ABSTRACT Large Eddy Simulation (LES) with a flamelet time scale combustion model is used to simulate  diesel combustion。 The flamelet time scale model uses a steady-state flamelet library for n-heptane indexed by mean mixture fraction, mixture fraction variance, and mean scalar dissipation rate。 In the combustion model, reactions proceed towards the flamelet library solution at a time scale associated with the slowest reaction。 This combination of a flamelet solution and a chemical time scale helps to account for unsteady mixing effects。 The turbulent sub-grid stresses are simulated using a one- equation, non-viscosity LES model called the dynamic structure model。 The model uses a tensor coefficient determined by the dynamic procedure and the subgrid kinetic energy。 The model has been expanded to include scalar mixing and scalar dissipation。 A new model for the conditional scalar dissipation has been developed to better predict local extinction。 Simulations of a Sandia jet flame and a heavy-duty diesel engine are used to develop and validate the models with comparisons to experimental results。 The models show  good comparison with the data and detailed analysis indicates terms within the model are physically reasonable。84566

INTRODUCTION

Internal combustion engines and gas turbines are required  to  improve  combustion  efficiency  and reduce

scales below the grid size are modeled using subgrid models。 In CFD, the transport equations are solved for filtered flow variables, such as filtered velocity and energy, which are obtained by a filtering process representing the local spatial averaging。 Filtering yields subgrid terms that can not be evaluated without help of subgrid models。

COMBUSTION MODEL

The goal of the LES combustion modeling is to evaluate the filtered reaction rate terms appearing in the species and energy conservation equations。  These terms contain subgrid chemistry-turbulence effects and thus can not be evaluated directly from resolved fields。 A flamelet approach originating from the laminar flamelet concept [1] is used to help with modeling these terms。 Many flamelet models for non-premixed combustion assume that a turbulent diffusion flame behaves locally as a steady, one-dimensional, laminar, and strained flame [2, 3]。 While this assumption works well in many practical applications, it fails to take into account the low Damkohler number regimes in which unsteady effects become important。 To account for unsteady effects at an economic computational cost, the Flamelet Time Scale (FTS) model [4], which is discussed in detail below, is applied to simulate combustion。

The transient flamelet equation is given in [1]:

pollutant emissions significantly。 Accurate prediction of pollutant emissions is dependent upon a good

understanding on the unsteady mixing process and combustion process。 Since conventional time-averaged methods, such as RANS, tend to smear out highly time- dependent features of the turbulent flow, Large Eddy Simulation (LES), based on spatial averaging, is becoming  a  better  choice  to  study  unsteady turbulent

where is mixture fraction and is called the scalar dissipation rate which accounts for mixing effects:

reacting flows, which are dominant in the engineering combustion devices。

The basic idea of LES is that all the scales larger than the computational grid size are directly solved by a space- and time- accurate method while the effects of

i i

Considering the computational cost, this flamelet equation is not solved interactively with the CFD code instead the OPPDIF code [5] is used to independently