Turbulent Fluctuations and Heat Transfer for Separated Flow Associated With a Double Step at Entrance to an Enlarged Flat Duct

1976 ◽  
Vol 98 (4) ◽  
pp. 588-593 ◽  
Author(s):  
N. Seki ◽  
S. Fukusako ◽  
T. Hirata
Keyword(s):  
Heat Transfer ◽  
Shear Layer ◽  
Empirical Equation ◽  
Separated Flow ◽  
Test Fluid ◽  
Uniform Heat Flux ◽  
Turbulent Fluctuation ◽  
Free Shear Layer ◽  
Height Ratio ◽  
Double Step

An experimental study on heat-transfer in separated, reattached, and redevelopment regions behind a double step at entrance to a flat duct is presented. Measurements of turbulent fluctuation in a free shear layer are made by using a hot-wire anemometer. The experiments are carried out under a condition of uniform heat flux with the test fluid of air. Reynolds number ranges approximately from 4 × 103 to 2.5 × 105 and a step height ratio h/L is varied between 0.035 and 7.0. It is found that the heat-transfer rate in the separated region is closely connected with the behavior of transported heat to be represented by the product of velocity and temperature fluctuations in the free shear layer. An empirical equation is also proposed for the local Nusselt number in the separated and reattached regions.

scholarly journals Instability patterns between counter-rotating disks

2003 ◽  
Vol 10 (3) ◽  
pp. 281-288 ◽  
Author(s):  
F. Moisy ◽  
T. Pasutto ◽  
M. Rabaud
Keyword(s):  
Aspect Ratio ◽  
Shear Layer ◽  
Particle Image ◽  
Shear Layer Instability ◽  
Free Shear Layer ◽  
Rotating Disks ◽  
Height Ratio ◽  
Aspect Ratios ◽  
Image Velocimetry ◽  
Local Reynolds Number

Abstract. The instability patterns in the flow between counter-rotating disks (radius to height ratio R/h from 3.8 to 20.9) are investigated experimentally by means of visualization and Particle Image Velocimetry. We restrict ourselves to the situation where the boundary layers remain stable, focusing on the shear layer instability that occurs only in the counter-rotating regime. The associated pattern is a combination of a circular chain of vortices, as observed by Lopez et al. (2002) at low aspect ratio, surrounded by a set of spiral arms, first described by Gauthier et al. (2002) in the case of high aspect ratio. Stability curve and critical modes are measured for the whole range of aspect ratios. From the measurement of a local Reynolds number based on the shear layer thickness, evidence is given that a free shear layer instability, with only weak curvature effect, is responsible for the observed patterns. Accordingly, the number of vortices is shown to scale as the shear layer radius, which results from the competition between the centrifugal effects of each disk.

Spatially Resolved Heat Transfer Coefficient in a Rib-Roughened Channel Under Coriolis Effects

2013 ◽  
Author(s):  
Ignacio Mayo ◽  
Tony Arts ◽  
Julien Clinckemaillie ◽  
Aude Lahalle
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Shear Layer ◽  
Rotation Number ◽  
Rectangular Cross Section ◽  
Flow Direction ◽  
Secondary Flows ◽  
Uniform Heat Flux ◽  
Flux Boundary Condition ◽  
Coriolis Forces

Heat transfer in a magnified rotating ribbed channel is studied by means of liquid crystal thermometry. The test section consists of four Plexiglas walls, forming a rectangular cross section, mounted on a large rotating disk together with the complete necessary measurement chain. The investigated wall is equipped with ribs perpendicular to the main flow direction, it is heated in such a way to achieve a uniform heat flux boundary condition. Facing the need of two-dimensional experimental heat transfer data, tets were carried out in order to quantify the convective heat transfer distribution on the wall between two consecutive ribs under rotating conditions. Different Rotation numbers (0, 0.06, 0.11 and 0.17) were tested at a Reynolds number of 15,000. For the selected heat flux and rotation rates, and based on previous aerodynamic and thermal investigations presented in open literature, no effect of buoyancy is expected, while the Coriolis forces play an important role in the determination of heat transfer. The rotating cases were performed in both senses of rotation in order to allow the studied wall to act as both a trailing and a leading side. At the highest Rotation number, the results confirm that heat transfer is enhanced up to 17% along the trailing side compared with the non-rotating case. This is due to the secondary flows and shear layer instability instigated by the Coriolis forces. On the other hand, heat transfer on the leading side is reduced up to 19% at the highest rotation number; this is caused by the stabilization of the shear layer and the contribution of the secondary flows.

Starting Flow and Heat Transfer Downstream of a Backward-Facing Step

1991 ◽  
Vol 113 (3) ◽  
pp. 583-589 ◽  
Author(s):  
F. K. Tsou ◽  
Shih-Jiun Chen ◽  
Win Aung
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Steady State ◽  
Shear Layer ◽  
Characteristic Time ◽  
Free Shear Layer ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Flow And Heat Transfer ◽  
Hot Wire Anemometry

Experiments are performed to study the starting process of heat transfer downstream of a backward-facing step. A Ludwieg tube wind tunnel is employed to produce the incompressible flow, which accelerates from a zero velocity to a steady state value with an accelerating period of 7 ms and a steady-state period of 12 ms. Hot-wire anemometry and heat flux gages are used to measure the flow and heat transfer history, respectively. The onset of transition in the free shear layer shows that the disturbance originates from the top corner of the step, then propagating to the free stream. The velocity and turbulence profiles in the free shear layer reach steady-state values after the leading edge disturbance traverses to the measurement locations. In regions upstream and far downstream of the step, heat flux history data suggest the transformation of the flow from laminar to transitional and finally to turbulent flow. Hot-wire anemometry measurements indicate high-frequency turbulence with a short characteristic time. In the recirculating region, however, a longer characteristic time is observed because of the existence of large-scale eddies. The dimensionless reattachment length (xr/H) is shown to increase with time from the bottom corner (xr/H = 0) in the laminar regime to a maximum value of 13.6 in the transitional regime, and decreases to a constant values of 7.6 in the turbulent regime. The steady-state flow field and heat transfer compare favorably with existing data obtained using steady-state techniques.

Heat Transfer in Turbulent Boundary-Layer Separation Over a Surface Cavity

1967 ◽  
Vol 89 (4) ◽  
pp. 335-340 ◽  
Author(s):  
R. L. Haugen ◽  
A. M. Dhanak
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Analytical Study ◽  
Separated Flow ◽  
Boundary Layer Separation ◽  
Eddy Diffusion ◽  
Uniform Heat Flux ◽  
Surface Cavity ◽  
Good Agreement

An experimental and analytical study is presented in this paper describing heat transfer in the region of separated flow over a two-dimensional rectangular cavity (of variable depth-width ratios) facing an oncoming turbulent boundary layer of variable thickness. The analysis, based on a prescription of eddy diffusion in the mixing region, predicts a heat transfer correlation, in terms of the foregoing variables, resulting in good agreement with the data. Experiments were performed with conditions of uniform temperature and uniform heat flux at the cavity walls and revealed no substantial difference between the two methods on the final correlation.

Augmentation of Laminar Forced-Convective Heat Transfer by the Application of a Transverse Electric Field

1989 ◽  
Vol 111 (2) ◽  
pp. 345-351 ◽  
Author(s):  
T. Fujino ◽  
Y. Yokoyama ◽  
Y. H. Mori
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Electric Field ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Heat Transfer Surface ◽  
Test Fluid ◽  
Uniform Heat Flux ◽  
Forced Convective Heat Transfer ◽  
Weakly Conducting

The effect of a uniform d-c electric field on laminar forced-convective heat transfer has been studied experimentally with a weakly conducting fluorocarbon refrigerant in the liquid state that is flowing in a channel confined by parallel-plate electrodes, one of which serves as a heat transfer surface of uniform heat flux. The dependencies of the heat transfer coefficient and the pressure drop on the sign and the magnitude of an applied voltage, the heat flux at the heat transfer surface, the electrical conductivity of the test fluid, etc. are presented, and the structure and the mechanism of the electroconvection causing the heat transfer enhancement are considered.

Forced Convection Heat Transfer in Channels With Rib Turbulators Inclined at 45 deg

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Giovanni Tanda ◽  
Roberto Abram
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Nusselt Number ◽  
Convection Heat Transfer ◽  
Local Heat Transfer ◽  
Uniform Heat Flux ◽  
Height Ratio ◽  
Smooth Channel ◽  
Rib Turbulators ◽  
The One

Local and average Nusselt numbers and friction factors are presented for rectangular channels with an aspect ratio of 5 and angled rib turbulators inclined at 45 deg with parallel orientations on one and two surfaces of the channel. The convective fluid was air, and the Reynolds number varied from 9000 to 35,500. The ratio of rib height to hydraulic diameter was 0.09, with the rib pitch-to-height ratio equal to 13.33 or 6.66. Experiments were based on the use of heating foils (for the attainment of uniform heat flux condition) and of the steady-state liquid crystal thermography (for the identification of isotherm lines and the reconstruction of local heat transfer coefficient). Local results showed quasiperiodic profiles of Nusselt number in the streamwise direction, whose features were strongly affected by the value of rib pitch and by the spanwise coordinate. For all the investigated geometries a heat transfer augmentation, relative to the fully developed smooth channel, was found; when inclined rib turbulators were placed on two opposite surfaces of the channel, the full-surface Nusselt number was higher (by 10–19%) than that for the one-ribbed wall channel, but pressure drop penalties also increased by a factor of about 3. For both the one- and two-ribbed wall channels, the best heat transfer performance for a constant pumping power, in the explored range of Reynolds number, was generally achieved by the larger rib pitch-to-height ratio (=13.33).

Free Shear Layer Behavior in Rotating Systems

1979 ◽  
Vol 101 (1) ◽  
pp. 117-120 ◽  
Author(s):  
P. H. Rothe ◽  
J. P. Johnston
Keyword(s):  
Shear Layer ◽  
Separated Flow ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Free Shear Layer ◽  
Rotation Direction ◽  
Rotating Systems ◽  
System Rotation ◽  
Steady Rate ◽  
Extensive Flow

Experiments are reported concerning turbulent separated flow downstream of a backward-facing step in a two-dimensional channel that was rotated at a steady rate about a spanwise axis. Reattachment distance is reported as a function of Reynolds number, rotation direction and number and passage aspect ratio. Extensive flow visualization films have been produced. It is demonstrated that turbulent motions in a free shear layer may be suppressed or enhanced by system rotation according to the sense of the rotation. Two-dimensional, spanwise vortices which have been observed in the free shear layer are found to be relatively insensitive to system rotation in the stabilizing direction. These vortices are believed to be important contributors to the high rates of free shear layer entrainment, even in stationary systems at moderate Reynolds numbers.

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