Title page for ETD etd-05112006-154758
|Type of Document
||Modified source-type flame model and vorticity generated by the flame and bluff bodies
||Aerospace and Ocean Engineering
|Grossman, Bernard M.
|Jakubowski, Antoni K.
|Mason, William H.
|O'Brien, Walter F. Jr.
|Schetz, Joseph A.
- Flame Research
- Turbulence Research
|Date of Defense
A numerical model is developed to simulate the wrinkled laminar flame sheet
flapping in weakly turbulent premixed combustion. The wrinkled laminar flame
sheet is represented by a discrete distribution of volume sources called source
disks. These source disks are utilized to produce the acceleration of combustion
prod ucts behind the flame sheet. The laminar flame speed is allowed to vary according
to flame stretch. A modified source model is proposed against the background
of the existing source model's physically unrealistic symmetric expansion
in both the upstream and the downstream directions. This flame model also includes
flame-generated vorticity which is associated with the increasing entropy
intrinsic to any system going through an irreversible process. The flame-generated
vorticity is treated as discrete vortex disks. Vorticity created on the surface of the
flame holder is computed with the vortex sheet method and diffuses into the surrounding
flow in the form of vortex disks. The freestream turbulence is simulated
by injecting vortex disks into an initially uniform freestream.
Flame-flow interactions are studied when a thin circular cylinder, a large circular
cylinder, and a flat plate normal to freestream are used as flame holders. Results
sho\v that the modified source model gives more accurate prediction of flame angle
than the existing source model does, the relative errors can be reduced by as
much as four times. The modified source model also produces velocity profiles
closer to those found in experiment, the deviations are cut by half at most sampling
points in the flow. The vorticity shed from a thin circular cylinder flame
stabilizer is found to only influence downstream regions very close to the cylinder.
The eddy shedding behind a bluff body flame holder is suppressed in reacting
flow simulations and the computed recirculating zone in a reacting flow is nearly
half as long as that in a cold flow. When the relative size of the flame holder is
one order of magnitude larger than the thickness of flame sheet, the vorticity shed
by the flame holder can no longer be neglected. Flame wrinkling and flame ex·
tinction caused by vortical fluid motion behind the flame holder are found
through numerical simulation.
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