Chemical Kinetic Simulations Research Articles

Fundamental Study on Hydrogen Low-NOx Combustion Using Exhaust Gas Self-Recirculation

Hydrogen is expected to be a next-generation energy source that does not emit carbon dioxide, but when used as a fuel, the issue is the increase in the amount of NOx that is caused by the increase in flame temperature. In this study, we experimentally investigated NOx emissions rate when hydrogen was burned in a hydrocarbon gas burner, which is used in a wide temperature range. As a result of the experiments, the amount of NOx when burning hydrogen in a nozzle mixed burner was twice as high as when burning city gas. However, by increasing the flow velocity of the combustion air, the amount of NOx could be reduced. In addition, by reducing the number of combustion air nozzles rather than decreasing the diameter of the air nozzles, a larger recirculation flow could be formed into the furnace, and the amount of NOx could be reduced by up to 51%. Furthermore, the amount of exhaust gas recirculation was estimated from the reduction rate of NOx, and the validity was confirmed by the relationship between adiabatic flame temperature and NOx calculated from the equilibrium calculation by chemical kinetics simulator software.

The spray ignition characteristics of ethanol blended with hydro-processed renewable diesel in a constant volume combustion chamber

In this study, the ignition characteristics of ethanol blended with hydro-processed renewable diesel (HRD) in a constant volume combustion chamber were conducted in both experiments and zero-dimensional (0-D) chemical kinetic simulations. The experimental results revealed that ignition delay (ID) increased with increasing ethanol concentration. The emissions of carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), and hydrocarbon (HC) under lean and stoichiometric conditions were studied at a chamber pressure (Pchamb) of 7 bar. It was found that CO2 emissions reduced with increasing ethanol concentration. However, opposite trend was observed for NOx emissions. Since ethanol has a lower heating value than HRD fuel, the combustion temperature was reduced with the use of the HRD-ethanol blend, which subsequently resulted in lower NOx formation. The addition of ethanol also increased the oxygen concentration which aided in promoting further oxidization of CO to form CO2, thereby reducing the amount of CO produced. Besides, the effect of the addition of ethanol in HRD on HC emission was also observed whereby increasing its ratio increased the oxygen concentration in fuel which had the advantage of reducing HC emission. A kinetic model of 144 species and 600 reactions for the ethanol blended with HRD was also formulated. The kinetic model reasonably reproduced the measured ID periods at initial pressures of 10–20 bar, temperatures of 600–818 K and equivalence ratios of 0.13–0.354, despite quantitative disagreement with the experiment at the lower temperatures. Additionally, the measured trend where the ID periods elongated with respect to the increase in the ethanol concentration was replicated by the kinetic model.

A numerical methodology based on CFD and plasma chemical kinetics simulations: a focus on the cyano radical

A numerical methodology based on CFD and plasma chemical kinetics simulations: a focus on the cyano radical

Optical diagnostics and chemical kinetic analysis on the dual-fuel combustion of methanol and high reactivity fuels

Dual-fuel combustion is an effective method to utilize methanol because of its high octane number and latent heat of evaporation. Two ways to use methanol are discussed. One is port-injected high reactivity fuel and directed-injected methanol, named as active-thermal atmosphere combustion (ATAC). The other is port-injected methanol and direct-injected high reactivity fuel, which is reactivity-controlled compression ignition (RCCI). In the current work, both methanol combustion modes were investigated in an optical engine. Several optically diagnostic methods were utilized to characterize flame features, including flame natural luminosity imaging, OH* natural luminosity imaging and two-color method by wavelength integration. Meantime, the chemical kinetics simulation was also combined to investigate the effects of active atmosphere and thermal atmosphere on methanol combustion. Results show that as for methanol ATAC, when the methanol is direct-injected during the low temperature heat release (LTHR) stage of n-heptane, it causes significant effects on the combustion phasing of the following high temperature heat release (HTHR). When the methanol is direct-injected during the HTHR stage of n-heptane, the single-peak HTHR turns to the double-peak HTHR. Besides, the yellow diffusion flame of methanol can also be observed. Affected by the in-cylinder vortex, the area and intensity of the OH* signal in the vortex squeeze zone are greater than those in the vortex stretch zone. The flame temperature of methanol maintains in 1800–2500 K at CA50 (the crank angle of 50% mixtures is burned completely), while the maximum KL factor maintains in 0.04–0.05. The thermal atmosphere created by the n-heptane combustion has the dominant effects on shortening the ignition delay of methanol. The effects of active atmosphere are limited. As for methanol/high reactivity fuel RCCI, because of the existence of homogeneous low reactivity methanol, the misfire condition occurs when the direct injection (DI) timing is late. With the DI timing of high reactivity fuel advancing, the combustion stability improves. But the flame development progress is mainly low-speed flame propagation, which results in long combustion duration. The combustion characteristics can be further improved by utilizing higher reactivity fuel, such as n-dodecane, specifically, the combustion phasing advances and the number of auto-ignition spots increases.

Laser-based detection of methane and soot during entrained-flow biomass gasification

Methane is one of the main gas species produced during biomass gasification and may be a desired or undesired product. Syngas CH4 concentrations are typically >5 vol-% (when desired) and 1–3 vol-% even when efforts are made to minimize it, while thermochemical equilibrium calculations (TEC) predict complete CH4 decomposition. How CH4 is generated and sustained in the reactor core is not well understood. To investigate this, accurate quantification of the CH4 concentration during the process is a necessary first step. We present results from rapid in situ measurements of CH4, soot volume fraction, H2O and gas temperature in the reactor core of an atmospheric entrained-flow biomass gasifier, obtained using tunable diode laser absorption spectroscopy (TDLAS) in the near-infrared (1.4 µm) and mid-infrared (3.1 µm) region. An 80/20 wt% mixture of forest residues and wheat straw was converted using oxygen-enriched air (O2>21 vol%) as oxidizer, while the global air-fuel equivalence ratio (AFR) was set to values between 0.3 and 0.7. Combustion at AFR 1.3 was performed as a reference. The results show that the CH4 concentration increased from 1 to 3 vol-% with decreasing AFR, and strongly correlated with soot production. In general, the TDLAS measurements are in good agreement with extractive diagnostics at the reactor outlet and TEC under fuel-lean conditions, but deviate significantly for lower AFR. Detailed 0D chemical reaction kinetics simulations suggest that the CH4 produced in the upper part of the reactor at temperatures >1700 K was fully decomposed, while the CH4 in the final syngas originated from the pyrolysis of fuel particles at temperatures below 1400 K in the lower section of the reactor core. It is shown that the process efficiency was significantly reduced due to the C and H atoms bound in methane and soot.

Development of an infrared laser absorption sensor for non-intrusive gas temperature measurements

Energetic materials have extremely high volumetric and specific energy densities, making them attractive and important in combustion systems. To improve their combustion performance, reliable temperature acquirement method is highly demanded. A laser sensor was developed for in situ and quantitative measurements of gas temperature. Scanned-wavelength direct absorption spectroscopy was used for line-of-sight temperature measurements. Multiple H2O absorption features in the near-infrared combination band (v1+v3) and mid-infrared fundamental band (v3) were selected to establish four absorption line pairs with good temperature sensitivity. Three infrared distributed feedback lasers (DFB) were used to cover the selected absorption lines. The accuracy and uncertainty of the sensor were first numerically evaluated in a wide temperature range of 1000–3000 ​K under different Gaussian white noise levels (5–20%). A free-space optical setup was established to experimentally evaluate the sensor performance by measuring benchmark laminar premixed flames, which were compared with additional thermocouple measurements, chemical kinetic modeling and computational fluid dynamics simulations. The good performance of the current sensor indicates the potential of being used in non-intrusive, in-situ and quantitative diagnostics of the energetic materials combustion.

Experimental and kinetic modeling studies of polyoxymethylene dimethyl ether (PODE) pyrolysis in jet stirred reactor

Polyoxymethylene dimethyl ethers (DMMn) is regarded as a potential alternative fuel for transportation utilization. The jet stirred reactor (JSR) pyrolysis of DMM1−3 was studied at temperatures of 450∼1080 K and pressures of 1.03 atm with the residence time (τ) of 2 s. A gas chromatography/mass spectrometry (GC–MS/FID) was used to detect and provide reliable pyrolysis experimental data. Seven key pyrolysis products were detected and measured in the present work, including fuel, methyl formate, methanol, acetaldehyde, methane, ethane and ethylene. A detailed kinetic model of DMM1−3 pyrolysis was elaborated. Both rate of production (ROP) and sensitivity analysis were performed to reveal the DMM1−3 pyrolysis processes. Results show that the chemical kinetic simulation can reproduce the consumption of fuel and most intermediate species reasonably well. With the increase of n from 1 to 3 inside DMM1−3, the reactivity increases evidently and the initial pyrolysis temperature decreases significantly, which is mainly resulted from different fuel-specific chain initiation reactions of DMM1−3. Methane and methyl formate are two detected key species with relatively significant concentrations. It was revealed that the fuel-specific chain initiation reactions of DMM1−3 can also significantly affect the production and consumption paths of methyl formate. In addition to the major production and consumption paths of formaldehyde, two key competitive channels were found to perform relatively high negative sensitivity to formaldehyde production of DMM2 and DMM3.

A comparative study of gasoline skeletal mechanisms under partial fuel stratification conditions using large eddy simulations

Partial fuel stratification (PFS) is a low temperature combustion strategy that can alleviate high heat release rates of traditional low temperature combustion strategies by introducing compositional stratification in the combustion chamber using a split fuel injection strategy. In this study, a three-dimensional computational fluid dynamics (CFD) model with large eddy simulations and reduced detailed chemistry was used to model partial fuel stratification at three different stratified conditions. The double direct injection strategy injects 80% of the total fuel mass at −300 CAD aTDC and the remaining 20% of the fuel mass is injected at three different timings of −160, −50, −35 CAD to create low, medium, and high levels of compositional stratification, respectively. The PFS simulations were validated using experiments performed at Sandia National Laboratories on a single-cylinder research engine that operates on RD5-87, a research-grade E10 gasoline. The objective of this study is to compare the performance of three different reduced chemical kinetic mechanisms, namely SKM1, SKM2, and SKM3, at the three compositional stratification levels and identify the most suitable mechanism to reproduce the experimental data. Zero-dimensional chemical kinetic simulations were also performed to further understand differences in performance of the three reduced chemical kinetic mechanisms to explain variations in CFD derived heat release profiles. The modeling results indicate that SKM3 is the most suitable mechanism for partial fuel stratification modeling of research-grade gasoline. The results also show that the autoignition event progresses from the richer to the leaner compositional regions in the combustion chamber. Notably, the leaner regions that have less mass per unit volume, can contribute disproportionately more toward heat release as there are more cells at leaner equivalence ratio ranges. Overall, this study illuminates the underlying compositional stratification phenomena that control the heat release process in PFS combustion.

Two-stage aerosol formation in low-temperature combustion

Low-temperature combustion is a promising strategy for reducing pollutant formation in internal combustion engines. However, there is a lack of understanding of how the chemistry governing the differences in ignition between low-temperature and conventional combustion affects the emission rates and physicochemical properties of particulate matter (aerosols). Here, we conducted combustion experiments in an atmospheric-pressure reactor controlled at constant equivalence ratio (ϕ = 2.3) and O2/N2 = 0.06, and at temperatures varied between 250 °C and 1035 °C. We used two fuels: toluene, which has high sooting propensity, and n-heptane, which has a comparatively lower sooting propensity but exhibits two-stage ignition that is not present in toluene combustion. We performed real-time measurements of aerosol size distributions, volatility, and light-absorption properties. We also performed offline molecular-size characterization. Aerosols emitted from both fuels were comprised of light-absorbing organics that are categorized as brown carbon. At the highest combustion temperature (1035 °C), the aerosol emissions from toluene combustion were a factor of 20 larger than n-heptane. The aerosol emissions from toluene combustion had more abundance of large molecular-size species, were less volatile, and were more light-absorbing than n-heptane. For both fuels, aerosol emission factors exhibited a steep drop with decreasing temperatures. However, there was a resurgence in aerosol emissions at lower temperatures with a peak at 290 °C for n-heptane combustion that was not observed for toluene. This is consistent with chemical kinetics simulations that show prominent two-stage ignition behavior for n-heptane, but not for toluene.

Chemical effects of anisole and toluene addition to n‑heptane on PAH characteristics in laminar premixed flames by LIF measurement and kinetic model

Anisole is a candidate renewable fuel that displays satisfying combustion characteristics, but its sooting characteristics are not well known. The goal of this study is to investigate the chemical effects of anisole and toluene on PAH formation in n-heptane laminar premixed flames, using LIF measurement and chemical kinetic simulation. To focus on chemical effects, the equivalence ratios, dilution ratios, and flame temperatures were kept nearly unchanged when anisole and toluene were blended separately into n-heptane flame. LIF experimental results indicated that PAH formation was promoted with the addition of anisole and toluene, and the effect of toluene was stronger than the promotional effect of anisole. The chemical kinetic model predicted the observed PAH tendencies well in the LIF experiments. Based on this model, reaction pathway and sensitivity analyses were performed to interpret the chemical effects. Results revealed that, due to their different molecular structures, the difference in chemical effects between anisole and toluene was notable in PAH growth processes. Anisole decomposed first via the O-CH3 bond dissociation reaction, and then proceeded to a CO elimination reaction to yield an important PAH precursor, C5H5, which contributed to PAH formation via styrene and indene reaction networks. In the toluene added flame, PAH formation was enhanced because of ring expansion reaction on the dehydrogenated branch of the toluene.

Chemical kinetic mechanisms and scaling of two-dimensional polymers via irreversible solution-phase reactions.

Two-dimensional (2D) polymers are extended networks of multi-functional repeating units that are covalently linked together but confined to a single plane. The past decade has witnessed a surge in interest and effort toward producing and utilizing 2D polymers. However, facile synthesis schemes suitable for mass production are yet to be realized. In addition, unifying theories to describe the 2D polymerization process, such as those for linear polymers, have not yet been established. Herein, we perform a chemical kinetic simulation to study the recent synthesis of 2D polymers in homogeneous solution with irreversible chemistry. We show that reaction sites for polymerization in 2D always scale unfavorably compared to 3D, growing as molecular weight to the 1/2 power vs 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane vs rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 106. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for the 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory the feasibility of producing two-dimensional polymers from irreversible polymerization in solution.

Chemical kinetics and density measurements of OH in an atmospheric pressure He + O2 + H2O radiofrequency plasma

This work presents experiments and modelling of OH densities in a radio-frequency driven atmospheric-pressure plasma in a plane-parallel geometry, operated in helium with small admixtures of oxygen and water vapour (He + O2 + H2O). The density of OH is measured under a wide range of conditions by absorption spectroscopy, using an ultra-stable laser-driven broad-band light source. These measurements are compared with 0D plasma chemical kinetics simulations adapted for high levels of O2 (1%). Without O2 admixture, the measured density of OH increases from 1.0 × 1014 to 4.0 × 1014 cm−3 for H2O admixtures from 0.05% to 1%. The density of atomic oxygen is about 1 × 1013 cm−3 and grows with humidity content. With O2 admixture, the OH density stays relatively constant, showing only a small maximum at 0.1% O2. The simulations predict that the atomic oxygen density is strongly increased by O2 addition. It reaches ∼1015 cm−3 without humidity, but is limited to ∼1014 cm−3 beyond 0.05% water content. The addition of O2 has a weak effect on the OH density because, while atomic oxygen becomes a dominant precursor for the formation of OH, it makes a nearly equal contribution to the loss processes of OH. The small increase in the density of OH with the addition of O2 is instead due to reaction pathways involving increased production of HO2 and O3. The simulations show that the densities of OH, O and O3 can be tailored relatively independently over a wide range of conditions. The densities of O and O3 are strongly affected by the presence of small quantities (0.05%) of water vapour, but further water addition has little effect. Therefore, a greater range and control of the reactive species mix from the plasma can be obtained by the use of well-controlled multiple gas admixtures, instead of relying on ambient air mixing.

Valine involved sulfuric acid-dimethylamine ternary homogeneous nucleation and its atmospheric implications

New Particle Formation (NPF) is considered as the dominant source of aerosol particles, influencing the environment and human health tightly. In recent years, valine (VAL) has been identified in field detection in urban areas, which reveals that VAL may play an important role in participating in atmospheric nucleation. However, there have been no categorical reports explaining how VAL participates in the multi-component nucleation with precursor Sulfuric Acid (SA) and the atmospheric organic amine Dimethylamine (DMA) so far. In this paper, quantum chemical calculation and cluster kinetic simulation have been studied to research the nucleation mechanism of (SA)x(DMA)y(VAL)z (0≤x, y, z ≤ 3) system. Structural and thermodynamic analysis at ωB97XD/6-31++G(d, p) theory level and basis set shows that VAL could form various stable structures through hydrogen bonds with SA and DMA, and proton transfer in ternary structure enhances the stability of system. However, in the clusters of SA-DMA-VAL system with large numbers of molecules, there has been steric hindrance between molecules of cluster due to the high proportion of inactive groups in VAL, which insulates the further formation of hydrogen bonds. With regard to the distribution of Gibbs free energy on VAL-DMA grid, the energy decreases rapidly along the diagonal with VAL and DMA in different numbers of SA molecules, predicting possible nucleation path along the diagonal. Besides, there is a synergistic effect of VAL in system SA-DMA-VAL, and VAL tends to replace the role of DMA in the ternary system. Evaporation rate analysis indicates that the ternary system containing SA is more stable. The simulating formation rate of SA-DMA-VAL system compared with the CLOUD experiment at CERN shows that the nucleation potential of SA-DMA-VAL is superior to that of SA-NH3-H2O, but inferior to that of SA-DMA-H2O, inferring that VAL may show more potential in growth processes than the initial nucleation processes in the light of the specific value of formation rate.

Quantitative investigation on the impact of injection timing on soot formation in a GDI engine with a customized sectional method

Soot engine-out emissions are no longer a prerogative of Diesel engines. Emission regulations related to Gasoline units aim to curb the soot emissions along with other pollutants. In this scenario, Computational Fluid Dynamics (CFD) is a very promising research and development tool to explore the influence of engine design and operational parameters, as well as of the fuel chemical nature, on the particulate matter formation. Among the soot models, the Sectional Method is an advanced resource to provide information on Particle Number, Particulate Mass and Particle Size Distribution. In this study, the Sectional Method is applied in conjunction with a customized soot library, where the source terms governing the soot sections transport equations are stored. The library is computed via chemical kinetics simulation of a 0D constant pressure reactor, which provides fuel-related coefficients for each individual source term over the entire range of conditions experienced by the 3D-CFD model. 3D-CFD simulations are then carried out for three different injection timings without case-by-case tuning. Numerical results are then compared to the experimental dataset by using a consistent methodology. A satisfactory agreement between 3D-CFD results and experimental measurements is reached for soot mass and particle numbers, while the particle size distribution function is only partially reproduced. Soot-related quantities are thoroughly analyzed for each of the examined injection strategies to understand the mechanisms leading to soot formation and emissions.

Numerical Investigation of the Combined Influence of Three-Plug Arrangement and Slot Positioning on Wankel Engine Performance

A numerical methodology for three-dimensional fluid dynamics and chemical kinetics simulation of the combustion and gas-exchange processes in the Wankel engine was developed and validated. Two approaches of performance enhancement were studied—the addition of a slot in the rear side of the rotor recess, and installation of a third plug in the trailing side of the working chamber, in addition to the two available plugs mounted in the leading side of the baseline engine. The obtained results showed that the suggested three-plug arrangement significantly improves the engine performance. Furthermore, positioning the trailing plug further from the passage between the trailing and leading sides is of preference for higher mean in-chamber pressures. Nevertheless, for maximum performance, the distance should be brought to an optimum as during the intake stroke there is a loss of inducted charge due to backflow from the trailing plug hole. For the three-plug arrangement the presence of a slot is necessary for the prevention of early flame quenching in the trailing side, while keeping the added volume to a minimum. Moreover, positioning the slot and the trailing plug off-center, results in higher flow intensity towards the leading plugs, and accordingly, to a higher combustion efficiency. For dual-plug ignition system (two plugs in the leading side) it is preferable to maintain minimum clearance in the trailing side.

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH3 with H2/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH3/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH3/air flame with various H2/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H2 has the greatest effects on increasing laminar burning velocities and net heat release rates of NH3/air flame, followed by SYN and CO. CO has the most significant effects on improving NH3/air adiabatic flame temperatures. The H2/CO/SYN additions can accelerate NH3 decomposition rates and promote the generation of H and NH2 radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH2 radicals. The reaction NH2 + NH <=> N2H2 + H and NH2 + NO <=> NNH + OH have remarkable positive effects on NH3 combustion. The mole fraction of OH × NH2 radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H2/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH3/air flame. The NH3 reaction pathways for NH3–H2/CO/SYN-air flames can be categorized mainly into NH3–NH2–NH–N–N2, NH3–NH2–HNO–NO(−N2O)–N2 and NH3–NH2(−N2H2)–NNH–N2. CO has the greatest influence on the proportions of three NH3 reaction routes.

Investigations on the effects of low temperature reforming of n-heptane/n-butanol blends on the flame development progress and combustion chemical kinetics

The effects of low temperature reforming (LTR) of n-heptane/n-butanol blends on the flame development progress and combustion characteristic were investigated in an optical engine. The reforming temperatures were set as 423 K, 523 K and 623 K. Three n-heptane/n-butanol blending mixtures with volume fractions of n-butanol of 30% (B30), 50% (B50) and 70% (B70) were utilized. The detection, experiment and simulation methods included the gas chromatograph (GC) detection, high-speed imaging and chemical kinetics calculation. The main results are the following. Firstly, the LTR products were initially detected by the GC system. With the reforming temperature increasing, the mole fractions of aldehydes decrease. With the volume fraction of n-butanol increasing, the mole fractions of carbon monoxide, methanol and aldehydes increase while other species decrease. Secondly, the LTR products combined with the direct-injected fuel were introduced into the optical engine. For B30 and B50, the combustion phasing delays with the reforming temperature increasing. For B70, the combustion phasing initially advances and then delays with the reforming temperature increasing. Thirdly, the chemical kinetics simulation shows that most of the LTR products can decrease the mixture reactivity and delay the ignition timing, while small parts of the LTR products have the opposite effects.

A Numerical Exploration of Engine Combustion Using Toluene Reference Fuel and Hydrogen Mixtures

Hydrogen-fueled internal combustion engines (H2ICEs) are capable of operating over a wide range of equivalence ratios: from ultra-lean mode to stoichiometric conditions. However, they provide maximum thermal efficiency and minimum NOx emissions if operated lean. Although NOx is produced, H2ICEs generate little or no CO, CO2, SO2, HC, or PM emissions. The main limitation to pure hydrogen fueling is power density. To overcome such an issue, mixtures of gasoline and hydrogen can be exploited, with small modifications to the engine feeding system. Due to the peculiar characteristics of hydrogen (in terms of thermophysical properties, molecular weight and propagating flame characteristics) care must be adopted when trying to address combustion using computational fluid dynamics (CFD) tools. In this work, we simulate the combustion of mixtures of toluene reference fuel (TRF) and hydrogen under largely different ratios. To simplify the problem, liquid and gaseous injections are neglected, and a premixed mixture at the inlet of the CFD domain is imposed. Due to the different laminar flame speeds of the mixture components, mass-fraction weighted in-house correlations based on chemical kinetics simulations are adopted. Outcomes are compared with those obtained using standard correlations and mixing rules available in most commercial CFD packages.

Verification of diesel spray ignition phenomenon in dual-fuel diesel-piloted premixed natural gas engine

Dual-fuel (DF) engines, in which premixed natural gas and air in an open-type combustion chamber is ignited by diesel-fuel pilot sprays, have been more popular for marine use than pre-chamber spark ignition (PCSI) engines because of their superior durability. However, control of ignition and combustion in DF engines is more difficult than in PCSI engines. In this context, this study focuses on the ignition stability of n-heptane pilot-fuel jets injected into a compressed premixed charge of natural gas and air at low-load conditions. To aid understanding of the experimental data, chemical-kinetics simulations were carried out in a simplified engine-environment that provided insight into the chemical effects of methane (CH4) on pilot-fuel ignition. The simulations reveal that CH4has an effect on both stages of n-heptane autoignition: the small, first-stage, cool-flame-type, low-temperature ignition (LTI) and the larger, second-stage, high-temperature ignition (HTI). As the ratio of pilot-fuel to CH4entrained into the spray decreases, the initial oxidization of CH4consumes the OH radicals produced by pilot-fuel decomposition during LTI, thereby inhibiting its progression to HTI. Using imaging diagnostics, the spatial and temporal progression of LTI and HTI in DF combustion are measured in a heavy-duty optical engine, and the imaging data are analyzed to understand the cause of severe fluctuations in ignition timing and combustion completeness at low-load conditions. Images of cool-flame and hydroxyl radical (OH*) chemiluminescence serve as indicators of LTI and HTI, respectively. The cycle-to-cycle and spatial variation in ignition extracted from the imaging data are used as key metrics of comparison. The imaging data indicate that the local concentration of the pilot-fuel and the richness of the surrounding natural-gas air mixture are important for LTI and HTI, but in different ways. In particular, higher injection pressures and shorter injection durations increase the mixing rate, leading to lower concentrations of pilot-fuel more quickly, which can inhibit HTI even as LTI remains relatively robust. Decreasing the injection pressure from 80 MPa to 40 MPa and increasing the injection duration from 500 µs to 760 µs maintained constant pilot-fuel mass, while promoting robust transition from LTI to HTI by effectively slowing the mixing rate. This allows enough residence time for the OH radicals, produced by the two-stage ignition chemistry of the pilot-fuel, to accelerate the transition from LTI to HTI before being consumed by CH4oxidation. Thus from a practical perspective, for a premixed natural gas fuel–air equivalence-ratio, it is possible to improve the “stability” of the combustion process by solely manipulating the pilot-fuel injection parameters while maintaining constant mass of injected pilot-fuel. This allows for tailoring mixing trajectories to offset changes in fuel ignition chemistry, so as to promote a robust transition from LTI to HTI by changing the balance between the local concentration of the pilot-fuel and richness of the premixed natural gas and air. This could prove to be a valuable tool for combustion design to improve fuel efficiency or reduce noise or perhaps even reduce heat-transfer losses by locating early combustion away from in-cylinder walls.

Effects of ethane addition on diesel-methane dual-fuel combustion in a heavy-duty engine

The present study is a continuation of the previous work by Ahmad et al. (2020), in which ethane (C2H6) enriched diesel-methane (CH4) dual-fuel (DF) combustion was experimentally investigated in a single-cylinder heavy-duty engine. Here, the experiments of ethane enriched DF combustion are carried out with new details together with supporting zero-dimensional (0D) and one-dimensional (1D) chemical kinetics simulations. Three port-fuel injected (PFI) gaseous blends of pure methane with varying ethane concentrations of 0%, 10%, and 20% are used as the main fuels. The PFI gaseous blend provides 97% of the total-fuel energy (TFE), which is ignited by a small 3% (TFE based) pilot diesel. Experiments are performed under lean condition (∅gas = 0.52) for two engine speeds while keeping the TFE and other operating conditions constant. Calculated results from 0D and 1D simulations under engine relevant conditions including theoretical combustion mode analysis (β-curve) are used to deepen the phenomenological understanding of the experimental results. The results reveal that adding ethane into pure methane has minor effects on the pilot-diesel ignition timing. However, ethane addition greatly enhances the ignitability of methane after the start of combustion. Ethane enriched gaseous blends yield higher thermal efficiency and reduce combustion duration compared to pure methane. According to combustion mode analysis, ethane tendency to promote spontaneous autoignition may be one of the reasons for improving overall combustion performance. It is observed that ethane enriched gaseous blends produce lower unburned methane (UB-CH4) and unburned hydrocarbons (THC) accompanied with higher nitrogen oxides (NOx) because of the higher combustion efficiency. Furthermore, ethane addition considerably helps to reduce cycle-to-cycle variations under lean conditions compared to pure methane.