scholarly journals Experimental and Analytical Study of Axial Turbulent Flows in an Interior Subchannel of a Bare Rod Bundle

1976 ◽  
Vol 98 (2) ◽  
pp. 262-268 ◽  
Author(s):  
P. Carajilescov ◽  
N. E. Todreas
Keyword(s):  
Velocity Field ◽  
Turbulent Flows ◽  
Length Distribution ◽  
Secondary Flows ◽  
Reynolds Stresses ◽  
Rod Bundle ◽  
Aspect Ratios ◽  
Fuel Elements ◽  
Velocity Turbulence ◽  
Rod Array

Reactor fuel elements generally consist of rod bundles with the coolant flowing axially through the bundles in the space between the rods. Heat transfer calculations form an important part in the design of such elements, which can only be carried out if information of the velocity field is available. A one-equation statistical model of turbulence is applied to compute the detailed description of velocity field (axial and secondary flows) and the wall shear stress distribution of steady, fully developed turbulent flows with incompressible, temperature-independent fluid, flowing through triangular arrays of rods with different aspect ratios (P/D). Also experimental measurements of the distributions of the axial velocity, turbulence kinetic energy, and Reynolds stresses were performed using a laser Doppler anemometer (LDA), operating in a “fringe” mode with forward scattering, in a simulated interior subchannel of a triangular rod array with P/D = 1.123 and L/DH = 77. From the experimental results, a new mixing length distribution is proposed. Comparisons between the analytical results and the results of this experiment as well as other experimental data available in the literature are presented. The results are in good agreement.

Turbulence Modeling of Axial Flow in a Bare Rod Bundle

1979 ◽  
Vol 101 (4) ◽  
pp. 628-634 ◽  
Author(s):  
J. G. Bartzis ◽  
N. E. Todreas
Keyword(s):  
Secondary Flow ◽  
Turbulent Flows ◽  
Axial Velocity ◽  
Nuclear Reactor ◽  
Turbulence Modeling ◽  
Three Dimensional ◽  
Axial Flow ◽  
Reynolds Stresses ◽  
Rod Bundle ◽  
Aspect Ratios

Temperature distribution within the rod bundle of a nuclear reactor is of major importance in nuclear reactor design. However temperature information presupposes knowledge of the hydrodynamic behavior of the coolant which is the most difficult part of the problem due to the complexity of the turbulence phenomena. In the present work a two equation turbulence model—a strong candidate for analyzing actual three dimensional turbulent flows—has been used to predict fully developed flow of infinite bare rod bundle of various aspect ratios (P/D). The model has been modified to take into account anisotropic effects of eddy viscosity. Secondary flow calculations have been also performed although the model seems to be too rough to predict the secondary flow correctly. Heat transfer calculations have been performed to confirm the importance of anisotropic viscosity in temperature predictions. Experimental measurements of the distribution of axial velocity, turbulent axial velocity, turbulent kinetic energy and radial Reynolds stresses were performed in the developing and fully developed regions. A two channel Laser Doppler Anemometer working in the reference mode with forward scattering was used to perform the measurements in a simulated interior subchannel of a triangular rod array with P/D = 1.124. Comparisons between the analytical results and the results of this experiment as well as other experimental data in rod bundle arrays available in the literature are presented. The predictions are in good agreement with the results for high Reynolds numbers.

On nonlinear K-l and K-ε models of turbulence

1987 ◽  
Vol 178 ◽  
pp. 459-475 ◽  
Author(s):  
Charles G. Speziale
Keyword(s):  
Asymptotic Expansion ◽  
Turbulent Flows ◽  
Nonlinear Model ◽  
Linear Models ◽  
Nonlinear Models ◽  
Turbulent Channel Flow ◽  
Secondary Flows ◽  
Separated Flows ◽  
Future Research ◽  
Reynolds Stresses

The commonly used linear K-l and K-ε models of turbulence are shown to be incapable of accurately predicting turbulent flows where the normal Reynolds stresses play an important role. By means of an asymptotic expansion, nonlinear K-l and K-ε models are obtained which, unlike all such previous nonlinear models, satisfy both realizability and the necessary invariance requirements. Calculations are presented which demonstrate that this nonlinear model is able to predict the normal Reynolds stresses in turbulent channel flow much more accurately than the linear model. Furthermore, the nonlinear model is shown to be capable of predicting turbulent secondary flows in non-circular ducts - a phenomenon which the linear models are fundamentally unable to describe. An additional application of this model to the improved prediction of separated flows is discussed briefly along with other possible avenues of future research.

scholarly journals Hydrodynamic behavior of a bare rod bunle

1977 ◽  
Author(s):  
Ιωάννης Μπαρτζής
Keyword(s):  
Secondary Flow ◽  
Turbulent Flows ◽  
Axial Velocity ◽  
Nuclear Reactor ◽  
Three Dimensional ◽  
Hydrodynamic Behavior ◽  
Rod Bundle ◽  
Aspect Ratios ◽  
Strong Candidate ◽  
High Reynolds

Temperature distributions within the rod bundle of a nuclear reactor is of major importance in nuclear reactor design. However temperature information presupposes knowledge of the hydrodynamic behavior of the coolant which is the most difficult part of the problem due to complexity of the turbulence phenomena. In the present work a 2-equation turbulence model - a strong candidate for analyzing actual three dimensional turbulent flows - has been used to predict fully developed flow of infinite bare rod bundle of various aspect ratios (P/D). The model has been modified to take into account anisotropic effects of eddy viscosity. Secondary flow calculations have been also performed although the mo^el seems to be too rough to predict the secondary flow correctly. Heat Transfer calculations have been performed to confirm the importance of anisotropic viscosity in temperature predictions. All numerical calculations for flow and heat have been performed by two computer codes developed in the present work which were based on the TEACH code [71]· Also experimental measurements of the distribution of axial velocity, turbulent axial velocity, turbulent kinetic energy and radial Reynoldsstresses were performed in the developing and fully developed regions. A 2-channel Laser Doppler Anemometer working on the Reference mode with forward scattering was used to perform the measurements in a simulated interior subchannel of a triangular rod array with P/D=1.124. Comparisons between the analytical results and the results of this experiment as well as other experimental data in rod bundle array available in literature are presented. The predictions are in good agreement with the results for the high Reynolds numbers.

scholarly journals A Comparison Between Measurements and Turbulence Models in a Turbine Cascade Passage

1988 ◽  
Author(s):  
Pietro Zunino ◽  
Marina Ubaldi ◽  
Antonio Satta ◽  
Enrico Peisino
Keyword(s):  
Turbulence Models ◽  
Turbine Rotor ◽  
Secondary Flows ◽  
Rotor Blades ◽  
Turbine Cascade ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Closure Models ◽  
Experimental Values ◽  
Velocity Turbulence

Detailed measurements of mean velocity, turbulence intensity and Reynolds stresses have been performed in the passage of a cascade of turbine rotor blades. By using the experimental values of the mean velocity, the turbulence quantities are computed with three different turbulence closure models. The results are analysed and compared with the experimental data. The capability of the closure models to describe the turbulence development associated with secondary flows in a turbine cascade is discussed.

scholarly journals Variable-Order Fractional Models for Wall-Bounded Turbulent Flows

Entropy ◽  
2021 ◽  
Vol 23 (6) ◽  
pp. 782
Author(s):  
Fangying Song ◽  
George Em Karniadakis
Keyword(s):  
Fractional Derivative ◽  
Turbulent Flows ◽  
Fractional Order ◽  
Variable Order ◽  
Universal Curve ◽  
Reynolds Stresses ◽  
Continuous Change ◽  
Mean Velocity ◽  
Symmetric Variable ◽  
Fractional Models

Modeling of wall-bounded turbulent flows is still an open problem in classical physics, with relatively slow progress in the last few decades beyond the log law, which only describes the intermediate region in wall-bounded turbulence, i.e., 30–50 y+ to 0.1–0.2 R+ in a pipe of radius R. Here, we propose a fundamentally new approach based on fractional calculus to model the entire mean velocity profile from the wall to the centerline of the pipe. Specifically, we represent the Reynolds stresses with a non-local fractional derivative of variable-order that decays with the distance from the wall. Surprisingly, we find that this variable fractional order has a universal form for all Reynolds numbers and for three different flow types, i.e., channel flow, Couette flow, and pipe flow. We first use existing databases from direct numerical simulations (DNSs) to lean the variable-order function and subsequently we test it against other DNS data and experimental measurements, including the Princeton superpipe experiments. Taken together, our findings reveal the continuous change in rate of turbulent diffusion from the wall as well as the strong nonlocality of turbulent interactions that intensify away from the wall. Moreover, we propose alternative formulations, including a divergence variable fractional (two-sided) model for turbulent flows. The total shear stress is represented by a two-sided symmetric variable fractional derivative. The numerical results show that this formulation can lead to smooth fractional-order profiles in the whole domain. This new model improves the one-sided model, which is considered in the half domain (wall to centerline) only. We use a finite difference method for solving the inverse problem, but we also introduce the fractional physics-informed neural network (fPINN) for solving the inverse and forward problems much more efficiently. In addition to the aforementioned fully-developed flows, we model turbulent boundary layers and discuss how the streamwise variation affects the universal curve.

Comments on the reconnexion rate of magnetic fields

1973 ◽  
Vol 9 (1) ◽  
pp. 49-63 ◽  
Author(s):  
E. N. Parker
Keyword(s):  
Velocity Field ◽  
Magnetic Fields ◽  
Turbulent Flows ◽  
Similarity Solutions ◽  
Neutral Point ◽  
Solar Photosphere ◽  
Complete Field ◽  
Fluid Equations ◽  
The Galaxy

The reconnexion rate of magnetic fields is crucial in understanding the fields found in turbulent flows in the solar photosphere and in the galaxy, and in flare phenomena. This paper examines the behaviour of magnetic fields in the neighbourhood of an X-type neutral point. The treatment is kinematical, specifying the velocity field v and constructing solutions to the hydromagnetic equation for B. The calculations demonstrate that the reconnexion rate is controlled by the diffusion in the near neighbourhood of the neutral point, and is not arbitrarily large, as has been suggested by similarity solutions of the complete field and fluid equations for vanishing diffusion

Stress Tensor Measurements within the Vaneless Diffuser of a Centrifugal Compressor

1989 ◽  
Vol 203 (1) ◽  
pp. 51-59 ◽  
Author(s):  
T M A Maksoud ◽  
M W Johnson
Keyword(s):  
Centrifugal Compressor ◽  
Sampling Technique ◽  
Phase Lock Loop ◽  
Tangential Component ◽  
Secondary Flows ◽  
Axial Component ◽  
Reynolds Stresses ◽  
Vaneless Diffuser ◽  
Cross Sectional ◽  
Impeller Outlet

Distributions of normal and shear (Reynolds) stresses inside the vaneless diffuser of a low-speed centrifugal compressor are presented. The measurements were made using a triple hot-wire system and a phase lock loop sampling technique. Results were obtained on cross-sectional planes at eight radial stations between the impeller outlet and the diffuser exit at three different flowrates. The turbulence was highly anisotropic and became more so as the flowrate was increased. The tangential component of turbulent intensity was found to be significantly smaller than either the radial or axial component. The blade wake observed at the diffuser inlet decays very rapidly due to the strong tangential Reynolds stresses generated by the opposed secondary flows on either side of the wake. The passage wake decays very much more slowly and is still identifiable at the diffuser discharge.

On the Oscillatory Instability of Closed-Loop Thermosyphons

1985 ◽  
Vol 107 (4) ◽  
pp. 826-832 ◽  
Author(s):  
K. Chen
Keyword(s):  
Turbulent Flows ◽  
Closed Loop ◽  
Numerical Solutions ◽  
Angular Frequency ◽  
Oscillatory Instability ◽  
Neutral Stability ◽  
Horizontal Segment ◽  
Aspect Ratios ◽  
The Stability ◽  
Laminar And Turbulent Flows

The stability of natural convection flows in single-phase closed-loop thermosyphons is investigated. The thermosyphons considered in the present analysis are fluid-filled tubes bent into rectangular shapes. The fluid is heated over the lower horizontal segment and cooled over the upper horizontal segment. Analytical and numerical solutions are presented for a range of loop aspect ratios and radii for both laminar and turbulent flows. It is found that the steady-state results for thermosyphons with different aspect ratios and radii can be expressed in terms of a single dimensionless parameter. When this parameter is less than a critical value, the flow is always stable. Above this critical point, oscillatory instability exists for a narrow range of a friction parameter. The calculated neutral stability conditions show that the flow is least stable when the aspect ratio of the loop approaches unity. The frequency of the convection-induced oscillation is slightly higher than the angular frequency of a fluid particle traveling along the loop.

Measurements of turbulent flows in a 2×2 rod bundle

2012 ◽  
Vol 249 ◽  
pp. 2-13 ◽  
Author(s):  
Shigeo Hosokawa ◽  
Taishi Yamamoto ◽  
Jun Okajima ◽  
Akio Tomiyama
Keyword(s):  
Turbulent Flows ◽  
Rod Bundle
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