due to the fact that the spray cause high strain rate, resulting larger turbulent kinetic energy。 To see the evolution of scalar dissipation rate more clearly, the maximum value for the scalar dissipation rate in the cylinder is shown versus crank angle as a simple line plot in Figure 10。

Figure 8。 Contours for mixture fraction (left) and mixture fraction variance (right) at a series of crank angles

Figure 7。 Pressure trace and heat release rate for baseline mode of Caterpillar 3400 series engine

Figure 9。 Contours of conditional scalar dissipation rate at a series of crank angles

Figure 8 shows the evolution of the mean mixture fraction over a series of crank angles during injection。 Fuel vapor originating from the droplets acts as a source term for mixture fraction, causing the mixture fraction to increase as time and the maximum value to be found in the spray core。 Due to the turbulent mixing process, the mixture fraction is convected and diffused away from the spray very quickly。 The evolution of the mixture fraction variance is also shown in Figure 8。 The transport of the mixture fraction variance is similar to the mean mixture fraction due to the similar transport process。 Contours of the conditional scalar dissipation rate, ˜st  , are shown  in

Shown in Figure 10 are the unconditional  mean, ˜  , along with conditional mean at the stoichiometric condition,  ˜st  。  As can be seen,  ˜  grows  rapidly after

the start of the injection (-9 ATDC) due to the spray generated turbulent kinetic energy。 When ignition occurs (about -3 ATDC), combustion induced expansion generates turbulence and enhances the mixing so as to increase ˜  。 After the end of injection (12 ATDC), the

source of turbulent kinetic energy drops significantly and

Figure  9。  ˜st

is generally large near the spray。 This     is

the mixture becomes nearly homogeneous, causing    ˜

rapidly   to   approach   zero。   The   conditional     scalar engine simulation。 The results show that the shape of

dissipation  rate

˜st

follows   the   similar   trend   to the

the conditional scalar dissipation rate in the engine is in accordance  with  the  relevant  physical  and    chemical

unconditional  scalar  dissipation  rate ˜    。  It  should   be

process inside the cylinder。

noticed   that   magnitude  of

than ˜ 。 This is qualitatively reasonable because  usually peaks at the reaction zone as a result of maximum gradient of mixture fraction normal to the flamelet surface。

The occurrence of local extinctions could be approximately predicted by comparing the local conditional   scalar   dissipation   rate   at   stoichiometric

ACKNOWLEDGMENTS

The authors would like to thank the support by the DOE SciDAC Program。

REFERENCEScondition ˜st

against the quenching value of  conditional1。

Peters,  N。,  “Laminar  Diffusion  Flamelet  Models inscalar  dissipation rate

˜st ,q   at  stoichiometric  condition

Non-premixed  Turbulent  Combustion”,  Progress inthat is found to be 94 seond-1 using OPPDIF solutions with n-Heptane mechanism。 From the magnitude of ˜st predicted here, local extinctions might not be happening under this baseline engine operating conditions because predicted values are significantly lower than the 摘要：大涡模拟（LES）的时间尺度和小火焰面燃烧模型是用来模拟柴油机的燃烧。小火焰的时间尺度模型对被平均混合分数指数、混合分数方差和平均标量耗散率所表征的正庚烷使用稳态小火焰面库。在燃烧模型中，趋于时间标尺下小火焰面库的模拟结果的反应过程与最慢的反应有关。小火焰面的模拟结果和化学时间尺度的结合有助于解释非稳态混合的影响。湍流次网格应力使用单方程、不考虑粘性的被称为动态结构模型的大涡模拟。该模型采用动态过程和次网格动能决定的张量系数。该模型已扩展到包含了标量混合和标量耗散。一个新的条件标量耗散模型已发展到能更好地预测局部熄火。Sandia射流火焰和重型柴油发动机的模拟是用来通过与实验结果进行比较来发展和验证模型的。该模型与实验数据和详细分析相符合，这表明在模型中的项在物理上是合理的。论文网