Multiple Flame Research Articles

Characteristics of rotating gliding arc induced thermal ignition of lean methane-air mixtures

The rotating gliding arc discharge (RGA) induced methane-air ignition process under the lean conditions is investigated by the high-speed videography and the numerical simulation, with the focus on the effects of dynamic discharge characteristics. The experimental and numerical results both indicate that the ignition process in the burner can be divided into two stages, including the waiting stage and the rapid flame propagation stage. The effective ignition time (τburner) including both stages decreases nonlinearly as the effective input ignition power (i.e. Pig/Pcomb) increases, following the exponential law, i.e. τburner=2.98 × 10−9(Pig/Pcomb)−7.299+81.75. It is because of that as the input power is relatively low τburner is governed by the duration of the waiting stage to decrease notably with the increment of the input power. However, as the input power is too large τburner becomes determined by the rapid flame propagation stage to be insensitive to the input power. The rotation frequency of the plasma sources can impact the τburner with that too low or too high frequency is detrimental to the ignition. The flame quenching is less likely to occur with the increase of the plasma volume. Based upon the experimental and numerical results, an RGA-induced ignition mechanism is proposed. It indicates that the two-staged ignition process is largely determined by the waiting stage, where multiple flame kernels are frequently initialized by the RGA to be locally quenched or propagate downstream/upstream with flow. Due to the recirculating flow, the energy released by the plasma and the ignited flame kernels can be accumulated upstream to reduce the local Ka number and enhance the flame propagation at the waiting stage. As Ka is lower than a critical value, the flame kernels can propagate rapidly to complete the ignition.

Applying Nanocomposite Coatings to Improve Orthopedic Alloys by Using Multiple Flame Spray

Applying Nanocomposite Coatings to Improve Orthopedic Alloys by Using Multiple Flame Spray

A novel multiple spark ignition strategy to achieve pure ammonia combustion in an optical spark-ignition engine

Ammonia is an attractive carbon-free fuel that has the potential to reduce the need for conventional hydrocarbons (HC) and reduce emissions of undesirable pollutants such as CO, CO2, particulates, and unburned hydrocarbons. However, ammonia has different combustion characteristics than conventional HC fuel. Ammonia is difficult to ignite and has a low combustion rate, resulting in large cyclic variations. In addition, nitrogen oxides (NOx) emissions in the exhaust tract are a major challenge when using ammonia as a fuel directly in engines. In this study, the effect of multiple spark ignition sites on the combustion of pure ammonia in an optical spark ignition engine (SI) was investigated. The experiment was conducted with four spark plugs mounted equidistantly on a special metal liner and one spark plug fitted at the top of the cylinder head. The multiple flames emitted from the different spark ignition sites were captured by natural flame luminosity (NFL) imaging. In the results, the conventional single spark ignition is compared with multiple ignitions. It was found that single spark ignition resulted in lower in-cylinder pressure, longer combustion duration, and higher combustion instability due to the poor ammonia fuel combustion rate. However, firing multiple spark plugs significantly improved combustion stability, increased engine power, and shortened the combustion period under the same operating conditions. In addition, the flame kernels produced by multiple ignition sites resulted in higher NOx emissions in the exhaust tract due to the higher temperatures in the cylinder. In addition, this study also investigated the effect of three different air–fuel equivalence ratios (λ) of 1.0, 1.2, and 1.4 on the combustion characteristics of ammonia fuel. The maximum NOx level was obtained for λ: 1.2 because the excess air in the mixture oxidizes the ammonia and provides abundant oxygen to generate more NOx. Further reducing the ammonia fuel and increasing the excess air to λ: 1.4 dramatically reduced NOx emissions due to the lower pressures and temperatures in the cylinder resulting from the longer combustion period.

Micro-explosive combustion and flame spread of Jatropha oil/2, 5-dimethylfuran (DMF) blended fuel droplet array

The micro-explosive combustion and flame spread characteristics of a multi-component fuel droplet array are investigated using high-speed backlight imaging technique. The multi-component blends consist of 2, 5-dimethylfuran (DMF) as a lower volatile component, while Jatropha oil is used as the higher volatile constituents. Three droplets (droplet array) with the same diameter are horizontally arranged at equal intervals using fiber support method. The effects of normalized droplet spacing (S/d0) and DMF concentration on multiple droplets micro-explosive combustion and flame spread are also analyzed. The results show that the interaction between multiple droplets combustion can not be ignored under small S/d0. It increases with the increase of DMF concentration and the decrease of S/d0. When the S/d0 is constant, the interaction between droplets depends on the duration of simultaneous combustion of multiple droplets and the number of droplets surrounded by the same flame. Due to the micro-explosion, the average flame spread velocity of Jatropha oil-DMF blended fuel droplet array decreases monotonously with S/d0 in normal gravity, which is different from that in microgravity. Four flame spread modes of droplet array containing micro-explosion are found: full-envelop flame, sequential full-envelop flame, semi-envelop flame to independent flame and independent sequential flame.

MMC-LES of spray combustion: Analysis of mixing timescales and flame structure

Large-eddy simulations (LES) of turbulent, pilot-stabilised, dilute acetone spray flames are performed using the sparse-Lagrangian multiple mapping conditioning approach (MMC-LES). The primary objective of this work is to assess the adaptability of the recently proposed dynamically evaluated molecular mixing timescale model for MMC-LES (Sharma et al., 2022, A fully dynamic mixing timescale model for the sparse Lagrangian multiple mapping conditioning approach, Combustion and Flame, 238, 111872), referred to as the dyn-aISO model, to the non-reacting and reacting two-phase flows. This work is the first application to perform a dynamic evaluation of the mixing timescale for turbulent spray combustion using the transported filtered density function (FDF) approach. A non-reacting case and four reacting flames with varying fuel mass loading are investigated, where the flame transitions from a diffusion flame to a premixed flame structure with a decrease in fuel loading. As is conventional for MMC-LES, a sparse resolution of one stochastic particle per six Eulerian finite-volume cells is used in this study, offering a much cheaper computing expense than the conventional transported FDF approach. The liquid evaporation is represented based on a non-equilibrium model at the droplet surface. The reaction chemistry is based on an augmented GRI 3.0 mechanism containing 39 species and 226 reactions to represent the oxidation of acetone. Predictions with the dyn-aISO model are compared with the constant coefficient anisotropic mixing time scale model and the isotropic C-K model. Thorough validation of liquid-phase statistics and gas-phase temperature is performed with available experimental data and popular studies from the literature. Analysis of the flame structure, with different timescale models, for both non-premixed and premixed type flames is also carried out. Quantification of the parameters affecting the timescale, such as model constants of the sub-Lagrangian-grid scale (slgs) mixture fraction variance (Cf) and its scalar dissipation rate (Scsgs−1Cν) is provided in comparison to their conventionally chosen static coefficient values. Predictions with the dyn-aISO model are quite good for the diffusion-type flames, while some discrepancies are noticed while predicting a premixed flame structure for lower liquid loading case having near stoichiometric jet-exit mixture composition.

Simulation of Low-Temperature Oxidation and Combustion of N-Dodecane Droplets under Microgravity Conditions

Fires are considered among the most dangerous accidents on manned spacecraft. That is why several programs of combustion experiments were implemented at the International Space Station (ISS) since 2008. In the experiments with n-heptane and n-dodecane droplet combustion, a new phenomenon was discovered, namely, the phenomenon of the radiative extinction of a burning droplet with subsequent multiple flashes of flame. In this paper, n-dodecane droplet ignition, combustion, radiative extinction, and subsequent low-temperature oxidation with multiple flashes of cool, blue, and hot flames under microgravity conditions are studied computationally. The mathematical model takes into account multiple elementary chemical reactions in the vicinity of a droplet in combination with heat and mass transfer in liquid and gas, heat release, convection, soot formation, and heat removal by radiation. The model is based on the non-stationary one-dimensional differential equations of the conservation of mass and energy in liquid and gas phases with variable thermophysical properties within the multicomponent diffusion concept in the gas phase. Calculations confirm the important role of the soot shell formed around the droplet and low-temperature reactions in the phenomenon of droplet radiative extinction with multiple flame flashes in the space experiment at the ISS. Calculations reveal the decisive role of the blue flame, arising due to the decomposition of hydrogen peroxide, in the multiple flame flashes. Calculations with forced ignition of the droplet reveal the effect of the ignition procedure on droplet evolution in terms of the timing and the number of cool, blue, and hot flame flashes, as well as in terms of the combustion rate constant of the droplet. Calculations with droplet self-ignition reveal the possible existence of new modes of low-temperature oxidation of droplets with the main reaction zone located very close to the droplet surface and with only partial conversion of fuel vapor in it.

Prediction of combustion instabilities based on the Green's function in a three-dimensional annular combustor with multiple flames

The prediction of combustion instabilities is essential for modern annular combustors fed by multiple flames. This paper develops a model based on Green's function to study the interaction of multiple flame responses with different types in a three-dimensional (3D) geometry. The flame responses are described by flame transfer functions and are recognized as monopole sources in 3D cavities. Then the dispersion relation equation in the form of matrix is performed with the condition of equality of pressure. Thermoacoustic complex frequencies, mode structure, as well as the stability behaviors in combustor can be obtained in this model and the analysis is accomplished using Rayleigh index. The results are compared to the values of existing theoretical models and a good agreement is achieved. Furthermore, the effects of non-identical flame responses on complicated slanted mode under the condition of asymmetric are studied. In addition, the combustion instabilities of radial mode can be predicted by this model when the radial position distributions of the heat sources move in the case of small ratio of inner wall to outer wall.

Inerting characteristics of ultrafine Mg(OH)2 on starch dust explosion flame propagation

Experiments on the flame propagation of starch dust explosion with the participation of ultrafine Mg(OH)2 in a vertical duct were conducted to reveal the inerting evolution of explosion processes. Combining the dynamic behaviors of flame propagation, the formation law of gaseous combustion products, and the heat dissipation features of solid inert particles, the inerting mechanism of explosion flame propagation is discussed. Results indicate that the ultrafine of Mg(OH)2 powders can cause the agglomeration of suspended dust clouds, which makes the flame combustion reaction zone fragmented and forms multiple small flame regions. The flame reaction zone presents non-homogeneous insufficient combustion, which leads to the obstruction of the explosion flame propagation process and the obvious pulsation propagation phenomenon. As the proportion of ultrafine Mg(OH)2 increases, flame speed, flame luminescence intensity, flame temperature and deflagration pressure all show different degrees of inerting behavior. The addition of ultrafine Mg(OH)2 not only causes partial inerting on the explosion flame, but also the heat dissipation of solid inert particles affects the acceleration of its propagation. The explosion flame propagation is inhibited by the synergistic effect of inert gas-solid phase, which attenuates the risk of starch explosion. The gas-solid synergistic inerting mechanism of starch explosion flame propagation by ultrafine Mg(OH)2 is further revealed.

Environmental fate of five brominated flame retardants co-exposure in a water-sediment-zebrafish microcosm system: Enrichment, removal, and metabolism mechanisms

Brominated flame retardants (BFRs) are widely used in electronic products. After electronics obsolescence, BFRs are gradually released and enriched in the environment, thus leading to frequent occurrence and detection. However, the environmental fate of multiple flame retardants in aquatic microcosms has not been explored, and it mainly focused on the environmental monitoring of BFRs and the toxicity of a single BFR to aquatic organisms. To fill the gap, based on actual investigation, a water-sediment-zebrafish microcosm system was co-exposed to five concentrations (0.2, 1, 5, 25, and 50 mg/kg) of pentabromotoluene (PBT), hexabromobenzene (HBB), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE), decabromodiphenyl ethane (DBDPE), and decabromodiphenyl ether (BDE-209) for the enrichment (35 days) and clearance (10 days) experiments. The results showed that the liver, intestine, and gill were the top three tissues enriched in BFRs, which were 7504.54, 6936.99, and 3532.97 ng/g, respectively. BFRs can also be transferred between a generation. The compositions of BFRs in the tissues of zebrafish were also different, and the average compositions of DBDPE and BDE-209 were 36.2% and 28.6%, respectively, indicating that DBDPE and BDE-209 were more easily absorbed by zebrafish. Additionally, the average scavenging half-lives of PBT, HBB, BTBPE, DBDPE, and BDE209 were 4.01, 4.09, 2.90, 3.72, and 3.75 days, respectively, suggesting that these chemicals barely lasted long in zebrafish. The metabolic pathways of the five BFRs were also probed, and gradual debromination occurred in the organisms. These observations would provide a basis for the study of the fate and toxicology of BFRs in the water environment.

Study of Explosion Characteristics and Mechanism of Sucrose Dust

In order to investigate the explosion mechanism of sucrose in the air atmosphere, the explosion intensity under different ignition delay times (IDT), powder input pressures (PIP), and concentrations were studied using a 20L-sphere. The sucrose particles were analyzed in a synchronized thermal analyzer (STA) and scanning electron microscope (SEM). The results are as follows: 1. The DSC curve has two endothermic peaks and one exothermic peak, respectively at T = 180.5 ℃, 510.2 ℃ and 582.6 ℃. 2. The explosion intensity varies with the experiment conditions. The maximum explosion pressure (Pmax) appears when IDT = 90 ms, PIP = 1.5 MPa and concentration = 625 g/m3. 3. The explosive mechanism is a homogeneous combustion mechanism based on particle surface pyrolysis and volatilization. Because of the decomposition, H2, CO, furfural, and other flammable gas-phase products are released, then surface burn appears, which leads to the crystal rupture on account of thermal imbalance, resulting in multiple flame points and a chain explosion. As the temperature of the 20L-sphere rises, more explosive products are released, which causes a rapidly expanding explosion and eventually forms the explosion. This paper can be used as a reference for the prevention of explosion accidents in sucrose production processing.

Recent advances in metal-family flame retardants: a review.

The use of polymer materials is inextricably linked to our manufacturing life. However, most of them are easily combusted in the air and the combustion process generates a large amount of toxic fumes and dangerous smoke. This can result in injuries and property damage, as well as limiting their use. It is essential to enhance the flame-retardant properties and smoke suppression performance by using multiple flame retardants. Metal-based flame retardants have a unique chemical composition. They are environmentally friendly flame retardants, which can impart good smoke suppression, flame retardancy to polymers and further reduce the production of toxic gases. The differences in the compounds formed between the transition metals and the main group metals make them act differently as flame retardants for polymers. As a result, this study presents the research progress and flame-retardant mechanism of flame-retardant polymers for flame retardants from different groups of metals in the periodic table of elements in a systematic manner. In view of the differences between the main group metals and transition metals, the mechanism of their application in flame retardant polymer materials is carefully detailed, as are their distinct advantages and disadvantages. And ultimately, prospects for the development of transition metals and main group metals are outlined. It is hoped that this paper will provide valuable references and insights for scholars in the field.

Multilayered premacular haemorrhage associated with central retinal vein occlusion

AbstractPurpose: We aimed to report a rare case of multilayered premacular haemorrhage complicating central retinal.vein occlusion (CRVO) and to describe its management and follow‐up.Methods: An observational case report of a 38‐year‐old man with no medical history who presented with sudden loss of vision in his left eye (LE) 4 days ago. The patient underwent complete ophthalmologic examination, fluorescein angiography (FA) and swept‐source optical coherence tomography (SS‐OCT).Results: Ophthalmologic examination of the right eye (RE) was unremarkable with a best‐corrected visual acuity (BCVA) of 20/20. In the LE, BCVA was limited to light perception (LP), intraocular pressure was 12 mmHg and anterior segment showed no abnormalities. Ophthalmoscopy showed dilated veins, multiple flame haemorrhages and a large dome‐shaped premacular haemorrhage with a double‐ring sign. SS‐OCT revealed a dome shaped clotted haemorrhage surrounded by hyperreflective dots under the internal limiting membrane (ILM) corresponding to a multilayered sub‐ILM haemorrhage. On FA, we noticed a delay in venous filling time and no areas of capillary‐non perfusion. Systemic work‐up with glaucoma screening were unremarkable. Nd:YAG laser hyaloidotomy and membranotomy of the ILM were performed. At one‐week follow‐up, there was evidence of the sub‐ILM and subhyaloid blood draining into the vitreous cavity. At 3 weeks following the laser treatment, persistence of clotted blood was noted. Pars plana vitrectomy with ILM peeling were performed. Two months post‐operatively, BCVA improved from LP to 20/25 with complete resolution of the haemorrhages on fundoscopy and SS‐OCT.Conclusions: Premacular haemorrhage associated with CRVO is a rare condition. SS‐OCT allows accurate diagnosis and non‐invasive follow‐up. Nd: YAG laser hyaloidotomy should be considered as the first line treatment. But if failed, vitrectomy with ILM peeling should not be delayed in order to avoid toxic damages to the retina.

Phenomenon and analysis of direct initiation of detonation using multiple turbulent flame jets

This work reports experimental investigations on the direct initiation of detonation using multiple turbulent flame jets, with a special focus on the arrangement schemes and fundamental physics in the initiation processes. Results show that the direct initiation of detonation can be achieved using turbulent jets even when the jet tube diameter is much smaller than the empirical critical tube diameter due to flame–shock–wall interactions. Conspicuous evidence has been shown that the probability of the direct initiation increases significantly near the detonatability limit using multi-jets compared to a single jet. These results are found to be closely related to several new phenomena observed when using multiple jets to initiate the detonation. They are: (1) unexpected rapid promotion of the final-stage flame acceleration in ignition tubes by multiple jets, which is attributed to the fact that the expanding precursor shock waves propagate back into the adjacent tube and interact with the flame; (2) enhancement of hot spot generation by multiple jets due to the precursor shock intersection and the formation of an induction zone; (3) obvious velocity loss of impinging jets initiation as a result of induced hot spots propagation in the burned gases.

Optical investigations on ignition and combustion characteristics of polyoxymethylene dimethyl ethers/methanol blends

ABSTRACT Internal combustion engines using carbon-neutral fuels are effective ways to achieve low CO2 emissions. Polyoxymethylene dimethyl ethers (PODE) and methanol are carbon-neutral fuels, both good alternative fuels for engine combustion. Dual-fuel combustion modes have been widely studied, but investigations on the ignition and combustion characteristics of PODE/methanol at blending mode are still lacking, especially in optical engines. In this study, the effects of methanol blending ratios (0, 10, 20%), intake temperatures (45, 85, 120°C), and injection timings (−60, −45, −30, −15 °CA aTDC) on the ignition and combustion behavior of PODE/methanol blends were optically studied. Synchronization measurements of in-cylinder pressure and high-speed photography were performed for combustion analysis. The results show that a reduced methanol blending ratio, increased intake temperature, and intermediate injection timing exhibit positive influences on flame initiation and combustion evolutions. Specifically, an optimized methanol blending ratio, intake temperature, and injection timing can shorten the ignition delay time by 11.2, 6.1, and 39.2 crank angles, respectively. Meanwhile, unlike methanol blending ratio and intake temperature, injection timing can significantly affect early-stage rapid combustion duration. Optical images show that multiple flame kernels are initially induced at local rich regions with shorter ignition delay time, while when ignition delay time becomes longer, flame kernels start to be discretely distributed throughout the combustion chamber. These observations demonstrate the significance of the competition between chemical reactivity and fuel stratification in ignition and combustion characteristics of PODE/methanol blends.

Deflagration-to-detonation transition and detonation propagation characteristics in a millimetre-scale spiral channel

The flame propagation processes involving deflagration-to-detonation transition (DDT) and detonation propagation in a novel millimetre-scale spiral channel (MSDC) are studied experimentally. The channel is of 3.0 mm in width and 5.2 m in length, embedded in a stainless-steel plate. In particular, a tiny gap of 0.02-mm width is reserved between the upper cover plate and the channel. Stoichiometric propane/hydrogen/air mixtures were adopted as the reactant. The combustion modes are investigated with various initial pressure (P0) and hydrogen blend ratio (XH), ranging from 100 to 500 kPa and 0.6 to 1.0, respectively. For lower P0 and XH, the flame gradually develops into an isobaric flame owing to significant thermal losses in narrow channels. With the increase of P0 and XH, the excess-enthalpy effect plays an important role, and a quantity of high-temperature burned gas in the inner turn enter the outer turn through the gap resulting in a spontaneous flame ahead of the wave front. It promotes the flame acceleration and the onset of local explosion. In subsequent detonation propagation process after DDT, multiple flame combination between the detonation wave and the secondary flame arise and the displacement speed of leading front dramatically increases, even more than 3 DCJ. Furthermore, benefited from excess enthalpy effect and the compact structure of MSDC, the displacement between the DDT location and the ignition spark is of the order of as small as 10 mm. It is of a great potential in application to miniature detonation engines, in which rapid DDT in narrow channels is of strong demand.

Profiling of multiple classes of flame retardants in house dust in China: Pattern analysis and human exposure assessment

Legacy [e.g., brominated- (BFRs)] and alternative [e.g., organophosphate- (OPFRs) and nitrogenous- (NFRs)] flame retardants have a propensity to migrate out of consumer products, and thus are dispersed in indoor microenvironments. In this study, simultaneous presence of 11 BFRs, 18 OPFRs and 11 NFRs were measured in house dust collected from Tianjin, China. OPFRs were found at the highest concentrations, with a median value of 3200 ng/g, followed by NFRs (2600) and BFRs (1600). Tris(2-butoxyethyl) phosphate (median: 1800 ng/g), melamine (1100), and BDE-209 (870) were the top three most abundant chemicals in the respective groups. Location-specific patterns of flame retardant concentrations were found with 30%, 20% and 10% of samples were predominated by OPFRs, NFRs and BFRs, respectively, and the remaining samples contained by two or more of the chemical groups occurring concurrently. Network and cluster analysis results indicated the existence of multiple sources of flame retardants in the indoor microenvironment. Estimated human daily intakes via indoor dust ingestion were approximately several tens of ng/kg bw/day and were below their respective reference dose values. Our results indicate widespread occurrence of multiple flame retardant families in indoor dust and suggest need for continued monitoring and efforts to reduce exposures through dust ingestion.

Numerical investigation of boundary layer flashback of CH[formula omitted]/H[formula omitted]/air swirl flames under different thermal boundary conditions in a bluff‐body swirl burner

One of the major challenges of applying hydrogen (H2) enriched fuels in industrial combustion systems is the risk of flashback that can cause considerable damage on combustors and affect pollutant emissions. While flashback has been extensively investigated, comparatively less analysis is made to understand the impact of thermal boundary conditions on flashback of bluff-body swirling H2 enriched flames. The aim of this work is to fill a part of this gap through large-eddy simulations of premixed 95%H2/5%CH4-air flame flashback in a bluff-body swirl burner. The results show that flashback characteristics are very sensitive to the bluff-body thermal boundary condition. At Twall=350 K, flashback is led by a large-scale swirling flame tongue (Mode I) that can cause the deflection of streamlines ahead of the flame front and is responsible for flashback Mode I, while a small region of negative velocity induced by small-scale flame bulges cannot lead to a net upstream propagation of the flame front. However, when treating the bluff-body as an adiabatic wall, the flashback is led by multiple small-scale flame bulges, and the circumferential motion of the lowest flame tip is negligible (Mode II). These small-scale bulges can cause large reverse flow pockets and formation of a stagnation point ahead of the preheat zone that facilitates flashback, similar to non-swirling channel flashback. This is the main mechanism responsible for flashback Mode II. At Twall=500 K, flashback is led by different structures, switching between Mode I (upstream propagation of swirling flame tongue) to Mode II (upstream propagation of non-swirling flame bulges). Furthermore, it is found that as the boundary heat loss increases, the axial flashback speed decreases, while the azimuthal flashback speed increases due to the presence of Mode I. The results open up the possibility of extending the flashback limit of H2 enriched swirling flames by controlling the thermal condition of bluff body.

Optimization of Pre-Chamber Geometry and Operating Parameters in a Turbulent Jet Ignition Engine

A turbulent jet ignition engine enables operation with lean mixtures, decreasing nitrogen oxide (NOX) emissions up to 92%, while the engine efficiency can be increased compared to conventional spark-ignition engines. The geometry of the pre-chamber and engine operating parameters play the most important role in the performance of turbulent jet ignition engines and, therefore, must be optimized. The initial experimental and 3D CFD results of a single-cylinder engine fueled by gasoline were used for the calibration of a 0D/1D simulation model. The 0D/1D simulation model was upgraded to capture the effects of multiple flame propagations, and the evolution of the turbulence level was described by the new K-k-ε turbulence model, which considers the strong turbulent jets occurring in the main chamber. The optimization of the pre-chamber volume, the orifice diameter, the injected fuel mass in the pre-chamber and the spark timing was made over 9 different operating points covering the variation in engine speed and load with the objective of minimizing the fuel consumption while avoiding knock. Two optimization methods using 0D/1D simulations were presented: an individual optimization method for each operating point and a simultaneous optimization method over 9 operating points. It was found that the optimal pre-chamber volume at each operating point was around 5% of the clearance volume, while the favorable orifice diameters depended on engine load, with optimal values around 2.5 mm and 1.2 mm at stoichiometric mixtures and lean mixtures, respectively. Simultaneous optimization of the pre-chamber geometry for all considered operating points resulted in a pre-chamber volume equal to 5.14% of the clearance volume and an orifice diameter of 1.1 mm.

An Enhanced Moth-Flame Optimization with Multiple Flame Guidance Mechanism for Parameter Extraction of Photovoltaic Models

How to accurately and efficiently extract photovoltaic (PV) model parameters is the primary problem of photovoltaic system optimization. To accurately and efficiently extract the parameters of PV models, an enhanced moth-flame optimization (EMFO) with multiple flame guidance mechanism is proposed in this study. In EMFO, an adaptive flame number updating mechanism is used to adaptively control the flame number, which enhances the local and global exploration capabilities of MFO. Meanwhile, a multiple flame guidance mechanism is designed for the full use of the position information of flames, which enhances the global diversity of the population. The EMFO is evaluated with other variants of the MFO on 25 benchmark functions of CEC2005, 28 functions of CEC2017, and 5 photovoltaic model parameter extraction problems. Experimental results show that the EMFO has obtained a better performance than other compared algorithms, which proves the effectiveness of the proposed EMFO. The method proposed in this study provides MFO researchers with ideas for adaptive research and making full use of flame population information.

Experimental study on multi-channel ignition of propane-air by transient repetitive nanosecond surface dielectric barrier discharge

Multi-channel ignition by nanosecond surface dielectric barrier discharge (nSDBD) offers potential for efficiency boost in spark ignition engines. In this paper, transient repetitive nSDBD ignition is proposed using a simple pulse forming circuit to make multi-channel ignition more feasible, considering that nSDBD can be enhanced by high frequency pulses and that convenient discharge energy control can be achieved by adjusting pulse number. Ignition experiments for propane/air mixtures were carried out in a constant volume combustion chamber under stoichiometric conditions at ambient initial temperature and pressure. Discharge channel transformation and flame propagation were captured by a high-speed camera. For 20 kHz negative pulses, discharge energy rose from 4 mJ to 8 mJ as pulse number increases from 1 to 12, and filaments grew from 2 mm to 12 mm. Partially overlapping filaments formed a tree-like discharge pattern with divergent roots and a concentrated middle part. Multiple flame kernels were generated in the middle region of the filaments and then greatly promoted by subsequent discharges, resulting in a petaloid flame. Ignition probability has been tested and the minimum ignition energy of repetitive nSDBD is obtained. The influence of discharge energy on flame propagation speed and combustion characteristics were analyzed.