Modeling Diesel Spray Flame Lift-Off Using Detailed Chemistry and a New Primary Breakup Model

The backward flow of the hot burned gas surrounding a diesel flame was found to be one of the factors reducing the set-off length (also called the lift-off length), that is, the distance from a nozzle exit into which a diffusion flame cannot intrude. In the combustion chamber of an actual diesel engine, the entrainment of the surrounding gas into a spray jet injected from a multi-hole nozzle is restricted by the combustion chamber walls and the adjacent spray jets, thus inducing the backward flow of the surrounding gas toward the nozzle exit. The emergence of this backward flow was measured by particle tracking velocimetry in the non-combusting condition. A new momentum theory for calculating the backward flow velocity was established by extending Wakuri’s momentum theory. Shadowgraph imaging in an optical engine successfully visualized the backward flow of the hot burned gas. The hot burned gas is re-entrained into the spray jet in the region of the set-off position and shortens the set-off length in comparison to that of a single free-spray flame which does not induce the backward flow.

Download Full-text

Modeling Diesel Spray Flame Lift-Off, Sooting Tendency and NOx Emissions Using Detailed Chemistry With Phenomenological Soot Model

ASME 2005 Internal Combustion Engine Division Spring Technical Conference ◽

10.1115/ices2005-1009 ◽

2005 ◽

Cited By ~ 26

Author(s):

Song-Charng Kong ◽

Yong Sun ◽

Rolf D. Reitz

Keyword(s):

Nox Reduction ◽

Diesel Spray ◽

Nox Emissions ◽

Soot Emission ◽

Detailed Chemistry ◽

Emission Process ◽

Start Of Injection ◽

Soot Emissions ◽

Lift Off ◽

Soot Model

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of n-heptane is coupled with a reduced NOx mechanism to simulate diesel fuel oxidation and NOx formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under PCCI-like conditions. Trends of heat release rate, NOx and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since NOx emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and NOx reduction for the engine conditions investigated in this study.

Download Full-text