A Numerical Study on Detailed Soot Formation Processes in Diesel Combustion

Abstract

This study simulates soot formation processes in diesel combustion using a large eddy simulation (LES) model, based on a one-equation subgrid turbulent kinetic energy model. This approach was implemented in the KIVA4 code, and used to model diesel spray combustion within a constant volume chamber. The combustion model uses a direct integration approach with a fast explicit ordinary differential equation (ODE) solver, and is additionally parallelized using OpenMP. The soot mass production within each computation cell was determined using a phenomenological soot formation model developed by Waseda University. This model was combined with the LES code mentioned above, and included the following important steps: particle inception during which acenaphthylene (A 2 R 5 ) grows irreversibly to form soot; surface growth with driven by reactions with C 2 H 2 ; surface oxidation by OH radical and O 2 attack; and particle coagulation.The results obtained using our new model are compared to those generated using a RANS (RNG k-epsilon) model, and also to experimental data from the engine combustion network (ECN) of Sandia National Laboratories. The sensitivity of the LES results to mesh resolution is also discussed. The results show that both RANS and LES simulations predict the dispersion and vapor penetration of the injected fuel fairly well. LES generally provides flow and spray characteristics in better agreement with experimental data than RANS. It is also shown that the phenomenological soot model is useful for investigating soot particle production and distribution. The LES model was better than the RANS model at describing instantaneous soot concentration contour.

Thermocouple temperature measurements in diesel spray flame for validation of in-flame soot formation dynamics

“…The minimum mesh size was set at 0.6 mm 3 cubic and extended along the spray axis reducing the spatial definition toward downstream in order to compensate the cost and the spatial resolution of the calculation. Additional details on the computational methods are available in Zhou et al 8 The boundary conditions for the simulation were set to be identical with the above-explained experiment as much as possible, except that the fuel in the simulation is n-tridecane while the one in the experiment is FTD fuel as indicated in Table 1. The simulated fuel has 5% lower calorific value and higher cetane number (90 for n-tridecane/simulation and 69 for FTD/experiment) with the same density.…”

“…The minimum mesh size was set at 0.6 mm 3 cubic and extended along the spray axis reducing the spatial definition toward downstream in order to compensate the cost and the spatial resolution of the calculation. Additional details on the computational methods are available in Zhou et al 8…”

“…Due to the progress of computing technologies and fundamental soot formation dynamics, 1,2 multidimensional numerical analyses of in-flame or incylinder soot processes employing reaction kinetics, either relatively simple or detailed ones depending on the objectives, are becoming feasible. [3][4][5][6][7][8][9] However, the validation of these numerical results with reliable experimental data under engine-equivalent conditions is indispensable. For effective validation, not only the quantitative accuracy of boundary conditions and measurements with wide and relevant ranges of conditions 10 but also the choice of appropriate measureable variable for the validation are crucial.…”

“…The measured temperatures were compared with conventional and novel ''scope-rod'' two-color temperatures, contrasted with previous laser spectroscopic measurements of in-flame PAHs [25][26][27] and predicted temperature and soot processes from large eddy simulation (LES) employing detailed chemical kinetics including soot formation. 8…”

Quantitative high-resolution transmission electron microscopy nanostructure analysis of soot oxidized in diesel spray flame periphery

“…Small fraction of error in the soot oxidation prediction therefore may result in significant error in the soot emission prediction, as discussed, for example, by Kamimoto et al 3 Present understanding on diesel in-flame soot oxidation is limited. For example, recent advanced simulation studies of diesel in-flame soot processes [4][5][6][7][8] generally employ rather classical soot oxidation models, such as the well-known ones by Nagle and Strickland-Constable 9 and Neoh et al 10 These models are based on low-pressure burner experiments conducted up to 40 years ago, and their applicability to the high-pressure and high-temperature in-flame conditions equivalent to modern diesel engines should be re-examined. As reviewed by Stanmore et al, 11 the high-temperature soot oxidation has been studied in laminar burner flames and shock tubes, and the results have been somewhat reflected to models for engine applications.…”