摘要 :
The fuel-ambient mixture in vaporized fuel jets produced by liquid sprays is fundamental to the performance and operation of engines. Unfortunately, experimental difficulties limit the direct measurement of local fuel-ambient mixt...
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The fuel-ambient mixture in vaporized fuel jets produced by liquid sprays is fundamental to the performance and operation of engines. Unfortunately, experimental difficulties limit the direct measurement of local fuel-ambient mixture, inhibiting quantitative assessment of mixing. On the other hand, measurement of global quantities, such as the jet penetration rate, is relatively straightforward. Simplified models to predict local fuel-ambient mixture have also been developed, based on these global parameters. However, experimental data to validate these models over a range of conditions is needed. In the current work, we perform measurements of jet global quantities such as vapor-phase penetration, liquid-phase penetration, spreading angle, and nozzle flow coefficients over a range of conditions in a high-temperature, high-pressure vessel. Using this data and other quantitative mixing measurements performed by Rayleigh scattering in the vaporized portion of the jet, we compare to an existing variable-radial-profile model for prediction of fuel mixture fraction during the steady period of injection. Results show that spreading angles based on measurement of the most sensitive outer boundary of the jet, by schlieren or Rayleigh-scatter imaging, are needed as inputs to the model to obtain a match between modeled and measured fuel jet penetration rates. By adjusting the model (with spreading angle) to match the measured penetration, the model predictions also produce local mixture fractions that are within the Rayleigh scattering experimental uncertainty. Using this same penetration-matching technique, accurate model predictions of mixture fraction are achieved for a range of ambient densities, fuel injector nozzle shapes, injection pressures, and types of fuels. Additionally, extrapolation of the mixing measurements suggests that a fuel spray has a smaller spreading angle in the near-field and transitions to a larger angle in the far-field jet.
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摘要 :
The fuel-ambient mixture in vaporized fuel jets produced by liquid sprays is fundamental to the performance and operation of engines. Unfortunately, experimental difficulties limit the direct measurement of local fuel-ambient mixt...
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The fuel-ambient mixture in vaporized fuel jets produced by liquid sprays is fundamental to the performance and operation of engines. Unfortunately, experimental difficulties limit the direct measurement of local fuel-ambient mixture, inhibiting quantitative assessment of mixing. On the other hand, measurement of global quantities, such as the jet penetration rate, is relatively straightforward. Simplified models to predict local fuel-ambient mixture have also been developed, based on these global parameters. However, experimental data to validate these models over a range of conditions is needed. In the current work, we perform measurements of jet global quantities such as vapor-phase penetration, liquid-phase penetration, spreading angle, and nozzle flow coefficients over a range of conditions in a high-temperature, high-pressure vessel. Using this data and other quantitative mixing measurements performed by Rayleigh scattering in the vaporized portion of the jet, we compare to an existing variable-radial-profile model for prediction of fuel mixture fraction during the steady period of injection. Results show that spreading angles based on measurement of the most sensitive outer boundary of the jet, by schlieren or Rayleigh-scatter imaging, are needed as inputs to the model to obtain a match between modeled and measured fuel jet penetration rates. By adjusting the model (with spreading angle) to match the measured penetration, the model predictions also produce local mixture fractions that are within the Rayleigh scattering experimental uncertainty. Using this same penetration-matching technique, accurate model predictions of mixture fraction are achieved for a range of ambient densities, fuel injector nozzle shapes, injection pressures, and types of fuels. Additionally, extrapolation of the mixing measurements suggests that a fuel spray has a smaller spreading angle in the near-field and transitions to a larger angle in the far-field jet.
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We investigate the mixing, penetration, and ignition characteristics of high-pressure n-dodecane sprays having a split injection schedule (0.5/0.5 dwell/0.5 ms) in a pre-burn combustion vessel at ambient temperatures of 750 K, 800...
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We investigate the mixing, penetration, and ignition characteristics of high-pressure n-dodecane sprays having a split injection schedule (0.5/0.5 dwell/0.5 ms) in a pre-burn combustion vessel at ambient temperatures of 750 K, 800 K and 900 K. High-speed imaging techniques provide a time-resolved measure of vapor penetration and the timing and progression of the first- and second-stage ignition events. Simultaneous single-shot planar laser-induced fluorescence (PLIF) imaging identifies the timing and location where formaldehyde (CH_2O) is produced from first-stage ignition and consumed following second-stage ignition. At the 900-K condition, the second injection penetrates into high-temperature combustion products remaining in the near-nozzle region from the first injection. Consequently, the ignition delay for the second injection is shorter than that of the first injection (by a factor of two) and the second injection ignites at a more upstream location near the liquid length. At the 750 K and 800 K conditions, high-temperature ignition does not occur in the near-nozzle region after the end of the first injection, though formaldehyde remains from first-stage reactions. Under these conditions, the second injection penetrates into cool-flame products that are slightly elevated in temperature (～100 K) relative to the ambient. This modest temperature increase and the availability of reactive cool-flame products reduces the first- and second-stage ignition delay of the second injection by a factor of approximately two relative to the first injection. At the 750-K ambient condition, high-temperature ignition of the first injection does not occur until the second injection enriches the very fuel-lean downstream regions.
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This work investigates the injection processes of an eight-hole direct-injection gasoline injector from the Engine Combustion Network (ECN) effort on gasoline sprays (Spray G). Experiments are performed at identical operating cond...
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This work investigates the injection processes of an eight-hole direct-injection gasoline injector from the Engine Combustion Network (ECN) effort on gasoline sprays (Spray G). Experiments are performed at identical operating conditions by multiple institutions using standardized procedures to provide high-quality target datasets for CFD spray modeling improvement. The initial conditions set by the ECN gasoline spray community (Spray G: Ambient temperature: 573 K, ambient density: 3.5 kg/m~3 (～6 bar), fuel: iso-octane, and injection pressure: 200 bar) are examined along with additional conditions to extend the dataset covering a broader operating range. Two institutes evaluated the liquid and vapor penetration characteristics of a particular 8-hole, 80° full-angle, Spray G injector (injector #28) using Mie scattering (liquid) and schlieren (vapor). Diffused back-illumination (DBI) imaging, which is the ECN standard liquid length diagnostic, was also used to provide a reference for the Mie scatter measurements. In addition to imaging the full liquid field, the DBI measurements included long-distance microscopy collection to permit characterization of near-nozzle, end-of-injection details. Interpretation of plume-to-plume variation was assisted by nozzle geometry measurements performed using optical microscopy and x-ray tomography. Results indicate that global spray parameters such as liquid and vapor penetration as well as spray angle are similar between the two facilities. The spray development and mixing is largely affected by charge gas conditions (mainly density). For instance, under the standard Spray G density, the individual plumes remained separated until the end of injection, while at higher ambient densities the plumes merged together. Spray development results, together with spray mechanical patternation supported the correlation with measured nozzle internal geometry. Long-distance microscopy measurements showed that the main flow was attracted toward the injector centerline after the end of injection, supporting the convergence of the plumes as observed in the spray angle measurements.
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Contamination from water and particulate matter in diesel or aviation fuel can cause complications even under normal operating conditions. Fuel contamination becomes a major problem in air transportation, where engine flame-out du...
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Contamination from water and particulate matter in diesel or aviation fuel can cause complications even under normal operating conditions. Fuel contamination becomes a major problem in air transportation, where engine flame-out due to injection system blockage or malfunction might have catastrophic consequences. The presence of contaminants in fuel has been linked to several incidents including commercial and military aircraft over the past decade. Unless they are detected and separated from the fuel, water or solid particles have several ways to reach the engine and cause troubles. Aviation fuel is commonly stored in large tanks and transferred frequently before it reaches the aircraft, increasing the risk for contamination. At the same time, gas bubbles may be present in the system. While harmless to the aircraft, gas bubbles have been the reason why contamination monitoring systems would fail, as the system would be triggered by gas bubbles, instead of detrimental contaminants. We developed a set of optical diagnostics to detect the presence of contaminants in fuel, and quantify their size and concentration. The system consists of a microscope imaging system, a multi-angle scattering diagnostic and an infrared line-of-sight extinction system. The objective of the microscope imaging system is to measure contaminants, producing particle size distributions that, in conjunction with the scattering data, provide real-time concentration measurements. The multi-angle scattering intensity information permits to separate the different kinds of particles; it is especially sensitive to detecting gas bubbles. While unable to separate water from the other types of particles, infrared extinction proved to be highly responsive at detecting water slugs, while providing redundant information about contamination levels. The different diagnostics enable the system to reliably measure concentrations for various particle types with unmatched measurement accuracy and precision.
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Spark-ignition direct-injection (SIDI) engines operating in a stratified, lean-burn regime offer improved engine efficiency, however, seemingly random fluctuations in stratified combustion that result in partial-burn or misfire pr...
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Spark-ignition direct-injection (SIDI) engines operating in a stratified, lean-burn regime offer improved engine efficiency, however, seemingly random fluctuations in stratified combustion that result in partial-burn or misfire prevent widespread implementation. Eliminating these poor combustion events requires detailed understanding of engine flow, fuel delivery, and ignition, but knowing the dominant cause is difficult because they occur simultaneously in an engine. In this study, the variability in fuel-air mixture linked to fuel injection hardware was addressed by experimentation in a near-quiescent pressure vessel at high-temperature conditions representative of late, stratified-charge injection. An 8-hole SIDI spray was interrogated using high-speed schlieren and Mie-scatter imaging from multiple, simultaneous views to acquire the vapor and liquid envelopes of the spray. The mixture fraction of vaporized sections of the spray was then quantified at a plane between plumes using Rayleigh scattering. Probability contours of the line-of-sight vapor envelope show little variability between injections whereas probability contours derived from planar, quantitative mixing measurements exhibit greater amounts of variability for lean-combustion-limit charge. The mixture field between plumes is characterized by multi-hole and end-of-injection dynamics that attract the plumes to each other and towards the injection axis.
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Reduced-order models typically assume that the flow through the injector orifice is quasi-steady. The current study investigates to what extent this assumption is true and what factors may induce large-scale variations. Experiment...
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Reduced-order models typically assume that the flow through the injector orifice is quasi-steady. The current study investigates to what extent this assumption is true and what factors may induce large-scale variations. Experimental data were collected from a single-hole metal injector with a smoothly converging hole and from a transparent facsimile. Gas, likely indicating cavitation, was observed in the nozzles. Surface roughness was a potential cause for the cavitation. Computations were employed using two engineering-level Computational Fluid Dynamics (CFD) codes that considered the possibility of cavitation. Neither computational model included these small surface features, and so did not predict internal cavitation. At steady state, it was found that initial conditions were of little consequence, even if they included bubbles within the sac. They however did modify the initial rate of injection by a few microseconds. Though the needle was never stationary, the mass discharge by the nozzle remained constant for most of the injection. The momentum discharge was more sensitive to lower needle lifts than the mass flow rate. An annular jet, that may follow either the needle surface or the sac wall, forms at low needle lift. The presence of this jet corresponds to a loss of momentum through the nozzle exit. The coefficient of area remains remarkably consistent during the early/late needle transient and is an important discovery.
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The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better unders...
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The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN “Spray A” configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (fv), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered.
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The mixing field of sprays injected into high temperature and pressure environments has been observed to be tightly connected to spreading angle, therefore linking vaporization and combustion processes to the angular dispersion of...
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The mixing field of sprays injected into high temperature and pressure environments has been observed to be tightly connected to spreading angle, therefore linking vaporization and combustion processes to the angular dispersion of the spray. Visualization of the Engine Combustion Network three-hole, Spray B diesel injector shows substantial variation in near-field spreading angle with respect to time compared to past measurements of the single-hole, Spray A injector. The source of these variations originating inside the nozzle, and the implications on mixing, evaporation, and combustion of the diesel plume, need to be understood. In this study, we characterize the ECN-target plume for a Spray B injector (Serial # 211201), which already benefits from extensive and detailed internal measurements of nozzle geometry and needle movement, while comparing to the single-hole Spray A with the same type of detailed geometry and understanding. We measure the spreading angle, liquid penetration, and vapor penetration with respect to time of the spray of interest using standardized diagnostics in a high-temperature, high-pressure capable optically accessible combustion chamber. High-speed Mie scattering and diffused back-illumination imaging (DBI) are applied for liquid penetration, and schlieren imaging, for vapor penetration. The measurements show that the near-field spreading angle is wide for the first 300 μs after the start of injection before dropping rapidly during a quasi-steady period and then increasing well before the end of injection. Changes in spreading angle are not coincident with needle motion throttling, suggesting more complicated internal flow transients. With DBI long-distance microscopy, a partially transparent region indicates that an intact liquid core at the nozzle exit occurs frequently in quasi-steady period, which is coincident with a narrow spreading angle. The liquid penetration measured by DBI is comparable to that of Mie-scattering using criteria and standardization already established by the ECN community for Spray A. The Spray B liquid and vapor penetration rates are slower than that of Spray A, showing responses connected to the transient spreading angle.
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This work investigates the effects of cavitation on spray characteristics by comparing measurements of liquid and vapor penetration as well as ignition delay and lift-off length. A smoothed-inlet, converging nozzle (nominal KS1.5)...
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This work investigates the effects of cavitation on spray characteristics by comparing measurements of liquid and vapor penetration as well as ignition delay and lift-off length. A smoothed-inlet, converging nozzle (nominal KS1.5) was compared to a sharp-edged nozzle (nominal K0) in a constant-volume combustion vessel under thermodynamic conditions consistent with modern compression ignition engines. Within the near-nozzle region, the K0 nozzle displayed larger radial dispersion of the liquid as compared to the KS1.5 nozzle, and shorter axial liquid penetration. Moving downstream, the KS1.5 jet growth rate increased, eventually reaching a growth rate similar to the K0 nozzle while maintaining a smaller radial width. The increasing spreading angle in the far field creates a virtual origin, or mixing offset, several millimeters downstream for the KS1.5 nozzle. Remarkably, this mixing offset appeared to globally influence the liquid penetration and lift-off stabilization location over a wide range of operating conditions. When this offset was removed, OH chemiluminescence-derived lift-off lengths for the two nozzles essentially collapsed. An Eulerian multiphase mixture model, with Large-Eddy Simulations (LES) combining internal and external flow predicted the trends in spreading angle in the region close to the injector. The K0 simulation showed cavitation zones along walls downstream of the nozzle inlet with some dispersion into the center of the jet before the nozzle exit. With a slightly diverging nozzle (as measured), the K0 simulation also indicated that low pressure zones draw ambient gas just inside the nozzle exit, which, combined with cavitation dynamics, should be considered as a potential contributor to the initial growth rate.
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