<P> The work presented m this thesis focused on using the hybrid Unsteady Reynolds-Averaged
Navier-Stokes (URANS)/Large Eddy Simulation (LES) methodology to
investigate the flow pulsation phenomenon in compound rectangular channels for
isothermal flows. The specific form of the hybrid URANS/LES approach that was used is
the Strelets (2001) version of the Detached Eddy Simulation (DES). It is of fundamental
interest to study the problem of flow pulsations, as it is one of the most important
mechanisms that directly affect the heat transfer occurring in sub-channel geometries
such as those in nuclear fuel bundles. The predictions associated with the heat transfer
and fluid flow in sub-channel geometry can be used to develop simplified physical
models for sub-channel mixing for use in broader safety analysis codes. The primary goal
of the current research work was to determine the applicability of the DES approach to
predict the flow pulsations in sub-channel geometries. It was of interest to see how
accurately the dynamics associated with the flow pulsations can be resolved from a
spatial-temporal perspective using the specific DES model. The research work carried out
for this thesis was divided into two stages. </p> <p> In the first stage of the research work, effort was concentrated to primarily
understand the field of sub-channel flow pulsations and its implications from both an
experimental and numerical point of view. It was noted that unsteady turbulence
modeling approaches have great potential in providing insights into the fundamentals of
sub-channel flow pulsations. It was proposed that for this thesis work, the Shear Stress
Transport (SST) based DES model be used to understand the dynamics associated with sub-channel flow pulsations. To the author's knowledge the DES-SST based turbulence
model has never been used for resolving the effects of sub-channel flow pulsations. Next,
the hybrid URANS/LES turbulence modeling technique was reviewed in great detail to
understand the philosophy of the hybrid URANS/LES technique and its ability to resolve
fundamental flows of interest. Effort was directed to understand the switching mechanism
(which blends the URANS region with the LES region) in the DES-SST model for fully
wall bounded turbulent flows without boundary layer separation. To the author's
knowledge, the DES-SST model has never been used on a fully wall bounded turbulent
flow problem without boundary layer separation. Thus, the DES-SST model was first
completely validated for a fully developed turbulent channel flow problem without
boundary layer separation. </p> <p> In the second stage of the research work, the DES-SST model was used to study
the flow pulsation phenomena on two rectangular sub-channels connected by a gap, on
which extensive experiments were conducted by Meyer and Rehme (1994). It was found
that the DES-SST model was successful in resolving significant portion of the flow field
in the vicinity of the gap region. The span-wise velocity contours, velocity vector plots,
and time traces of the velocity components showed the expected cross flow mixing
between the sub-channels through the gap. The predicted turbulent kinetic energy showed
two clear peaks at the edges of the gap. The dynamics of the flow pulsations were
quantitatively described through temporal auto-correlations, spatial cross-correlations and
power spectral functions. The numerical predictions were in general agreement with the
experiments in terms of the quantitative aspects. From an instantaneous time scale point of view, the DES-SST model was able to identify different flow mixing patterns. The
pulsating flow is basically an effect of the variation of the pressure field which is a
response to the instability causing the fluid flow pulsations. Coherent structures were
identified in the flow field to be comprised of eddies, shear zones and streams. Eddy
structures with high vorticity and low pressure cores were found to exist near the vicinity
of the gap edge region. A three dimensional vorticity field was identified and found to
exist near the gap edge region. The instability mechanism and the probable cause behind
the quasi-periodic fluid flow pulsations was identified and related to the inflectional
stream-wise velocity profile. Simulations were also performed with two different channel
lengths in comparison to the reference channel length. Different channel length studies
showed similar statistical description of the flow field. However, frequency independent
results were not obtained. In general, simulations performed using the DES-SST model
were successful in capturing the effects of the fluid flow pulsations. This modeling
technique has great potential to be used for actual rod bundle configurations. </p> / Thesis / Doctor of Philosophy (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/19459 |
| Date | 07 1900 |
| Creators | Home, Deepayan |
| Contributors | Lightstone, Marilyn, Mechanical Engineering |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
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