DYNAMICS OF LARGE SCALE FLOW AND ITS EFFECT ON HEAT TRANSFER IN TURBULENT CONVECTION

2018 ◽  
Author(s):  
Arnab Kumar De ◽  
P. K. Mishra
Keyword(s):  
Heat Transfer ◽  
Large Scale ◽  
Turbulent Convection ◽  
Large Scale Flow

Coherent large–scale flow structures in turbulent convection

2008 ◽  
pp. 606-608
Author(s):  
C. Resagk ◽  
R. du Puits ◽  
E. Lobutova ◽  
A. Maystrenko ◽  
A. Thess
Keyword(s):  
Large Scale ◽  
Turbulent Convection ◽  
Flow Structures ◽  
Large Scale Flow

Numerical Simulation of Natural Convection in Stationary and Rotating Cavities

2004 ◽  
Author(s):  
Zixiang Sun ◽  
Alistair Kifoil ◽  
John W. Chew ◽  
Nicholas J. Hills
Keyword(s):  
Heat Transfer ◽  
Centrifugal Force ◽  
Large Scale ◽  
Transfer Rates ◽  
Buoyancy Driven Flows ◽  
Gravity Driven ◽  
Gravity Driven Flow ◽  
Large Scale Flow ◽  
Rayleigh Numbers ◽  
Good Agreement

In compressor inter-disc cavities with a central axial throughflow it is known that the flow and heat transfer is strongly affected by buoyancy in the centrifugal force field. As a step towards developing CFD methods for such flows, buoyancy-driven flows under gravity in a closed cube and under centrifugal force in a sealed rotating annulus have been studied. Numerical simulations are compared with the experimental results of Kirkpatrick and Bohn (1986) and Bohn et al (1993). Two different CFD codes have been used and are shown to agree for the stationary cube problem. Unsteady simulations for the stationary cube show good agreement with measurements of heat transfer, temperature fluctuations, and velocity fluctuations for Rayleigh numbers up to 2 × 1010. Similar simulations for the rotating annulus also show good agreement with measured heat transfer rates. The CFD results confirm Bohn et al’s results, showing reduced heat transfer and a different Rayleigh number dependency compared to gravity-driven flow. Large scale flow structures are found to occur, at all Rayleigh numbers considered.

Large-scale flow in turbulent convection: a mathematical model

1986 ◽  
Vol 170 ◽  
pp. 385-410 ◽  
Author(s):  
L. N. Howard ◽  
R. Krishnamurti
Keyword(s):  
Mathematical Model ◽  
Large Scale ◽  
Laboratory Experiments ◽  
Boussinesq Equations ◽  
Time Dependent ◽  
Heteroclinic Orbits ◽  
Turbulent Convection ◽  
Mean Flow ◽  
Dependent Flow ◽  
Large Scale Flow

A mathematical model of convection, obtained by truncation from the two-dimensional Boussinesq equations, is shown to exhibit a bifurcation from symmetrical cells to tilted non-symmetrical ones. A subsequent bifurcation leads to time-dependent flow with similarly tilted transient plumes and a large-scale Lagrangian mean flow. This change of symmetry is similar to that occurring with the advent of a large-scale flow and transient tilted plumes seen in laboratory experiments on turbulent convection at high Rayleigh number. Though not intended as a description of turbulent convection, the model does bring out in a theoretically tractable context the possibility of the spontaneous change of symmetry suggested by the experiments.Further bifurcations of the model lead to stable chaotic phenomena as well. These are numerically found to occur in association with heteroclinic orbits. Some mathematical results clarifying this association are also presented.

Some Observations on the Computational Sensitivity of Rotating Cavity Flows

2020 ◽  
Author(s):  
Tom Hickling ◽  
Li He
Keyword(s):  
Heat Transfer ◽  
Large Scale ◽  
Internal Flow ◽  
Systematic Investigation ◽  
Small Scale ◽  
Rotating Cavity ◽  
Ekman Layers ◽  
Inertial Wave ◽  
Wall Flow ◽  
Large Scale Flow

Abstract Heat transfer inside rotating cavities plays an important role in gas turbine engineering. Flows in both compressors and turbine internal flow cavities exhibit self-generated large-scale inertial wave structures, and buoyancy effects are often important. Across the open literature on the topic, there seems to be no clear consensus on what the most suitable modelling fidelity is — although it is a widely held opinion that URANS approaches are less suitable than LES, many authors have succeeded in getting reasonable heat transfer results with URANS. There is also little knowledge of the validity of hybrid URANS/LES type approaches (such as DES) when it comes to predicting the heat transfer in these flows, and furthermore, on the sensitivity of the flow model validity to local driving aerothermal mechanisms in different parts of the cavity. This paper presents the results of a systematic investigation of a rotating cavity with axial throughflow at a Grashof number of 3 × 109. It is found that, for the case investigated, the disk Ekman layers remain laminar. This causes the disk heat transfer to be relatively insensitive to the modelling fidelity used with URANS, DES, and LES giving similar results. The effect of the disk thermal boundary condition is also investigated — it is found to have a significant effect on the direction of the near-wall flow at high radii, despite the large-scale flow structure within the cavity remaining essentially unchanged. This feedback of the disk heat transfer to the near-disk aerodynamics implies that conjugate heat transfer computations of rotating cavities may be worth investigating. On the shroud, URANS fails to resolve the heat transfer enhancement from small-scale buoyancy driven streaks, whilst these are captured by LES. DES also captures these streaks, as the URANS layer within which they are located returns a very small eddy viscosity, and behaves in a similar manner to LES.

scholarly journals Large-scale flow generation in turbulent convection

1981 ◽  
Vol 78 (4) ◽  
pp. 1981-1985 ◽  
Author(s):  
R. Krishnamurti ◽  
L. N. Howard
Keyword(s):  
Large Scale ◽  
Turbulent Convection ◽  
Flow Generation ◽  
Large Scale Flow

High-Rayleigh-number convection of pressurized gases in a horizontal enclosure

2002 ◽  
Vol 469 ◽  
pp. 1-12 ◽  
Author(s):  
A. S. FLEISCHER ◽  
R. J. GOLDSTEIN
Keyword(s):  
Heat Transfer ◽  
High Pressure ◽  
Rayleigh Number ◽  
Flow Field ◽  
Large Scale ◽  
Data Set ◽  
Video Images ◽  
Large Scale Flow ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

High-pressure gases are used to study high-Rayleigh-number Rayleigh–Bénard convection in cylindrical horizontal enclosures. The Nusselt–Rayleigh heat transfer relationship is investigated for 1×109 < Ra < 1.7×1012. Schlieren video images of the flow field are recorded through optical viewports in the pressure vessel. The data set is well correlated by Nu = 0.071Ra0.328. The schlieren results confirm the existence of a large-scale flow that periodically interrupts the ascending and descending plumes. The intensity of both the plumes and the large-scale flow increases with Rayleigh number.

scholarly journals Aspect ratio dependence of heat transfer and large-scale flow in turbulent convection

2010 ◽  
Vol 655 ◽  
pp. 152-173 ◽  
Author(s):  
J. BAILON-CUBA ◽  
M. S. EMRAN ◽  
J. SCHUMACHER
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Large Scale ◽  
Turbulent Convection ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Cylindrical Cell ◽  
Aspect Ratios ◽  
Pod Analysis

The heat transport and corresponding changes in the large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection are studied by means of three-dimensional direct numerical simulations as a function of the aspect ratio Γ of a closed cylindrical cell and the Rayleigh number Ra. The Prandtl number is Pr = 0.7 throughout the study. The aspect ratio Γ is varied between 0.5 and 12 for a Rayleigh number range between 107 and 109. The Nusselt number Nu is the dimensionless measure of the global turbulent heat transfer. For small and moderate aspect ratios, the global heat transfer law Nu = A × Raβ shows a power law dependence of both fit coefficients A and β on the aspect ratio. A minimum of Nu(Γ) is found at Γ ≈ 2.5 and Γ ≈ 2.25 for Ra = 107 and Ra = 108, respectively. This is the point where the LSC undergoes a transition from a single-roll to a double-roll pattern. With increasing aspect ratio, we detect complex multi-roll LSC configurations in the convection cell. For larger aspect ratios Γ ≳ 8, our data indicate that the heat transfer becomes independent of the aspect ratio of the cylindrical cell. The aspect ratio dependence of the turbulent heat transfer for small and moderate Γ is in line with a varying amount of energy contained in the LSC, as quantified by the Karhunen–Loève or proper orthogonal decomposition (POD) analysis of the turbulent convection field. The POD analysis is conducted here by the snapshot method for at least 100 independent realizations of the turbulent fields. The primary POD mode, which replicates the time-averaged LSC patterns, transports about 50% of the global heat for Γ ≥ 1. The snapshot analysis enables a systematic disentanglement of the contributions of POD modes to the global turbulent heat transfer. Although the smallest scale – the Kolmogorov scale ηK – and the largest scale – the cell height H – are widely separated in a turbulent flow field, the LSC patterns in fully turbulent fields exhibit strikingly similar texture to those in the weakly nonlinear regime right above the onset of convection. Pentagonal or hexagonal circulation cells are observed preferentially if the aspect ratio is sufficiently large (Γ ≳ 8).

scholarly journals How heat transfer efficiencies in turbulent thermal convection depend on internal flow modes

2011 ◽  
Vol 676 ◽  
pp. 1-4 ◽  
Author(s):  
KE-QING XIA
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Thermal Convection ◽  
Large Scale ◽  
Internal Flow ◽  
Global Transport ◽  
Large Scale Flow ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard ◽  
Flow States

How internal flow states can influence the global transport properties in a turbulent system has always been an intriguing question. Weiss & Ahlers (J. Fluid Mech., this issue, vol. 676, 2011, pp. 5–40) have provided an example by measuring the instantaneous Nusselt number in turbulent Rayleigh-Bénard convection and correlating it to the different modes of large-scale flow.

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