scholarly journals Numerical Investigation on Turbulent Flow and Heat Transfer of Rectangular Channels With Elliptic Scale-Roughened Walls

2012 ◽  
Author(s):  
Feng Zhou ◽  
Ivan Catton
Keyword(s):  
Heat Transfer ◽  
Flow Direction ◽  
Flow Characteristics ◽  
Spanwise Direction ◽  
Pressure Drops ◽  
Nusselt Numbers ◽  
Flow Mixing ◽  
Rectangular Channels ◽  
Smooth Channel ◽  
Fluid Flow Characteristics

In the present paper, six new types of rectangular channels with elliptic scale-roughened walls for heat transfer enhancement, which include elongated scale cases (Pt/Pl = 0.3, 0.5, 0.7) and squeezed scale cases (Pt/Pl = 1.43, 2, 3.33), are proposed. Heat transfer and fluid flow characteristics for sixteen different scale-roughened models (with the scale height varying in the range from 1mm to 2.5mm) are predicted numerically using commercial CFD code, Ansys CFX, with the Reynolds number ranging from 5000 to 15000. The turbulent model employed is the k-ω based Shear-Stress-Transport (SST) model with automatic wall function treatment. It is found that the elliptic scales with their long axis oriented perpendicular to the flow direction enhance the heat transfer performance considerably, while the scales elongated in the flow direction have lower Nusselt numbers and pressure drops compared to the circular scale-roughened channels. It is also found that the scale-shaped roughness strongly spins the flow in the spanwise direction, which breaks the near wall boundary layers continuously and enhances the bulk flow mixing. With the flow marching in a spiral pattern, Nusselt number ratios between the squeezed scale-roughened and smooth channel flows (Nu/Nu∞) could be augmented to be within the range of 6.1 to 8.1, which is a 50% improvement over the circular scale-roughened channels.

scholarly journals A Numerical Investigation of Turbulent Flow and Heat Transfer in Rectangular Channels With Elliptic Scale-Roughened Walls

2013 ◽  
Vol 135 (8) ◽  
Author(s):  
Feng Zhou ◽  
Ivan Catton
Keyword(s):  
Heat Transfer ◽  
Heat Transfer Enhancement ◽  
Flow Direction ◽  
Flow Characteristics ◽  
Volume Averaging ◽  
Spanwise Direction ◽  
Transfer Enhancement ◽  
Pressure Drops ◽  
Flow Mixing ◽  
Rectangular Channels

In the present paper, rectangular channels with six types of elliptic scale-roughened walls for heat transfer enhancement are numerically studied. Heat transfer and fluid flow characteristics for sixteen different scale-roughened models (with the scale height varying in the range from 1 mm to 2.5 mm) are numerically predicted using commercial computational fluid dynamics (CFD) code, Ansys cfx. The turbulent model employed is the k–ω based shear–stress transport (SST) model with automatic wall function treatment. In the performance evaluation, we use a “universal” porous media length scale based on volume averaging theory (VAT) to define the Reynolds number, Nusselt number, and friction factor. It is found that heat transfer performance is most favorable when the elliptic scales are oriented with their long axis perpendicular to the flow direction, while the scales elongated in the flow direction have lower Nusselt numbers and pressure drops compared with the circular scale-roughened channels. Results indicate that the scale-shaped roughness strongly spins the flow in the spanwise direction, which disrupts the near-wall boundary layers continuously and enhances the bulk flow mixing. With the flow marching in a more intense spiral pattern, a 40% improvement of heat transfer enhancement over the circular scale-roughened channels is observed.

Flow and Heat Transfer Characteristics in Rectangular Channels With Staggered Transverse Ribs on Two Opposite Walls

2007 ◽  
Vol 129 (12) ◽  
pp. 1732-1736 ◽  
Author(s):  
Rong Fung Huang ◽  
Shyy Woei Chang ◽  
Kun-Hung Chen
Keyword(s):  
Heat Transfer ◽  
Flow Field ◽  
Turbulence Intensity ◽  
Flow Characteristics ◽  
Channel Height ◽  
Primary Concern ◽  
Height Ratio ◽  
Nusselt Numbers ◽  
Characteristic Flow ◽  
Rectangular Channels

The flow characteristics and the heat transfer properties of the rectangular channels with staggered transverse ribs on two opposite walls are experimentally studied. The rib height to channel height ratio ranges from 0.15 to 0.61 (rib height to channel hydraulic diameter ratio from 0.09 to 0.38). The pitch to rib height ratio covers from 2.5 to 26. The aspect ratio of the rectangular channel is 4. The flow characteristics are studied in a water channel, while the heat transfer experiments are performed in a wind tunnel. Particle image velocimetry (PIV) is employed to obtain the quantitative flow field characteristics. Fine-wire thermocouples imbedded near the inner surface of the bottom channel wall are used to measure the temperature distributions of the wall and to calculate the local and average Nusselt numbers. Using the PIV measured streamline patterns, various characteristic flow modes, thru flow, oscillating flow, and cell flow, are identified in different regimes of the domain of the rib height to channel height ratio and pitch to rib height ratio. The vorticity, turbulence intensity, and wall shear stress of the cell flow are found to be particularly larger than those of other characteristic flow modes. The measured local and average Nusselt numbers of the cell flow are also particularly higher than those of other characteristic flow modes. The distinctive flow properties are responsible for the drastic increase of the heat transfer due to the enhancement of the momentum, heat, and mass exchanges within the flow field induced by the large values of the vorticity and turbulence intensity. Although the thru flow mode is conventionally used in the ribbed channel for industrial application, the cell flow could become the choice if the heat transfer rate, instead of the pressure loss, is the primary concern.

scholarly journals NUMERICAL STIMULATION OF RAYLEIGH BERNARD CONVECTION IN WAVY ENCLOSURES

2012 ◽  
pp. 274-277
Author(s):  
S. SUBHA
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Flow Characteristics ◽  
Practical Interest ◽  
Nusselt Numbers ◽  
Numerical Stimulation ◽  
Vertical Walls ◽  
Finite Volume Approach ◽  
Fluid Flow Characteristics ◽  
Enclosed Space

Enclosures are frequently encountered in practice, and heat transfer through them is of practical interest. Heat transfer in enclosed space is complicated by the fact that fluid in the enclosure, in general, does not remain constant. The fluid adjacent to the hotter surface rises and the fluid adjacent to the cooler one falls, setting a rotionary motion within the enclosure that enhances the heat transfer through the enclosure. This paper describes a numerical predication of heat transfer and fluid flow characteristics inside an enclosure bounded by horizontal wavy walls and two periodic straight vertical walls. Governing equation were discretized using an implicit finite difference method, based on finite volume approach. Simulation was carried out for a range of Rayleigh number (104-106) and Aspect ratio (0.35-0.75) for the fluid having Prandtl number 0.71. Results are presented by streamlines, isotherms and local Nusselt numbers. It is observed that flow and thermal field inside the enclosure are affected by the shape of enclosure and heat transfer rate increases as Rayleigh number increase.

scholarly journals Investigation of Thermal-Flow Characteristics of Pipes with Helical Micro-Fins of Variable Height

Energies ◽  
2021 ◽  
Vol 14 (8) ◽  
pp. 2048
Author(s):  
Piotr Bogusław Jasiński ◽  
Michał Jan Kowalczyk ◽  
Artur Romaniak ◽  
Bartosz Warwas ◽  
Damian Obidowski ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Evaluation Criteria ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Working Fluid ◽  
Pressure Drops ◽  
Nusselt Numbers ◽  
Heat Transfer Efficiency ◽  
Friction Factors ◽  
Regular Roughness

The results of numerical investigations of heat transfer and pressure drops in a channel with 30° helical micro-fins are presented. The main aim of the analysis is to examine the influence of the height of the micro-fins on the heat-flow characteristics of the channel. For the tested pipe with a diameter of 12 mm, the micro-fin height varies within the range of 0.05–0.40 mm (with 0.05 mm steps), which is equal to 0.4–3.3% of its diameter. The analysis was performed for a turbulent flow, within the range of Reynolds numbers 10,000–100,000. The working fluid is water with an average temperature of 298 K. For each tested geometry, the characteristics of the friction factor f(Re) and the Nusselt number Nu(Re) are shown in the graphs. The highest values of Nusselt numbers and friction factors were obtained for pipes with the micro-fins H = 0.30 mm and H = 0.35 mm. A large discrepancy is observed in the friction factors f(Re) calculated from the theoretical relationships (for the irregular relative roughness values shown in the Moody diagram) and those obtained from the simulations (for pipes with regular roughness formed by micro-fins). The PEC (Performance Evaluation Criteria) heat transfer efficiency analysis of the geometries under study is also presented, taking into account the criterion of the same pumping power. The highest PEC values, reaching 1.25, are obtained for micro-fins with a height of 0.30 mm and 0.35 mm and with Reynolds numbers above 40,000. In general, for all tested geometries and for large Reynolds numbers (above 20,000), the PEC coefficient reaches values greater than 1, while for lower Reynolds numbers (less than 20,000), its values are less than 1.

Heat Transfer and Flow Phenomena in a Swirl Chamber Simulating Turbine Blade Internal Cooling

1998 ◽  
Author(s):  
C. R. Hedlund ◽  
P. M. Ligrani ◽  
H.-K. Moon ◽  
B. Glezer
Keyword(s):  
Heat Transfer ◽  
Turbine Blade ◽  
Flow Characteristics ◽  
Leading Edge ◽  
Internal Cooling ◽  
Vortex Pairs ◽  
Nusselt Numbers ◽  
Swirl Chamber ◽  
Flow Phenomena ◽  
Energy Balances

Heat transfer and fluid mechanics results are given for a swirl chamber whose geometry models an internal passage used to cool the leading edge of a turbine blade. The Reynolds numbers investigated, based on inlet duct characteristics, include values which are the same as in the application (18000–19400). The ratio of absolute air temperature between the inlet and wall of the swirl chamber ranges from 0.62 to 0.86 for the heat transfer measurements. Spatial variations of surface Nusselt numbers along swirl chamber surfaces are measured using infrared thermography in conjunction with thermocouples, energy balances, digital image processing, and in situ calibration procedures. The structure and streamwise development of arrays of Görtler vortex pairs, which develop along concave surfaces, are apparent from flow visualizations. Overall swirl chamber structure is also described from time-averaged surveys of the circumferential component of velocity, total pressure, static pressure, and the circumferential component of vorticity. Important variations of surface Nusselt numbers and time-averaged flow characteristics are present due to arrays of Görtler vortex pairs, especially near each of the two inlets, where Nusselt numbers are highest. Nusselt numbers then decrease and become more spatially uniform along the interior surface of the chamber as the flows advect away from each inlet.

A Numerical Study of Heat Transfer and Flow Structure in Channels With Miniature V Rib-Dimple Hybrid Structure on One Wall

2018 ◽  
Author(s):  
Peng Zhang ◽  
Yu Rao ◽  
Yanlin Li
Keyword(s):  
Heat Transfer ◽  
Turbulent Flow ◽  
Numerical Simulations ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Hybrid Structure ◽  
Transfer Enhancement ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Flow Mixing

This paper presents a numerical study on turbulent flow and heat transfer in the channels with a novel hybrid cooling structure with miniature V-shaped ribs and dimples on one wall. The heat transfer characteristics, pressure loss and turbulent flow structures in the channels with the rib-dimples with three different rib heights of 0.6 mm, 1.0 mm and 1.5 mm are obtained for the Reynolds numbers ranging from 18,700 to 60,000 by numerical simulations, which are also compared with counterpart of a pure dimpled and pure V ribbed channel. The results show that the overall Nusselt numbers of the V rib-dimple channel with the rib height of 1.5 mm is up to 70% higher than that of the channels with pure dimples. The numerical simulations show that the arrangement of the miniature V rib upstream each dimple induces complex secondary flow near the wall and generates downwashing vortices, which intensifies the flow mixing and turbulent kinetic energy in the dimple, resulting in significant improvement in heat transfer enhancement and uniformness.

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