Publications

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To provide a better understanding of the fluid mechanical mechanisms governing entrainment in decelerating jets, we performed a large eddy simulation (LES) of a transient air jet. The ensemble-averaged LES calculations agree well with the available measurements of centerline velocity, and they reveal a region of increased entrainment that grows as it propagates downstream during deceleration. Within the temporal and spatial domains of the simulation, entrainment during deceleration temporarily increases by roughly a factor of two over that of the quasi-steady jet, and thereafter decays to a level lower than the quasi-steady jet. The LES results also provide large-structure flow details that lend insight into the effects of deceleration on entrainment. The simulations show greater growth and separation of large vortical structures during deceleration. Ambient fluid is engulfed into the gaps between the large-scale structures, causing large-scale indentations in the scalar jet boundary. The changes in the growth and separation of large structures during deceleration are attributed to changes in the production and convection of vorticity. Both the absolute and normalized scalar dissipation rates decrease during deceleration, implying that changes in small-scale mixing during deceleration do not play an important role in the increased entrainment. Hence, the simulations predict that entrainment in combustion devices may be controlled by manipulating the fuel-jet boundary conditions, which affect structures at large scales much more than at small scales. © 2012 American Institute of Physics.

Post-injection strategies aimed at reducing engine-out emissions of unburned hydrocarbons (UHC) were investigated in an optical heavy-duty diesel engine operating at a low-load, low-temperature combustion (LTC) condition with high dilution (12.7% intake oxygen) where UHC emissions are problematic. Exhaust gas measurements showed that a carefully selected post injection reduced engine-out load-specific UHC emissions by 20% compared to operation with a single injection in the same load range. High-speed in-cylinder chemiluminescence imaging revealed that without a post injection, most of the chemiluminescence emission occurs close to the bowl wall, with no significant chemiluminescence signal within 27 mm of the injector. Previous studies have shown that over-leaning in this near-injector region after the end of injection causes the local equivalence ratio to fall below the ignitability limit. With a carefully selected post-injection, mixtures close to the injector show significant chemiluminescence emission, indicating more complete combustion of those regions, likely due to increased local equivalence ratios. Simultaneous planar laser-induced fluorescence (PLIF) of OH with 284-nm excitation and PLIF of combined formaldehyde and poly aromatic hydrocarbons (PAH) with 355-nm excitation were employed to identify the regions of first- and second-stage ignition, as well as providing some indication of local equivalence ratios. The laser diagnostics show that without a post injection, regions close to the injector show formaldehyde fluorescence late in the cycle without detectable OH fluorescence, indicating that these regions do not achieve second-stage ignition, and therefore likely contribute to UHC emissions. Persistence of formaldehyde fluorescence late in the cycle is also consistent with fuel-lean mixtures. With a carefully selected post injection, strong OH fluorescence appears in the near-injector regions, indicating that they are likely enriched by the post-injection such that they reach second-stage ignition and more complete oxidation. The reduction observed in the exhaust UHC emission is therefore attributed to the enrichment mechanism of the near-injector regions by the close-coupled post-injection. © 2011 SAE International.

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Laser-induced incandescence (LII) of soot has commonly been implemented using Nd:YAG harmonics at 532 or 1064 nm. Recent atmospheric-pressure flame studies have shown that significant C2 and C3 fluorescence interference can arise at fluences as low as 0.2 J/cm2 at 532 nm and 1064 nm. This paper explores LII fluorescence interference in a low-temperature combustion (LTC) diesel engine. Results show that the spectral and spatial distributions of LII for mature soot are similar at 532 and 1064 nm. Closer to the onset of soot formation, however, the 532-nm LII spectrum has strong blue-shifted broadband emission compared to 1064-nm LII, but without any clear evidence of C2 or C3 fluorescence, even at high fluences. The 532-nm laser-induced emission is initially distributed over most of the diesel jet, while the 1064-nm signal is negligible. We speculate that the broadband blue-shifted signal for 532-nm LII is most likely fluorescence from soot precursor species like polycyclic aromatic hydrocarbons (PAHs), whereas the 1064-nm signal is primarily true soot LII. These results suggest that for LTC diesel engines, the emission arising from 532-nm excitation close to the onset of soot formation will likely contain significant broadband fluorescence interference relative to the true LII signal.

Low temperature combustion (LTC) strategies, which can mitigate emissions of particulate matter (PM) and nitrogen oxides (NOx) from diesel engines, typically have longer ignition delays compared to conventional diesel operation. With extended ignition delays, more time is available for premixing, which reduces PM formation. The effect of varying ignition delay on the spatial and temporal evolution of soot in LTC diesel jets is studied by imaging the natural soot luminosity, while the in-cylinder soot mass and temperature are measured using two-color soot thermometry. Ignition delay in the engine is controlled by adjusting the intake air temperature while keeping the same charge density at TDC. This allowed us to study sooting characteristics at various ignition delays while keeping the same diesel jet penetration for all the cases. Results show a 95% decrease in the total in-cylinder soot mass as ignition delay increases from 3 to 15 crank angle degrees (CAD) at an engine speed of 1200 RPM. Furthermore, the structure of the sooty regions in the jet is strongly affected by the ignition delay. For a short ignition delay of 3 CAD, soot formation originates downstream in the jet, 25 mm from the injector. After the end of injection, the sooty region first spreads back to the injector and then it is rapidly oxidized in the near-injector region within a few crank angle degrees. This suggests that rapid mixing occurs in the near injector mixtures just after the end of injection, which promotes soot oxidation. For a longer ignition delay of 15 CAD, soot first appears farther downstream in the jet, and it does not spread back to the injector. Indeed, soot never forms in the jet near the injector when the ignition delay is long, indicating that those regions do not promote soot formation, likely because they become too fuel-lean during the ignition delay.

Diesel injection parameters effect on liquid-phase diesel spray penetration after the end-of-injection (EOI) is investigated in a constant-volume chamber over a range of ambient and injector conditions typical of a diesel engine. Our past work showed that the maximum liquid penetration length of a diesel spray may recede towards the injector after EOI at some conditions. Analysis employing a transient jet entrainment model showed that increased fuel-ambient mixing occurs during the fuel-injection-rate ramp-down as increased ambient-entrainment rates progress downstream (i.e. the entrainment wave), permitting complete fuel vaporization at distances closer to the injector than the quasi-steady liquid length. To clarify the liquid-length recession process, in this study we report Mie-scatter imaging results near EOI over a range of injection pressure, nozzle size, fuel type, and rate-of-injection shape. We then use a transient jet entrainment model for detailed analysis. Results show that an increased injection pressure correlates well with increasing liquid length recession due to an increased entrainment wave speed. Likewise, an increased nozzle size, with higher jet momentum and faster entrainment wave, enhances the liquid length recession. A low-density, high-volatility fuel does not decrease the strength of the entrainment wave; however, it decreases the steady liquid length causing the entrainment wave to reach the liquid spray tip earlier, which ultimately results in faster liquid length recession. A slow ramp down in injection rate causes a weaker entrainment wave so that the liquid length recession occurs even prior to injector close.

Unlike conventional diesel engines, which have a negative ignition dwell, many strategies for low-emissions diesel combustion operate with a positive ignition dwell mode, where the ignition delay exceeds the injection duration. Although nitrogen oxides and particulate matter emissions can be reduced by operating with a positive ignition dwell, unburned hydrocarbon and carbon monoxide emissions typically increase. Sources of these emissions can stem from characteristics of the fuel spray after the end of injection, which may differ significantly from the main injection period where most spray models have been developed. To provide fundamental details of spray mixing during the end-of-injection transient, we have studied liquid-phase spray penetration and evaporation using simultaneous high-speed shadowgraph and Mie-scatter imaging for a single-hole, common-rail injector. Experiments were conducted over a wide range of ambient temperature and density in a constant-volume vessel. The experiments show that during the injection-rate ramp-down, the liquid penetration decreases (recedes towards the injector) from the quasi-steady-state distance for most diesel conditions. A transient jet entrainment model, coupled with the assumption of mixing-limited spray vaporization and direct measurement of the vaporized jet spreading angle, shows that this behavior is caused by a slower fuel delivery interacting with an increased rate of ambient entrainment during the injection-rate ramp-down. This increased mixing travels downstream as an "entrainment wave", permitting complete vaporization at distances closer to the injector than the quasi-steady liquid length. The position of the entrainment wave relative to the quasi-steady liquid length determines how far, and how quickly, the liquid recedes towards the injector. The tendency of recession increases with increasing ambient temperature and density because the transit time of the entrainment wave to the liquid length is shorter than the injection-rate ramp-down transient. Alternatively, the liquid-length recession is zero for conditions with low ambient temperature or density because the entrainment wave does not reach the quasi-steady liquid length until after the end of the injection-rate ramp-down. Copyright © 2008 by the Japan Society of Mechanical Engineers.

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A late-injection, high exhaust-gas recirculation rate, low-temperature combustion strategy is investigated in a heavy-duty diesel engine using a suite of optical diagnostics: chemiluminescence for visualization of ignition and combustion, laser Mie scattering for liquid-fuel imaging, planar laser-induced fluorescence (PLIF) for both OH and vaporfuel imagings, and laser-induced incandescence for soot imaging. Fuel is injected at top dead center when the in-cylinder gases are hot and dense. Consequently, the maximum liquid-fuel penetration is 27 mm, which is short enough to avoid wall impingement. The cool flame starts 4.5 crank angle degrees (CAD) after the start of injection (ASI), midway between the injector and bowl rim, and likely helps fuel to vaporize. Within a few CAD, the cool-flame combustion reaches the bowl rim. A large premixed combustion occurs near 9 CAD ASI, close to the bowl rim. Soot is visible shortly afterward, along the walls, typically between two adjacent jets. OH PLIF indicates that premixed combustion first occurs within the jet and then spreads along the bowl rim in a thin layer, surrounding soot pockets at the start of the mixing-controlled combustion phase near 17 CAD ASI. During the mixing-controlled phase, soot is not fully oxidized and is still present near the bowl rim late in the cycle. At the end of combustion near 27 CAD ASI, averaged PLIF images indicate two separate zones. OH PLIF appears near the bowl rim, while broad-band PLIF persists late in the cycle near the injector. The most likely source of broad-band PLIF is unburned fuel, which indicates that the near-injector region is a potential source of unburned hydrocarbons. Copyright © 2008 by ASME.

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Based on a phenomenological model of diesel combustion and pollutant-formation processes, a number of fuel additives that could potentially reduce in-cylinder soot formation by altering combustion chemistry have been identified. These fuel additives, or ''combustion modifiers'', included ethanol and ethylene glycol dimethyl ether, polyethylene glycol dinitrate (a cetane improver), succinimide (a dispersant), as well as nitromethane and another nitro-compound mixture. To better understand the chemical and physical mechanisms by which these combustion modifiers may affect soot formation in diesel engines, in-cylinder soot and diffusion flame lift-off were measured, using an optically-accessible, heavy-duty, direct-injection diesel engine. A line-of-sight laser extinction diagnostic was employed to measure the relative soot concentration within the diesel jets (''jetsoot'') as well as the rates of deposition of soot on the piston bowl-rim (''wall-soot''). An OH chemiluminescence imaging technique was utilized to measure the lift-off lengths of the diesel diffusion flames so that fresh oxygen entrainment rates could be compared among the fuels. Measurements were obtained at two operating conditions, using blends of a base commercial diesel fuel with various combinations of the fuel additives. The ethanol additive, at 10% by mass, reduced jet-soot by up to 15%, and reduced wall-soot by 30-40%. The other fuel additives also affected in-cylinder soot, but unlike the ethanol blends, changes in in-cylinder soot could be attributed solely to differences in the ignition delay. No statistically-significant differences in the diesel flame lift-off lengths were observed among any of the fuel additive formulations at the operating conditions examined in this study. Accordingly, the observed differences in in-cylinder soot among the fuel formulations cannot be attributed to differences in fresh oxygen entrainment upstream of the soot-formation zones after ignition.