Two-Stage Ignition and NTC Phenomenon in Diesel Engines

Two-stage ignition and NTC phenomenon in diesel sprays is investigated by performing 3-D two-phase reacting flow simulations in a dual-fuel engine. Spray processes modeled include fuel atomization, droplet distortion, droplet drag, turbulent dispersion, droplet interactions in terms of collision and coalescence, vaporization, and spray-wall interaction. A validated reaction mechanism is implemented in the CFD solver, which has previously been validated for both evaporating and reacting sprays. For single-fuel cases, the effect of temperature on two-stage ignition is examined by varying the start of injection (SOI). While results indicate global similarities between the two-stage ignition processes in diesel sprays and spatially homogeneous mixtures, there are also noticeable differences between them due to temporally and spatially evolving temperature and species fields in the spray case. For instance, both the first- and second-stage ignition delays are higher for the spray cases compared to homogeneous mixtures. Second, while ignition delay for homogeneous mixtures exhibits a NTC region, that for sprays indicate a ZTC region. Moreover, the first- and second-stage ignitions for the spray occur over a wide φ range and at multiple locations in the spray, implying a spatially wide ignition kernel. Additionally, while the chemical ignition delays are strongly influenced by the injection timing, the physical delays are essentially independent of this parameter. Results with dual fuel indicate that the two-stage ignition behavior remains intact even at high molar fractions of methane. The addition of methane increases ignition delays for both sprays and homogeneous mixtures, and can be attributed to the reduction in O2 and the chemical effect of methane. The sensitivity analysis indicated that the chemical effect is primarily due to reaction CH4 + OH= CH3 + H2O

Effect of Nozzle Orifice Geometry on Spray, Combustion, and Emission Characteristics under Diesel Engine Conditions

Diesel engine performance and emissions are strongly coupled with fuel atomization and spray processes, which in turn are strongly influenced by injector flow dynamics. Modern engines employ micro-orifices with different orifice designs. It is critical to characterize the effects of various designs on engine performance and emissions. In this study, a recently developed primary breakup model (KH-ACT), which accounts for the effects of cavitation and turbulence generated inside the injector nozzle is incorporated into a CFD software CONVERGE for comprehensive engine simulations. The effects of orifice geometry on inner nozzle flow, spray, and combustion processes are examined by coupling the injector flow and spray simulations. Results indicate that conicity and hydrogrinding reduce cavitation and turbulence inside the nozzle orifice, which slows down primary breakup, increasing spray penetration, and reducing dispersion. Consequently, with conical and hydroground nozzles, the vaporization rate and fuel air mixing are reduced, and ignition occurs further downstream. The flame lift-off lengths are the highest and lowest for the hydroground and conical nozzles, respectively. This can be related to the rate of fuel injection, which is higher for the hydroground nozzle, leading to richer mixtures and lower flame base speeds. A modified flame index is employed to resolve the flame structure, which indicates a dual combustion mode. For the conical nozzle, the relative role of rich premixed combustion is enhanced and that of diffusion combustion reduced compared to the other two nozzles. In contrast, for the hydroground nozzle, the role of rich premixed combustion is reduced and that of non-premixed combustion is enhanced. Consequently, the amount of soot produced is the highest for the conical nozzle, while the amount of NOx produced is the highest for the hydroground nozzle, indicating the classical tradeoff between them

Liftoff and blowout characteristics of laminar syngas nonpremixed flames

Liftoff and blowout behavior of nonpremixed syngas flames is investigated using a time-accurate CFD code with a detailed description of transport and chemistry. Lifted flames are established in coflowing laminar jets using N2 dilution in the fuel jet. Results focus on the effects of syngas composition and temperature on the liftoff, stabilization, and the edge (triple) flame structure. For a given syngas mixture, as the N2 dilution exceeds certain value, the flame lifts off from the burner rim and propagates along the stoichiometric mixture fraction line, and its structure changes from diffusion to double flame. With further dilution, the flame liftoff height increases rapidly, the base structure transitions from double to triple flame, and its stabilization involves a balance between the triple flame speed and local flow velocity. The temporal evolution of propagating jet flame also exhibits a similar behavior. The transition from diffusion to double and then to triple flame is examined using state relationships in mixture fraction coordinate. As H2 fraction in syngas and/or temperature is increased, the N2 dilution required for flame liftoff and blowout increases. The ratio of the triple flame speed to the unstretched premixed flame speed also increases with the increase in H2 fraction. For H2 fraction above 30%, the flame liftoff and blowout become less sensitive to syngas composition and temperature

Fuel Unsaturation Effects on NOx and PAH Formation in Spray Flames.

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine 6 conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the 7 detailed CRECK mechanism2. The reduced mechanism and spray models are validated against the shock tube 8 ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network [26]. 9 3-D simulations are performed using the CONVERGE software to examine the structure and emission 10 characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate 11 that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich 12 premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further 13 downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in 14 the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in 15 the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a 16 double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane 17 flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene 18 flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species 19 can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to scission 20 reactions resulting from the presence of double bond in 1-heptene

Fuel Unsaturation Effects on NOx and PAH Formation in Spray Flames

The effect of fuel unsaturation on NOx and PAH formation in spray flames is investigated at diesel engine conditions. The directed relation graph methodology is used to develop a reduced mechanism starting from the detailed CRECK mechanism . The reduced mechanism and spray models are validated against the shock tube ignition data and high-fidelity, non-reacting and reacting spray data from the Engine Combustion Network [26]. 3-D simulations are performed using the CONVERGE software to examine the structure and emission characteristics of n-heptane and 1-heptene spray flames in a constant-volume combustion vessel. Results indicate that the combustion under diesel engine conditions is characterized by a double-flame structure with a rich premixed reaction zone (RPZ) near the flame stabilization region and a non-premixed reaction zone (NPZ) further downstream. Most of NOx is formed via thermal NO route in the NPZ, while PAH species are mainly formed in the RPZ. A small amount of NO is also formed via prompt route in the RPZ, and via N2O intermediate route in the region outside NPZ, and via NNH intermediate route in the region between RPZ and NPZ. The presence of a double bond leads to higher flame temperature and thus higher NO in 1-heptene flame than that in n-heptane flame. It also leads to the increased formation of PAH species, implying increased soot emission in 1-heptene flame than that in n-heptane flame. Reaction path analysis indicate that the increased formation of PAH species can be attributed to the significantly higher amounts of 1,3-butadiene and allene formed due to scission reactions resulting from the presence of double bond in 1-heptene

Effects of Fuel Reactivity and Injection Timing on Diesel Engine Combustion and Emissions

Recent strategies for simultaneously reducing NOx and soot emissions have focused on achieving nearly premixed, lowerature combustion (LTC) in diesel engines. A promising approach in this regard is to vary fuel reactivity in order to control the ignition delay and optimize the level of premixing and reduce emissions. The present study examines such a strategy by performing 3-D simulations in a single-cylinder of a diesel engine. Simulations employ the state-of-the-art two-phase models and a validated semi-detailed reaction mechanism. The fuel reactivity is varied by using a blend of n-heptane and iso-octane, which represent surrogates for gasoline and diesel fuels, respectively. Results indicate that the fuel reactivity strongly influences ignition delay and combustion phasing, whereas the start of injection (SOI) affects combustion phasing. As fuel reactivity is reduced, the ignition delay is increased and the combustion phasing is retarded. The longer ignition delay provides additional time for mixing, and reduces equivalence ratio stratification. Consequently, the premixed combustion is enhanced relative to diffusion combustion, and thus the soot emission is reduced. NOx emission is also reduced due to reduced diffusion combustion and lower peak temperatures caused by delayed combustion phasing. An operability range is observed in terms of fuel reactivity and SOI, beyond which the mixture may not be sufficiently well mixed, or compression ignited. The study demonstrates the possibility of finding an optimum range of fuel reactivity, SOI, and EGR for significantly reducing engine out emissions for a given load and speed

Evaluation of Chemical-Kinetics Models for n-Heptane Combustion Using a Multidimensional CFD Code

Computational fluid dynamics (CFD)-based predictions are presented for nonpremixed and partially premixed flames burning vaporized n-heptane fuel. Three state-of-the-art chemical kinetics models are incorporated into a time-dependent, two-dimensional, CFD model known as UNICORN. The first mechanism is the San Diego (SD) mechanism (52 species and 544 reactions), the second one is the Lawrence Livermore National Laboratory (LLNL) mechanism (160 species and 1540 reactions), and the third one is the National Institute of Standards and Technology (NIST) mechanism (197 species and 2926 reactions). Soot model based on acetylene, and radiation model based on optically thin media assumption are included. Twodimensional calculations are made for the detailed structures of nonpremixed and partially premixed flames, strain-induced extinction and diffusion-controlled autoignition and the results are compared with the available experimental data. Diffusion-controlled autoignition characteristics are also compared with the ignition delay times calculated in homogeneous stoichiometric mixture of n-heptane and air. Through the simulation of complete flowfields between the opposing fuel and air ducts reasons for the flame curvature seen in some experiments are explained. Compared to the traditional one-dimensional models for opposing-jet flames, two-dimensional simulations are found to give results closer to the experimental values when the flames are highly stretched. While LLNL mechanism predicted extinction of a nonpremixed flame better, NIST mechanism predicted the autoignition behavior in the flowfield established by the opposing jets of fuel and heated air better. However, all three mechanisms predicted both the nonpremixed and partially premixed n-heptane flames very well. Surprisingly, SD mechanism with less than one-third of the species used in the other two mechanisms predicted flame structures with nearly the same accuracy. Comparisons made with the available experimental data could not suggest which mechanism is better in predicting the minor species concentrations. Computations also could not predict the temperature rise detected in the experiments in the premixed-combustion zone of a partially premixed flame when it was subjected to a moderately high stretch rate

Fuel and diluent property effects during wet compression of a fuel aerosol under RCM conditions

Wet compression of fuel aerosols has been proposed as a means of creating gas-phase mixtures of involatile diesel-representative fuels and oxidizer + diluent gases for rapid compression machine (RCM) experiments. The intent of this study is to investigate the effects of fuel and diluent gas properties on the wet compression process, specifically to: (a) explore a range of fuels which could have applicability in aerosol RCM experiments, and (b) fundamentally understand how fuel and diluent gas properties affect the wet compression process and assess which ones are most important. Insight gained from this work can be utilized to aid the design and successful operation of aerosol RCMs. A spherically-symmetric, single-droplet wet compression model is used where n-Heptane, n-dodecane, 2,2,4,4,6,8,8- heptamethylnonane (isocetane), n-hexadecane (cetane) and n-eicosane are investigated as the dieselrepresentative fuels, while comparisons are made to water droplets. Nitrogen, neon and argon are selected as the gas-phase diluents while the oxidizer is considered to be oxygen at atmospheric concentrations. Initial droplet diameters of d0 = 3 and 8μm are used based on results of previous studies where the overall compression time is set to 15.3ms with the maximum volumetric compression ratio 13.4. An overall equivalence ratio of ϕ = 1.0 is used. It is shown that under these conditions, involatile fuels up to ~ n-hexadecane appear to be candidates for aerosol RCM experiments. However, the use of small droplets (d0 < 5μm) will be necessary in order to ensure complete vaporization and adequate gas-phase mixing in advance of low temperature chemical reactivity. Fuels with higher boiling points might not be useable unless extremely small droplets (d0 < 1μm) and low pressures (e.g., P0 < 0.5bar) are employed along with longer compression times. In addition, the boiling curve (i.e., saturation pressure) and Lf are found to be the dominant fuel properties while the density-weighted mass diffusivity, ρgDg, which controls the rate of gas phase mass diffusion, and thus compositional stratification, generally plays a secondary role. The heat capacity and molar mass are the dominant diluent properties that affect the near-droplet and ‘far-field’ conditions. The gas-phase mixture Lewis number (Leg) contributes to either greater compositional (Leg>1) or thermal (Leg<1) stratification. For large hydrocarbons and oxygenated hydrocarbons that are representative of diesel fuels Leg ~ 3-5, and therefore compositional stratification could be significant; this characteristic has the potential to complicate interpretation of ignition/oxidation data acquired from these machines

The association between insomnia symptoms and cardiovascular risk factors in patients who complete outpatient cardiac rehabilitation

Objective The present study investigated whether completion of an exercise-based cardiac rehabilitation (CR) program was associated with improvements in both traditional cardiovascular risk factors and insomnia symptoms, and whether degree of improvement in insomnia symptom severity was associated with degree of improvement in cardiovascular risk. Methods Participants (N = 80) with cardiovascular disease completed a 12-week outpatient CR program involving supervised moderate-intensity exercise sessions held twice weekly. Insomnia symptom severity, blood pressure, body mass index, psychological distress, and lipid profile were measured at baseline and after completion of the program. Results Nearly 40% reported mild to moderate insomnia symptom severity at baseline. There were improvements in insomnia symptom severity, anxious and depressive symptoms, low-density lipoprotein levels, triglycerides, and total cholesterol from baseline to post-program. After statistical adjustment for age, sex, and functional capacity, greater improvement in insomnia symptom severity was associated with greater improvements in total cholesterol and symptoms of anxiety and depression. Conclusions Completion of CR may contribute to improved sleep that, in turn, is associated with improvements on some indices of cardiovascular risk. Future research should examine the direction of the association between insomnia and cardiovascular risk, including whether efforts to alleviate insomnia may bolster the cardiovascular benefits of CR

Tailored Bifunctional Polymer for Plutonium Monitoring

Monitoring of actinides with sophisticated conventional methods is affected by matrix interferences, spectral interferences, isobaric interferences, polyatomic interferences, and abundance sensitivity problems. To circumvent these limitations, a self-supported disk and membrane-supported bifunctional polymer were tailored in the present work for acidity-dependent selectivity toward Pu­(IV). The bifunctional polymer was found to be better than the polymer containing either a phosphate group or a sulfonic acid group in terms of (i) higher Pu­(IV) sorption efficiency at 3–4 mol L^–1 HNO₃, (ii) selective preconcentration of Pu­(IV) in the presence of a trivalent actinide such as Am­(III), and (iii) preferential sorption of Pu­(IV) in the presence of a large excess of U­(VI). The bifunctional polymer was formed as a self-supported matrix by bulk polymerization and also as a 1–2 μm thin layer anchored on a microporous poly­(ether sulfone) by surface grafting. The proportions of sulfonic acid and phosphate groups in both the self-supported disk and membrane-supported bifunctional polymer were found to be the same as expected from the mole proportions of monomers in polymerizing solutions used for syntheses. α radiography by a solid-state nuclear track detector indicated fairly homogeneous anchoring of the bifunctional polymer on the surface of the membrane. Pu­(IV) preconcentrated on a single bifunctional bead was used for determination of the Pu isotopic composition by thermal ionization mass spectrometry. The membrane-supported bifunctional polymer was used for preconcentration and subsequent quantification of Pu­(IV) by α spectrometry using the absolute efficiency at a fixed counting geometry. The analytical performance of the membrane-supported-bifunctional-polymer-based α spectrometry method was found to be highly reproducible for assay of Pu­(IV) in a variety of complex samples