Turbulent separated convection flow adjacent to backward-facing step—effects of step height

Simulations of turbulent convection flow adjacent to a two dimensional backward-facing step are presented to explore the effects of step height, step inclination angle, a mounted rib and Prandtl number on velocity field and heat transfer. Reynolds number and duct’s height downstream from the step are kept constant at Re0 = 28000 and H = 0.19m, respectively. Uniform and constant heat flux of qw = 270W/m2 is specified at the stepped wall downstream from the step, while other walls are treated as adiabatic. The selection of the values for these parameters is motivated by the fact that measurements are available for this geometry and they can be used to validate the flow and heat transfer simulation code. The simulated results compare very well the measurements. The primary and secondary recirculation regions increase in size as the step height increases. The friction coefficient becomes smaller in magnitude with the increase of the step height. The peak Stanton number becomes smaller as the step height increases. The reattachment location becomes longer as the step inclination angle increases. With increase of the step inclination angle, the secondary recirculation region disappears. The peak friction coefficient inside the primary recirculation region becomes smaller as the step inclination angle decreases. Installation of a baffle on the upper wall causes the primary recirculation region to become smaller. The Stanton number decreases as the Prandtl number increases.

Download Full-text

Mixed Convection Adjacent to 3-D Backward-Facing Step

10.1115/imece2000-1408 ◽

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.

Download Full-text

The Effect of Backward-facing Step Height on Instability Growth and Breakdown in Swept-Wing Boundary-Layer Transition

45th AIAA Fluid Dynamics Conference ◽

10.2514/6.2015-2776 ◽

2015 ◽

Cited By ~ 2

Author(s):

Jenna L. Eppink ◽

Richard W. Wlezien ◽

Rudolph A. King ◽

Meelan M. Choudhari

Keyword(s):

Boundary Layer ◽

Boundary Layer Transition ◽

Step Height ◽

Swept Wing ◽

Backward Facing Step ◽

Layer Transition ◽

Instability Growth

Download Full-text

Numerical Simulation of Mixed Convective Flow Over a Three-Dimensional Horizontal Backward Facing Step

Journal of Heat Transfer ◽

10.1115/1.2005272 ◽

2005 ◽

Vol 127 (9) ◽

pp. 1027-1036 ◽

Cited By ~ 32

Author(s):

J. G. Barbosa Saldana ◽

N. K. Anand ◽

V. Sarin

Keyword(s):

Convective Flow ◽

Three Dimensional ◽

Step Length ◽

Expansion Ratio ◽

Step Height ◽

Streamwise Direction ◽

Backward Facing Step ◽

Thermal Features ◽

Mixed Convective Flow ◽

Thermally Conducting

Laminar mixed convective flow over a three-dimensional horizontal backward-facing step heated from below at a constant temperature was numerically simulated using a finite volume technique and the most relevant hydrodynamic and thermal features for air flowing through the channel are presented in this work. The channel considered in this work has an aspect ratio AR=4, and an expansion ratio ER=2, while the total length in the streamwise direction is 52 times the step height (L=52s) and the step length is equal to 2 times the step height (l=2s). The flow at the duct entrance was considered to be hydrodynamically fully developed and isothermal. The bottom wall of the channel was subjected to a constant high temperature while the other walls were treated to be adiabatic. The step was considered to be a thermally conducting block.

Download Full-text

The Effect of the Backward-Facing Step Height on Flow in a Channel with Rectangular Cross Section

PAMM ◽

10.1002/pamm.200510251 ◽

2005 ◽

Vol 5 (1) ◽

pp. 549-550 ◽

Cited By ~ 1

Author(s):

Pavel Jonáš ◽

Oton Mazur ◽

Václav Uruba

Keyword(s):

Cross Section ◽

Rectangular Cross Section ◽

Step Height ◽

Backward Facing Step

Download Full-text

Experiments on Backward-Facing Step Flows Preceding a Filter

Volume 2: Fora, Parts A and B ◽

10.1115/fedsm2007-37204 ◽

2007 ◽

Author(s):

S. Yao ◽

C. Krishnamoorthy ◽

F. W. Chambers

Keyword(s):

Reynolds Number ◽

Turbulent Flow ◽

Separated Flow ◽

Reynolds Numbers ◽

Velocity Profiles ◽

Step Height ◽

Reattachment Point ◽

Backward Facing Step ◽

Air Filters ◽

Air Filter

The resistance of automotive air filters alters upstream pressure gradients and thereby affects flow separation, the velocity distributions over the filter, and the performance of the filter. Air filters provide a resistance sufficient to alter flows, but not enough to make face velocities uniform. The backward-facing step flow is an archetype with a separation that resembles those found in automotive air filter housings. To gain insight to the problem of separation and filters, experiments were conducted measuring velocity fields for air flows in a 10:1 aspect ratio rectangular duct with a backward-facing step with and without the resistance of an air filter mounted downstream. The expansion ratio for the step was 1:2. The filter was mounted 4.25 and 6.75 step heights downstream of the step; locations both upstream and downstream of the nominal 6 step-height no-filter reattachment point. Experiments were performed at four Reynolds numbers between 2000 and 10,000. The Reynolds numbers were based on step height and inlet maximum velocity. The inlet velocity profiles at the step were developed. A Laser Doppler Anemometer (LDA) was used to measure velocity profiles and map separated regions between the step and the filter. The results indicate that the filter tends to decrease the streamwise velocity on the non-separated side of the channel and increase it on the separated, step, side compared to the no-filter flow. Non-separated flow tends to separate due to the deceleration and separated flow reattaches before the filter, whether the filter is placed at 4.25 or 6.75 step heights. The literature shows that without a filter the reattachment location depends on the Reynolds number in the laminar and transitional regimes, but is constant for turbulent flow. However, the area of the reversed flow may vary with Reynolds number for turbulent flow. With the filter at 4.25 step heights, the area of reversing flow is reduced significantly, and the Reynolds number has little effect on the main properties of the flow. With the filter at 6.75 step heights, the reversing flow area decreases as the Reynolds number increases though the reattachment point is fixed just upstream of the filter.

Download Full-text

Effects of a baffle on separated convection flow adjacent to backward-facing step

International Journal of Thermal Sciences ◽

10.1016/j.ijthermalsci.2008.05.015 ◽

2009 ◽

Vol 48 (3) ◽

pp. 618-625 ◽

Cited By ~ 37

Author(s):

J.H. Nie ◽

Y.T. Chen ◽

H.T. Hsieh

Keyword(s):

Convection Flow ◽

Backward Facing Step

Download Full-text

Study of Laminar Forced Convection of Radiating Gas Over an Inclined Backward Facing Step Under Bleeding Condition Using the Blocked-Off Method

Journal of Heat Transfer ◽

10.1115/1.4003607 ◽

2011 ◽

Vol 133 (7) ◽

Cited By ~ 14

Author(s):

A. B. Ansari ◽

S. A. Gandjalikhan Nassab

Keyword(s):

Forced Convection ◽

Gas Flow ◽

Radiation Heat Transfer ◽

Simple Algorithm ◽

Temperature Fields ◽

Convection Flow ◽

Backward Facing Step ◽

Cartesian Coordinate ◽

Volume Method ◽

Laminar Forced Convection

This paper presents a numerical investigation for laminar forced convection flow of a radiating gas over an inclined backward facing step in a horizontal duct subjected to bleeding condition. The fluid is treated as a gray, absorbing, emitting, and scattering medium. The two-dimensional Cartesian coordinate system is used to simulate flow over inclined surface by considering the blocked-off region in regular grid. The governing differential equations consisting the momentum and energy are solved numerically by the computational fluid dynamics techniques to obtain the velocity and temperature fields. Discretized forms of these equations are obtained by the finite volume method and solved using the SIMPLE algorithm. Since the gas is considered as a radiating medium, convection, conduction, and radiation heat transfer mechanisms take place simultaneously in the gas flow. For computation of the radiative term in the gas energy equation, the radiative transfer equation is solved numerically by the discrete ordinate method to find the radiative heat flux distribution inside the radiating medium. The effects of bleeding coefficient, inclination angle, optical thickness, albedo coefficient, and the radiation-conduction parameter on the flow and temperature distributions are carried out.

Download Full-text