Title:
Simulations of vitiated bluff body stabilized flames
Simulations of vitiated bluff body stabilized flames
Author(s)
Smith, Andrew Gerard
Advisor(s)
Menon, Suresh
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Abstract
Bluff bodies have a wide range of applications where low-cost, light weight methods are needed to stabilize flames in high-speed flow. The principles of bluff body flame stabilization are straightforward, but many details are not understood; this is
especially true in vitiated environments where measurements are difficult to obtain. Most work has focused on premixed flames but changing application requirements
are now driving studies on non-premixed gaseous and spray flames. This thesis aims
to improve the understanding of vitiated, bluff body stabilized flames, specifically on
non-premixed, spray flames, through the use of Large Eddy Simulation (LES).
The single flameholder facility at Georgia Tech was chosen as the basis for the
simulations in this thesis. The flameholder was a rectangular bluff body with an
aerodynamic leading edge with discrete liquid fuel injectors embedded just upstream
of the trailing edge in a configuration described as “close-coupled.” The liquid phase
was modeled using a Lagrangian particle approach where discrete fuel droplets were
injected into the domain. Experimental data was used to tune model parameters as
well as the stripped droplet velocities and sizes. The discharge coefficient needed to
be taken into account to achieve the correct fuel jet penetration.
The experiments were conducted over a range of global equivalence ratios; lean
equivalence ratios, φ global ≈ 0.5, exhibited symmetric flame shedding and conversely
large scale sinusoidal B ́ernard/von-K ́arm ́an shedding was observed when the equiva-
lence ratio was near unity. Reacting flow LES were computed at these two fuel flow
rates to improve understanding of the different flame dynamics. LES were first com-
pleted using a quasi-laminar subgrid turbulence-chemistry interaction model. Span-
wise averaging of instantaneous and time-averaged LES results were compared with experimental high- and low-speed imaging and showed the LES was in qualitative
agreement at both fuel flow rates. At phi_global ≈ 0.5, the fuel jet did not penetrate as
far into the crossflow compared to phi_global ≈ 0.95 and thus more fuel was delivered to
the shear layers of the bluff body resulting in higher heat release in the shear layers
for the low fuel flow rate. The heat release damped the large sinusoidal structures
via gas expansion and baroclinic torque generation. Higher fuel jet penetration in the
phi_global ≈ 0.95 case meant less fuel was delivered to the shear layers and so less heat
release occurred directly behind the bluff body so the large scale sinusoidal shedding
was not damped. The impact of the subgrid turbulence-chemistry interaction model
on the flame dynamics was tested by comparing the quasi-laminar LES with LES
using the subgrid linear eddy model (LEMLES). The flame structure predicted with
LEMLES matched that of the quasi-laminar LES, at both fuel flow rates in the near-
field behind the bluff body but deviated farther downstream. A flame edge analysis
showed little sensitivity to the choice of subgrid model in the region x < 4D.
A high-order hybrid finite-difference solver with consisting of a WENO upwind
method and compact central scheme was implemented to assess the effects of the
numerical method. A series of test cases was used to verify, validate and compare
several of the available spatial and temporal methods before the high fuel flow rate
bluff body case was run. For the simple test cases the higher-order methods were
clearly more efficient but for more complex cases the differences between the second-
order and high-order methods are smaller.
To test the hypothesis that the fuel jet penetration was the main factor in the flame
dynamics another configuration with a modified fuel injector diameter was simulated.
The injector size was chosen to match the spray penetration of phi_global ≈ 0.5 case
while maintaining the fuel flow rate of the phi_global ≈ 0.95 case. The results confirmed
the hypothesis as the flame dynamics of this configuration match the original low fuel
flow rate case.
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Date Issued
2016-05-17
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Text
Resource Subtype
Dissertation