层流小火焰燃烧模型

Examples of CFD calculations based on the flamelet approach are shown for a Diesel engine and for a gas turbine combustor.1 IntrFra bibliotekduction
Technical processes in turbulent combustion are in general subdivided into two classes: premixed or non-premixed combustion. For example, combustion in homogeneous charge, spark-ignition engines occurs under premixed conditions. On the other hand, combustion in a Diesel engine or in furnaces essentially takes place under nonpremixed conditions.
fluctuation
vα = v¯α + v0α :
(1)
This decomposition is then introduced into the Navier-Stokes equations to derive equations The non-linear convective terms in the Navier-Stokes equations are quadratic. Consequently
Slow chemistry is not very often of practical interest: there are a few situations like low NOx burners which operate with strong exhaust gas recirculation and therefore lower temperature where the chemistry is slow compared to turbulent mixing. Also, in the post-flame region of a spark ignition engine NOx production is slow while the temperature field is nearly homogeneous. On the other hand, reacting flows with fast chemistry occur in nearly all the applications mentioned above. The reason is simple: for combustion to be stable and efficient, the temperature must be high and therefore the chemical time scales are short. Therefore engines are designed such that only at
1
premixed
fast chemistry
spark-ignition engines
slow chemistry
NOx formation in post-flame regions
partially and non-premixed Diesel engines gas turbine engines combustion in furnaces
v0αv0βv0γ. The equations for
for the
the unknown correlations triple correlations contain
fourth order correlations and so on. This is called the closure problem of turbulent flow. All these correlations are
Rather than trying to solve the most general problem, classical closure models consider only the first two mo-
Another criterion to classify turbulent combustion is related to the ratio of turbulent to chemical time scales. If turbulence is very intense the turbulent time is likely to be short. Chemical time scales become short if the temperature is high and they become long with decreasing temperature. The case of short turbulent and long chemical time scales is simply called slow chemistry while the case of a comparatively long turbulent and short chemical time scale is called fast chemistry.
moments of the joint probability density function Pvα; vβ; vγ. Although a transport equation for Pvα; vβ; vγ can be derived, its closure is by no means evident.
The Use of Flamelet Models in
CFD-Simulations
N. Peters Institut für Technische Mechanik
RWTH Aachen ERCOFTAC-Centre
Germany
Abstract
The state-of-the-art of combustion models for implementation in CFD codes is reviewed. Existing closure assumptions suffer in general from an insufficient understanding of the principles of closure hypotheses in turbulent flows. It is shown that for fast chemistry not only the chemical source term but also the scalar transport is effected by chemistry. Therefore the gradient transport hypothesis is also not valid for reacting scalars. Models based on pdf transport require in addition a closure for the molecular diffusion term. These may therefore be justified in the limit of slow chemistry only.
Fig. 1 shows a diagram where combustion applications are classified with respect to both criteria mentioned above.
2 Governing Equations
2.1 Principles of Turbulent Modeling and Closure
It is clear, that in addition to premixed and non-premixed combustion, partially premixed combustion plays, at least locally, an important role in technical applications. An important example are modern gas turbine combustion chambers where fuel rich regions are used for flame stabilization but, in order to minimize NOx formation, most of the combustion occurs under premixed fuel lean conditions. Similarly, combustion in direct injection or stratified charge spark-ignition engines occurs under partially premixed conditions.
tfeorrmthseomf tehaenfovramluev0αvvα0β.
ov0αccvu0βrwinhitchheweqilul athtieonnscofonrtav¯iαn.trTiphleerecfoorrreeliattiisoncsonovfethneiefnotrtmo
derive transport equations
low NOx-burners
Figure 1: Classification of combustion applications.
limit conditions, i.e. at very high engine speeds, turbulent time scales may become comparable to chemical time scales.