Acceleration Of Flame Front Research Articles

The Synergy between Methanol M100 and Plasma-Assisted Ignition System PAI to Achieve Increasingly Leaner Mixtures in a Single-Cylinder Engine

Currently, conventional spark–ignition engines face challenges in meeting the ever-growing demands of customers and increasingly stringent regulations regarding pollutant emissions. A combination of innovative strategies and carbon-neutral fuels is deemed necessary in order to further reduce fuel consumption and minimize engine emissions. The present work aims to assess the performance of combustion strategies using low-carbon-content fuel, such as methanol M100, ignited by a plasma-assisted igniter (PAI) under ultra-lean conditions. The experimental campaign is conducted on a single-cylinder research engine at 1000 rpm and low loads, moving up to the engine lean stable limits. The specific purpose of this work is to determine the benefits brought by the proposed strategy, referred to as M100–PAI, which compared market gasoline E5 ignited by the PAI system and conventional spark. The synergy between M100 (methanol) and Plasma-Assisted Ignition (PAI) in internal combustion engines yielded notable benefits. This combination significantly improved combustion stability if compared to the other combinations tested, by extending the lean stable limit to λ = 2.0, reducing cycle-to-cycle variability, and facilitating faster flame front acceleration, resulting in enhanced homogeneity. These enhancements, obtained with the combination M100–PAI, contributed to higher fuel efficiency, showing a 10% efficiency gain over the combination E5–gasoline spark ignition. The findings highlight the potential of innovative combustion strategies using low-carbon fuels and advanced ignition systems to meet stringent emissions regulations while improving engine performance.

Влияние объема водородно-воздушной газовой смеси, типа и энергии инициирования на распространение сферического фронта пламени

The investigation considered a spherical flame front propagating in free space filled with a hydrogen-air mixture containing 15 % hydrogen. The experiments were conducted inside latex shells. We initiated combustion in the centre of the volume, using either a 1 mJ spark or a nichrome wire explosion releasing 5 J of energy. In the case of the 7 m3 and larger shells, we recorded video data on the flame front using an InfraTec ImageIR 8320 infrared camera featuring a 2.0--5.7 μm spectral ranges and a rate of 300 frames per second. In the case of the 0.003 m3 and smaller shells, we visualised the processes by means of an IAB-451 schlieren system. We recorded these images using a Videosprint G/2 visible-wavelength camera at a rate of 1000 frames per second. We identified the differences in flame front acceleration dynamics for the same combustible mixture composition and flame initiation energy. We found that the distribution function specifics concerning the pre-exponential factors in the power law of flame front acceleration indicate that individual random perturbations have a considerable effect at the initial stage of quasi-spherical flame front formation. We compared flame front propagation for the cases when the initiation energy was 1 mJ and 5 J respectively and determined that the initiation energy also affects the process

Acceleration of hydrogen/air flames in a cylindrical envelope

Flame front behavior during hydrogen/air deflagration in initially quiescent mixtures in cylindrical envelopes was experimentally studied using ionization gauges and infrared photography. Hydrogen/air mixtures with hydrogen content from 12% to 30% were filled in the polyethylene envelope of 4.5 m3 and ignited with exploding wire of 5 J energy. The dependences of the flame front position on time were obtained. Dynamics of the flame front was analyzed on harmonic instabilities development. The mechanisms for the creation of turbulence are discussed. Flame front acceleration is analyzed using Kolmogorov law.

Influence of sound absorbing surfaces on acoustic oscillations and flame acceleration in hydrogen–air mixture

The frequency spectrum of acoustic disturbances that are emitted by accelerating flame front in an air-hydrogen mixture within an axially symmetric channel with a uniform cross section is experimentally determined. The effect of acoustic disturbances that are reflected from the closed end of the combustion chamber on the flame front acceleration is studied. It is revealed that the frequency spectrum of generated acoustic disturbances under experiment conditions has maximums at frequencies close to 250, 800, and 1500 Hz.

Sectioning of Potential Explosion Domains to Reduce the Overall Severity of a Vapor Cloud Explosion

Medium scale vapour cloud explosion experiments demonstrated that the severity of a vapour cloud explosion (VCE) can be reduced substantially by initiating counter fires within a vapour cloud as soon as a developing flame can be detected within the vapour cloud. For instance, experiments with two simultaneous ignitions at opposite sides of the used symmetrical test rig resulted in explosion pressures which were reduced by a factor of three to five compared to single point ignition at one side.A later re-analysis of the test findings drew the attention to a flame acceleration mechanism, which had been known before, but its contribution to the severity of a VCE had not been fully appreciated. The generally accepted theory is that flame acceleration occurs as the flame passes obstacles. In addition to that, the contribution of the expanding combustion gas has been found to play a paramount role as well.A big part of the strong reduction of the explosion severity with two-point ignition could be related to the fact that the two developing flames were acting against each other, creating a kind of expansion barrier against each other. This prevented the expanding combustion gas to push into the rig, which otherwise would have added to the flame speed.In the past, walls in a congested area had been assumed to increase the severity of a VCE. The test findings suggest, however, that barriers could be implemented in a way such that they slow down the acceleration of a flame front.It is expected that such barriers can be effective in particular when they are installed perpendicular to the anticipated direction of flame front acceleration. Process equipment is typically installed in rows 8 to 15 m wide, often extending over a length of more than 50 m, with access corridors at both sides. Horizontal equipment such as drums and heat exchangers are typically arranged perpendicular to the longitudinal direction.For such an arrangement, a flame can be assumed to accelerate preferentially in the longitudinal direction, perpendicular to the obstacles in its way, and to speed up as it passes gaps between and below equipment, pushed through the gaps by the expanding gas behind. By closing such gaps or by installing barriers, the longitudinal hot gas expansion could be partly suppressed and its contribution to the flame speed would be reduced substantially. In addition, the barriers would force the hot gas to vent towards the uncongested space aside and above the equipment and they would force the flame front to “take a detour” around the barriers. Together, these effects would reduce the burning rate.Hence, properly arranged barriers are expected to subdivide larger congested domains into smaller explosion cells, where less severe explosions occur. Because of the time needed for the flames to proceed from one cell to another, the pressure waves from these cells would be spread out over time, resulting in a lower overall explosion pressure, but continuing over a longer time period.

The effect of reflected acoustic disturbances on flame front acceleration

The frequency spectrum of acoustic disturbances that are emitted by accelerating flame front in an air–hydrogen mixture within an axially symmetric channel with a uniform cross section is experimentally determined. The effect of acoustic disturbances that are reflected from the closed end of the combustion chamber on the flame front acceleration is studied. It is revealed that the frequency spectrum of generated acoustic vibrations under experiment conditions has maximums at frequencies close to 250, 800, and 1500 Hz. The average flame velocity is 200 m/s. Spectra of generated acoustic vibrations and the flame front velocity depending on the material of the closed end of the axially symmetric channel are obtained for the stoichiometric air–hydrogen mixture.

Generic event trees and probabilities for the release of different types of hazardous materials

To simplify quantitative risk analysis, the initiating events leading to loss of containment are normally described using generic hypotheses. For example, the following hypothesis is applied to the loss of containment from a storage tank: instantaneous release of the complete inventory, continuous release of the complete inventory in 10 min, and continuous release from a hole with a diameter of 10 mm. Once the initiating events have been specified, the corresponding event trees must be drawn to establish the sequences from each initiating event to the diverse final outcomes or accident scenarios, which will depend on the properties of the released material or on other specific factors. In this paper we propose, in a systematic way, a set of short generic event trees for the main loss of containment scenarios involving different types of hazardous materials. Even though most of them have been taken from the literature (BEVI Reference Manual), we have modified some of them, added the corresponding intermediate probabilities (immediate ignition, delayed ignition, flame front acceleration, etc.) obtained from a literature review and expert judgment, and associated the use of each event tree to the hazardous properties of the material (flammability, volatility and toxicity) and to its category according to EC labeling directives.

Minimizing Uncertainty in Vapour Cloud Explosion Modelling

There have been significant advances in consequence modelling of releases of hazardous materials in the last decade, but the use of simple models for predicting overpressures from vapour cloud explosions (VCEs) continues to elude safety practitioners. The TNO multi-energy model has generally been regarded as the best model available to date, for a rapid assessment of explosion overpressures and positive phase durations. This model requires two major assumptions to be made—the level of congestion in the plant that provides obstacles for flame front acceleration, and the explosion efficiency. The former enables the selection of an appropriate charge strength from the family of parametric curves in the model. While there has been some guidance available for making relevant assumptions, there is still a high level of uncertainty in the model results, and the drag load obtained cannot be confidently applied as the basis for structural design. Flame acceleration models based on computational fluid dynamics (CFD) have been developed for gas explosion modelling in offshore oil and gas facilities. Sensitivity analysis is conducted on these models, using various cloud sizes and ignition point locations, and the results are processed into an exceedence curve based on event frequencies, in order to obtain the best estimate of peak overpressures and drag loads for design purposes. In this paper, the TNO multi-energy model has been applied to two different configurations of topsides of offshore installations. In these applications the assumptions regarding explosion efficiency and charge strength selections have been calibrated against the results obtained through the CFD modelling, by obtaining a match of the explosion over-pressures. The paper comments on the findings from this exercise in order to give guidance on the application of the TNO multi-energy method and on the selection of model parameters with relation to the equipment lay-out and level of equipment congestion.

Local flame front disturbance development under acceleration

The development of a local flame front disturbance in an accelerating gas flow was explored using high-speed schlieren photography. The movement of the local flame front was examined in detail, and, based on these results, the flow field around the disturbed flame front was inferred. From this, it was confirmed that in an accelerating flow field, the unburned gas penetrates into the burned gas region at a velocity about 30 times higher than the laminar burning velocity. Flame front acceleration generates intense shear flow layers across disturbed flame fronts.

Vented gas deflagrations a detailed mathematical model tuned on a large set of experimental data

A detailed mathematical model based on conservation laws and physicochemical relationships has been developed to simulate vented gas deflagrations. The main phenomena involved are described using a priori relationships whenever well-established quantitative theories are available. In the case of flame front acceleration due to flame wrinkling in closed vessels or to turbulence induced by vent opening, where such theories do not exist two empirical relationships have been introduced. These have been turned by comparison with a collection of about 160 literature experimental data, covering a wide range of values for vessel volume (0.001–199 m 3), initial pressure (0.1–0.4 MPa), and bursting pressure (0.1–2.96 MPa) and including various vessel shapes and fuel-air compositions. The obtained average relative error in the maximum explosion pressure values is equal to 10.4%, comparable with the uncertainty inherent to the vented deflagration data considered. A dedicated computer algorithm has been developed to provide a fast running simulation program (i.e., about 10 seconds of CPU time on a VAX 8650 for a typical deflagration process).

The Measurement of Propagation of Fire in Vertically Suspended Fabric; Upward Acceleration of Flame Front

Methods and instruments are described for determining the upward acceleration of a flame front in a vertically suspended fabric. The phenomenon of flame propagation is analyzed. Analysis of experimental results would indicate that the upward acceleration of flame front is a useful measure of the relative flammability hazard of textiles.