A generic formulation for modeling the sub-grid combustion in
compressible, high Reynolds number, two-phase, reacting flows has
been developed and validated. A sub-grid mixing/combustion model
called Linear Eddy Mixing (LEM) model has been extended to
compressible flows and used inside the framework of Large Eddy
Simulation (LES) in this LES-LEM approach. The LES-LEM approach is
based on the proposition that the basic mechanistic distinction
between the convective and the molecular effects should be
preserved for accurate prediction of the complex flow-fields such
as those encountered in many combustion systems. In LES-LEM, all
the physical processes such as molecular diffusion, small and
large scale turbulent convection and chemical reaction are modeled
separately but concurrently at their respective time scales. This
multi-scale phenomena is solved using a two-scale numerical
approach, wherein molecular diffusion, small scale turbulent
convection and chemical reaction are grouped as small scale
processes and the convection at the (LES grid) resolved scales are
deemed as the large scale processes. Small-scale processes are
solved using a hybrid finite-difference Monte-carlo type approach
in a one-dimensional domain. Large-scale advection on the
three-dimensional LES grid is modeled in a Lagrangian manner that
conserves mass.
Liquid droplets (represented by computational parcels) are tracked
using the Lagrangian approach wherein the Newton's equation of
motion for the discrete particles are integrated explicitly in the
Eulerian gas field.
Drag effects due to the droplets on the gas phase and the heat
transfer between the gas and the liquid phase are explicitly
included. Thus, full coupling is achieved between the two phases
in the simulation.
Validation of the compressible LES-LEM approach is conducted by
simulating the flow-field in an operational General Electric
Power Systems' combustor (LM6000). The results predicted using
the proposed approach compares well with the experiments and a
conventional (G-equation) thin-flame model.
Particle tracking algorithms used in the present study are
validated by simulating droplet laden temporal mixing layers.
Comparison of the energy growth in the fundamental and
sub-harmonic mode in the presence and absence of the droplets
shows excellent agreement with spectral DNS.
Finally, to test the ability of the present two-phase LES-LEM in
simulating partially premixed combustion, a LES of freely
propagating partially premixed flame in a droplet-laden isotropic
turbulent field is conducted. LES-LEM along with the spray models
correctly captures the flame structure in the partially premixed
flames. It was found that most of the fuel droplets completely
vaporize before reaching the flame, and hence provides a
continuous supply of reactants, which results in an intense
reaction zone similar to a premixed flame. Some of the droplets
that did not evaporate completely, traverse through the flame and
vaporize suddenly in the post flame zone. Due to the strong
spatial variation of equivalence ratio a broad flame similar to a
premixed flame is realized. Triple flame structure are also
observed in the flow-field due to the equivalence ratio
fluctuations.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/5274 |
| Date | 24 November 2003 |
| Creators | Sankaran, Vaidyanathan |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
| Format | 4280138 bytes, application/pdf |
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