Investigation of soot formation in n-dodecane spray flames using LES and a discrete sectional method

Considering stricter regulations on soot emissions, the detailed soot modeling approaches facilitating prediction of soot particle size distributions (PSD) are increasingly in demand. In this context, the transient evolution of soot is numerically investigated for two high-pressure turbulent sprays from the Engine Combustion Network (ECN), namely, Spray C (SC) and Spray D (SD). The 900-K ambient temperature (Tam) sprays are studied. This is because the two cases tend to produce a similar amount of soot at Tam=900K, despite the significantly different spray development. To predict the soot formation with information on PSD, a discrete sectional method is applied within the large-eddy simulation (LES) framework. The applied modeling strategy favorably captures the experimentally observed similar soot mass for SC and SD in the characteristic field-of-view (FOV) frustum. Moreover, the transient dynamics of soot within the FOV frustum is well captured, and the onset of soot is well predicted. It is observed that for SC and SD, soot formation is more prominent in fuel-rich (2&lt;π&lt;4) and high-temperature (T&gt;1500K) regions. Despite the stronger fuel dilution in the downstream area of the spray, soot is predominantly present in the head of the spray during the whole combustion progress, corresponding to larger particle sizes and higher soot number density. Although the spray development of SC and SD are different, the FOV approach bridges the two cases. The unique correlation between soot mass and FOV volume recognized in experiments was found to hold for the complete quasi-steady sooting region. Moreover, PSD analysis suggests very similar soot size and number density with respect to the FOV volume. This is attributed to the similar LOL for SC and SD in the normalized coordinates, and the same FOV volume corresponding to similar locations in the normalized coordinates.</p

Combustion characteristics of oxymethylene dimethyl ether-diesel blends:An experimental investigation using a constant-volume combustion chamber

The combustion characteristics of oxymethylene dimethyl ether (OMEx) and its blends with diesel have been investigated using a multi-hole injector in a constant-volume combustion chamber. The results show OMEx addition can reduce the ignition delay, especially when blends of more than 50 vol% are used, at low chamber temperature. Two individual heat-release peaks are observed for OMEx during premixed combustion at 750 K, due to a pronounced low-temperature heat-release phase. The chamber temperature of 800 K can be regarded as a transition point for the behavior of burn duration as well as maximum ROHR peak, mostly caused by combustion regime transition from premixed- to diffusion combustion. It appears that there is an approximate linear relation between maximum ROHR peak and the time at which this peak occurs with injection pressure. The ignition delay of OMEx is almost insensitive to a decrease in ambient oxygen concentration. And the premixed ROHR profile, due to its high oxygen content, is very similar and only ignition delay and burn duration increase slightly. Additionally, comparisons of natural luminosity results for OMEx and diesel indicate that OMEx produces near-zero soot values. Luminosity is expected to be caused by chemiluminescence alone, which increases with injection pressure

A comprehensive methodology for computational fluid dynamics combustion modeling of industrial diesel engines

Combustion control and optimization is of great importance to meet future emission standards in diesel engines: increase in break mean effective pressure at high loads and extension of the operating range of advanced combustion modes seem to be the most promising solutions to reduce fuel consumption and pollutant emissions at the same time. Within this context, detailed computational fluid dynamics tools are required to predict the different involved phenomena such as fuel-air mixing, unsteady diffusion combustion and formation of noxious species. Detailed kinetics, consistent spray models and high quality grids are necessary to perform predictive simulations which can be used either for design or diagnostic purposes. In this work, the authors present a comprehensive approach which was developed using an open-source computational fluid dynamics code. To minimize the pre-processing time and preserve results' accuracy, algorithms for automatic mesh generation of spray-oriented grids were developed and successfully applied to different combustion chamber geometries. The Lagrangian approach was used to describe the spray evolution while the combustion process is modeled employing detailed chemistry and, eventually, considering turbulence-chemistry interaction. The proposed computational fluid dynamics methodology was first assessed considering inert and reacting experiments in a constant-volume vessel, where operating conditions typical of heavy-duty diesel engines were reproduced. Afterward, engine simulations were performed considering two different load points and two piston bowl geometries, respectively. Experimental validation was carried out by comparing computed and experimental data of in-cylinder pressure, heat release rate and pollutant emissions (NOx, CO and soot)

Effects of oxygen enrichment on diesel spray flame soot formation in O₂/Ar atmosphere

In this study, diesel spray combustion at oxygen-enriched conditions (oxygen volume fraction of 21–70 %) with argon dilution is experimentally investigated in a constant-volume combustion chamber. Optical diagnostics are employed to study flame development, stabilization, and soot formation at oxygen-enriched conditions. To further verify the experimental observations, two-stage Lagrangian simulations are used to analyze the effects of oxygen on the formation and oxidation of soot precursors, polycyclic aromatic hydrocarbons. Results show that replacing nitrogen in air by argon leads to a 50 % reduction of the flame lift-off length, an increased soot flame temperature by 300 K, and higher soot concentrations. Flame morphology and structure still follow the classic conventional diesel combustion model in the oxygen range of 21–40 %, while changes are observed when oxygen levels are higher than 50 %. The width and length of the soot flame are shortened, and chemiluminescence from intermediate species like CO dominates the flame natural luminosity at the spray head, where the flame temperature reaches near 3000 K. Soot reduction mechanisms at high-degree oxygen-enrichment conditions are investigated. The intrinsic mixing-limited combustion of diesel sprays leads to unavoidable fuel-rich areas locally, but the shortened flame lift-off length and sufficient oxygen supply confines soot-forming conditions to a smaller, upstream region. The residence time of fuel parcels in this confined soot-forming area is shortened due to the larger local spray velocity. Thereafter, fuel parcels enter a high-temperature fuel-lean region, where the formed soot is oxidized rapidly.</p

Effects of oxygen enrichment on diesel spray flame soot formation in O₂/Ar atmosphere

In this study, diesel spray combustion at oxygen-enriched conditions (oxygen volume fraction of 21–70 %) with argon dilution is experimentally investigated in a constant-volume combustion chamber. Optical diagnostics are employed to study flame development, stabilization, and soot formation at oxygen-enriched conditions. To further verify the experimental observations, two-stage Lagrangian simulations are used to analyze the effects of oxygen on the formation and oxidation of soot precursors, polycyclic aromatic hydrocarbons. Results show that replacing nitrogen in air by argon leads to a 50 % reduction of the flame lift-off length, an increased soot flame temperature by 300 K, and higher soot concentrations. Flame morphology and structure still follow the classic conventional diesel combustion model in the oxygen range of 21–40 %, while changes are observed when oxygen levels are higher than 50 %. The width and length of the soot flame are shortened, and chemiluminescence from intermediate species like CO dominates the flame natural luminosity at the spray head, where the flame temperature reaches near 3000 K. Soot reduction mechanisms at high-degree oxygen-enrichment conditions are investigated. The intrinsic mixing-limited combustion of diesel sprays leads to unavoidable fuel-rich areas locally, but the shortened flame lift-off length and sufficient oxygen supply confines soot-forming conditions to a smaller, upstream region. The residence time of fuel parcels in this confined soot-forming area is shortened due to the larger local spray velocity. Thereafter, fuel parcels enter a high-temperature fuel-lean region, where the formed soot is oxidized rapidly.</p

Heavy-Duty Diesel Engine Spray Combustion Processes: Experiments and Numerical Simulations

open6siA contemporary approach for improving and developing the understanding of heavy-duty Diesel engine combustion processes is to use a concerted effort between experiments at well-characterized boundary conditions and detailed, high-fidelity models. In this paper, combustion processes of n-dodecane fuel sprays under heavy-duty Diesel engine conditions are investigated using this approach. Reacting fuel sprays are studied in a constant-volume pre-burn vessel at an ambient temperature of 900 K with three reference cases having specific combinations of injection pressure, ambient density and ambient oxygen concentration (80, 150 & 160 MPa - 22.8 & 40 kg/m 3 -15 & 20.5% O 2 ). In addition to a free jet, two different walls were placed inside the combustion vessel to study flame-wall interaction. Experimentally, low- and high-temperature reaction product distributions are imaged simultaneously using single-shot planar laser-induced fluorescence (PLIF) of formaldehyde and high-speed line-of-sight imaging of the chemically-excited hydroxyl radical (OH). Interference of soot incandescence in experimental OH∗ recordings is assessed to improve interpretation of the results. Interference by poly-cyclic aromatic hydrocarbon (PAH) LIF and soot radiation is mostly evaded by evaluating flame structures shortly after ignition for one of the studied cases, but presumably included in others. Simulations were performed using a recently developed computational fluid dynamics (CFD) methodology with detailed chemistry and turbulence-chemistry interaction. Apart from the capability to model flame structures and combustion indicators based on optical diagnostics, heat-release rate trends are predicted accurately at varying boundary conditions. Significant variation in the distribution of low-temperature combustion products under heavy-duty operating conditions are explained using both CFD simulations and a one-dimensional jet model.openMaes, Noud; Dam, Nico; Somers, Bart; Lucchini, Tommaso; D'Errico, Gianluca; Hardy, GillesMaes, Noud; Dam, Nico; Somers, Bart; Lucchini, Tommaso; D'Errico, Gianluca; Hardy, Gille