Burning Limit Research Articles

Hydrogen addition to a commercial self-aspirating burner and assessment of a practical burner modification strategy to improve performance

The ability for existing burners to operate safely and efficiently on hydrogen-blended fuels is a primary concern for the many industries looking to adopt hydrogen as an alternative fuel. This study investigates the efficacy of increasing fuel injector diameter as a simple modification strategy to extend the hydrogen-blending limits before flashback. The collateral effects of this modification are quantified with respect to a set of key performance criteria. The results show that the unmodified burner can sustain up to 50 vol% hydrogen addition before flashback. Increasing the fuel injector diameter reduces primary aeration, allowing for stable operation on up to 100% hydrogen. The flame length, visibility and radiant heat transfer properties are all increased as a result of the reduced air entrainment with a trade-off reported for NOx emissions, where, in addition to the effects of hydrogen, reducing air entrainment further increases NOx emissions.

Optical Study on Spark Plug Gap in Extending Methane Lean Combustion Limits under High Ignition Energy Conditions

<div>Lean combustion has the potential to achieve high thermal efficiency for internal combustion engines. However, natural gas (NG) engines often suffer from slow burning rates and large cyclic variations when adopting lean combustion. In this study, the effects of spark plug gaps (SPGs) on methane lean combustion are optically investigated under high ignition energy conditions. Synchronization measurements of in-cylinder pressure and high-speed photography are performed for combustion analysis. The results show that large SPGs with high ignition energy exhibit great improvement in engine combustion stability and power capability. Under ultra-lean conditions, a large SPG with a high ignition energy of 150–200 mJ can extend the lean limit to 1.55. Combustion images indicate that this is contributed by the enlarged initial flame kernel, which promotes early flame propagation. Besides, an empirical criterion is adopted to quantify the underlying mechanism, and the results confirm that a more stable early flame development with a faster burning rate can be obtained by a larger SPG and higher ignition energy under lean conditions. Therefore, a large SPG is an effective way to improve combustion stability and thermal efficiency for NG engines.</div>

Investigation on the performance of a by-product hydrogen-fueled spark-ignition engine with Miller cycle under various lean conditions

The direct combustion of by-product hydrogen is a promising way for reducing costs and greenhouse emissions. Coke oven gas (COG) which is mainly composed of H2, CH4 and CO is a major source of by-product hydrogen that is suitable to be adopted by internal combustion engines. This paper investigated the COG combustion in a spark-ignition engine with Miller cycle. To get an overview of by-product hydrogen combustion properties, two kinds of COG compositions with the maximum and minimum hydrogen fractions were tested. The excess air ratio (λ) was raised from stoichiometric to the lean limit to explore the capability of COG on improving the engine performance under a common-driving speed of 1500 rpm and part load conditions. The test results indicated that the engine fueled by COG with higher hydrogen fractions could gain concentrated heat release period, extended lean burn limit, reduced HC and CO2 emissions, and enhanced working capability only at ultra-lean conditions. The properly increased λ availed improving engine efficiency and lowering HC emissions. The peak indicated thermal efficiency of 38.6% was acquired at the λ of 1.2, while the lowest HC emissions of only 32 ppm was gained at the λ of 1.4. Under ultra lean conditions, NO emission dropped to near zero, and CO from the by-product hydrogen-fueled engine was negligible for all tested ranges. An interesting trend of peak pressure correlated crank angle versus λ was also found in this study.

Preparation and performance characterization of functionalized boron-based energetic-microcapsules with uniform size

Energetic metal and metalloid nano-powders are highly promising as high-energy fuel or energetic additive of propellants. Specifically, boron has extremely high gravimetric and volumetric calorific values. However, the difficulty of ignition, slow combustion and uncomplete energy release limit its application. Herein, we developed an effective strategy to fabricate the functionalized energetic-microcapsules (nB@M−CS) with uniform size (with coefficient of variance lower than 3%) using droplet microfluidic technology, in which the boron nanoparticles (nBs) are uniformly encapsulated in chitosan (CS), with organometallic catalyst (M = Mo, Fe and Co) homogeneously dispersed inside. During combustion, nB@M−CS can overcome the agglomeration of nBs by generating micro-explosion to release and ignite the inner nBs, and the low loading (<2 wt%) and high dispersion of metal catalyst can shorten the mass transfer distance and enhance the contact area between nBs and M catalyst. These can significantly reduce the thermal oxidation temperature and accelerate the combustion rate. Specifically, nB@Mo-CS shows much lower initial oxidation temperature (393 °C) than the previously reported B particles, and exhibits the combustion heat and maximum pressurization rate increased by 114.3% and 7.3 times, respectively, compared with nBs. This work shows the great potential for designing and synthesizing functionalized propellant using microfluidic technology.

Lazarus Stars: Numerical investigations of stellar evolution with star-lifting as a life extension strategy

Abstract The aging and gradual brightening of the Sun will challenge Earth’s habitability in the next few billion years. If life exists elsewhere in the Universe, the aging of its host star similarly poses an existential threat. One solution, which we dub a Lazarus star, is for an advanced civilization to remove (or star-lift) mass from their host star at a rate that offsets the increase in luminosity, keeping the flux on the habitable planet(s) constant and extending the lifetime of their star. While this idea has existed since 1985 when it was first proposed by Criswell, numerical investigations of star-lifting have been lacking. Here, we use the stellar evolution code MESA to find mass vs. age and $\dot{M}$vs. age relations which would hold the flux on surrounding planets constant. We explore initial mass ranging from 0.2 M⊙ to 1.2 M⊙. For most stars with a mass initially below about 0.4M⊙, we find that star-lifting increases their main-sequence lifetimes up to 500 Gyr until they approach the hydrogen burning limit and star-lifting is no longer possible. For more massive stars, star-lifting increase main-sequence lifetimes by 1 Gyr to 100 Gyr, though they still enter the red-giant phase. For example, a Sun-like star has a main-sequence lifetime which can be increased by up to 3 Gyr. This requires a mass-loss rate of about 0.05 MCeres per year. We compare star-lifting to other survival strategies and briefly discuss methods for detecting these engineered stars.

DMPP-3: confirmation of short-period S-type planet(s) in a compact eccentric binary star system, and warnings about long-period RV planet detections

ABSTRACT We present additional HARPS radial velocity observations of the highly eccentric (e ∼ 0.6) binary system DMPP-3AB, which comprises a K0V primary and a low-mass companion at the hydrogen burning limit. The binary has a 507 d orbital period and a 1.2 au semimajor axis. The primary component harbours a known 2.2 M⊕ planet, DMPP-3A b, with a 6.67-d orbit. New HARPS measurements constrain periastron passage for the binary orbit and add further integrity to previously derived solutions for both companion and planet orbits. Gaia astrometry independently confirms the binary orbit and establishes the inclination of the binary is 63.89 ± 0.78°. We performed dynamical simulations that establish that the previously identified ∼800 d RV signal cannot be attributed to an orbiting body. The additional observations, a deviation from strict periodicity, and our new analyses of activity indicators suggest the ∼800 d signal is caused by stellar activity. We conclude that there may be long-period planet ‘detections’ in other systems, which are similar misinterpreted stellar activity artefacts. Without the unusual eccentric binary companion to the planet-hosting star, we could have accepted the ∼800 d signal as a probable planet. Further monitoring of DMPP-3 will reveal which signatures can be used to most efficiently identify these imposters. We also report a threshold detection (0.2 per cent FAP) of a ∼2.26 d periodicity in the RVs, potentially attributed to an Earth-mass S-type planet interior to DMPP-3A b.

Enhancing ammonia combustion with minimum hydrogen blended in presence of self-excited intermittent pulsating oscillations

In this work, we propose and test a partial premixed fuel injection design of NH3−H2−O2 with double ring-shaped inlets to enhance ammonia combustion in an open-ended combustor by generating and sustaining pulsating combustion oscillations. Emphasis is being placed on determining the minimum amount of hydrogen being blended with ammonia in the presence of such self-excited pulsating oscillations. With the numerical model validated by comparing with experimental and theoretical data, we identify and systemically investigate three key thermodynamic parameters. They are shown to strongly affect the thermal, combustion, and emission performances. These parameters include the following: (1) total fuel mass flow rate ṁf; (2) mass fraction of hydrogen ω̇H2; and (3) the temperature TH of a heat exchanger implemented downstream of the combustor. It is interesting to observe that intermittent pulsating oscillations are sustained by such ammonia–hydrogen combustion. Furthermore, comparison is conducted between the present results and those with the classical single ring-shaped fuel inlet under the same flow and operating conditions. It is found that the exothermic heat of the proposed double-ring inlets is increased by 98.7% on average. The frequency of such intermittent oscillations is shown to increase with the decreased NH3 proportion. When pure hydrogen is supplied and passing through the outer ring inlet, the combustion limit can be greatly expanded, even if the inlet mass fraction of hydrogen is very small. The minimum hydrogen blended with ammonia is shown to be 0.1% to achieve a sustainable combustion and large-amplitude oscillations. The NO emission is found to be decreased, and H2O is shown to increase. The present study open ups an approach to enhance ammonia combustion by improving its flammability limit with the minimal hydrogen blended.

Experimental investigation of coal particle size on the kinetic properties of coal oxidation and spontaneous combustion limit parameters

Investigating the behavior of coal spontaneous combustion (CSC) is of great significance to the prevention of coal mine fires. and the dangerous area. In this paper, we firstly analyzed the CO concentration variation at different coal sample particle sizes based on the programmed heating experiments, and initially determined the critical temperature (TC) and xerochasy temperature (TX), the two key characteristic temperatures in the CSC process. Afterwards, the three oxidation stages of the coal spontaneous combustion process were divided as per TC and TX, and the apparent activation energy of the corresponding stages was obtained. Conclusively, the variation patterns of the coal spontaneous combustion limit parameters (minimum float coal thickness hmin, upper limit air leakage intensity Qmax and lower limit oxygen concentration cmin) were analyzed. It was found that the smaller the particle size, the greater the CO concentration, oxygen consumption rate and exothermic intensity of the coal samples. An exponential function was fitted between the exothermic intensity and the coal temperature. The apparent activation energy of the oxidation kinetic parameter increases as the coal spontaneously combusts, and is positively correlated with the particle size. This is mainly due to the fact that the larger the particle size, the smaller the contact area between the coal and oxygen and the corresponding reduction in the active material in the coal, resulting in a greater apparent activation energy required for the coal oxygen reaction. Grain size plays a non-negligible role in the spontaneous combustion limit parameters of coal. The smaller the particle size, the smaller the minimum floating coal thickness and the lower oxygen concentration limit, and the greater the upper air leakage intensity. The relationship between coal spontaneous combustion limit parameters and grain size is not only a quadratic function, but also an exponential function. The smaller the grain size of the relict coal in the mining area, the greater the risk of coal spontaneous combustion. The content of this study can provide some support for predicting and preventing the risk of coal spontaneous combustion.

Explosion and vented explosion behaviors of low-concentration gas in large-scale pipes

The extraction, transmission, and use of low-concentration gas in coal mines are often challenged by potential accidents due to proximity to the combustion and explosion concentration limit. Water sealing of fire barriers and explosion venting devices guarantee the safe transmission of low-concentration gas. The propagation and venting of gas explosion under different concentrations are experimentally investigated in a DN500 mm pipe explosion propagation test system. The water sealing of fire barrier vented explosion field evolution of methane-air mixture at the stoichiometric concentration (9.5 vol%) is examined through Fluent numerical simulation to reveal the water sealing of fire barrier further vented explosion pattern of gas transmission pipelines under low concentrations. The explosion overpressure and flame propagation of low-concentration gas in pipes are observed and correlated. Based on their coupling relationship, the design basis for the installation location of the explosion venting device is derived. The vented explosion characteristics of low-concentration gas during pipe transmission are summarized, proving that this explosion venting technique enhances both the explosion venting device and explosion parameters. Our observations and analysis can offer useful clues to underground gas explosion and prevention in coal mines.

Investigation Effects of Pre-chamber Volume Variations on the Performance of a Natural Gas Lean-Burn Engine

&lt;div&gt;The pre-chamber ignition system accelerates combustion efficiently by supplying multiple ignition points, high ignition energy, and strong turbulent disturbance. This system expands the lean combustion limit and improves combustion stability on natural gas engines. This work studied the effects of pre-chamber volume variations on combustion, performance, and emission behaviors of a natural gas lean-burn engine under experimental and numerical methods. Results show an increase in the pre-chamber volume from 0.3% to 4.4% of compression volume can increase the in-cylinder pressure in single-stage combustion. The energy and exergy efficiency of the engine Model-1.3% increased up to 43.7% and 41.9%, respectively, which are the highest values among the prepared models. Simultaneously, the model heat loss with the maximum pre-chamber volume was two times higher than the minimum pre-chamber volume. The exhaust gas exergy of Model-1.3% and Model-2.1% are the lowest values, approximately 29% of the fuel exergy. Increasing the volume of the pre-chamber from 1.3% of compression volume led to a reduction in emissions. An increase in the pre-chamber volume increases the exit velocity of gases from the pre-chamber bores, and turbulence in the entrance, exit, and inside the bores, which generates uniform and isothermal heat across the cylinder. Therefore, a pre-chamber with a compression volume below 5% stabilizes the combustion. Under stable working conditions, a pre-chamber volume can be defined such that the spark ignition (SI) engine would have the highest energy and exergy efficiencies with the lowest emission.&lt;/div&gt;

AF Lep b: The lowest-mass planet detected by coupling astrometric and direct imaging data

Aims. Using the direct-imaging technique, we searched for low-mass companions around the star AF Lep, which presents a significant proper-motion anomaly (PMa) signal obtained from the comparison of HIPPARCOS and Gaia eDR3 catalogs. Methods. We observed AF Lep in two epochs with VLT/SPHERE using its subsystems IFS and IRDIS in the near-infrared, covering wavelengths ranging from the Y to the K spectral bands (between 0.95 and 2.3 μm). We then reduced the data using the high-contrast imaging techniques angular differential imaging (ADI) and spectral differential imaging in order to be able to retrieve the signal from low-mass companions of the star. Results. A faint companion was retrieved at a separation of ~0.335″ from the star and with a position angle of ~70.5° in the first epoch and with a similar position in the second epoch. This corresponds to a projected separation of ~9 au. The extracted photometry allowed us to estimate a mass for the companion of between 2 and 5 MJup. This mass is in good agreement with astrometric measurements of the dynamic mass of the companion, which give 5.2–5.5 MJup. This is the first companion with a mass well below the deuterium burning limit that was discovered by coupling direct imaging with PMa measurements. Orbital fitting done using the orvara tool allowed us to further confirm the companion mass and to define its main orbital parameters.

Formation of products upon ignition, combustion and melting of mixtures of high-entropy alloy FeNiCoCrCu with titanium and carbon

The dependence of the ignition temperature, combustion rate and composition of the resulting products on the concentration of Ti + C in mixtures with powder of a high-entropy alloy (HEA) FeNiCoCrCu and the initial mixture of metals forming it (MIX) has been studied. HEA was obtained by mechanical activation (MA) of a mixture of metal powders in argon. At the melting temperature, the high-entropy FeNiCoCrCu alloy decomposes into several phases, but the basis of the HEA alloy, as well as the alloy obtained by melting and crystallizing MIX, is a 5-component phase with an average formula Cu1.2Fe1.4Ni1.4Co1.4Cr. In addition, 5, 4, and 3-component phases with averaged formulas Cu2Ni2Co2Fe2Cr, Cu3Ni3Co2.9Fe2.5Cr, Cu4.8Ni4.5Co4.6Fe4.2Cr, Cu40Fe2Ni4Co2C, Cr12.5Fe3.2Co2.6Ni and Co3.2 Fe3,5 Cr are present in small amounts in the binder. Experiments on the ignition and combustion of mixtures of MIX and HEA with Ti + C were carried out in argon at atmospheric pressure. The combustion rate, ignition temperature, and maximum temperature reached in the thermal explosion of MIX and HEA mixtures with Ti + C increase with increasing Ti + C concentration. Due to the low exothermicity of the mixtures, the experiments were carried out at an initial temperature of 500 °С. At this initial temperature, the combustion limit of the samples occurs when the Ti + C concentration in the HEA and MIX mixtures is less than 30 %. Based on the results of scanning electron microscopy, the volume concentration of the number of titanium carbide (TiC) particles in molten samples was calculated. In an alloy with a HEA binder, the number of TiC particles per unit volume is 1.5-3.0 times greater than in an alloy with a MIX binder, and their size is correspondingly smaller. With an increase in the concentration of Ti + C from 30 to 40 % in a mixture with HEA, the number of TiC particles per unit volume decreases. In a mixture with MIX, the number of TiC particles per unit volume passes through a minimum. This is due to two opposite processes: on the one hand, the probability of the generation of TiC particles increases, on the other hand, their coagulation occurs.

NICMOS Kernel-phase Interferometry. II. Demographics of Nearby Brown Dwarfs

Star formation theories have struggled to reproduce binary brown dwarf population demographics (e.g., frequency, separation, and mass ratio). Kernel-phase interferometry is sensitive to companions at separations inaccessible to classical imaging, enabling tests of these theories at new physical scales below the hydrogen burning limit. We analyze the detections and sensitivity limits from our previous kernel-phase analysis of archival HST/NICMOS surveys of field brown dwarfs. After estimating physical properties of the 105 late-M to T dwarfs using Gaia distances and evolutionary models, we use a Bayesian framework to compare these results to a model companion population defined by log-normal separation and power-law mass-ratio distributions. When correcting for Malmquist bias, we find a companion fraction of and a separation distribution centered at au, smaller and tighter than seen in previous studies. We also find a mass-ratio power-law index that strongly favors equal-mass systems: depending on the assumed age of the field population (0.9–3.1 Gyr). We attribute the change in values to our use of kernel-phase interferometry, which enables us to resolve the peak of the semimajor axis distribution with significant sensitivity to low-mass companions. We confirm the previously seen trends of decreasing binary fraction with decreasing mass and a strong preference for tight and equal-mass systems in the field-age substellar regime; only % of systems are wider than 20 au and % of systems have a mass ratio q < 0.6. We attribute this to turbulent fragmentation setting the initial conditions followed by a brief period of dynamical evolution, removing the widest and lowest-mass companions, before the birth cluster dissolves.

Experimental investigation of methane content on the oxidation characteristics and spontaneous combustion limit parameters of coal

ABSTRACT The methane contained in a coal seam is closely related to the oxidative properties of the coal and serves as an important basis for accurately evaluating the coal spontaneous combustion (CSC) behavior. In this paper, programmed temperature rise experiments were conducted on three coal samples with different methane contents. The sensitivity of oxidation products, olefin products, and their production rates to methane content were investigated. Also, the contribution made by methane content to the characteristic temperature point and CSC limit parameters were qualitatively assessed. When the methane content is 2.7 cm3/g, 4.5 cm3/g, and 6.8 cm3/g, the critical temperatures (CT) are 70°C, 80°C, and 100°C, respectively, and the dry cracking temperatures (DCT) are 100°C, 110°C, and 130°C, respectively. The higher the methane content, the greater the CT and DCT points during coal oxidation, which is because high concentration of methane will inhibit coal oxygen chemical reaction. The methane content is positively correlated with the minimum floating coal thickness (h min) and the lower limit oxygen concentration (C min), while the upper limit air leakage intensity (Q max) decreases with the increasing methane content. Coal samples with high methane content can withstand higher safe float thicknesses and oxygen concentrations and require less air leakage to carry away the heat accumulated by the residual coal in the mining area, making the CSC risk lower.

Dependence of polyethylene combustion dynamics in a 1 m&lt;sup&gt;3&lt;/sup&gt; chamber on particle size

Introduction. The results of a standard study on the explosion hazard of polyethylene air suspensions (PES) can contribute to the theory of turbulent combustion. For example, analysis of polydispersity data and values of the PES lean combustion limit in a 1 m3 chamber helped to identify the maximum size of explosive particles d*m,t ≈ 100 µm (Poletaev, 2014). In this work, a relationship was obtained between the dynamics of PES combustion in a 1 m3 chamber and the average particle size of the suspension, which is understood as the average particle size of its explosive fraction d*50.Initial data. Well-known findings of a study on the explosion of 28 polyethylene specimens in a 1 m3 chamber were used. Continuous functions of specimen particles distribution by size, necessary for calculating d*50, were represented using the Rosin-Rammler distribution.Combustion dynamics. The dynamics of PES turbulent combustion in a 1 m3 chamber is described by the maximum rate of air suspension burnout Ub. Ub was calculated according to the formula (Kumar, 1992) intended for gas-air mixtures by substituting PES explosion parameters into this formula.Results and its discussion. The graph, describing the dependence of the complex d*50Ub on d*50, is provided. The averaged value of the complex (≈ 45 µm · (m/s)) is constant in the range 40 µm &lt; d*50 &lt; 90 µm. The latter is typical for the product of the particle size and the normal velocity of laminar flame in liquid aerosols (Myers, 1986), which indicates similarity between the effect of particle dispersion and dynamics of turbulent and laminar combustion of the aforementioned heterogeneous mixtures.Conclusions. The dispersive capacity of an explosive polydisperse polyethylene specimen is determined by the average particle size of the explosive fraction of the specimen d*50. The similarity of combustion patterns indicates the proximity of propagation mechanisms typical for turbulent flame, typical for PES, and laminar flame, typical for liquid aerosols.

High Compression Ratio Active Pre-chamber Single-Cylinder Gasoline Engine with 50% Gross Indicated Thermal Efficiency.

Active pre-chamber turbulent jet ignition with a high compression ratio has been demonstrated to be an effective method for significantly enhancing engine thermal efficiency. A dual modification of the combustion chamber and the pre-chamber was performed on an AVL 5400 single-cylinder Miller engine to achieve stable ultra-lean burn at a high compression ratio, and a breakthrough of 51.10% gross indicated thermal efficiency was achieved at the compression ratio of 16.4 and λ = 2.236. Spark ignition and pre-chamber turbulent jet ignition exhibit significant performance diversities under lean burn conditions. Pre-chamber turbulent jet ignition is able to significantly expand the lean burn limit of spark ignition to λ = 2.7 (CoVIMEP < 5%) at only the expense of an increased HC emission, while apparently reducing fuel consumption and nitrogen oxide emissions. With an increase in the compression ratio from 13.6 to16.4, spark ignition and pre-chamber turbulent jet ignition exhibit contradictory performance laws. The engine performance of a spark ignition engine decreases significantly as the compression ratio increases, whereas a pre-chamber jet ignition engine can still operate reliably at a high compression ratio with ultra-lean combustion. Within the scope of the test, the performance of the pre-chamber jet ignition engine is enhanced by a greater compression ratio. This improvement is primarily attributable to the reduction of heat transfer loss and exhaust energy loss under ultra-lean combustion, as determined by an analysis of the structure of power losses.

Effects of trifluoroiodomethane and pentafluoroethane on combustion characteristics of flammable refrigerant propane

For the environmentally friendly refrigerant R290, the flammability is the biggest concern for its applications. In this paper, the inhibition of trifluoroiodomethane (R13I1) and pentafluoroethane (R125) on laminar burning velocity (LBV), lower flammable limit (LFL) and heat of combustion (HOC) of R290 have been researched. The laminar burning velocities were obtained at elevated temperature by an externally heated diverging channel method, and the lower flammable limits were measured in a glass tube under different humidity conditions. R13I1 showed a greater inhibition effect than R125 on the flammability of R290. The LBV decreased exponentially and the LFL increased markedly with addition of R13I1. The computation of LBV for R290/R13I1 mixtures showed an average relative error within 7.4%. The inhibition effectiveness of R13I1 on the LFL was reduced in wet air, mainly by the chemical effect of water molecules. And the linear relationship was found at LFL between calculated adiabatic flame temperature and the proportion of R13I1. The model was proposed to calculate the LFL and demonstrated a good prediction accuracy of 3.2% for LFL of R290/R13I1 mixtures. This study set out to investigate the inhibition of R13I1 and R125 on the flammability of R290 and assess the flammability of R290/R13I1 mixtures to provide low GWP and less flammable alternative refrigerants in air conditioners or heat pumps.

Flow and Combustion Characteristics of Wave Rotor–Trapped Vortex Combustor System

Breaking through the limit of conventional compression and combustion, wave rotor and trapped vortex combustors are able to improve the thermal efficiency of gas turbines. Detailed two-dimensional numerical simulations based on Ansys Fluent were performed to study the flow and combustion characteristics of the wave rotor–trapped vortex combustor system. The calculated pressure characteristics agree with the experimental results giving a relative error for average pressure of 0.189% at Port 2 and of 0.672% at Port 4. The flow stratification characteristics and the periodic fluctuations were found to benefit the zonal organized combustion in the trapped vortex combustor. For the six cases of different rotor speeds, as the rotor speed increased, the oxygen mass fraction at the combustor inlet rose and then fell. The proportion of exhaust gas recirculation fell at first and then rose, and the combustion mode became unstable with the dominant frequencies of the fluctuations increasing.

Numerical research of the in-cylinder natural gas stratification in a natural gas-diesel dual-fuel marine engine

The urgent need for decarbonization and emission reduction for marine engines is driving the development of alternative low-carbon fuels. One of the best alternative fuels for engines is natural gas, a clean energy source with vast reserves and a low price. However, low-pressure injection dual-fuel engines exhibit poor combustion characteristics at low engine loads, and extinction is likely to occur in the fuel mixture's dilution region. Thus, the Low Pressure Post Injection (LPPI) strategy was proposed in this paper. The major objective of the LPPI mode is to increase monocirculation combustion stability under low to moderate engine loads by achieving a reasonable distribution of NG in the combustion chamber. LPPI was compared with the Low Pressure Injection (LPI) strategy. Results indicate that the LPPI mode could successfully raise the swirl ratio in the cylinder upto 60.7 percent while perfectly avoiding the NG leakage phenomena. Additionally, with the aid of radial-wall distribution of NG and an improved swirl ratio in LPPI mode, the combustion duration is reduced by 33.6 percent, and the lean burn limit of the dual-fuel marine engine can be expanded to 0.30. Although local higher combustion temperature caused an large increase in NOx emission which is more than three times than LPI NOx emission, LPPI mode still meets Tier III NOx emission requirements.

A methodology to initialize tumble flow fields for fast 3D-CFD simulations of pent-roof SI engines

The demand of powertrain technologies able to efficiently employ several non-fossil fuels types, as bio or synthetic ones, compels to develop simplified strategies to accelerate the engines industrial optimization. High-tumble ultra-lean SI engines are currently an attractive option, thanks to their fuel flexibility and the potential of extending the lean combustion limit with novel ignition strategies. This work presents a methodology to initialize tumble flow fields at closed-valves inside pent-roof SI engines, without the need of simulating the gas exchange process. First, assuming an elliptical-shaped vortex, a velocity field is initialized into the cylinder according to the target tumble ratio at inlet valve closing (IVC). Then, the flow field is evolved at fixed piston position for a time duration proportional to the integral time scale of turbulence. This process is essential to adapt the vortex morphology to the surrounding geometrical details, despite some kinetic energy is lost due to frictions. To compensate this last effect, the velocity field is re-scaled at the end of the process to match the initial tumble ratio. Finally, the parameters affecting the tumble initialization are calibrated by comparing the development of the flow-field during the compression stroke with the results from a full-cycle simulation in a pent-roof SI engine. The methodology is validated on the Darmstadt optical engine. First, the numerical velocity fields are compared against PIV measurements at different crank-angles. Then, the combustion prediction is also evaluated, to assess the quality of the initialized tumble flow field. The achieved results are satisfactory, demonstrating the industrial applicability of the presented methodology.