Determination of modeled luminosity-based and pressure-based ignition delay times of turbulent spray combustion (original) (raw)
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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.
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.
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.
Ignition, lift-off, and soot formation studies in n-dodecane split injection spray-flames
International Journal of Engine Research, 2017
A close-coupled double injection strategy with two 0.5-ms injections separated by a 0.5-ms dwell is implemented. Studies are performed in a constant volume pre-burn type combustion vessel over two ambient temperatures (900 and 800 K) at constant density (22.8 kg/m3) with 15% O2 by volume in the ambient. The aim of this work is to investigate the establishment and dependence of ignition delay and flame stabilization on the ambient temperature conditions especially for the main injection, and thereby investigating eventual soot production. Simultaneous schlieren and planar laser -induced fluorescence experiments as well as three-dimensional Reynolds-averaged numerical simulation computational fluid dynamic modeling with chemical kinetics in every computational fluid dynamic cell were performed. It was observed experimentally that at 900 K, the second injection is injected in a high-temperature combustion recessed ambient of the first injection whereas at 800 K it is injected in a low ...
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, 2017
1 The influence of internal nozzle flow characteristics over ignition delay, and flame lift-off of reacting direct-injection sprays is studied experimentally for three fuels using two different nozzle geometries. This is a continuation of previous work by the authors, where, evaporative and non-evaporative, isothermal spray developments were studied experimentally for the same nozzle geometries and fuels. Current study reports the ignition delay through Schlieren technique, and flame lift-off length through OH* chemiluminescence visualization. The nozzle geometries consist of a conical nozzle and a cylindrical nozzle with 8.6 % larger outlet diameter when compared to the conical nozzle. The three fuels considered are n-heptane, n-dodecane and a three-component surrogate to better represent the physical and chemical properties of diesel fuel. Reacting spray is found to penetrate faster than non-reacting spray due to combustion induced acceleration after ignition. Higher oxygen concentration, and ambient temperature enhance the reactivity leading to higher spray tip penetration. Injection pressure does not affect
Numerical Simulations of Constant-Volume Spray Combustion of n-Heptane with Chemical Kinetics
Indian Journal of Science and Technology, 2017
Objectives: A reduced toluene reference fuel (TRF) mechanism of multi-component nature from the literature is utilized to simulate constant-volume spray combustion of n-heptane. The approach allows a preliminary assessment of fuel kinetic model and computational fluid dynamics (CFD) formulations in a simplified computational domain before integrating them in complex engine simulations. Methods: The operating conditions vary in ambient densities between 14.8 kg/m 3 and 30 kg/m 3 with initial oxygen concentrations ranging from 10% to 21%. The CFD models are first calibrated to replicate spray penetration lengths of the non-reacting condition. The tuned numerical models are then applied to simulate the combustion and soot formation events of reacting sprays. The soot model employed is the multi-step Moss-Brookes model with updated oxidation models. Findings: The relative errors for ignition delay and lift-off length predictions are within 35% and 22% respectively. Furthermore, simulated soot volume fraction contours agree qualitatively with the experimental soot clouds. Computed peak soot locations, however, are found to be further downstream axially as compared to the experimental results across all test cases. Application: Good agreement with experimental spatial soot distributions allows the incorporation of both fuel and soot models in engine configurations.
Combustion and Flame, 2015
An n-dodecane spray flame (Spray A from Engine Combustion Network) was simulated using a δ function combustion model along with a dynamic structure large eddy simulation (LES) model to evaluate its performance at engine-relevant conditions and to understand the transient behavior of this turbulent flame. The liquid spray was treated with a traditional Lagrangian method and the gas-phase reaction was modeled using a δ function combustion model. A 103-species skeletal mechanism was used for the n-dodecane chemical kinetic model. Significantly different flame structures and ignition processes are observed for the LES compared to those of Reynolds-averaged Navier-Stokes (RANS) predictions. The LES data suggests that the first ignition initiates in a lean mixture and propagates to a rich mixture, and the main ignition happens in the rich mixture, preferably less than 0.
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.
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.