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Recent interest in non-equilibrium 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 non-equilibrium 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.

Transient plasma ignition (TPI) has recently been attracting more attention due to the combination of its much larger volume ignition kernels and the generation of active radicals, which stabilizes the initial stage of the combustion by the enhancement of flame propagation rates compared with conventional spark ignition. In this study, TPI early flame propagation characteristics were investigated for near-atmospheric stoichiometric and lean mixtures of methane and air. Transient plasma was generated by applying nanosecond (pulse duration 10 – 12 ns), high-voltage (6 – 10 kV) pulses between two opposed pin-to-pin configurations with a gap distance of 3 mm residing within a custom-built constant volume combustion vessel. The delay to ignition and peak pressure as a function of voltage, air-fuel mixture rate (ϕ = 0.6 – 1.0), and pulse repetition frequency (PRF) of 1 – 10 kHz were evaluated. The early flame propagation and the discharge streamer phenomena were investigated via high-speed schlieren images as well as examining nanosecond pulses’ voltage-current waveform sequences. It is showed that a leaner mixture could reduce the ignition voltage range. The experiment results showed that higher PRF has potential advantages such as higher peak pressure, shorter ignition delay, faster flame propagation speed, smaller energy consumption, and leaner condition compared to conventional spark ignition.

In the present study, the performance and emissions characteristics of three low-temperature plasma (LTP) ignition systems were compared to a more conventional strategy that utilized a high-energy coil (93 mJ) inductive spark igniter. All experiments were performed in a single-cylinder, optically accessible, research engine. In total, three different ignition systems were evaluated: (1) an Advanced Corona Ignition System (ACIS) that used radiofrequency (RF) discharges (0.5–2.0 ms) to create corona streamer emission into the bulk gas via four-prong electrodes, (2) a Barrier Discharge Igniter (BDI) that used the same RF discharge waveform to produce surface LTP along an electrode encapsulated completely by the insulator, and (3) a Nanosecond Repetitive Pulse Discharge (NRPD) ignition system that used a non-resistor spark plug and positive DC pulses (∼10 nanoseconds width) for a fixed frequency of 100 kHz, with the operating voltage-controlled to avoid LTP transition to breakdown. For the LTP ignition systems, pulse energy and duration (or number) were varied to optimize efficiency. A single 1300 revolutions per minute (rpm), 3.5 bar indicated mean effective pressure (IMEP) homogeneous operating point was evaluated. Equivalence ratio (ϕ) sweeps were performed that started at stoichiometric conditions and progressed toward the lean limit. Both the ACIS and NRPD ignition systems extended the lean limit (where the variation of IMEP < 3%) limit (ϕ = 0.65) compared to the inductive spark (ϕ = 0.73). The improvement was attributed to two related factors. For the ACIS, less spark retard was required as compared to spark ignition due to larger initial kernel volumes produced by four distinct plasma streamers that emanate into the bulk gas. For the NRPD ignition system, additional pulses were thought to add expansion energy to the initial kernel. As a result, initial flame propagation was accelerated, which accordingly shortens early burn rates.

Stringent oxides of nitrogen (NOx) and particulate matter (PM) emission standards and regulations, increased consumer concern about greenhouse gas (GHG) emissions and cost-effective competition with battery-electric vehicles (BEV) have forced the automotive engine manufacturers to search for novel and innovative strategies. A viable approach for clean, high-efficiency operation for modern spark-ignited (SI) gasoline or natural gas engines is downsized boosted ultra-lean operation. Ultra-lean ignition process, however, gives rise to some serious challenges. Ignition becomes increasingly difficult with the leaner mixture. Moreover, engine operation becomes susceptible to misfires, cycle-to-cycle variability, reduced efficiency, increase unburned hydrocarbon (UHC) emission, etc. Turbulent jet ignition (TJI)—where pre-chamber combustion generated turbulent jet consisting of hot combustion products and active radicals ignite a lean mixture—is a robust yet straightforward ignition strategy that has potential to solve challenges encountered at lean-burn operation. The current study explores the fundamentals as well as the scalability aspect of the TJI strategy. Even though the TJI system is easy to implement, the fluid mechanics and chemical kinetics processes involved in TJI are incredibly complicated due to the complex coupling between turbulence and chemistry. Ignition and flame propagation behavior of TJI were studied in custom-built, optically accessible, constant-volume vessel for lean (\(\phi = 0.55 - 0.95\)) methane/air using simultaneous high-speed schlieren and OH* chemiluminescence imaging. Detailed characteristics of two distinct types of ignition mechanisms—jet ignition and flame ignition—and their dependence on the critical thermophysical and geometric parameters were discussed. The impact of four significant parameters—initial pressure, nozzle diameter, pre-chamber equivalence ratio and pre-chamber volume—was carefully examined. An in-depth discussion of various competing pre-chamber processes on TJI emphasized the necessity of non-dimensionalization. The non-dimensionalization of governing equations for compressible reacting flow produced four dimensionless numbers associated with TJI—Reynolds, Schmidt, Prandtl and Damkohler number. Assuming the transport coefficients are independent of temperature and kinematic transport coefficients are directly proportional to the temperature yielded unity Schmidt and Prandtl number. The remainder two non-dimensional numbers—Reynolds and Damkohler number, regulated the TJI mechanisms of premixed methane/air. Correlations were developed for TJI using Reynolds and Damkohler number. Lastly, the crucial factors and challenges in developing a scalable TJI system were addressed. These results provide useful insights about the TJI fundamentals and serve as a guideline for future pre-chamber design and optimization.

Chemical kinetics effects on fundamental laminar combustion processes were investigated for 20% methanol-blended gasoline and 20% hydrogen-blended methanol for a range of equivalence ratios, \( 0.5 < \phi < 2 \) using a detailed reaction mechanism comprising of 2027 species and 8619 reactions. Fundamental laminar combustion properties—flame speed, flame thickness, minimum ignition energy, ignition, and extinction strain rates of the two fuel blends were compared with base fuels, methanol, gasoline, and hydrogen. Mean extinction strain rates were nearly twice the ignition strain rates which indicated a hysteresis behaviour of laminar flames. An in-depth analysis of the thermal and chemical flame structure of lean fuel blends was conducted to explore the significance of intermediate products and radical species on flame behaviour. Chemical kinetics caused a greater impact on the ignition process compared to extinction. A sensitivity analysis of ignition and extinction strain rates showed the importance of key reactions and their role in the improvement of laminar flame characteristics of fuel blends. The sensitivity coefficients for ignition were greater compared to extinction signifying a stronger influence of chemical kinetics on the ignition process compared to extinction. The knowledge obtained from this study can be leveraged in creating tailored fuel blends with superior laminar combustion competencies enabling higher efficiency and lesser pollutant emission for automotive applications.

Low-temperature combustion (LTC) is a desirable mode of fuel utilization that ensures low particulate matter (PM) and oxides of nitrogen (NOx) emissions. Low-temperature combustion kinetics of five different methanol-blended gasoline containing 10, 20, 30, 40, and 50% of methanol by volume, respectively, were compared with neat gasoline. Methanol addition in gasoline enhanced low-temperature combustion behavior compared to neat gasoline due to the improved evaporation and mixing characteristics. Unlike the combustion efficiency and power, which showed marginal improvement; carbon monoxide (CO), and NOx emissions dropped significantly with the increase in methanol/gasoline ratio. The effect of methanol/gasoline ratio on different low-temperature chemical pathways was investigated to find an optimum value in terms of combustion performance and emission. Operating strategies to reduce and mitigate unburned methanol emission are discussed, which arose at higher methanol concentrations. Dimethyl ether (DME) is an attractive fuel for compression ignition engines. Since it exhibits low-temperature chemistry, it is particularly suited for homogeneous charge compression ignition (HCCI) Engines. Active radicals generated by low-temperature transient plasma (TP) discharge offer a viable approach to control the combustion phasing of an HCCI engine. In this study, the low-temperature kinetics of DME was explored employing a non-equilibrium plasma ignition strategy using zero-dimensional modeling. A sensitivity analysis of the first and second-stage ignition delays was carried out. The negative temperature coefficient (NTC) regime revealed interesting distinctions between DME and other low molecular weight alkanes. The presence of a thermal bottleneck was found to decrease the efficiency of plasma assistance.

Dilute combustion, either using exhaust gas recirculation or with excess air, is considered a promising strategy to improve the thermal efficiency of internal combustion engines. However, the dilute air-fuel mixture, especially under intensified turbulence and high-pressure conditions, poses significant challenges for ignitability and combustion stability, which may limit the attainable efficiency benefits. In-depth knowledge of the flame kernel evolution to stabilize ignition and combustion in a challenging environment is crucial for effective engine development and optimization. To date, a comprehensive understanding of ignition processes that result in the development of fully predictive ignition models usable by the automotive industry does not yet exist. Spark-ignition consists of a wide range of physics that includes electrical discharge, plasma evolution, joule-heating of gas, and flame kernel initiation and growth into a self-sustainable flame. In this study, an advanced approach is proposed to model spark-ignition energy deposition and flame kernel growth. To decouple the flame kernel growth from the electrical discharge, a nanosecond-pulsed high-voltage discharge is used to trigger spark-ignition in an optically accessible small ignition test vessel with a quiescent mixture of air and methane. Initial conditions for the flame kernel, including its thermodynamic state and species composition, are derived from a plasma-chemical equilibrium calculation. The geometric shape and dimension of the kernel are characterized using a multi-dimensional thermal plasma solver. The proposed modeling approach is evaluated using a high-fidelity computational fluid dynamics procedure to compare the simulated flame kernel evolution against flame boundaries from companion Schlieren images.

The ignition characteristics of a hot turbulent jet impinging on a flat plate surrounded by an ultra-lean premixed H2/air was studied both experimentally and numerically. The hot turbulent jet was generated by burning a small quantity of stoichiometric H2/air mixture in a separate small volume called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushed the combustion products into the main chamber through a small nozzle (0.75–4.5 mm in diameter) in the form of a hot turbulent jet, which then impinged on a flat plate. Six different plates with varying impinging heights and angles were used. Two important parameters controlling the impinging characteristics of the jet, the ratio of the impinging distance to the nozzle diameter, H / D and the impinging angle, θ were examined. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration/impinging and ignition process inside the main combustion chamber. Results illustrate the existence of two distinct types of ignition mechanisms. If the impinging distance is short and the hot turbulent jet hits the plate with high enough momentum, the temperature increases around the stagnation point and ignition starts from this impinging region. However, if the impinging distance is long, the hot turbulent jet mixes with the ambient unburned H2/air in the main chamber and ignites the mixture at the upstream from the plate. For such type of ignition, the impinging plate has a minimum role on main chamber ignition. Employing the stagnation point ignition, a leaner limit of H2/air in the main chamber was achieved. Numerical modeling of the turbulent hot jet impingement process was carried out to explain the impinging jet ignition mechanism. It was found that H / D ratio was the controlling parameter between the two ignition mechanisms. The limiting H / D ratio was found to be 21.6, below which ignition occurred via jet impingement. Unlike the H / D ratio, the impinging angle did not affect the ignition mechanism; however, it affected the main chamber burn time.

A detailed investigation on the ignition characteristics of ultra-lean premixed H2/air mixtures by multiple hot turbulent jets in a dual combustion chamber system was carried out. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition processes inside the main combustion chamber. The focus was on the effects of the spark location and fuel/air equivalence ratio within the pre-chamber on the ignition pattern of the main-chamber mixture. The results show that multiple jets resulted in similar lean flammability limit of H2/air as the single jets, given that the total nozzle areas are the same. However, the ignition probability improved significantly near the lean flammability limit using multi-jets compared to single jets. Additionally, depending on the spark location and pre-chamber equivalence ratio, either the side jet, the middle jet, or all the jets can initiate ignition in the main chamber. The ignition characteristics of straight and angled multi-jets were compared, and angled jets produced shorter main chamber burn time. Numerical modeling of the flame propagation process within the pre-chamber was carried out to explain the behavior of individual jets in a multi-jet system. It was found that depending on the pre-chamber spark position, the flame shape inside the pre-chamber changes drastically. The effective L / D ratio governs the flame dynamics in the pre-chamber and subsequently determines which jet (side, middle, or all) will first ignite the main chamber.

Nanosecond pulsed discharges have attracted the attention of engine manufacturers due to the possibility of attaining distributed ignition sites that accelerate burn rates while resulting in very little electrode erosion. Multidimensional modeling tools currently capture the electrical structure of such discharges accurately, but resolving the chemical structure remains a challenging problem owing to the disparity of time-scales in streamer propagation (nanoseconds) and ignition phenomena (microseconds). The purpose of this study is to extend multidimensional results toward resolving the chemical structure in the wake of streamers (or the afterglow) by using a batch reactor model (BRM). This can afford the use of very detailed chemical kinetic information. The full nonequilibrium nature of the electrons is taken into account, along with fast gas heating, shock wave propagation, and thermal diffusion. The results shed light on ignition phenomena brought about by such discharges.

An experiment was developed to investigate the ignition mechanisms of premixed CH4/air and H2/air mixtures using a turbulent hot jet generated by pre-chamber combustion. Simultaneous high-speed Schlieren and OH∗ chemiluminescence imaging were applied to visualize the jet penetration and ignition process inside the main combustion chamber. Results illustrate the existence of two ignition mechanisms: jet ignition and flame ignition. The former produced a jet comprising of only hot combustion products from pre-chamber combustion. The latter produced a jet full of wrinkled turbulent flames and active radicals. A parametric study was performed to understand the effects of pressure, temperature, equivalence ratio along with geometric factors such as orifice diameter and spark position on the ignition mechanisms and probability. A global Damkohler number was defined to remove parametric dependency. The limiting Damkohler number, below which ignition probability is nearly zero, was found to be 140 for CH4/air and 40 for H2/air. Lastly, the ignition outcomes were plotted on the turbulent premixed combustion regime diagram. All non-ignition cases fell within the broken reaction zone regime, whereas flame and jet ignition mostly fell within the thin reaction zone regime. These results can provide useful guidelines for future pre-chamber design and optimization.

Low Surface Brightness (LSB) galaxies have very diffuse, low surface density stellar disks which appear faint in optical images. They are very rich in neutral hydrogen (HI) gas, which extends well beyond the stellar disks. Their extended HI rotation curves and stellar disks indicate that they have very massive dark matter (DM) halos compared to normal bright galaxies. Hence, LSB galaxies may represent valuable laboratories for the indirect detection of DM. In this paper, we search for WIMP annihilation signatures in four LSB galaxies and present an analysis of nearly nine years of data from the Fermi Large Area Telescope (LAT). Above 500 MeV, no excess emission was detected from the LSB galaxies. We obtain constraints on the DM cross-section for different annihilation channels, for both individual and stacked targets. In addition to this, we use radio data from the Very Large Array (VLA) radio telescope in order to derive DM constraints, following a multiwavelength approach. The constraints obtained from the four considered LSB galaxies are nearly 3 orders of magnitude weaker than the predicted limits for the thermal relic abundances and the combined limits achieved from Fermi-LAT observations of dwarf spheroidal galaxies. Finally, we discuss the possibility of detecting emission from LSB galaxies using the upcoming ground-based $\gamma$-ray and radio observatories, namely the Cherenkov Telescope Array (CTA) and the Square Kilometre Array (SKA)., Comment: To be published in Monthly Notices of the Royal Astronomical Society (accepted in 2020 December 8)

The aggressive move of modern spark ignited (SI) gasoline engines toward challenging dilute and boosted combustion regimes necessitates the development of new and robust ignition devices. Transient plasma ignition (TPI)-where high-energy, short-pulsed, non-equilibrium plasma are used to ignite flammable mixtures-is of interest to engine developers due to the generation of distributed ignition sites that accelerate initial burn rates and reduced electrode erosion that results from the avoidance of breakdown. However, transient plasma igniter development has been slowed by a lack of foundational understanding of the associated ignition mechanisms. In particular, multi -physics TPI modeling at engine-relevant conditions is complicated by the vast range of chemical time-scales (fs - ms) and lack of suitable experimental post-discharge data of the fuel/oxidizer products at relevant high-pressure conditions.

The main reason that hot turbulent jet ignition has become attractive to gas engine manufacturers is that hot jet ignition can achieve faster burn rates due to the ignition system producing multiple, distributed ignition sites, which has greater likelihood igniting a lean mixture compared to spark ignition. This leads to better thermal efficiency and low NOx production. Compared to conventional spark ignition, a hot jet has a much larger surface area leading to multiple ignition sites on its surface which can enhance the probability of successful ignition and cause faster flame propagation and heat release. Over the last few decades, pre-chamber jet ignition had technologically advanced from conceptual design phase to actual engines. The early designs developed by Gussak [1–4], Oppenheim [5, 6], Wolfhard [7], and Murase [8] showed the promise of lean ignition by a hot turbulent jet. Later, Ghoneim and Chen [9], Pitt [10], Yamaguchi [11], Elhsnawi [12], Sadanandan [13], Toulson [14, 15], Gholamisheeri [16], Attard [17], Perera [18], Carpio [19], Shah [20], Karimi [21], Thelen [22], and Biswas [23, 24] further investigated in detail the parametric effects and fundamental physics of turbulent jet ignition in laboratory scale prototype combustors and at engine-relevant conditions. All these studies support that turbulent jet ignition possesses several advantages over traditional spark ignition during ultra-lean combustion such as higher ignition probability, faster burn rates, and multiple ignition kernels.

Turbulent jet ignition can reliably be used to ignite an ultra-lean fuel/air mixture as illustrated in previous chapters. This ignition technique can be utilized in various applications ranging from pulse detonation engines, wave rotor combustor explosions, to supersonic combustors and natural gas engines. Compared to a conventional spark plug, the hot jet has a much larger surface area leading to multiple ignition sites on its surface which can enhance the probability of successful ignition and cause faster flame propagation and heat release. In short, turbulent jet ignition has many advantages over conventional ignition system.

In recent years gas engine manufacturers have faced stringent emission regulations on oxides of nitrogen (NOx) and unburned hydrocarbons (UHC) [1, 2]. Operating internal combustion engines at ultra-lean conditions can reduce NOx emissions and also improve thermal efficiency [3, 4]. An approach that can potentially solve the challenge of igniting ultra-lean mixtures is to use a reacting/reacted hot turbulent jet to ignite the ultra-lean mixture instead of a conventional electric spark [5–10]. The hot turbulent jet is produced by burning a small amount of stoichiometric or near-stoichiometric fuel/air mixture in a small volume separated from the main combustion chamber called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushes combustion products into the main combustion chamber in the form of a hot reacting/reacted turbulent jet, which then ignites the ultra-lean mixture in the main combustion chamber. Compared to conventional spark ignition, the hot turbulent jet has a much larger surface area containing numerous ignition kernels over which ignition can occur. Hot jet ignition has the potential to enable the combustion system to operate near the fuel’s lean flammability limit, leading to ultralow emissions.

Combustion at small scales (micro- and mesoscales) is gaining increasing attention these days due to the wide spectrum of potential applications in sensors, actuators, portable electronic devices, rovers, robots, unmanned air vehicles, thrusters, industrial heating devices, and, furthermore, heat and mechanical backup power sources for air-conditioning equipment in hybrid vehicles and direct ignition (DI) engines as well [1–3]. Combustion of hydrocarbon fuels is more attractive to manufacturers of miniature power devices because the energy density of hydrocarbons is several times higher than modern batteries [4]. Microscale combustion physics is quite different from those at larger length scales. For example, flame propagation through narrow channels has unique characteristics, e.g., the increasing effects of flame–wall interaction and molecular diffusion [5–10]. In small-scale combustion systems, the surface-to-volume (S/V) ratio is large, which leads to more heat loss and thus causes flame extinction more easily.

Motivated by the fact that turbulent jets from straight nozzles could ignite a lean (0.5 < ϕ < 0.9) main chamber reliably as discussed in Chap. 2, we wanted to explore the possibility to reach ultra-lean limit using supersonic jets. The same experimental setup that uses a dual-chamber design (a small pre-chamber resided within the big main chamber) was used except the straight nozzles were replaced by converging or converging-diverging (C-D) nozzles. The primary focus was to reveal the characteristics of supersonic jet ignition, in comparison to subsonic jet ignition. Another intention behind supersonic jets was from ignition delay standpoint; a high-speed jet could well reduce the ignition delay. Simultaneous high-speed schlieren photography and OH* chemiluminescence were applied to visualize the supersonic jet penetration and ignition processes in the main chamber. Infrared imaging was used to characterize the thermal field of the hot jet. Numerical simulations were carried out using the commercial CFD code, Fluent 15.0, to characterize the transient supersonic jet, including spatial and temporal distribution of species, temperature and turbulence parameters, velocity, Mach number, turbulent intensity, and so on. The present work focuses on the effect of supersonic jets on lean flammability limits.

NCC or Neurocysticercosis is one of the clinical syndromes produced by infection with larval stage of tapeworm Taenia Solium (Pork tapeworm). It is the commonest parasitic infection affecting the central nervous system in human beings. Though seizure disorders are common in NCC in adults, sudden death is rarely reported. In our present case study one young male footpath dweller of 35 years of age and known to be a chronic alcoholic, died suddenly after a drinking bout. The dead body was sent to Kolkata police morgue for autopsy examination as a medico-legal case. Save a minor degree of cirrhotic changes in the liver, abdominal and thoracic viscera were unremarkable. Unexpectedly the brain parenchyma was found to be studded with numerous cystic lesions, which on Histopathological examination was confirmed to be the lesions of Neurocysticercosis. Cardiovascular events originating from coronary atherosclerosis are regarded worldwide as commonest cause of sudden death, more so in alcoholics. Neurological causes, particularly vascular events are not rare too. Meticulous autopsy, supported by ancillary investigations, occasionally leads an autopsy surgeon to such surprising outcomes.

Dead body recovered from water stained with mud is commonly thought to be a drowning case. One such case was brought to Kolkata Police morgue for medicolegal autopsy. Apparently it was a simple case of drowning but when autopsy was started, the actual fact was revealed something else. Injuries over the face, asphyxia signs and findings of external genitalia were sufficient to draw the opinion and the case turned to be a homicide with sexual assault. Such crimes are increasing day by day in every part of our country. All such victims, irrespective of their socioeconomic status deserve the justice. Here we are going to discuss the case to highlight how a meticulous post mortem examination can be a first ray of light in a dark field of justice. Language: en