solve the steady flamelet equation governing an opposed jet diffusion flame and solutions are stored as a flamelet library。 A chemical mechanism for n-heptane
solution。 This time scale is currently implemented based on a one-step irreversible reaction for diesel combustion:
with 44 species and 112 reactions [6] is used for
generating the library for diesel combustion while the mechanism for methane with 29 species and 49 reactions [7] is used for the library for simulating methane-air combustion。 The mean mass fractions stored in the library are determined from averaging the instantaneous OPPDIF solution with a joint PDF for
and as:
chem fuel o2 A
where A = 1。54e+10 and EA=77。3 kJ/mol。 These values are determined from a one-step reaction for tetradcane [8]。
Since this combustion model essentially uses a steady- state flamelet library displaced by a time scale, it is not applicable to predict the auto-ignition delay in diesel
engines。 For engine simulations, the Shell model [9] is adopted to handle auto-ignition process。
Equation (3) is simplified by assuming statistical independence of and 。 This allows the mass fraction
to be averaged as:
LES TURBULENCE MODELS
For non-premixed turbulent combustion, the fuel must be mixed with oxidizer at the molecular level before they
can react。 Thus, the turbulent mixing plays a critical role and is mostly a controlling process。 To account for mixing, the parameters which serve as inputs for the
where the probability distribution function (PDF) P() is
assumed to be a beta distribution and P() is assumed
Flamelet Time Scale combustion model, the mean mixture fraction, ˜ , mixture fraction variance, ~"2 , and
to be a lognormal distribution。 P()
and P()
can be
scalar dissipation rate, ˜ , need to be obtained from
approximately constructed with knowing their
corresponding first and the second moments。 The second moment of is assumed to be unity。 Thus,
determination of these PDF’s only requires the mean mixture fraction, ˜ , mixture fraction variance, ~"2 , and mean scalar dissipation rate ˜ 。 These quantities can be determined from the flow field with the help of LES turbulence models that will be discussed in the
subsequent section。 As a summary, the flamelet library can be generated as mean species mass fractions indexed by three key parameters:
solving the flow field。 Due to the filtering on the original equations, some subgrid scale terms appear that can not be solved directly。 These unresolved terms are modeled using a dynamic LES modeling approach without assuming an eddy viscosity [10]。 With this approach, the subgrid quantities are dynamically determined from the resolved field such that the universal model coefficients that are used in the traditional zero-equation model [11] or one-equation model [12] are not necessary。
SUBGRID STRESS TENSOR MODEL
A one-equation non-viscosity LES model called the dynamic structure LES model [10] is used to simulate the unclosed terms in the LES momentum equation: 柴油机大涡中小火焰模型模拟英文文献和中文翻译(2):http://www.youerw.com/fanyi/lunwen_100459.html