In regarding to the efficiency of KIVA for LES, from authors’ experience, it usually takes roughly 40~50%
st
f (
; Lean , Rich )
(37)
more time to run a LES simulation than RANS using the
same mesh。 The extra computational time mainly comes from the test filtering process used by the dynamic LES
where the subscript st means evaluating at the stoichiometric condition。 Taking the average of both sides of Equation (37) using the PDF of :
Lean Richmodels。RESULTS AND DISCUSSIONS
LES simulations of a turbulent reacting methane-air jet
flame, known as the Sandia D flame [22], and a heavy-
st f (alternatively,;Lean ,Rich )
duty diesel engine, Caterpillar 3400 series [23], have been performed。
JET FLAME SIMULATION
The experimental setup consists of a main fuel jet of diameter 7。2 mm surrounded by a burnt gas jet of
st Lean Rich
f (
;Lean ,Rich )
diameter 18 mm。 The main jet composition is 25% methane and 75% air。 The Reynolds number based on the inlet of fuel stream is 22,400。 The flame is stabilized
Employing the Chebyshev inequality to seek the possible limiting value of mixture fraction within 96% confidence level:
above the burner tip by the co-flow jet。
A three-dimensional Cartesian mesh is used for the LES simulation。 A plane of this mesh is shown in Figure 1。
The grid cells are non-uniform in the radial and axial directions to refine the regions near the axis and burner exit while saving computation in other regions。 The mesh has dimensions approximately 15 cm x 15 cm x 60
Hence, the boundaries in the mixture fraction space can
be estimated as:
cm and consists of 227,475 cells。 The top boundary condition is continuative outflow while the bottom boundary is a specified inflow。 The side boundaries are set as free-slip walls。 Specifically, the inflow velocity profile is approximated using a 1/7th power law。 In order to simulate the turbulent behavior of flow entering the domain, the subgrid kinetic energy is perturbed at the
inlet by imposing on the mean profile an unsteady sine wave having amplitude of 20% of the mean。
Figure 1。 Cutting plane view of computational mesh for Sandia flame D
Figure 2 shows an instantaneous temperature field of the simulated jet。 Also shown in this figure is the surface of the stoichiometric mixture fraction, which indicates the reaction zone。 The temperature distribution in Figure 2 qualitatively agrees with the fundamental experimental observation on the jet flame。 It can be seen that high temperature region are coincident with the reaction zone。
between LES results and ensemble-averaged experimental results, the LES data must be time- averaged since LES yields time-dependent results。 The LES results are thus averaged over a sufficient long time, say 0。02 second, after the startup transients have vanished (0。09 second)。 Figure 3 and Figure 4 show the axial distribution of temperature and radial temperature distribution at several downstream locations respectively。 The agreements are generally good especially for the relatively coarse mesh used here。
Figure 3。 Centerline temperature versus axial distance from jet exit
Figure 2。 Instantaneous temperature [K] distribution, the black curve represent stoichiometric contour 柴油机大涡中小火焰模型模拟英文文献和中文翻译(6):http://www.youerw.com/fanyi/lunwen_100459.html