Ignition Dynamics Research Articles

On the critical pressure pulse width for ignition of hydrogen–air and dimethyl ether–air mixtures

Under certain situations, such as a water hammer event, a volume of gas can undergo a rapid compression immediately followed by a rapid expansion, which could lead to auto-ignition under certain conditions. The goal of the present study was to investigate under which critical conditions such a pressure pulse could lead to an ignition event. To simulate the pressure pulse, a simplified Gaussian pressure pulse model was implemented in Cantera. Hydrogen–air and dimethyl ether–air mixtures were employed to study the ignition dynamics of chemical systems with a single and a double step of heat release. The numerical simulations were performed for three peak pressures of 101.3, 1013.2, and 5066 kPa, for a range of compression ratios between 5 and 45, and various pulse widths, which determine the characteristics of the pressure pulse. Depending on the width of the Gaussian pulse, either ignition or chemical quenching were observed. Critical ignition, just at the limit between ignition and quenching, is the result of the competition between chemical reaction and volumetric expansion due to the imposed specific volume changes. This competition can be summarized by the concept of critical pressure pulse width, which is an extension of the critical decay rate concept. To further investigate the mechanisms responsible for critical ignition and quenching, quantitative thermo-chemical analyses were performed for sub-critical, critical and super-critical conditions. A Damköhler number was defined as the ratio of characteristic time of expansion to ignition delay time to characterize the competition between ignition and quenching. A simplified non-dimensional pressure pulse model for which the derivative of the naperian logarithm of density was assumed to follow a linear law was established. It can be employed to derive the critical conditions for which ignition fails. The evolution of non-dimensional temperature was obtained numerically and showed the existence of a critical pulse width for successful ignition and that the location of ignition delay time moves from the expansion phase to the compression phase as the non-dimensional pulse width is increased.

Low-temperature filamentary plasma for ignition-stabilized combustion

In this work, an experimental study investigating dielectric barrier discharge (DBD) plasma-assisted ignition in methane-air flows at near-atmospheric pressure is presented. Discharges are produced using 10 ns duration high voltage pulses at maximum 3 kHz pulse repetition rate. The goal is to check the feasibility of plasma as an ignition source for ignition-stabilized combustion in a wide range of equivalence ratio, pressure, and flow speed conditions. To that end, four important characteristics are investigated: 1) Ignition dynamics, 2) Minimum number of pulses required for ignition, 3) Plasma energy per pulse, gas temperature, and the effective reduced electric field, and 4) Plasma NOx production. In continuous plasma mode, we observe an elongated flame outside of the discharge region when a methane-air mixture flows through the discharge. The elongated flame is due to repetitively ignited kernels moving downstream with the flow. High-speed intensified imaging is used to explore plasma morphology and ignition dynamics. Plasma has a filamentary nature which is perceived as moving with the flow due to pulse-to-pulse memory effects. For fuel-air mixtures, ignition is followed by kernel expansion and splitting, flame propagation, and consecutive ignition. The time delay between consecutive ignition events is found to be a function of flow speed, because ignition is achieved only when a previously ignited kernel moves out of the plasma region, which happens faster at higher flow speeds. We performed optical emission spectroscopy in burst mode to further characterise plasma and ignition parameters. In air, plasma gas temperature and hence the reduced electric field increases because of pulse-to-pulse effects. By comparing the air plasma temperature with the methane auto-ignition temperature, we conclude that the observed ignition is low-temperature ignition. Parametric studies of NOx measurements suggest that plasma NOx emissions are higher for low flow speeds, high pulse repetition rates, and low-pressure conditions. Overall, our DBD filamentary plasma is found to be a fast, repetitive, and low-temperature ignition source that can be used for ignition-stabilized combustion at near atmospheric conditions.

Plasma Scalpels: Devices, Diagnostics, and Applications.

The plasma scalpel is an application of gas discharges in electrosurgery. This paper introduces the device structure and physicochemical parameters of the two types of plasma scalpels, namely, a single-electrode Ar discharge device (argon plasma coagulation) and a two-electrode discharge device in normal saline. The diagnostic methods, including the voltage and current characteristics, optical emission spectroscopy, electron spin resonance, and high-speed imaging, are introduced to determine the critical process parameters, such as the plasma power, the gas temperature, the electron density, and the density of active species, and study the ignition dynamics of the plasma discharges in water. The efficacy of the plasma scalpel is mainly based on the physical effects of the electric current and electric field, in addition to the chemical effects of high-density energetic electrons and reactive species. These two effects can be adjusted separately to increase the treatment efficacy of the plasma scalpel. Specific guidance on further improvements of the plasma scalpel devices is also provided.

A Most Reactive Mixture Analysis of Ozone- and Temperature-Enhanced n-Heptane Droplet Autoignition and Cool Flame Burning in Microgravity

ABSTRACT Numerical simulations are used to study effects of ozone (O3) and thermal enhancement on autoignition and subsequent burning characteristics of isolated n-heptane droplets in microgravity conditions. Particular focus is given to effects of O3 on low temperature combustion, or cool flames. Levels of ambient O3 mole fraction and ambient temperature are varied from 0 to 0.03 and 400 K to 750 K, respectively. A comparison between two low temperature chemistry enhancement strategies – O3 addition and temperature increase – are investigated on an equivalent energy addition basis to evaluate relative effects on combustion characteristics. Results of this study are analyzed using physical and mixture fraction approaches because of unique insights offered by these frameworks. Ignition and ensuing flame propagation are investigated via a most reactive mixture analysis, evaluated using homogeneous reactor simulations of mixture compositions and temperatures surrounding the droplet. Lastly, a Damköhler number analysis is performed to explore oscillatory ignition dynamics.

Hypergolic ignition of paraffin-based hybrid rocket fuels by sprays of liquid oxidizer

An experimental study is conducted to measure the ignition delay of ammonia borane doped paraffin wax with sprays of white fuming nitric acid as oxidizer. The injection pressure and injector diameter are varied to investigate the effects of spray characteristics on the ignition dynamics. A range of Weber (We) number for the oxidizer jet covering 100 < We <2700 and Reynolds (Re) number 1300 < Re <8900 is investigated, whereas the We number at the droplet scale varies from 0.8 < Wedrop <24.1. The sprays are characterized using phase-doppler anemometry to obtain the size and speed distribution of the oxidizer droplets across multiple positions. In addition to conventional high-speed imaging, the hypergolic ignition is also observed using Schlieren imaging and a high-speed mid-infrared camera. The results show that the ignition delay of a hypergolic hybrid fuel under spray conditions is unaffected by the mean droplet size of the oxidizer. The ignition delay decreases with increasing We number at the droplet scale and increasing droplet velocity. The variability in the ignition delay also decreases with Weber number and droplet velocity. The values obtained for the ignition delay when using a spray are generally higher than those from simple oxidizer drop experiments, highlighting the need to investigate the hypergolic ignition in configurations representative of engine conditions.

Simultaneous side-wall-schlieren and -emission imaging of autoignition phenomena in conventional and constrained-reaction-volume shock-tube experiments

Simultaneous schlieren and emission imaging through the side wall of a round shock tube is reported for application to experimental autoignition studies. A round, optically accessible shock tube featuring windows designed as aberration-corrected cylindrical lenses provides a large side-wall field of view compatible with both emission and schlieren imaging. Autoignition experiments are reported for non-dilute propane–oxygen–argon mixtures (ϕ=1 C3H8 in 21% O2, 79% Ar) at temperatures (T5) spanning the range 1,203–1,438 K and pressures near 1 atm. Experiments are performed using both conventional and constrained-reaction-volume (CRV) filling, allowing for the most detailed comparison to date of ignition dynamics between these two types of experiments. When T5 exceeds 1,290 K, strong ignition is observed to initiate in the immediate proximity of the end wall regardless of the filling strategy. In lower-T5 experiments, mild ignition is observed to initiate spontaneously at remote locations, which often exceeding the imaging FOV in conventional experiments but are constrained within about 5 cm of the end wall in CRV experiments. Measured ignition delay times (IDTs) are compared to predictions from three kinetic mechanisms. IDTs simulated using the San Diego and USC Mech II mechanisms are of the same magnitude as the measurements but exhibit a lower activation energy; the propane submechanism from NUIG 1.1, meanwhile, systematically over predicts observed IDTs by roughly a factor of two. Comparisons are also made between the experimental measurements and predictions from a recently introduced 1-D axial-temperature-gradient model describing remote ignition in shock tubes. Considering the effect of a supposed dT5/dz=150 K/m gradient, the 1-D model functionally predicts the remote ignition observed in T5< 1,290 K experiments and suggests the use of CRV filling may significantly reduce the error introduced into IDT measurements by remote ignition at low-T5 conditions.

Impact of chamber back pressure on the ignition dynamics of hydrogen enriched premixed flames

The impact of chamber back pressure and H2-enrichment on flashback of premixed CH4−H2-Air flames triggered by ignition dynamics is investigated. Different back pressures are imposed by varying the exhaust flow blockage with perforated plates of different porosities. Experiments measuring the ignition transient, from kernel formation to stable flames and flashback, with increasing levels of H2-enrichment covering a wide range of operating conditions are conducted. Time-series of pressure, velocity and heat release rate are examined to characterise the ignition process. It is shown that the final state of the flame is highly modified by the over pressure caused by the volumetric expansion of the hot gases. Three different scenarios are identified: Soft ignition with direct stabilisation, transient flashback and permanent flashback. The magnitude of the over pressure generated at ignition is significantly amplified by small increases in mean back pressure which, in turn, modifies the flowrate through the injector thereby promoting flashback. In addition to its role as a pressure amplifier, increasing the combustor back pressure also affects the time lag between the over pressure and the peak in heat release rate just after ignition. The impact of H2-enrichment on flashback is investigated by keeping both the bulk injection velocity Ub and laminar burning velocity Sl0 constant for flames with 0% to 100%H2-content. It is shown that the magnitude of the over pressure approximately scales with Ub/Sl0 and is independent of the total thermal power and H2 enrichment. Although the mean flow conditions and over pressures are kept constant, the stabilisation mode directly depends on H2-content. This shows that flashback triggered by the ignition dynamics cannot be fully described by classical kinematic arguments and a new mechanism is presented.

Effect of Strong Azimuthal Swirl on Ignition and Light-Around in an Annular Combustor

Abstract In this paper, we investigate the effect of strong azimuthal swirl on ignition dynamics in a laboratory-scale annular combustor. Bulk azimuthal swirl was produced by a novel angled injector configuration, producing swirling jet flames oriented downwards toward the combustor backplane and in the azimuthal direction, replicating a simplified version of the SAFRAN spinning combustor concept. To provide more realistic flow conditions, the design included rich-quench-lean (RQL) staging via a circumferential distribution of dilution ports and an effusion cooled combustor backplane. High-speed imaging and an azimuthal array of photomultipliers to measure OH* chemiluminescence were used to characterize the ignition dynamics for different injector velocities and global equivalence ratios. The mass flows through the injectors, dilution ports, and effusion cooled backplane were independently metered so that the injector equivalence ratio and global equivalence ratio could be separately controlled. The light-around times were found to have no clear correlation with the injector velocity since the rich injector equivalence ratio meant the flame burned in a nonpremixed mode even though the global equivalence ratio was lean due to the RQL staging. However, it was found that lower injector velocities extended the lean ignition limit based on the global equivalence ratio. The ignition sequence during light-around (order in which the injectors are ignited) was found to be highly repeatable, igniting each consecutive injector in the anticlockwise direction (the direction of bulk swirl). In rare cases, the ignition sequence was observed to branch in both directions. Finally, in an effort to extend the lean ignition limit, the effect of azimuthal staging was investigated. Two configurations were tested. In the first configuration, the injectors on one half of the annulus were operated at a fixed equivalence ratio whereas the other half of the annulus was operated at a different equivalence ratio. In the second configuration, every second injector had the same equivalence ratio. Both configurations extended the lean extinction limit but the first configuration was the most effective.

Smoldering ignition and emission dynamics of wood under low irradiation

AbstractWood is one of the longest‐standing and sustainable construction and building materials, and has gained a new renaissance for high‐rise buildings to achieve global carbon neutrality. However, wood can sustain both flaming and smoldering fires, and numerous timber structure fires have raised fire safety to be a public concern. This work investigates the smoldering ignition of wood blocks under long‐lasting low‐intensity irradiation and the robustness of smoldering fire after the removal of irradiation. We found a smoldering ignition map including three regimes, (i) no ignition, (ii) unsustained smoldering, and (iii) self‐sustained smoldering. The minimum irradiation for smoldering ignition of wood is about 5.5 kW/m2 after heating for hours. Without sufficient and in‐depth preheating, smoldering ignition cannot self‐sustain without irradiation. The criteria for self‐sustained smoldering on thick wood include the minimum surface temperature of 350 ± 20°C, the minimum smoldering front thickness of 30 ± 5 mm, and the minimum mass flux of 3.8 ± 0.4 g/m2 s before the irradiation is terminated. The CO/CO2 ratio of the smoldering wood under low irradiation varies between 0.1 and 0.2. This work helps evaluate the fire risk of wood materials and understand their burning behaviors under real fire scenarios.

Spontaneous Ignition of Cryo-Compressed Hydrogen in a T-Shaped Channel System

Sudden releases of pressurised hydrogen may spontaneously ignite by the so-called “diffusion ignition” mechanism. Several experimental and numerical studies have been performed on spontaneous ignition for compressed hydrogen at ambient temperature. However, there is no knowledge of the phenomenon for compressed hydrogen at cryogenic temperatures. The study aims to close this knowledge gap by performing numerical experiments using a computational fluid dynamics model, validated previously against experiments at atmospheric temperatures, to assess the effect of temperature decrease from ambient 300 K to cryogenic 80 K. The ignition dynamics is analysed for a T-shaped channel system. The cryo-compressed hydrogen is initially separated from the air in the T-shaped channel system by a burst disk (diaphragm). The inertia of the burst disk is accounted for in the simulations. The numerical experiments were carried out to determine the hydrogen storage pressure limit leading to spontaneous ignition in the configuration under investigation. It is found that the pressure limit for spontaneous ignition of the cryo-compressed hydrogen at temperature 80 K is 9.4 MPa. This is more than 3 times larger than pressure limit for spontaneous ignition of 2.9 MPa in the same setup at ambient temperature of 300 K.

The evolution and structure of ignited high-pressure cryogenic hydrogen jets

The anticipated upscaling of hydrogen energy applications will involve the storage and transport of hydrogen at cryogenic conditions. Understanding the potential hazard arising from leaks in high-pressure cryogenic storage is needed to improve hydrogen safety. The manuscript reports a series of numerical simulations with detailed chemistry for the transient evolution of ignited high-pressure cryogenic hydrogen jets. The study aims to gain insight of the ignition processes, flame structures and dynamics associated with the transient flame evolution. Numerical simulations were firstly conducted for an unignited jet released under the same cryogenic temperature of 80 K and pressure of 200 bar as the considered ignited jets. The predicted hydrogen concentrations were found to be in good agreement with the experimental measurements. The results informed the subsequent simulations of the ignited jets involving four different ignition locations. The predicted time series snapshots of temperature, hydrogen mass fraction and the flame index are analyzed to study the transient evolution and structure of the flame. The results show that a diffusion combustion layer is developed along the outer boundary of the jet and a side diffusion flame is formed for the near-field ignition. For the far-field ignition, an envelope flame is observed. The flame structure contains a diffusion flame on the outer edge and a premixed flame inside the jet. Due to the complex interactions between turbulence, fuel-air mixing at cryogenic temperature and chemical reactions, localized spontaneous ignition and transient flame extinguishment are observed. The predictions also captured the experimentally observed deflagration waves in the far-field ignited jets.

Experimental investigation on the ignition dynamics of an annular combustor with multiple centrally staged swirling burners

The ignition behaviors of an annular combustor consisting of 16 centrally staged swirling burners are experimentally investigated in this work. This research is mainly focused on the light-round mechanism of burner-burner flame propagation. The swirling flow structure of the staged burner and the flow interaction between multiple burners in the annular combustor are well measured via the particle image velocimetry method. Two high speed cameras are applied to analyze the light-round process from the side view and the top view. The light-round time, ignition and extinction limits, flame propagating pattern, and dynamics of flame leading point are analyzed. Increasing the equivalence ratio, the light-round time decreases gradually. A more complicated “sawtooth” pattern of flame propagation is discovered during the burner to burner flame propagation, compared to that with non-staged burners. The trajectories of the flame leading points are moving in a “zigzag” pattern during the light-round process. The trajectories of the anti-clockwise leading point are near the inside wall, while the trajectories of the clockwise one are closer to the outside wall. For various equivalence ratios and airflow rates, the circumferential flame speeds of the clockwise flame front are constantly faster than the anti-clockwise one. In addition, the two flame speeds and their differences increase with larger equivalence ratio. These characteristics are very different from those in an annular combustor with non-staged burners.

Ignition dynamics of radio frequency discharge in atmospheric pressure cascade glow discharge

A cascade glow discharge in atmospheric helium was excited by a microsecond voltage pulse and a pulse-modulated radio frequency (RF) voltage, in which the discharge ignition dynamics of the RF discharge burst was investigated experimentally. The spatio-temporal evolution of the discharge, the ignition time and optical emission intensities of plasma species of the RF discharge burst were investigated under different time intervals between the pulsed voltage and RF voltage in the experiment. The results show that by increasing the time interval between the pulsed discharge and RF discharge burst from 5 μs to 20 μs, the ignition time of the RF discharge burst is increased from 1.6 μs to 2.0 μs, and the discharge spatial profile of RF discharge in the ignition phase changes from a double-hump shape to a bell-shape. The light emission intensity at 706 nm and 777 nm at different time intervals indicates that the RF discharge burst ignition of the depends on the number of residual plasma species generated in the pulsed discharges.

A Review of Current Understanding of the Underlying Physics Governing the Interaction, Ignition and Combustion Dynamics of Multiple-Injections in Diesel Engines

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;This work is a comprehensive technical review of existing literature and a synthesis of current understanding of the governing physics behind the interaction of multiple fuel injections, ignition, and combustion behavior of multiple-injections in diesel engines. Multiple-injection is a widely adopted operating strategy applied in modern compression-ignition engines, which involves various combinations of small pre-injections and post-injections of fuel before and after the main injection and splitting the main injection into multiple smaller injections. This strategy has been conclusively shown to improve fuel economy in diesel engines while achieving simultaneous NO&lt;sub&gt;X&lt;/sub&gt;, soot, and combustion noise reduction - in addition to a reduction in the emissions of unburned hydrocarbons (UHC) and CO by preventing fuel wetting and flame quenching at the piston wall. Despite the widespread adoption and an extensive literature documenting the effects of multiple-injection strategies in engines, little is known about the complex interplay between the underlying flow physics and combustion chemistry involved in such flows, which ultimately governs the ignition and subsequent combustion processes thereby dictating the effectiveness of this strategy. In this work, we provide a comprehensive overview of the literature on the interaction between the jets in a multiple-injection event, the resulting mixture, and finally the ignition and combustion dynamics as a function of engine operational parameters including injection duration and dwell. The understanding of the underlying processes is facilitated by a new conceptual model of multiple-injection physics. We conclude by identifying the major remaining research questions that need to be addressed to refine and help achieve a design-level understanding to optimize advanced multiple-injection strategies that can lead to higher engine efficiency and lower emissions.&lt;/div&gt;&lt;/div&gt;

Computational study of the Premixed Charge Compression Ignition combustion in a Rapid Compression Expansion Machine: Impact of multiple injection strategy on mixing, ignition and combustion processes

Combustion processes operating under the low-temperature (LTC) are a promising alternative for internal combustion engines, achieving high efficiency and low emissions within the legislative framework. Under this field of study, the concept of Premixed Charge Compression Ignition (PCCI) was introduced to control the combustion phase by varying the injection strategy. The present research, based on the aforementioned combustion strategy, employs Computational Fluid Dynamics (CFD) to study the mixing, ignition and initial combustion processes in a Rapid Compression and Expansion Machine (RCEM). The operating conditions of the PCCI strategy are achieved through the split injection technique, which separate the injection into several shots. In this case, the injection strategy consists of a long base injection and two very short, consecutive post injections delivered when the piston is close to the top dead center. Three different operating conditions have been considered which differ mainly in the duration and the starting time of the first injection. Large Eddy Simulations (LES) are selected to account for the effects of turbulence. Results are validated against experimental data, showing good agreement between both approaches. The effect of the first injection on the second one in multiple injection strategy is appreciated as the penetration of the subsequent fuel sprays is faster. Differences in the local mixture field and ignition dynamics are observed as a function of varying operating conditions. The ignition process is highly influenced by the operating strategies affecting the distribution of the mixture and thus the location of the low temperature oxidation. In the current computational study, in addition to the evaporating fuel scalar, the corresponding source term due to evaporation has also been provided to three passive transported scalars, namely three mixture fractions, one for each injection event. This allows for discerning the fuel stemming from each one of the pulses in the overall distribution, thus offering new insights into the effect of the injection strategy.

The Period of Ignition Delay for Methane-Air Mixture with Hydrogen and Ethylene Additives

The article is devoted to estimating the intensifying efficiency of methane-air ignition by adding a small amount of hydrogen and/or ethylene. It presents features of the experimental determination of the ignition delay period for fuel-air mixtures using shock installation and methods of processing empirical data. The testing of the known ignition kinetic models for methane, hydrogen, and ethylene with air was carried out. The results of test calculations were compared with those previously published, as well as original experiments. The kinetic model was chosen to provide the minimum discrepancy between the calculated and experimental data. The regularities of the effect of hydrogen and ethylene additives on the ignition dynamics of the methane-air mixture for the range of initial pressures from 1 to 8 bar at temperatures from 900 to 1100 K were obtained with the use of non-stationary numerical modeling. Methane-air mixtures with the mass fraction of additives not exceeding 10% were studied. The quantitative indicators of possible reduction in the ignition delay period of methane-air mixtures were detected.

Ignition dynamics and combustion mode transitions in a rocket-based combined cycle combustor operating in the ramjet/scramjet mode

Cavity ignition was investigated in a rocket-based combined cycle combustor. The Mach number, total pressure, and total temperature of the inflow were 2.92, 2.60 MPa, and 1650 K, respectively. A backward-facing step was employed to replace the rocket engine that was shut down. The expansion fan and the reattachment shock wave resulting from the backward-facing step distorted the mainstream. The combustor was 40 mm in height (H) at the entrance. Three typical cases with d/H=2.0, 4.5, and 7.5 were reported in this study, where d was the distance between the backward-facing step and the cavity. Schlieren images and flame chemiluminescence images were captured to reveal the ignition processes. The ignition process in the case with d/H=2.0 was similar to the conventional cavity ignition because only the rear of the cavity was subjected to the expansion fan. Two combustion modes, namely, the local combustion mode and the cavity stabilized scramjet mode, were witnessed in this case. The first combustion mode was characterized by a small reaction zone in the fore part of the cavity. The primary reaction zone of the second mode was in the region near the cavity ramp. The rapid flame spreading in the cavity shear layer led to the transition between these two combustion modes. In the case with d/H=4.5, the reattachment shock wave enlarged the cavity recirculation flow. The expansion fan decreased the static pressure upstream of the cavity, which made the combustor more sensitive to backpressure. Although the first two combustion modes of this case were similar to those of the case with d/H=2.0, a new combustion mode (unsteady jet-wake stabilized scramjet mode) was observed in this case. The mode transition between the cavity stabilized scramjet mode and the unsteady jet-wake stabilized scramjet mode was attributed to the backpressure resulting from the heat release. The backpressure gradually pushed the reaction zone upstream. In the case with d/H=7.5, the cavity was not directly exposed to the expansion fan or the reattachment shock wave. However, the ignition process in this case was more complex. The combustor successively experienced the aforementioned three combustion modes and eventually settled into the ramjet mode. The mode transition between the last two modes was the result of thermal choking. The flame propagated upstream rapidly when the combustor was chocked.

On the flame stabilization of turbulent lifted hydrogen jet flames in heated coflows near the autoignition limit: A comparative DNS study

Three-dimensional direct numerical simulations of turbulent lifted hydrogen jet flames in heated coflows are performed with a detailed H2/air chemical mechanism to understand their ignition dynamics and stabilization mechanisms. Turbulent lifted jet flames with four different coflow temperatures, Tc, between 750 K and 1100 K are investigated by examining the instantaneous/time-averaged values and conditional means of heat release rate and species critical to ignition, and by performing a displacement speed analysis and a local combustion mode analysis with an indicator, α. Although Tc at 950 K is higher than the autoignition limit, the flame is primarily stabilized by flame propagation rather than autoignition, while at 1100 K, flame stabilization is found to be highly affected by autoignition. The local combustion mode analysis further reveals that at 950 K, even if a local ignition mode with |α|<1 first appears in the near field of the jet, it develops into a local extinction mode with α<−1 as local temperature decreases due to the excessive mixing of heated coflow and cold H2 within vortical structures, which inhibits the ignition kernel development upstream of the flamebase. At 1100 K, however, a local ignition mode prevails upstream of the flamebase. To further identify the effect of a vortex on the early development of an ignition kernel in a mixing layer between the heated coflow and cold H2, a series of two-dimensional DNSs are performed, varying several vortex parameters and air temperature, as a reference for the more complicated corresponding 3-D turbulent DNS cases. The results substantiate that the development of a vortex in the mixing layer tends to retard the autoignition within the vortex, especially when its temperature is slightly above the autoignition limit.

Influence of the diesel pilot injector configuration on ethanol combustion and performance of a heavy-duty direct injection engine

Thanks to its properties and production pathways, ethanol represents a valuable alternative to fossil fuels, with potential benefits in terms of CO2, NOx, and soot emission reduction. The resistance to autoignition of ethanol necessitates an ignition trigger in compression-ignition engines for heavy-duty applications, which in the current study is a diesel pilot injection. The simultaneous direct injection of pure ethanol as main fuel and diesel as pilot fuel through separate injectors is experimentally investigated in a heavy-duty single cylinder engine at a low and a high load point. The influence of the nozzle hole number and size of the diesel pilot injector on ethanol combustion and engine performance is evaluated based on an injection timing sweep using three diesel injector configurations. The tested configurations have the same geometric total nozzle area for one, two and four diesel sprays. The relative amount of ethanol injected is swept between 78 – 89% and 91 – 98% on an energy basis at low and high load, respectively. The results show that mixing-controlled combustion of ethanol is achieved with all tested diesel injector configurations and that the maximum combustion efficiency and variability levels are in line with conventional diesel combustion. The one-spray diesel injector is the most robust trigger for ethanol ignition, as it allows to limit combustion variability and to achieve higher combustion efficiencies compared to the other diesel injector configurations. However, the two- and four-spray diesel injectors lead to higher indicated efficiency levels. The observed difference in the ethanol ignition dynamics is evaluated and compared to conventional diesel combustion. The study broadens the knowledge on ethanol mixing-controlled combustion in heavy-duty engines at various operating conditions, providing the insight necessary for the optimization of the ethanol-diesel dual-injection system.

Flow dynamics of laser-induced breakdown at a fuel–oxidizer interface and its effect on ignition

Motivated by experimental observations, we analyze the ignition dynamics following a laser-induced breakdown near a distinct fuel–oxidizer interface using detailed simulations of the flow following a model energy deposition. Cases simulated with a detailed H2-O2 combustion model show the dependence of ignition time, and its success or failure, on the post-breakdown kernel geometry. For asymmetric kernels, ignition can occur by ejections of hot gas that propagate several times the kernel size. The extreme conditions induced near the laser focus lead to near-complete dissociation, allowing heat release by radical recombination to be a primary mechanism by which the ejected gas maintains ignition temperatures as it advects. For kernels close to ehe interface, a pronounced hydrodynamic interaction between the density gradient and expanding kernel results in ignition-suppressing flow that repels hot gas away so completely in some cases that ignition fails. Mechanisms for a wide range of parameters are studied with a four-species reduced combustion model, and we show that this ignition-adverse flow response, particularly salient in hydrogen–oxygen cases, is absent for heavier fuels. For disparate fuel and oxidizer molecular weights, the varied flow response due to the breakdown location is a primary determinant of outcome.