Evaluation and Impact of Mixing Phenomena & Injection Strategy on Ducted Fuel Injection Combustion
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Godbold, Conner Waite
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Abstract
Diesel engines have long served society as powerplants for both transportation and power generation. While these engines possess high simple-cycle thermal efficiencies due to their high compression ratios and low pumping work, they are plagued by relatively high production of harmful emissions—such as particulate matter and oxides of nitrogen—per unit energy released. This elevated emission production per unit heat release stems from the non-premixed nature of the combustion process.
One promising solution, demonstrated in early experiments to significantly reduce particulate matter formation, is Ducted Fuel Injection (DFI). DFI is believed to function akin to the nozzle of a Bunsen burner, where the fuel draws air into the fuel injector, thereby promoting leaner combustion. To this end, a hollow cylindrical body is positioned in front of the fuel injector within a diesel engine combustion chamber, and the fuel spray passes through it during the injection/combustion event. Existing literature has consistently shown that DFI increases ignition delay and lift-off length while concurrently decreasing particulate matter formation.
This thesis pursued three primary objectives: experimentally investigating DFI's soot mitigation mechanisms, experimentally examining DFI's effect on lift-off length (LOL), and assessing pilot injection impacts on DFI's ignition characteristics and soot reduction.
High-speed optical diagnostics under diesel engine conditions addressed the first two objectives, examining non-reacting mixing and reacting flow interactions across various duct geometries. Note that for the ducts tested and described herein, ``D'' indicates the inner diameter, ``L'' indicates the duct length, and ``G'' indicates the gap distance from the injector tip to the duct. D3L16G2.6, D2L16G2.6, D1L16G2.6, D2L8G2.6, and D1L8G2.6 ducts with a 90 µm injector orifice were used for this study.
Regarding non-reacting mixing, variations in injection pressure minimally influenced mixing fields. Larger duct diameters increased upstream air entrainment, and the influence of duct length depended on diameter. A one-dimensional mass and momentum conservation based jet-pumping model was developed, to better understand the measured results. This model showed that larger duct exit diameters entrain more air due to having lower dynamic pressures at duct exit. Jet-pumping air mass flow rate initially rose with inlet diameter before asymptotically declining due to decreasing vacuum levels at the duct inlet.
Reacting flow results aligned with literature, showing DFI increased LOL and ignition delay (ID), as well as decreased spatially integrated natural luminosity (SINL) compared to free-spray conditions. Three different flame stabilization modes were measured: detached, near-nozzle, and upstream of duct exit. The D1 and D3 configurations consistently stabilized flames in detached and upstream positions, respectively, while the D2 flame stabilization mode varied with injection pressures and chamber temperatures. Detached flames demonstrated nearly linear SINL reduction with increased LOL. When near-nozzle or upstream flames transitioned to detached, SINL dropped significantly.
Non-dimensional analysis was used to evaluate if the yielded flame stabilization mode could be predicted. Higher injection pressures and lower chamber pressures favored flame detachment. A scalar dissipation rate estimation and chemical timescale calculations via constant-pressure reactors was used to calculated an effective Damköhler number ($\text{Da}$). Plotting duct exit-to-LOL measurements against $Da$ revealed that Da could effectively predict which flame stabilization mode would occur for a given configuration and condition.
The mixing data was then used to better interpret the reacting SINL measurements. A positive relationship was found between SINL and mass flow weighted cross-sectionally averaged equivalence ratio at LOL, as expected. Flames upstream of the duct exit did not align with the other configuration's results, which is likely due to in-duct combustion elevating the dynamic pressure at duct exit, causing the non-reacting mixing field to be non-indicative of the reacting one.
Addressing the third objective, an experimental and numerical study examined pilot injections' influence on DFI's premixed heat-release spike and soot reduction. Pilot injections shortened ID and decreased peak rate-of-heat-release (ROHR) in both free-spray and DFI, though DFI metrics only reduced to free-spray no-pilot levels due to no combustion recession. Pilot injections decreased peak SINL under DFI, coinciding with increased spray-head penetration rates, likely due to the spray propagating through the pilot injection's residual. Numerical studies corroborated the spray penetration rate finding, and showed that the use of pilot injections lowered the average mixture fraction between LOL and spray tip. This likely contributed to reduced peak SINL by limiting residence time for soot growth.
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Date
2025-07-22
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Text
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Dissertation