Clément Chartier

Analysis of Smokeless Spray Combustion in a Heavy-Duty Diesel Engine by Combined Simultaneous Optical Diagnostics

A heavy-duty diesel engine operating case producing no engine-out smoke was studied using combined simultaneous optical diagnostics. The case was close to a typical low-load modern diesel operating point without EGR. Parallels were drawn to the conceptual model by Dec and results from high-pressure combustion vessels. Optical results revealed that no soot was present in the upstream part of the jet cross-section. Soot was only observed in the recirculation zones close to the bowl perimeter. This indicated very slow soot formation and was explained by a significantly higher air entrainment rate than in Decs study. The local fuel-air equivalence ratio, Φ, at the lift-off length was estimated to be 40% of the value in Decs study. The lower Φ in the jet produced a different Φ-T history, explaining the soot results. The increased air entrainment rate was mainly due to smaller nozzle holes and increased TDC density. Furthermore, increased injection pressure was believed to reduce the residence time in the jet, thus reducing the soot formation. OH was detected at the periphery of the jet, upstream of the location where fuel started to react on the jet centerline. The OH region extended relatively far into the jet, further supporting the conclusion of a less fuel-rich jet in the current case. Partially oxidized fuel (POF) was found at the center of the jet, downstream of the lift-off position. This indicated that the temperature needed to start chemical reactions inside the jet had not been obtained at the lift-off position. The high-temperature reaction zone at the periphery thus added heat over a distance before POF was observed on the centerline.

Air-Entrainment in Wall-Jets Using SLIPI in a Heavy-Duty Diesel Engine

Mixing in wall-jets was investigated in an optical heavy-duty diesel engine with several injector configurations and injection pressures. Laser-induced fluorescence (LIF) was employed in non-reacting conditions in order to quantitatively measure local equivalence ratios in colliding wall-jets. A novel laser diagnostic technique, Structured Laser Illumination Planar Imaging (SLIPI), was successfully implemented in an optical engine and permits to differentiate LIF signal from multiply scattered light. It was used to quantitatively measure local equivalence ratio in colliding wall-jets under non-reacting conditions. Mixing phenomena in wall-jets were analyzed by comparing the equivalence ratio in the free part of the jet with that in the recirculation zone where two wall-jets collide. These results were then compared to φ predictions for free-jets. It was found that under the conditions tested, increased injection pressure did not increase mixing in the wall-jets. Comparisons with free-jet predictions further indicated that mixing in wall-jets is less effective than in free-jets for identical conditions and downstream distances. The confined nature of the wall-jet in the optical engine is suspected to be the reason for these observations. A rapid leaning-out of the jet after end of injection was observed for all cases, but this enhanced mixing was not transmitted to the wall-jet. (Less)

Mixing in wall-jets in a heavy-duty diesel engine : A les study

The paper presents a large eddy simulation investigation on the effect of fuel injection pressure on mixing, in an optical heavy-duty diesel engine. Recent investigation on impinging wall jets at constant-volume and quiescent conditions exhibited augmented air entrainment in wall jets with increasing injection pressure, when compared with a free jet. The increased mixing rates were explained as owing to enhanced turbulence and vortex formation in the jet-tip in the recirculation zone. A recent investigation carried out in an optical heavy-duty diesel engine indicated however a negligible effect of injection pressure on the mixing in the engine environment. The effect of enhanced turbulence and vortex formation of the jet-tip in the recirculation zone is believed weaker than the effect of engine confinement, due to the presence of fuel from adjacent jets limiting the mixing the fuel with the ambient gas. The aims of this paper are to investigate this issue and to look into more details about the nature of the mixing process in diesel engines. Two fuel injection pressures, 2000 bar and 2500 bar for a constant swirl level of 0.5 and inter-jet angles 45° and 135° were employed. The simulations were performed using OpenFoam, accounting for the mesh-motion and fuel spray treatment. The results exhibit the same trend found in the optical experiments. It was shown that increasing the mass flow rate by increasing injection velocity resulted in augmented turbulence levels in the recirculation zone; however, nearly injection pressure independent equivalence ratio was exhibited during quasi-stationary flow conditions.