Shorter Lift-off Length Research Articles

Numerical investigation of n-dodecane ECN spray and combustion characteristics using the one-way coupled Eulerian-Lagrangian approach

This work investigated the spray and combustion characteristics of the Engine Combustion Network (ECN) n-dodecane injections using a one-way coupled Eulerian-Lagrangian approach. The actual X-ray measured injector geometries from the Argonne National Laboratory were adopted for Eulerian simulations. The multi-phase flow was modeled using the Volume-of-Fluid (VoF) method with the high-resolution interface capturing scheme to track the fluid-gas interface. The generated parcel data from the Eulerian simulations were mapped and utilized in the Lagrangian simulations to ensure fidelity. The simulation results agree well with the experimental data in terms of the projected density, mass flow rates, spray penetrations, mixture distributions, and ignition delays. The diverging nozzle channel in the Spray C injector resulted in cavitation and air entrainment, which reduced the mass flow rate. Because of the low oxygen concentration, the low-temperature combustion (LTC) reactions made a significant contribution to the heat release process. In comparison, similar jet-flame structures were observed for the Spray A, C, and D cases, consisting of the upstream LTC region, midstream intermediate-temperature combustion region, downstream fuel-rich high-temperature combustion (HTC) region, and downstream premixed HTC region. The higher ambient temperature led to a more advanced combustion phasing but shorter lift-off length. As a result, the mixing time was reduced to result in a higher concentration of acetylene. In addition, upon increasing the injection pressure, no significant difference was observed in the jet flame structure. On the other hand, the higher injection velocity resulted in a higher flame tip and broader HTC region.

An optical study on spray and combustion characteristics of ternary hydrogenated catalytic biodiesel/methanol/n-octanol blends; part П: Liquid length and in-flame soot

Methanol has been considered as a promising alternative fuel for combustion engines. However, it is quite challenging to use it directly in compressed ignition engines because of its ignition issues, especially for low load conditions. In this research, methanol was blended with hydrogenated catalytic biodiesel (HCB) using n-octanol as co-solvent. A fundamental study on spray and combustion characteristics of two ternary blends (68% HCB+17% octanol+ 15% methanol by volume; 58% HCB+17% octanol+ 25% methanol by volume) and the pure HCB was carried out within a constant volume combustion chamber equipped with a single-hole injector. As in Part Ⅰ, the spray morphology, ignition delay, and flame lift-off length have been studied in detail. This part focuses on liquid length and in-flame soot formation of reacting sprays, which were quantified utilizing a diffused back-illumination extinction imaging technique. There is an overlapping area between the fuel liquid phase and flame for all three fuels under all operating points. The reduction on liquid length after ignition is more noticeable for pure HCB than other blends, because of its shorter lift-off length. The results show that the liquid length increases with increasing fraction of methanol in the mixtures, which is mainly governed by the high latent heat of vaporization of methanol. Furthermore, extra methanol addition brings a considerable reduction on in-flame soot production because of the leaner fuel combustion at longer flame lift-off length, as well as the more oxygen content within the blends.

Transient Flame Development in a Constant-Volume Vessel Using a Split-Scheme Injection Strategy

Multiple-injection strategies are characterized by a complex and transient interplay between high- and low-temperature reactions. Tracking low-temperature reaction products such as formaldehyde (CH2O) is particularly important to understand ignition phenomena and the so-called “combustion recession” that is observed in experiments. Experimentally, it is often difficult to discriminate between formaldehyde and other species such as poly-aromatic hydrocarbons, which is why a selective excitation approach is used in this work. Simultaneous high-speed imaging of the chemically-excited hydroxyl radical (OH*) is used to improve indication of flame location and second stage ignition. During experiments in a constant-volume vessel, two 0.5-ms injections of n-dodecane, separated by 0.5-ms dwell time, are injected into a 900-K ambient. The global flame development is characterized based on high-speed diagnostics, followed by an investigation into the spatial distribution of formaldehyde at four different times after start-of-injection (aSOI). Results show significant influence of the first injection on characteristics of the second. Ignition delay and lift-off location of the second injection are prominently reduced, while flame penetration is greatly enhanced by the wake of the first injection. Formaldehyde structure is observed during both end-of-injection transients, reaching as far upstream as 6 mm from the nozzle. Even after the second injection, the flame structure still appears to be influenced by the first, with a shorter lift-off length and compressed formaldehyde structure. Based on the selective excitation procedure, it becomes clear that the interpretation of laser-induced fluorescence (LIF) images obtained by 355-nm excitation alone is prone to ambiguity.

Large eddy simulation of n-Dodecane spray combustion in a high pressure combustion vessel

Autoignition and stabilization of n-Dodecane spray combustion under diesel engine like conditions are investigated using large eddy simulation and detailed chemical kinetics. The Spray A cases of Engine Combustion Network (ECN) with ambient temperatures of 900K and 1000K are considered. Two-stage ignition behavior is predicted in the studied conditions. It is found that the first-stage ignition occurs on the fuel-lean mixture, whereas the second-stage ignition starts on the fuel-rich mixture. The first stage ignition in the fuel-lean mixture promotes the first and the second stage ignition in the fuel-rich mixture owing to rapid turbulent mixing. Two mechanisms, autoignition and flame propagation coupling with the low temperature ignition, are used to explain the lift-off position and stabilization of the combustion process. They compete with each other, and their relative importance depends on the ambient temperature. The ambient temperature is shown to affect the soot emission in the flame through its influences on the lift-off length and the reaction zone structure. Higher ambient temperature results in a shorter lift-off length, which gives rise to higher soot emission due to the lower air entrainment to the fuel-rich zone in front of the flame. In the lower temperature case, the flame is stabilized by an autoignition induced flame front where a considerable amount of fuel is oxidized to CO at the leading front of the flame. Consequently, it reduces the soot formation in the flame.

Auto-ignition of diesel spray using the PDF-Eddy Break-Up model

This work presents the development and implementation of auto-ignition modelling for DI diesel engines by using the PDF-Eddy Break-Up (PDF-EBU) model. The key concept of this approach is to combine the chemical reaction rate dealing with low-temperature mode, and the turbulence reaction rate governing the high-temperature part by a reaction progress variable coupling function which represents the level of reaction. The average reaction rate here is evaluated by a probability density function (PDF) averaging approach. In order to assess the potential of this developed model, the well-known Shell ignition model is chosen to compare in auto-ignition analysis. In comparison, the PDF-EBU ignition model yields the ignition delay time in good agreement with the Shell ignition model prediction. However, the ignition kernel location predicted by the Shell model is slightly nearer injector than that by the PDF-EBU model leading to shorter lift-off length. As a result, the PDF-EBU ignition model developed here are fairly satisfactory in predicting the auto-ignition of diesel engines with the Shell ignition model.