Ignition Phase Research Articles

Relation between hydroxyl and formaldehyde in a direct-injection heavy-duty diesel engine

Formaldehyde and the hydroxyl radical have been recorded simultaneously in a diesel spray in an optically-accessible heavy-duty diesel engine. Formaldehyde (CH 2O) was excited by 355 nm radiation, while hydroxyl (OH) was excited around 284 nm. Both laser beams were focussed into a sheet, of which the overlap while traversing the spray was monitored. The experimental results have been compared to simulations. The ignition phase was modeled by a set of homogeneous reactors, while the diffusion combustion phase was simulated by a flamelet approach, viz. a rich partially premixed counterflow diffusion flame. Two situations were tested, one in which the combustion started only after the end of injection (short and early injection) and one with a longer injection duration where injection and combustion partly overlapped in time. In the former, OH initially shows up far from the injector and then quickly proceeds towards the injector. The position of OH varied significantly from cycle to cycle, suggesting that the combustion is partly governed by turbulence. When combustion and injection do partly overlap, the flame structure is much more stable. In both cases, OH and CH 2O generally form complementing spatial distributions, indicating that CH 2O is consumed locally during the hot combustion.

HiPER laser architecture principles

The HiPER project [1] is in a preliminary phase and a risk assessment is being conducted for a generic HiPER beamline concept – this is based on an anticipated requirement for a single beamline (or focal spot unit) drawn from the best available knowledge we have to date of the fusion physics requirement. For the moment the general architecture is to first order independent of the specific beamline implementation (i.e compression phase or ignition phase) as the fundamental requirements are very similar. In this paper we are describing the laser beamline as a bundle of unit beams (or beamlets) and we are explaining that optical zooming is quite possible especially in the case of shock ignition.

Optical and electrostatic potential investigations of electrical breakdown phenomena in a low-pressure gas discharge lamp

The ignition phase is a critical stage in the operation of gas discharge lamps where the neutral gas enclosed between the electrodes undergoes a transformation from the dielectric state to a conducting phase, eventually enabling the production of light. The phenomena occurring during this phase transition are not fully understood and the related experimental studies are often limited to local optical measurements in environments prone to influencing these transient phenomena. In this work unipolar ignition phenomena at sub-kilovolt levels are investigated in a 3 Torr argon discharge tube. The lamp is placed in a highly controlled environment so as to prevent any bias on the measurements. A fast intensified CCD camera and a specially designed novel electrostatic probe are used simultaneously so as to provide a broad array of measured and computed parameters which are displayed in space–time diagrams for cross comparisons. Experiments show that three distinct phases exist during successful ignitions: upon the application of voltage a first ionization wave starts from the active electrode and propagates in the neutral gas towards the opposite electrode. A local front of high axial E field strength is associated with this process and causes a local ionization to occur, leading to the electrostatic charging of the lamp. Next, a second wave propagates from the ground electrode back towards the active electrode with a higher velocity, and in this process leads to a partial discharging of the lamp. This return stroke draws a homogeneous plasma column which eventually bridges both electrodes at the end of the wave propagation. At this point both electrode sheaths are formed and the common features of a glow discharge are observed. The third phase is an increase in the light intensity of the plasma column until the lamp reaches a steady-state operation. Failed ignitions present only the first phase where the first wave starts its propagation but extinguishes in the lamp, leading to a charge memory effect. It is found that the full propagation of this first wave is a requirement for successful lamp ignition. Differences in the properties of the waves were observed depending on the voltage polarity, and it was estimated that a photoelectric effect at the wall is the most likely source of electrons for the ionization wave of positive polarity. Finally, a simple model of the first ionization wave is developed and used to analyse the fundamental differences between processes occurring at negative and at positive polarity. From this study three conditions are developed for the successful unipolar ignition of lamps and the relations between them are derived.

Performance Evaluation of Second-Throat Diffuser for High-Altitude-Test Facility

The performance of a second-throat ejector―diffuser system employed in high-altitude testing of large-area-ratio rocket motors is considered under various steady and transient operating conditions. When the diffuser attains started condition, supersonic flow fills the entire inlet section and a series of oblique shock cells occurring in the diffuser duct seal the vacuum environment of the test chamber against backflow. The most sensitive parameter that influences the stagnation pressure needed for diffuser starting is the second-throat diameter. Between the throat and exit diameters of the nozzle, there exists a second-throat diameter value that corresponds to the lowest stagnation pressure for starting. When large radial/axial gaps exist between the nozzle exit and diffuser duct, significant reverse flow occurs for the unstarted cases, which spoils the vacuum in the test chamber. However, the starting stagnation- pressure value remains unaffected by the axial/radial gap. Numerical simulations establish that it is possible to arrive at an optimum diffuser geometry that facilitates early functioning of the high-altitude-test facility during motor ignition phase. The predicted axial variations of static pressure and temperature along the diffuser for the testing of a cryogenic upper-stage motor agree well with available experimental data.

Dynamics of reactive high-power impulse magnetron sputtering discharge studied by time- and space-resolved optical emission spectroscopy and fast imaging

Time- and space-resolved optical emission spectroscopy and fast imaging were used for the investigation of the plasma dynamics of high-power impulse magnetron sputtering discharges. 200 μs pulses with a 50 Hz repetition frequency were applied to a Cr target in Ar, N2, and N2/Ar mixtures and in a pressure range from 0.7 to 2.66 Pa. The power density peaked at 2.2–6 kW cm−2. Evidence of dominating self-sputtering was found for all investigated conditions. Up to four different discharge phases within each pulse were identified: (i) the ignition phase, (ii) the high-current metal-dominated phase, (iii) the transient phase, and (iv) the low-current gas-dominated phase. The emission of working gas excited by fast electrons penetrating the space in-between the electrodes during the ignition phase spread far outwards from the target at a speed of 24 km s−1 in 1.3 Pa of Ar and at 7.5 km s−1 in 1.3 Pa of N2. The dense metal plasma created next to the target propagated in the reactor at a speed ranging from 0.7 to 3.5 km s−1, depending on the working gas composition and the pressure. In fact, it increased with higher N2 concentration and lower pressure. The form of the propagating plasma wave changed from a hemispherical shape in Ar, to a droplike shape extending far from the target in N2. An important N2 emission rise in the latter case was detected during the transition at the end of the metal-dominated phase.

Time- and space-resolved quantitative LIF measurements of formaldehyde in a heavy-duty diesel engine

Formaldehyde (CH 2O) is a characteristic species for the ignition phase of diesel-like fuels. As such, the spatio-temporal distribution of formaldehyde is an informative parameter in the study of the ignition event in internal combustion engines, especially for new combustion modes like homogeneous charge compression ignition (HCCI). This paper presents quantitative data on the CH 2O distribution around diesel and n-heptane fuel sprays in the combustion chamber of a commercial heavy-duty diesel engine. Excitation of the 4 0 1 band (355 nm) as well as the 4 0 1 2 0 1 band (339 nm) is applied. We use quantitative, spectrally resolved laser-induced fluorescence, calibrated by means of formalin seeding, to distinguish the contribution from CH 2O to the signal from those of other species formed early in the combustion. Typically, between 40% and 100% of the fluorescence in the wavelength range considered characteristic for formaldehyde is in fact due to other species, but the latter are also related to the early combustion. Numerical simulation of a homogeneous reactor of n-heptane and air yields concentrations that are in reasonable agreement with the measurements. Formaldehyde starts to be formed at about 2 °CA (crank angle degrees) before the rise in main heat release. There appears to be a rather localised CH 2O formation zone relatively close to the injector, out of which formaldehyde is transported downstream by the fuel jet. Once the hot combustion sets in, formaldehyde quickly disappears.

Characteristics of pulse-modulated radio-frequency atmospheric pressure glow discharge

In this letter, the electrical and optical characteristics of pulse-modulated radio-frequency atmospheric pressure glow discharge are presented. The 13.56 MHz glow discharge is modulated with pulses at repetition frequency of 100 kHz. It is shown that the discharge during power on has similar characteristics to that of discharge without pulsing, except during the discharge ignition and extinguishment phase. The spatio-temporal evolution of discharge in phases of ignition and extinguishment is studied by time-resolved imaging and optical emission spectrograph.

Influence of liquid additives in electro-erosive micromachining (EDMM)

Studies at the Otto-von-Guericke-University of Magdeburg in cooperation with oelheld GmbH in Stuttgart have analyzed the influence of selected additives in dielectric working fluids for electrical discharge machining (EDM) on the ignition and breakdown phases of the plasma channel. The electrical breakdown in the hydrocarbon base fluid without additives is determined by the formation of the external Helmholtz-Layer as an interface between electrode surface and dielectric medium. The influence of the different additive molecules on the formation of the Helmholtz-Layer and on the ignition and discharge mechanism will be discussed.

Discharge ignition characteristics of pulsed radio-frequency glow discharges in atmospheric helium

An experimental study of radio-frequency (15 MHz) glow discharges in atmospheric helium modulated by pulses with repetition frequency of 500 kHz and duty cycle of 6% and 8% is presented in this paper. In each discharge burst, the discharge is restricted to operate in ignition phase with duration of one or two radio-frequency cycles. The ignition characteristics in terms of spatial-temporal evolution of discharge interelectrode structure and optical emission intensity are investigated by time resolved imaging. Optical emission intensities at lines of 706 and 777 nm are used to capture clearly the temporal evolution of energetic electrons and active specie of atom oxygen generated in discharge.

Study of HCCI-DI combustion and emissions in a DME engine

HCCI combustion demonstrates the capability of simultaneously reducing NO x and PM emissions and having a high brake thermal efficiency. However, there are still many challenges such as combustion control to overcome before full HCCI operation can be used reliably over the full engine operation range. Recently, the HCCI-DI compound combustion concept is presented, which is a compromise to full HCCI in that only a portion of the fuel is premixed and a portion of combustion is still controlled by the direct injection timing. Investigations towards HCCI-DI combustion in a DME engine were carried out in this paper. HCCI engine performances were presented to make a comparison. The peak in-cylinder pressure and the maximum heat release rate for HCCI-DI were lower than those for HCCI combustion and they decreased with a decrease in port DME aspiration quantity. Moreover, combustion duration was longer for HCCI-DI combustion and it would elongate with a decrease in port DME aspiration quantity. Engine experimental results showed HCCI-DI combustion could extend the operating range with a comparatively high brake thermal efficiency in comparison to HCCI combustion. CO and HC emission for HCCI-DI were lower than those of HCCI engine. As for NO x emissions for HCCI-DI operation, it decreased remarkably at low loads with an increase in port DME aspiration quantity, while showed an increasing trend at high loads. To control the ignition and combustion phase of HCCI, the effect of cooled EGR on HCCI-DI was evaluated. As a result, NO x emission decreased and the engine’s operating range enlarged for HCCI-DI combustion with cooled EGR.

Efficiency of catalytic processes for the reduction of CO and VOC emissions from wood combustion in domestic fireplaces

Pollutant characterization of domestic fireplaces, according to two paces of functioning (normal and low-charge phase) was performed. Two catalysts supported on cordierite or metal were placed in the exhaust of two domestic fireplaces (old and new generation) in order to reduce gaseous pollutants. Active phase of catalysts is composed of noble metals (Pd, Pt) and cerium. Methane was the dominant compound of the released Volatile Organic Compounds (VOC, 80% b. v.). Products resulting from incomplete combustion (CO and other VOC) did not represent more than 6% of the initial carbon content in wood. Lower concentration of CO in the exhaust was obtained with the new generation fireplace as compared to the older one with mean concentrations of CO normalized for 13% oxygen b.v. equal to 0.12% and 0.3%, respectively. Emission of VOC is also drastically reduced for new generation fireplace. The presence of a catalyst induced a decrease of the CO and VOC emission factors during ignition and low-charge phases by factors ranging from 65% to 70%. The abatement of VOC for the old generation fireplace was better in the presence of metal as compared to cordierite, with efficiency values of 65% and 30%, respectively. The new fireplace was the one on which the addition of the cleanup implements had most impact. Besides the introduction of a catalyst, a heating system of the fume was set up below the catalyst. This heating system allowed a faster activation of the catalyst, particularly during ignition and low-charge phases. Best abatements were obtained with the heated metallic support with values close to 80% and 94% for VOC and CO respectively.

A Comparative Study of Medium-Voltage Power Converter Topologies for Plasma Torch Under Dynamic Operating Conditions

This paper compares several medium-voltage power conversion systems for plasma torch application. The thermal plasma torch rated for 3 MW, 2 kA with the physical size of 1 m long is selected. The dynamic characteristics of the dc arc in a plasma torch are investigated using advanced 3-D magnetohydrodynamics simulation. The arc voltage noise of plusmn10% due to the periodic pressure wave is modeled as a sawtooth waveform in arc power loss term. The parameters of the Cassie-Mayr arc model are calculated based on 3-D simulation. A 12-pulse thyristor rectifier and two-phase staggered three-level step-down dc-dc converter having 12-pulse front-end diode rectifier are compared under the same dynamic operating condition. The three-level dc-dc converter has superior dynamic performance over the 12-pulse thyristor rectifier under the existence of arc disturbance noise. This power converter topology provides higher ignition voltage around 5 kV during ignition phase and higher arc stability.

Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration

The control of ignition in a rocket engine is a critical problem for combustion chamber design. Delayed ignition may lead to high-amplitude pressure fluctuations that can damage the burner (strong ignition), whereas early ignition may fail. This paper describes a numerical study of a strong ignition sequence observed in a laboratory-scale single-injector rocket chamber ignited by a laser and fueled with gaseous oxygen and hydrogen. OH-emission images, Schlieren pictures, and pressure measurements make it possible to follow the flame propagation experimentally. The present large eddy simulation (LES) approach includes shock treatment, a six species–seven reaction chemical scheme for H 2–O 2, and a model for the energy deposition by a laser. Flame/turbulence interaction is modeled with the thickened flame concept. LES is used to compute both the filling phase (during which the gaseous hydrogen and oxygen mix) and the ignition phase. The flame location and structure, as well as the temporal evolution of the chamber pressure obtained numerically, are in good agreement with the experiment. The use of complex chemistry in the computation also allows the comparison of LES data with experimental OH-images and shows that the sensitivity of the CCD camera used to record the spontaneous emission of the OH ∗ radical is not high enough to properly locate the flame front in rich regions. The combined experimental and numerical results lead to a more detailed analysis of the ignition processes and its coupling with flow rate oscillations in the H 2 and O 2 feeding lines.

Electrothermal-Chemical Ignition Research on 120-mm Gun in Korea

Electrothermal-chemical (ETC) propulsion with the control of a combustion process yet requires large electrical energies for an appreciable effect. Referring to the changed tendency of focusing ETC researches on the early ignition phase, the Agency for Defense Development (ADD), Korea, has been studying an electrothermal ignition for a 120-mm gun using a capillary plasma injector. Under the condition of a limited capillary geometry, design efforts with various liner materials have been done to achieve robust capillary discharges capable of transferring energies around 100 kJ. Electrical properties were estimated with the help of plasma theories and compared with experimental results. Experiments using a closed vessel and a gun-chamber simulator have been done to investigate the burning behaviors and the early phase of an electrothermal ignition, respectively. The closed vessel was designed for both a plasma and a conventional ignition. The burning rates could be obtained up to 70 MPa. The gun-chamber simulator was designed in the shape of the chamber of a 120-mm gun except for an acrylic chamber wall withstanding up to 30 MPa. Through prior tests using the closed vessel and the gun-chamber simulator, appropriate electrical energies and propellant have been selected. Firings of the full-scale 120-mm gun have been done using JA2 propellant. Electrical pulses of 1-2.5-ms duration in the energy from 20 to 120 kJ were applied. Aside from ignition capability, the results showed a possibility of a control on the peak pressure. Currently, ADD, Korea, continues the research to enhance the efficiency of the plasma injector, to compact the pulse forming network, and to investigate the electrothermal ignition properties of LOVA propellant.

Low Mach Number Modeling of Type Ia Supernovae. III. Reactions

We continue the description of a low Mach number hydrodynamics algorithm for reacting, full star flows. Here we demonstrate how to accurately incorporate reactions using a second-order accurate Strang-splitting technique. We also improve the fidelity of the model by allowing the base state to evolve in response to large-scale convection as well as large-scale heating, taking care to account for the compositional changes to the base state as well. The new algorithm is tested via comparisons with a fully compressible code and shown to be in good agreement. The resulting code, MAESTRO, once extended to incorporate a spherically symmetric base state, will be used to study the convection and ignition phases of Type Ia supernovae.

Search for the Optimizing Control Method of Compound Charge Compression Ignition (CCCI) Combustion in an Engine Fueled with Dimethyl Ether

Taking advantage of HCCI combustion and in-cylinder direct-injection combustion for an engine fueled with dimethyl ether (DME), a new concept combustion system, namely, a compound charge compression ignition (CCCI) combustion system by port aspiration and direct injection of DME, is proposed. A diesel engine was modified to carry out this test. Combustion and emission characteristics were investigated in different premixed fuel ratios and fuel delivery advance angles. To further optimize the emissions and improve the DME fuel consumption in a CCCI engine, port fuel design concept and port aspiration of CO2 were employed. The experimental results show that the CCCI combustion process comprises HCCI combustion, premixing combustion, and diffusion combustion. The combustion characteristic is mainly decided by a premixed fuel ratio and CO2 concentration in air charge. In comparison to a homogeneous charge compression ignition (HCCI) combustion mode, CCCI combustion can extend the operating range with low NOx, hydrocarbon (HC), and CO emissions. To control the ignition and combustion phase of HCCI, the effect of liquid petroleum gas (LPG) added to DME fuel was evaluated. As a result, NOx emissions decreased and thermal efficiency was improved with the mix of LPG. It is of interesting to note that HCCI−MK (modulated kinetics) combustion for CCCI combustion engines was observed at a 14°CA before top dead center (BTDC) fuel delivery advance angle with an appropriate CO2 concentration in air charge, and NOx emissions could be lowered to near-zero levels.

Oscillatory secular modes: the thermal micropulses

Stars in the narrow mass range of about 2.5 and 3.5 solar masses can develop a thermally unstable He-burning shell during its ignition phase. We study, from the point of view secular stability theory, these so called thermal micropulses and we investigate their properties; the thermal pulses constitute a convenient conceptual laboratory to look thoroughly into the physical properties of a helium-burning shell during the whole thermally pulsing episode. Linear stability analyses were performed on a large number of 3 solar-mass star models at around the end of their core helium-burning and the beginning of the double-shell burning phase. The stellar models were not assumed to be in thermal equilibrium. The thermal mircopulses, and we conjecture all other thermal pulse episodes encountered by shell-burning stars, can be understood as the nonlinear finite-amplitude realization of an oscillatory secular instability that prevails during the whole thermal pulsing episode. Hence, the cyclic nature of the thermal pulses can be traced back to a linear instability concept.

Prescribed burn documentation and fire danger ratings: A case study

The six standard relative numerical ratings of the Canadian Forest Fire Weather Index (FWI) System associated with a prescribed fire previouly reported on in the literature (Woodard et al., 1983) have been recomputed based on all the available weather data, in addition to being updated in terms of the current edition of the FWI System. This case study approach has been taken in order to illustrate the importance that fire danger rating information should play in the formal prescribed burn documentation process. A key feature in this particular retrospective analysis was the manner in which the previous day’s fuel moisture codes, which are required as starting values for FWI System calculations, were estimated (in the absence of pre-burn fire weather monitoring) for the actual site using the observations from nearby fire weather network stations. This information was combined with the on-site fire weather observations on the day the prescribed fire took place in order to quantify the burning conditions in terms of past and present weather influences that prevailed just prior to and/or during the ignition phase of the prescribed burning operation according to the three fuel moisture codes and three fire behavior indexes comprising the FWI System. The values of the six components of the FWI System associated with the 1983 Ram Mountain prescribed fire are as follows:

Simulating thermal explosion of octahydrotetranitrotetrazine-based explosives: Model comparison with experiment

A model comparison with measurements for the thermal, chemical, and mechanical behaviors in a thermal explosion experiment is presented. Confined high explosives (HEs) are heated at a rate of 1°C∕h until an explosion is observed. The heating, ignition, and deflagration phases are modeled using an arbitrarily Lagrangian-Eulerian (ALE3D) code that can handle a wide range of time scales that vary from a structural to a hydrodynamic time scale. During the preignition phase, quasistatic mechanics and diffusive thermal transfer from a heat source to the HE are coupled with the finite chemical reactions that include both endothermic and exothermic processes. Once the HE ignites, a hydrodynamic calculation is performed as a burn front propagates through the HE. Two octahydrotetranitrotetrazine (HMX)-based explosives, LX-04 and LX-10, are considered, whose chemical-thermal-mechanical models are constructed based on measurements of thermal and mechanical properties along with small-scale thermal explosion measurements. The present HMX modeling work shows the violence calculations with thermal predictions associated with a confined thermal explosion test. The simulated dynamic response of HE confinement during the explosive phase is compared to measurements in larger scale thermal explosion tests. The explosion temperatures for both HEs are predicted to within 1°C. Calculated and measured wall strains provide an indication of vessel pressurization during the heating phase and violence during the explosive phase.

Breakdown process and corona to spark transition between metal and liquid electrodes

Investigations of the processes of breakdown and formation of corona and spark between a metal electrode and an electrolyte surface are presented. It is found that a value of the breakdown voltage does not change with increasing of the conductivity of a solution in the range of 50 ÷ 500 µS/cm and the voltage of spark generation does not depends on the polarity of the applied voltage for different shapes of a metal electrode. Depending on the interelectrode distance and the applied voltage a different type of discharge is found. For distances higher than 6 mm an impulse corona discharge with a frequency up to 7.8 kHz and duration of 60 ns is generated. The ignition phase of the discharge and spark generation are studied with a high-speed CCD camera. It is found that corona-to-spark transition takes place during 0.9 ÷ 1 µs.