Large eddy simulation of a reacting spray flame with multiple realizations under compression ignition engine conditions (original) (raw)
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Large Eddy Simulation of Turbulent Spray Combustion
The two-phase filtered mass density function (FMDF) method is employed for large eddy simulation (LES) of high speed evaporating and combusting n-heptane sprays using simple (global) and complex (skeletal) chemical kinetic mechanisms. The resolved fluid velocity and pressure fields are obtained by solving the filtered compressible Navier–Stokes equations with high-order Eulerian finite difference methods. The liquid spray and gas scalar (temperature and species mass fractions) fields are both obtained by Lagrangian stochastic models. The chemistry calculation is accelerated by incorporating the parallel in situ adaptive tabulation (ISAT) method. There are two-way interactions among Eulerian and Lagrangian fields. Simulations of evaporating sprays with and without combustion indicate that the two-phase LES/FMDF results are consistent and compare well with the available experimental data at different gas temperatures and oxygen concentrations. The spray controlled flame tends to move away from a diffusion flame structure toward a premixed one as the oxygen concentration decreases and/or the ambient gas temperature increases because of changes in spray-induced turbulence and mixing. The LES/FMDF results for ignition delay show more sensitivity to the chemical kinetic model at lower gas temperatures due to slower reaction and stronger turbulence–chemistry interactions. The liftoff length is less sensitive to the kinetics.
Flow, Turbulence and Combustion, 2016
Accurate modelling of spray combustion process is essential for efficiency improvement and emissions reduction in practical combustion engines. In this work, both unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and large eddy simulations (LES) are performed to investigate the effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame in a constant volume chamber at realistic conditions. The non-reacting spray process is first simulated with URANS to investigate the effects of entrainment gas-jet model on the penetration characteristics and fuel vapor distributions. It is found that the droplet motion near the nozzle has significant influence on the fuel vapor distribution, while the liquid penetration length is controlled by the evaporation process and insensitive to gas-jet model. For the case considered, both URANS with the gas-jet model and large eddy simulations can properly predict the vapor penetration. For the combustion characteristics, it is found that LES yields better predictions in the global combustion characteristics. The URANS with gas jet model Zhuyin Ren
Fuel, 2021
Numerical simulations using large eddy simulation (LES) and Unsteady Reynolds Averaged Navier-Stokes (URANS) are carried out to identify the underlying mechanisms that govern the early soot evolution process in an n-dodecane spray flame at 21% O 2 by molar concentration. A two-equation phenomenological soot model is used here to simulate soot formation and oxidation. Both ignition delay time (IDT) and lift-off length (LOL) are found to agree with experimental measurements. The transient evolution of soot mass, in particularly the soot spike phenomenon, is captured in the present LES cases, but not in the URANS cases. Hence, a comparison of numerical results from LES and URANS simulations is conducted to provide a better insight of this phenomenon. LES is able to predict the rapid increasing soot mass during the early stage of soot formation due to having a large favorable region of equivalence ratio (ϕ > 1.5) and temperature (T > 1800 K) for soot formation. This favorable region increases and then decreases to reach a quasi-steady state in the LES case, while it continues to increase in the URANS simulation during the early time. In addition, the soot spike is a consequence of the competition between soot formation and oxidation rates. The time instance when the total soot mass reaches peak value coincides with the time instance when the total mass of soot precursor reaches a plateau. The soot spike is formed due to the continuous increase of oxidizing species in the LES case which leads to a more dominant oxidation process than the formation process.
Turbulent spray combustion simulations based on a new skeletal mechanism for n-dodecane
A study of turbulent spray combustion of n-dodecane was conducted using computational fluid dynamics simulations. We report a new skeletal mechanism based on the reduction of a detailed kinetic reaction mechanism for high pressure conditions (50-60 bar), temperatures from 750 to 2500 K, and a range of equivalence ratios from 0.5 to 1.5. The skeletal mechanism has 85 species and 266 reactions. The mechanism was implemented in a computational fluid dynamic code to model the combustion of n-dodecane in a high pressure (60 bar) and temperature (900 K) constant volume chamber. A dynamic structure turbulence model with fine mesh size was utilized. Both first-stage low-temperature combustion, or cool-flame, and second-stage high-temperature combustion were observed due to the decrease in the gas temperature surrounding the spray caused by the fuel evaporative cooling. The species mass fraction histories were studied numerically to find a correlation between first-stage and second-stage combustion and species consumption. Species mass fractions, combustion chamber pressure, and combusting n-dodecane vapor penetration histories were studied computationally, and the results were compared with experiments to find a numerical equivalent to the light-based activated OH chemiluminescence ignition delay experiment.
High Fidelity Simulations of Turbulent Spray Combustion
A high–fidelity two-phase large eddy simulation (LES)/filtered mass density function (FMDF) model is developed and used for detailed simulations of turbulent spray breakup, evaporation and combustion. The spray is simulated with Lagrangian droplet transport, stochastic breakup, wake, collision/coalescence and finite rate heat and mass transfer submodels. The spray model is used together with the compressible, Eulerian LES gas flow model for velocity and pressure fields and the two-phase Lagrangian FMDF for the scalar (species mass fraction and enthalpy) field. There are two-way couplings between all Lagrangian and Eulerian models. The numerical results for non-reacting and reacting sprays are compared with the available experimental data for global spray variables such as the spray penetration length, ignition delay and flame lift-off lengths. It is shown that the two-phase LES/FMDF results are consistent and compare well with the experimental data.
Study of Turbulent Spray Combustion of N-Dodecane Fuel
Volume 2: Emissions Control Systems; Instrumentation, Controls, and Hybrids; Numerical Simulation; Engine Design and Mechanical Development, 2015
Turbulent spray combustion of n-dodecane fuel was studied numerically in current paper. The ignition delay, lift-off length, combustion chamber pressure rise, fuel penetration and vapor mass fraction were compared with experimental data. n-Dodecane kinetic model was studied by using a recently developed mechanism. The combustion chamber pressure rise was modeled and compared with experiments; the result was corrected for speed-of-sound to find the ignition delay timing in comparison with pressure-based ignition delay measurement. Species time histories and reaction paths at low and high temperature combustion are modeled and studied at two conditions, 900 K and 1200 K combustion chamber temperatures. The modeled species mass histories were discussed to define the firststage and total ignition delay timings. Among all of the studied species in this work, including OH, Hydroperoxyalkyl mass history can be utilized to determine the exact timing of luminosity-based ignition delay. Moreover, n-dodecane vapor penetration can be used to determine the luminosity-based ignition delay.
International Journal of Heat and Mass Transfer, 2016
Turbulent spray combustion of n-dodecane was studied computationally in a constant volume combustion chamber, with a special emphasis on determining an analogous definition of experimental luminosity-based ignition delay time for computational fluid dynamics simulations. This modeling study was conducted over a range of initial combustion chamber temperatures varying from 900 K to 1200 K and gas density of 22.8 kg/m 3 , using Large Eddy Simulation of turbulence, multizone combustion model, adaptive mesh refinement, and a skeletal n-dodecane chemical kinetic model. The spray and jet penetrations, combustion chamber pressure rise, fuel vapor mass fraction, and flame lift-off length were modeled and compared with the experimental data. Among all of the key species and spray characteristics studied, the modeled ignition delay times based on the n-dodecane vapor penetration, the hydroperoxyalkyl (QOOH) and hydroxyl (OH) mass history were shown to better match with the experimental results, and hence can be utilized in simulations to accurately determine the luminosity-based ignition delay times.
2023
Here, a finite-rate chemistry large-eddy simulation (LES) solver is utilized to investigate dual-fuel (DF) ignition process of n-dodecane spray injection into a methane-air mixture at engine-relevant ambient temperatures. The investigated configurations correspond to single-fuel (SF) φ CH 4 = 0 and DF φ CH 4 = 0.5 conditions for a range of temperatures. The simulation setup is a continuation of the work by Kahila et al. (2019, Combustion and Flame) with the baseline SF spray setup corresponding to the Engine Combustion Network (ECN) Spray A configuration. First, ignition is investigated at different ambient temperatures in 0D and 1D studies in order to isolate the effect of chemistry and chemical mechanism selection to ignition delay time (IDT). Second, 3D LES of SF and DF sprays at three different ambient temperatures is carried out. Third, a reaction sensitivity analysis is performed to investigate the effect of ambient temperature on the most sensitive reactions. The main findings of the paper are as follows: (1) DF ignition characteristics depend on the choice of chemical mechanism, particularly at lower temperatures. (2) Addition of methane to the ambient mixture delays ignition, and this effect is the strongest at lower temperatures. (3) While the inhibiting effect of methane on low-and high-temperature IDT's is evident, the time difference between these two stages is shown to be only slightly dependent on temperature. (4) Reaction sensitivity analysis indicates that reactions related to methane oxidation are more pronounced at lower temperatures. The provided quantitative results indicate the strong ambient temperature sensitivity of the DF ignition process.
Analysis of LES-based combustion models applied to an acetone turbulent spray flame
Combustion Science and Technology, 2018
Two different combustion models are analyzed for the prediction of an acetone turbulent diluted spray flame. Simulations are conducted in the Large Eddy Simulation (LES) framework, coupled with the Flamelet Generated Manifold (FGM) chemistry reduction method. To represent the polydispersed spray the Eulerian-Lagrangian specification is applied. Both combustion models consist of the Artificially Thickened Flame (ATF) and the presumed PDF approach. Effects of the evaporative cooling and the presence of droplets into the combustion modeling are accounted for. Results achieved with both models are validated against experimental data. These consist in statistical data of droplets velocities, liquid volumetric flux, a characteristic diameter, and temperature. A general good agreement with experimental data is observed. Analysis of simulations results allow deeper interpretation of additional flame features, for instance the double flame structure. As an outcome, the concept of the burning potential is introduced in this paper to assist the interpretation of the underlying mechanisms to the occurrence of different flame modes.
Large-eddy simulation on the influence of injection pressure in reacting Spray A
Combustion and Flame
The Engine Combustion Network (ECN) Spray A target case corresponds to high-pressure liquid fuel injection in conditions relevant to diesel engines. Following the procedure by Wehrfritz et al. (2016), we utilize large-eddy simulation (LES) and flamelet generated manifold (FGM) methods to carry out an injection pressure sensitivity study for Spray A at 50, 100 and 150 MPa. Comparison with experiments is shown for both non-reacting and reacting conditions. Validation results in non-reacting conditions indicate relatively good agreement between the present LES and experimental data, with some deviation in mixture fraction radial profiles. In reacting conditions, the simulated flame lift-off length (FLOL) increases with injection pressure, deviating from the experiments by 4-14%. Respectively, the ignition delay time (IDT) decreases with increasing injection pressure and it is underpredicted in the simulations by 10-20%. Analysis of the underlying chemistry manifold implies that the observed discrepancies can be explained by the differences between experimental and computational mixing processes.