameters. We monitor the behavior of the discharge by measuring (i) the optical emission intensity (OEI) of a selected Ar-I spectral line, (ii) the electron density at the center of the plasma (using a hairpin probe) as well as (iii) the electrical characteristics (by voltage and current probes). It is found that at relatively large ζ (i.e., at low disparity between the two voltage amplitudes), for both the ‘step-up’ and ‘step-down’ cases, these parameters evolve relatively smoothly with time upon changing the voltage amplitude, and the ignition process strongly depends on the duration of the zero-voltage period. At low ζ (i.e., at high disparity between the voltage amplitudes), an abnormal evolution of the parameters can be observed during the low-voltage period for both cases. Specifically, the voltage amplitude and the modulus of the system impedance increase to a higher value, while the relative phase, φ vi, between the voltage and the current approaches 90°, resulting in a reduction of the power deposition and the OEI. The enhanced voltage amplitude decreases to a steady-state value, accompanied by a decline of φ vi, and an abnormal increase of the current amplitude and the electron density after some time, of which the duration increases with the decrease of ζ. The ζ-dependent evolution of the electron density during the low-voltage period was found to significantly affect the subsequent ignition process and electron power absorption mode at the beginning of the high-voltage period.

Circulating Mitochondrial DNA and Inter-Organelle Contact Sites in Aging and Associated Conditions.

Mitochondria are primarily involved in cell bioenergetics, regulation of redox homeostasis, and cell death/survival signaling. An immunostimulatory property of mitochondria has also been recognized which is deployed through the extracellular release of entire or portioned organelle and/or mitochondrial DNA (mtDNA) unloading. Dynamic homo- and heterotypic interactions involving mitochondria have been described. Each type of connection has functional implications that eventually optimize mitochondrial activity according to the bioenergetic demands of a specific cell/tissue. Inter-organelle communications may also serve as molecular platforms for the extracellular release of mitochondrial components and subsequent ignition of systemic inflammation. Age-related chronic inflammation (inflamm-aging) has been associated with mitochondrial dysfunction and increased extracellular release of mitochondrial components—in particular, cell-free mtDNA. The close relationship between mitochondrial dysfunction and cellular senescence further supports the central role of mitochondria in the aging process and its related conditions. Here, we provide an overview of (1) the mitochondrial genetic system and the potential routes for generating and releasing mtDNA intermediates; (2) the pro-inflammatory pathways elicited by circulating mtDNA; (3) the participation of inter-organelle contacts to mtDNA homeostasis; and (4) the link of these processes with senescence and age-associated conditions.

On the Issue of Calculating the Magnitude of the Potential Fire Risk of Emergencies on Gas Distribution Networks

An approach is proposed to determine the value of the conditional probability of human injury at a certain point of the territory as a result of the implementation of any scenario for the development of fire-hazardous situations on gas distribution networks associated with their rupture and subsequent gas ignition. Based on the analysis of the causes and factors determining the outcomes of accidents, taking into account the peculiarities of technological processes of natural gas transportation, typical accident scenarios are identified. For each scenario, the values of the probability of occurrence of emergency situations are calculated. A method for determining the probability of finding people in the residential area of gas pipelines is proposed.

Coke-assisted microwave ignition and combustion behaviors of anthracite in a switching atmosphere

Low volatile coal has a large reserve but faces significant challenges in industrial applications. Due to the rapid and selective heating process, microwave-based technology is generally applied in various studies of carbonaceous materials. This work verified a novel non-contact microwave ignition of anthracite mixed with coke, where typical events of ignition or combustion were recorded by a digital camera. Video image analysis showed that the weakening of O2 to the tiny spark discharges between coke particles failed microwave heating. Hence a switching atmosphere (100% N2 firstly, then O2 at a certain proportion added) was used to ensure rapid microwave heating and subsequent ignition. Ignition of mixed anthracite & coke took place in the middle of the sample once the volatile flame appeared, which following the sample locally turned red-hot. The time for the flame burning was prolonged by the microwave power and especially the increase of O2 proportion. Gas products analysis showed a large amount of CO and CO2 instead of CH4 and H2O during the flame burning, which suggested that extra gasification reaction, where CO as product, supported the existence of flame. Moreover, the plasma generated by the violent discharge further expanded the range of the flame, with clear spectral peaks at 589 nm, 766 nm, and 770 nm. In addition, the distribution of coke in the mixture for effective ignition was also discussed. This study may provide a new idea for the application of low volatile coals.

Experimental study of the ignition of lean methane/air mixtures using inductive and NRPD ignition systems in the pre-chamber and turbulent jet ignition in the main chamber

The optimization of the combustion process in spark ignition engines has led to novel strategies to ignite lean and low reactivity mixtures, such as the application of turbulent jet ignition and Nanosecond Repetitively Pulsed Discharge (NRPD) ignition systems and Turbulent Jet Ignition (TJI). In the present experimental work, an optically accessible setup was used to investigate the ignition occurrence and early flame development in the pre-chamber and subsequent main chamber ignition of methane/air mixtures at density conditions relevant to engine application. Two schlieren setups coupled with fast recording cameras allow the visualization of both the pre-chamber and main chamber.NRPD ignition with different pulse patterns in terms of pulse number and repetition frequency is applied in the pre-chamber. Conventional inductive ignition and NRPD-assisted ignition are compared for the first time in terms of flame development in the pre-chamber and hot jet ignition in main chamber. The effect of different air to fuel ratios is assessed in both laminar and turbulent pre-chamber conditions.Results show that while NRPD is always advantageous when compared to standard inductive ignition, the higher amount of energy added by increasing the number of pulses only affects the ignition success in very lean conditions, thus showing that increased energy is not the main governing mechanism for successful ignition. Irrespectively of the initial conditions (laminar or turbulent) in the pre-chamber, it is shown that an optimum jet velocity exists that ensures increased ignition probability in the main chamber, thus suggesting that mixing plays a major role in main chamber ignition. The combination of both techniques, NRPD and TJI, allowed assessing the optimum conditions in terms of number and frequency of NRPD pulses for a reliable ignition in lean conditions.

Measurement and modeling of the near-nozzle ambient gas entrainment of high-pressure diesel sprays

The ambient gas entrainment of the diesel spray affects the fuel evaporation and the subsequent ignition and pollutant formation processes. In this work, the gas entrainment process in the breakup length of the diesel spray is investigated by the high-speed micro-particle tracking velocimetry (micro-PTV) technique. With the high frequency of the micro-PTV technique of 28 kHz, the images of the tracer particle and the spray boundary are captured simultaneously in one shot. The experimental results show that during the quasi-steady and transient states, the gas entrainment velocity increases as the injection pressure rises. However, the ambient density has no impact on the entrainment velocity in the breakup length. During the quasi-steady state, the local gas entrainment rate shows a linear dependence on the spray axial distance (z), and the cumulative gas entrainment rate shows a z2 dependence. A theoretical zero-dimensional (0-D) model is proposed to estimate the cumulative gas entrainment rate during the quasi-steady state, and it is validated with the experimental results. The results suggest that the gas entrainment in the breakup length is probably dominated by the breakup process of the liquid core.