Flame surface density (FSD) models are often used in premixed turbulent combustion modeling to provide closure in large eddy simulations (LES) and Reynolds-averaged simulations. In the present study, data obtained from direct numerical simulation, with reduced chemistry, of flames developing from ignition kernels in lean methane-air mixtures is used to study the accuracy of the FSD modeling terms for LES applications. This study is conducted at elevated temperature and pressure conditions relevant to lean-burn natural gas engines. The closure models for four terms in the FSD transport equation are studied. It is observed that the propagation term as resolved by the LES grid adequately models the actual propagation term without additional sub-grid scale modeling. On the other hand, the closure of the curvature term requires a sub-grid scale model. Two sub-grid scale curvature models are studied and it is seen that there are differences between the actual and the modeled curvature terms during the early evolution of the ignition kernel. The agreement improves as the developing flame approaches a statistically steady-state. The modeling of the sub-grid convection term is analyzed and differences between the actual and the modeled terms are observed to grow during the transient evolution of the ignition kernel. A sub-grid scale model for the closure of the strain rate term shows disagreement at both early and late stages of kernel growth. In general, the sensitivity of the strain rate and curvature models to the filter width is found to be greater than for convection and propagation.

In this work, we use a dissipative-particle-dynamics-based model for two-phase flows to simulate the breakup of liquid nanocylinders. Rayleigh's criterion for capillary breakup of inviscid liquid cylinders is shown to apply for the cases considered, in agreement with prior molecular dynamics ͑MD͒ simulations. Also, as shown previously through MD simulations, satellite drops are not observed, because of the dominant role played by thermal fluctuations which lead to a symmetric breakup of the neck joining the two main drops. The parameters varied in this study are the domain size, cylinder radius, thermal length scale, viscosity, and surface tension. The breakup time does not show the same scaling dependence as in capillary breakup of liquid cylinders at the macroscale. The time variation of the radius at the point of breakup agrees with prior theoretical predictions from expressions derived with the assumption that thermal fluctuations lead to breakup.

Dissipative-particle dynamics ͑DPD͒, a particle-based fluid-simulation approach, is employed to simulate isothermal pressure-driven flow across a sphere in compliant cylindrical channels. The sphere is represented by frozen DPD particles, while the surrounding fluid is modeled using simple fluid particles. The channel walls are made up of interconnected finite extensible nonlinear elastic bead-spring chains. The wall particles at the inlet and outlet ends of the channel are frozen so as to hinge the channel. The model is assessed for accuracy by computing the drag coefficient C D in shear flow past a uniform sphere in unbounded flow, and comparing the results with those from correlations in literature. The effect of the aspect ratio of the channel, i.e., the ratio of the sphere diameter d to the channel diameter D, on the drag force F D on the sphere is investigated, and it is found that F D decreases as decreases toward the values predicted by the correlations as approaches zero. The effect of the elasticity of the wall is also studied. It is observed that as the wall becomes more elastic, there is a decrease in F D on the sphere.

Direct numerical simulations (DNS) with reduced chemistry are carried out to study the influence of flow velocity and length scales on flames developing from flame kernels in lean methane/air mixtures. This study is conducted at elevated temperature and pressure conditions relevant to lean-burn natural gas engines. There are two effects which come into consideration:the interaction of turbulence with the kernel and the interaction with the local flame. A range of scales is selected so that interactions in different regimes can be studied. A flame surface density (FSD) approach is used to determine the evolution of flame area and study how the length and velocity scales enhance or diminish flame development. From the perspective of engine performance, it is observed that shorter length scales lead to faster growth of flame kernels. The ratio of kernel diameter to length scale ratio is identified as an independent controlling variable and its effect is studied by varying the initial size of the flame kernel. Smaller ratios result in a faster increase in the flame area. It is also observed that larger velocity scales lead to higher rates of heat release.

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