Effects of aspect ratio on turbulent heat transfer around a downward facing step

2002 ◽  
Author(s):  
Terukazu Ota ◽  
HongGong Fu ◽  
Hiroyuki Yoshikawa
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat

scholarly journals Turbulent flow through a high aspect ratio cooling duct with asymmetric wall heating

2018 ◽  
Vol 860 ◽  
pp. 258-299 ◽  
Author(s):  
Thomas Kaller ◽  
Vito Pasquariello ◽  
Stefan Hickel ◽  
Nikolaus A. Adams
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Turbulent Flow ◽  
Secondary Flow ◽  
High Aspect Ratio ◽  
Heated Wall ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Wall Heating ◽  
Flow Through

We present well-resolved large-eddy simulations of turbulent flow through a straight, high aspect ratio cooling duct operated with water at a bulk Reynolds number of $Re_{b}=110\times 10^{3}$ and an average Nusselt number of $Nu_{xz}=371$. The geometry and boundary conditions follow an experimental reference case and good agreement with the experimental results is achieved. The current investigation focuses on the influence of asymmetric wall heating on the duct flow field, specifically on the interaction of turbulence-induced secondary flow and turbulent heat transfer, and the associated spatial development of the thermal boundary layer and the inferred viscosity variation. The viscosity reduction towards the heated wall causes a decrease in turbulent mixing, turbulent length scales and turbulence anisotropy as well as a weakening of turbulent ejections. Overall, the secondary flow strength becomes increasingly less intense along the length of the spatially resolved heated duct as compared to an adiabatic duct. Furthermore, we show that the assumption of a constant turbulent Prandtl number is invalid for turbulent heat transfer in an asymmetrically heated duct.

Heat Transfer and Friction in a Low-Aspect-Ratio Rectangular Channel With Staggered Perforated Ribs on Two Opposite Walls

1995 ◽  
Vol 117 (4) ◽  
pp. 843-850 ◽  
Author(s):  
Jenn-Jiang Hwang ◽  
Tong-Miin Liou
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Rectangular Channel ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Transfer Enhancement ◽  
Constant Flow Rate ◽  
Low Aspect Ratio ◽  
Flow Parameters ◽  
Constant Friction

Experiments are conducted to study the turbulent heat transfer and friction in a low-aspect-ratio rectangular channel in which two opposite walls are roughened by perforated ribs. The perforated ribs are arranged in a staggered manner. Effects of perforated rib open-area ratio (β = 10, 22, 38, 44, and 50 percent), rib pitch-to-height ratio (PR = 5, 10, 15, and 20), rib height-to-channel hydraulic diameter ratio (H/De = 0.063 and 0.081), rib alignment (staggered and symmetric), and Reynolds number (10,000 ≤ Re ≤ 50,000) are examined. It is found that approximately the same heat transfer enhancement and pressure drop penalty are obtained between symmetric and staggered rib arrangements. A permeability criterion is proposed by tracing heat transfer coefficient distributions, which compares well with previous flow-visualization results. Results also show that ribs with β = 44 percent give the best thermal performance under either the constant friction power or the constant flow rate constraint. Roughness functions for friction and heat transfer are further developed in terms of rib and flow parameters.

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).

The Characteristics of Turbulent Heat Transfer in a Curved Rectangular Duct With Various Aspect Ratios

2010 ◽  
Author(s):  
Hang Seok Choi ◽  
Tae Seon Park
Keyword(s):  
Heat Transfer ◽  
Aspect Ratio ◽  
Turbulent Flow ◽  
Secondary Flow ◽  
Turbulent Heat Transfer ◽  
Turbulent Heat ◽  
Rectangular Duct ◽  
Flow And Heat Transfer ◽  
Aspect Ratios ◽  
Turbulent Characteristics

The turbulent flow fields of a parallel plate or channel with spatially periodic condition have been widely investigated by many researchers. However the rectangular or square curved duct flow has not been fundamentally scrutinized in spite of its engineering significance, especially for cooling device. Hence, in the present study large eddy simulation is applied to the turbulent flow and heat transfer in a rectangular duct with 180° curved angle varying its aspect ratio. The turbulent flow and the thermal fields are calculated and the representative vortical motions generated by the secondary flow are investigated. From the results, the secondary flow has a great effect on the heat and momentum transport in the flow. With changing the aspect ratio, the effect of the geometrical variation to the secondary flow and its influence on the turbulent characteristics of the flow and heat transfer are studied.

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