Measurements in Buoyancy-Assisting Separated Flow Behind a Vertical Backward-Facing Step

1993 ◽  
Vol 115 (2) ◽  
pp. 403-408 ◽  
Author(s):  
B. J. Baek ◽  
B. F. Armaly ◽  
T. S. Chen
Keyword(s):  
Nusselt Number ◽  
Buoyancy Force ◽  
Heated Wall ◽  
Laser Doppler Velocimeter ◽  
Force Parameter ◽  
Reattachment Length ◽  
Reattachment Point ◽  
Backward Facing Step ◽  
Temperature Distributions ◽  
Laminar Mixed Convection

Measurements of velocity and temperature distributions in buoyancy-assisting laminar mixed convection boundary-layer flow over a vertical, two-dimensional backward-facing step are reported. The leading surface upstream of the step and the step itself were adiabatic, and the surface downstream of the step was heated and maintained at a uniform temperature. A laser-Doppler velocimeter and a cold-wire anemometer were utilized to measure simultaneously the velocity and the temperature distributions in the recirculation and the reattached region downstream of the step. Flow visualization was used to study the flow and to measure the reattachment length for different free-stream velocities (0.37 m/s ≤ u0 ≤ 0.72 m/s), wall temperature differences (10°C ≤ ΔT ≤ 30°C), and step heights (0.38 cm ≤ s ≤ 1 cm). Results show that for a given step height the reattachment length decreases as the buoyancy force parameter, Grs/Res2, increases. The Nusselt number at the heated wall downstream of the step increases and the location of its maximum value moves closer to the step as the buoyancy force parameter increases. For the present experimental range, it is found that the location of the maximum Nusselt number occurs downstream of the reattachment point and the distance between the reattachment point and the location of the maximum Nusselt number increases as the buoyancy force parameter increases. Predicted behavior agrees favorably with the measured results.

Laminar Natural Convection Flow Over a Vertical Backward-Facing Step

1995 ◽  
Vol 117 (4) ◽  
pp. 895-901 ◽  
Author(s):  
H. I. Abu-Mulaweh ◽  
B. F. Armaly ◽  
T. S. Chen
Keyword(s):  
Natural Convection ◽  
Free Stream ◽  
Heated Wall ◽  
Laser Doppler Velocimeter ◽  
Convection Flow ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Natural Convection Flow ◽  
Temperature Distributions ◽  
Laminar Natural Convection

Measurements and predictions of laminar boundary-layer air flow in natural convection over a vertical two-dimensional backward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. The experiment was carried out for the ranges of step heights 3.5 mm ≤ s ≤ 9 mm, temperature differences of 5.8°C ≤ ΔT ≤ 23°C between the heated wall and the free stream (corresponding to 2.238 × 107 < Grxi < 8.877 × 107), and reference velocities of 0.24 m/s ≤ u* ≤ 0.47 m/s. Laser-Doppler velocimeter and cold-wire anemometer were utilized to measure, respectively, the velocity and the temperature distributions simultaneously. Flow visualization was also performed to determine the reattachment length. Measurements compare favorably with predictions. These results reveal that the step height significantly affects the velocity and temperature distributions, the friction coefficient, and the rate of heat transfer downstream of the backward-facing step.

Mixed Convection Adjacent to 3-D Backward-Facing Step

2000 ◽  
Author(s):  
A. Li ◽  
B. F. Armaly
Keyword(s):  
Heat Flux ◽  
Nusselt Number ◽  
Mixed Convection ◽  
Buoyancy Force ◽  
Three Dimensional ◽  
The Other ◽  
Convection Flow ◽  
Backward Facing Step ◽  
Grashof Number ◽  
Recirculating Flow

Abstract Results from three-dimensional numerical simulation of laminar, buoyancy assisting, mixed convection airflow adjacent to a backward-facing step in a vertical rectangular duct are presented. The Reynolds number, and duct geometry were kept constant at Re = 200, AR = 8, ER = 2, and S = 1 cm. Heat flux at the wall downstream from the step was kept uniform, but its magnitude was varied to cover a Grashof number (Gr) range between 0.0 to 4000. All the other walls in the duct were kept at adiabatic condition. The flow, upstream of the step, is treated as fully developed and isothermal. The relatively small aspect ratio of the channel is selected specifically to focus on the developments of the three-dimensional mixed convection flow in the separated and reattached flow regions downstream from the step. The presented results focus on the effects of increasing the buoyancy force, by increasing the uniform wall heat flux, on the three-dimensional flow and heat transfer characteristics. The flow and thermal fields are symmetric about the duct’s centerline. Vortex generated near the sidewall, is the major contributor to the three dimensional behavior in the flow domain, and that feature increases as the Grashof number increases. Increasing the Grashof number results in an increase in the Nusselt number, the size of the secondary recirculating flow region, the size of the sidewall vortex, and the spanwise flow from the sidewall toward the center of the channel. On the other hand, the size of the primary reattachment region decreases with increasing the Grashof number. That region lifts away and partially detaches from the downstream wall at high Grashof number flow. The maximum Nusselt number occurs near the sidewalls and not at the center of the channel. The effects of the buoyancy force on the distributions of the three-velocity components, temperature, reattachment region, friction coefficient, and Nusselt number are presented, and compared with 2-D results.

Heat Transfer and Fluid Flow Characteristics of Separated Flows Encountered in a Backward-Facing Step Under the Effect of Suction and Blowing

2007 ◽  
Vol 129 (11) ◽  
pp. 1517-1528 ◽  
Author(s):  
E. Abu-Nada ◽  
A. Al-Sarkhi ◽  
B. Akash ◽  
I. Al-Hinti
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Fluid Flow ◽  
Coefficient Of Friction ◽  
Local Nusselt Number ◽  
Bottom Wall ◽  
Reattachment Length ◽  
Backward Facing Step ◽  
Recirculation Bubble ◽  
Suction And Blowing

Abstract Numerical investigation of heat transfer and fluid flow over a backward-facing step (BFS), under the effect of suction and blowing, is presented. Here, suction/blowing is implemented on the bottom wall (adjacent to the step). The finite volume technique is used. The distribution of the modified coefficient of friction and Nusselt number at the top and bottom walls of the BFS are obtained. On the bottom wall, and inside the primary recirculation bubble, suction increases the modified coefficient of friction and blowing reduces it. However, after the point of reattachment, mass augmentation causes an increase in the modified coefficient of friction and mass reduction causes a decrease in modified coefficient of friction. On the top wall, suction decreases the modified coefficient of friction and blowing increases it. Local Nusselt number on the bottom wall is increased by suction and is decreased by blowing, and the contrary occurs on the top wall. The maximum local Nusselt number on the bottom wall coincides with the point of reattachment. High values of average Nusselt number on the bottom wall are identified at high Reynolds numbers and high suction bleed rates. However, the low values correspond to high blowing rates. The reattachment length and the length of the top secondary recirculation bubble are computed under the effect of suction and blowing. The reattachment length is increased by increasing blowing bleed rate and is decreased by increasing suction bleed rate. The spots of high Nusselt number, and low coefficient of friction, are identified by using contour maps.

scholarly journals Development and Validation of Nusselt Number Correlations for Mixed Convection in an Arc-Shape Cavity

2019 ◽  
Vol 9 (1) ◽  
pp. 526-531
Keyword(s):  
Nusselt Number ◽  
Convective Flow ◽  
Buoyancy Force ◽  
Close Agreement ◽  
Analytical Study ◽  
Numerical Data ◽  
Force Parameter ◽  
Arc Shape ◽  
Mixed Convective Flow ◽  
Development And Validation

The analytical study has been performed to investigate the combined effects of lid movement and buoyancy force parameter on mixed convective flow in an arc-shape cavity. The dimensional analysis based on Buckingham π-Theorem is used in the present study. It results in correlations for Nusselt number in terms of non dimensionalized parameters, viz. Re, Pr, Gr, θ etc. The correlations developed are validated against the experimental data of horizontal arc- shape cavity and numerical data of inclined arc-shape cavity obtained from open literature. The correlation developed in the present study for horizontal arcshape cavity is valid for wide ranges of Re varying from 30 to 1500 and Gr from 0 to 107. In inclined arc-shape cavity it is valid for Re varying from 30 to 1500, Gr from105 to 107 and inclination angle from 150to 600.The close agreement in the comparison between predicted results by correlation developed in the present study and reported Nu correlation shows the validity of the correlation.

scholarly journals Uncertainties Analysis on the Prediction of Flow and Heat Transfer of Liquid Sodium with CFD Technology

2020 ◽  
Vol 2020 ◽  
pp. 1-13
Author(s):  
Zhanwei Liu ◽  
Xinyu Li ◽  
Tenglong Cong ◽  
Rui Zhang ◽  
Lingyun Zheng ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Friction Coefficient ◽  
Liquid Sodium ◽  
Heat Transfer Characteristics ◽  
Backward Facing Step ◽  
Heating Wall ◽  
Transfer Characteristics ◽  
Cfd Models ◽  
Flow And Heat Transfer

The prediction of flow and heat transfer characteristics of liquid sodium with CFD technology is of significant importance for the design and safety analysis of sodium-cooled fast reactor. The accuracies and uncertainties of the CFD models should be evaluated to improve the confidence of the numerical results. In this work, the uncertainties from the turbulent model, boundary conditions, and physical properties for the flow and heat transfer of liquid sodium were evaluated against the experimental data. The results of uncertainty quantization show that the maximum uncertainties of the Nusselt number and friction coefficient occurred in the transition zone from the inlet to the fully developed region in the circular tube, while they occurred near the reattachment point in the backward-facing step. Furthermore, in backward-facing step flow, the maximum uncertainty of temperature migrated from the heating wall to the geometric center of the channel, while the maximum uncertainty of velocity occurred near the vortex zone. The results of sensitivity analysis illustrate that the Nusselt number was negatively correlated with the thermal conductivity and turbulent Prandtl number, while the friction coefficient was positively correlated with the density and Von Karman constant. This work can be a reference to evaluate the accuracy of the standard k-ε model in predicting the flow and heat transfer characteristics of liquid sodium.

Direct numerical simulation of a turbulent jet impinging on a heated wall

2015 ◽  
Vol 764 ◽  
pp. 362-394 ◽  
Author(s):  
T. Dairay ◽  
V. Fortuné ◽  
E. Lamballais ◽  
L.-E. Brizzi
Keyword(s):  
Heat Transfer ◽  
Numerical Simulation ◽  
Nusselt Number ◽  
Direct Numerical Simulation ◽  
Temperature Fields ◽  
Heated Wall ◽  
Small Scale ◽  
Turbulent Regime ◽  
High Heat Transfer ◽  
Radial Evolution

AbstractDirect numerical simulation (DNS) of an impinging jet flow with a nozzle-to-plate distance of two jet diameters and a Reynolds number of 10 000 is carried out at high spatial resolution using high-order numerical methods. The flow configuration is designed to enable the development of a fully turbulent regime with the appearance of a well-marked secondary maximum in the radial distribution of the mean heat transfer. The velocity and temperature statistics are validated with documented experiments. The DNS database is then analysed focusing on the role of unsteady processes to explain the spatial distribution of the heat transfer coefficient at the wall. A phenomenological scenario is proposed on the basis of instantaneous flow visualisations in order to explain the non-monotonic radial evolution of the Nusselt number in the stagnation region. This scenario is then assessed by analysing the wall temperature and the wall shear stress distributions and also through the use of conditional averaging of velocity and temperature fields. On one hand, the heat transfer is primarily driven by the large-scale toroidal primary and secondary vortices emitted periodically. On the other hand, these vortices are subjected to azimuthal distortions associated with the production of radially elongated structures at small scale. These distortions are responsible for the appearance of very high heat transfer zones organised as cold fluid spots on the heated wall. These cold spots are shaped by the radial structures through a filament propagation of the heat transfer. The analysis of probability density functions shows that these strong events are highly intermittent in time and space while contributing essentially to the secondary peak observed in the radial evolution of the Nusselt number.

Pulsating Flow in a Channel With a Backward-Facing Step

1997 ◽  
Vol 50 (11S) ◽  
pp. S232-S236
Author(s):  
Alvaro Valencia
Keyword(s):  
Vortex Breakdown ◽  
Separation Zone ◽  
Pulsating Flow ◽  
Steady Flows ◽  
Reattachment Length ◽  
Flow Conditions ◽  
Backward Facing Step ◽  
Primary Vortex ◽  
Inlet Flow ◽  
Inlet Velocity

The incompressible laminar flow in a channel with a backward-facing step is studied for steady cases and for pulsating inlet flow conditions. For steady flows, the influrnce of the inlet velocity profile, the height of the step, and the Reynolds number on the reattachment length is investigated. A parabolic entrance profile was used for pulsating flow. It was found with amplitude of oscillation of one by Re = 100 that the primary vortex breakdown through one pulsatile cycle and the wall shear stress in the separation zone varied markedly with pulsating inlet flow.

Laminar Mixed Convection Adjacent to Three-Dimensional Backward-Facing Step

2001 ◽  
Vol 124 (1) ◽  
pp. 209-213 ◽  
Author(s):  
A. Li and ◽  
B. F. Armaly
Keyword(s):  
Nusselt Number ◽  
Mixed Convection ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Flow Region ◽  
The Other ◽  
Backward Facing Step ◽  
Number Range ◽  
Grashof Number ◽  
Recirculation Flow

Simulations of three-dimensional laminar buoyancy-assisting mixed convection adjacent to a backward-facing step in a vertical rectangular duct are presented to demonstrate the influence of Grashof number on the distributions of the Nusselt number, and the reverse flow regions that develop adjacent to the duct’s walls. The Reynolds number, and duct’s geometry are kept constant: heat flux at the wall downstream from the step is kept uniform but its magnitude varied to cover a Grashof number range of 0–4000; all the other walls in the duct are kept at adiabatic condition; and the flow, upstream of the step, is treated as fully developed and isothermal. Increasing the Grashof number results in increasing the Nusselt number; the size of the secondary recirculation flow region adjacent to the stepped wall; the size of the reverse flow region adjacent to the sidewall and the flat wall; and the spanwise flow from the sidewall toward the center of the duct. On the other hand, the size of the primary recirculation flow region adjacent to the stepped wall decreases and detaches partially from the heated stepped wall as the Grashof number increases. Details are presented and discussed.

Sign in / Sign up

Export Citation Format

Share Document