Cinematographic stereoscopic PIV, combined with Taylor's frozen flow hypothesis, is used to generate three-dimensional (3D) quasi-instantaneous pseudo volumes
of the three-component (3C) velocity field in the far field of turbulent nonpremixed
jet flames at jet exit Reynolds number Reδ in the range 8,000-15,300. The effect of heat release, however, lowers the local (i.e., based on local properties) Reynolds number to the range 1,500-2,500. The 3D data enable computation of all nine components of the velocity gradient tensor ∇u from which the major 3D kinematic
quantities, such as strain rate, vorticity, dissipation and dilatation, are computed.
The volumetric PIV is combined with simultaneously acquired 10 Hz OH planar
laser-induced fluorescence (PLIF). A single plane of the OH distribution is imaged
on the center-plane of the volume and provides an approximate planar representation
of the instantaneous reaction zone. The pseudo-volumes are reconstructed from
temporally and spatially resolved kilohertz-rate 3C velocity field measurements on
an end-view plane (perpendicular to the jet flame axis) invoking Taylor's hypothesis.
The interpretation of the measurements is therefore twofold: the measurements provide
a time-series representation of all nine velocity gradients on a single end-view plane or, after volumetric reconstruction, they offer a volumetric representation, albeit
approximate, of the spatial structure of the flow. The combined datasets enable
investigation of the fine-scale spatial structure of turbulence, the effect of the reaction
zone on these structures and the relationship between the jet kinematics and the
reaction zone. Emphasis is placed on the energy dissipation field and on the presence
and role of dilatation. Statistics of the components of the velocity gradient tensor
and its derived quantities show that these jet flames exhibit strong similarities to incompressible
turbulent flows, such as in the distribution of the principal strain rates
and strain-vorticity alignment. However, the velocity-gradient statistics show that
these jet flames do not exhibit small-scale isotropy but exhibit a strong preference
for high-magnitude radial gradients, which are attributed to regions of strong shear
induced by the reaction zone. The pseudo volumes reveal that the intense-vorticity
field is organized in two major classes of structures: tube-like away from the reaction
zone (the classical worms observed in incompressible turbulence) and sheet-like in
the vicinity of the local reaction zone. Sheet-like structures are, however, the dominant
ones. Moreover, unlike incompressible turbulence where sheet-like dissipative
structures enfold, but don't coincide with, clusters of tube-like vortical structures, it
is observed that the sheet-like intense-vorticity structures tend to closely correspond
to sheet-like structures of high dissipation. The primary reason for these features is
believed to be due to the stabilizing effect of heat release on these relatively low local
Reynolds number jet flames. It is further observed that regions of both positive and
negative dilatation are present and tend to be associated with the oxidizer and fuel
sides of the OH zones, respectively. These dilatation features are mostly organized in
small-scale, short-lived blobby structures that are believed to be mainly due to convection
of regions of varying density rather than to instantaneous heat release rate.
A model of the dilatation field developed by previous researchers using a flamelet
approximation of the reaction zone was used to provide insights into the observed
features of the dilatation field. Measurements in an unsteady laminar nonpremixed
jet flame where dilatation is expected to be absent support the simplified model and
indicate that the observed structure of dilatation is not just a result of residual noise
in the measurements, although resolution effects might mask some of the features of
the dilatation field. The field of kinetic energy dissipation is further investigated by
decomposing the instantaneous dissipation field into the solenoidal, dilatational and
inhomogeneous components. Analysis of the current measurements reveals that the
effect of dilatation on dissipation is minimal at all times (it contributes to the mean
kinetic energy dissipation only by about 5-10%). Most of the mean dissipation
arises from the solenoidal component. On average, the inhomogeneous component
is nearly zero, although instantaneously it can be the dominant component. Two
mechanisms are believed to be important for energy dissipation. Near the reaction
zone, where the stabilizing effect of heat release generates layers of laminar-like shear
and hence high vorticity, solenoidal dissipation (which is proportional to the enstrophy)
dominates. In the rest of the ow the inhomogeneous component dominates in
regions subjected to complex systems of nested vortical structures where the mutual
interaction of interwoven vortical structures in intervening regions generates intense
dissipation. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/ETD-UT-2009-05-161 |
| Date | 03 September 2009 |
| Creators | Gamba, Mirko |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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