Heat Transfer Enhancement of Separated Convection Flow Adjacent to Backward-Facing Step With Baffle

2006 ◽  
Author(s):  
Jianhu Nie ◽  
Yitung Chen ◽  
Lijian Sun ◽  
Hsuan-Tsung Hsieh
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Friction Coefficient ◽  
Three Dimensional ◽  
Constant Heat Flux ◽  
Convection Flow ◽  
Spanwise Direction ◽  
Streamwise Direction ◽  
Backward Facing Step ◽  
Laminar Forced Convection

Simulations of three-dimensional laminar forced convection adjacent to inclined backward-facing step in rectangular duct are presented to examine effects of the baffle on flow and heat transfer distributions. The step height is maintained as constant. A baffle is mounted onto the upper wall and its distance from the backward-facing step is varied. The inlet flow is hydrodynamically steady and fully developed with uniform temperature. The bottom wall is heated with constant heat flux, while other walls are maintained as being thermally adiabatic. Velocity, temperature, Nusselt number, and friction coefficient distributions are presented. A baffle mounted onto the upper wall increases the magnitude of maximum Nuselt number at the stepped wall. One segment of the xu-line developing close to the backward-facing step becomes shorter with the decrease of the distance of the baffle from the backward-facing step. It becomes more relatively uniform in the spanwise direction as the distance decreases. The other segment developing adjacent to the sidewall moves further downstream as the baffle moves in the streamwise direction. The maximum Nusselt number does not appear at the center of the duct, as one may expect. It develops near the sidewall, and it moves further downstream as the location of the baffle moves in the streamwise direction. The friction coefficient at the stepped wall decreases as the distance of the baffle from the inlet increases.

Simulation of three-dimensional laminar forced convection flow of a radiating gas over an inclined backward-facing step in a duct under bleeding condition

2012 ◽  
Vol 227 (2) ◽  
pp. 332-345 ◽  
Author(s):  
M Atashafrooz ◽  
SA Gandjalikhan Nassab
Keyword(s):  
Heat Transfer ◽  
Forced Convection ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Numerical Procedure ◽  
Discrete Ordinates ◽  
Convection Flow ◽  
Backward Facing Step ◽  
Forced Convection Flow ◽  
Laminar Forced Convection

This study presents a numerical analysis of three-dimensional laminar forced convection flow of a radiating gas over an inclined backward-facing step in a rectangular duct under bleeding condition. The fluid is treated as a gray, absorbing, emitting, and scattering medium. The three-dimensional Cartesian coordinate system is used to solve the governing equations which are the conservations of mass, momentum, and energy. These equations are solved numerically using the computational fluid dynamic techniques to obtain the temperature and velocity fields, while the blocked-off method is employed to simulate the incline surface. 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, besides the convective and conductive terms in the energy equation, the radiative term also presented. For computation of this term, the radiative transfer equation is solved numerically by the discrete ordinates method to find the divergence of radiative heat flux distribution inside the radiating medium. By this numerical procedure, the role of radiation heat transfer on convection flow of a radiating gas which has many engineering applications (for example in heat exchangers and combustion chambers) is studied in detail. Beside, the effects of bleeding coefficient, albedo coefficient, optical thickness, and the radiation–conduction parameter on heat transfer behavior of the system are investigated. Comparison of numerical results with the available data published in the open literature shows a good agreement.

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.

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.

Parametric Study of Turbulent Separated Convection Flow Over a Backward-Facing Step in a Duct

2007 ◽  
Author(s):  
Jianhu Nie ◽  
Yitung Chen ◽  
Robert F. Boehm ◽  
Hsuan-Tsung Hsieh
Keyword(s):  
Heat Transfer ◽  
Friction Coefficient ◽  
Prandtl Number ◽  
Inclination Angle ◽  
Stanton Number ◽  
Step Height ◽  
Recirculation Region ◽  
Convection Flow ◽  
Backward Facing Step ◽  
Simulation Code

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.

Experimental Study on the Effect of Low Acute Side Angles on Heat Transfer in Rhombic Shaped Microchannel

2018 ◽  
Vol 141 (2) ◽  
Author(s):  
Sunil K. Dwivedi ◽  
Sandip K. Saha
Keyword(s):  
Heat Transfer ◽  
Boundary Condition ◽  
Experimental Study ◽  
Heat Flux ◽  
Nusselt Number ◽  
Three Dimensional ◽  
Constant Heat Flux ◽  
Numerical Results ◽  
Transfer Model ◽  
Design And Optimization

This experimental study on rhombic shaped microchannels was conducted to understand the effect of a low acute side angle on the Nusselt number and compare the results with the published numerical results for H1 (axially constant heat flux and circumferentially constant temperature) and H2 (constant axial and circumferential wall heat flux) boundary conditions. The hydraulic and heat transfer characteristics of the rhombic geometry with a side angle of 30 deg for different mass flow rates and heat flux inputs are obtained using a three-dimensional (3D) conjugate heat transfer model, which is validated with the experimental results. It is found that the average Nusselt number obtained from the experimental and numerical results can be approximated closely with that computed using the H1 boundary condition. The local Nusselt number of hydrodynamically and thermally developed regions obtained from the numerical analysis is compared with a correlation for the H1 boundary condition. These results will be useful in design and optimization of a rhombic shaped microchannel for electronic cooling applications.

Reattachment of Three-Dimensional Flow Adjacent to Backward-Facing Step

2003 ◽  
Vol 125 (3) ◽  
pp. 422-428 ◽  
Author(s):  
J. H. Nie ◽  
B. F. Armaly
Keyword(s):  
Three Dimensional ◽  
Separated Flow ◽  
Streamwise Velocity ◽  
Convection Flow ◽  
Backward Facing Step ◽  
Dimensional Flow ◽  
Line Region ◽  
Two Dimensional Flow ◽  
Streamwise Velocity Component ◽  
Laminar Forced Convection

Numerical simulations for incompressible three-dimensional laminar forced convection flow adjacent to backward-facing step in rectangular duct are performed to examine the reattachment region of the separated flow on the stepped wall. The feasibility of utilizing the two-dimensional flow definition and the limiting streamline definition for identifying the reattachment line/region was examined. The downwash and the “jet-like” flow that develops near the sidewall creates significant spanwise flow adjacent to the stepped wall, making it difficult to identify a reattachment line/region both numerically and experimentally. The use of the line/region that identifies the location on a plane adjacent to the stepped wall where the gradient of the mean streamwise velocity component is zero ∂u/∂y|y=0=0 is recommended for code and apparatus validation of three-dimensional separated flow.

scholarly journals Heat Transfer on a Film-Cooled Blade — Effect of Hole Physics

1998 ◽  
Author(s):  
Vijay K. Garg ◽  
David L. Rigby
Keyword(s):  
Heat Transfer ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Spanwise Direction ◽  
Computational Domain ◽  
Streamwise Direction ◽  
Flow Through ◽  
Flow Domain ◽  
Cooled Blade ◽  
Coolant Velocity

A multi-block, three-dimensional Navier-Stokes code has been used to study the within-hole and near-hole physics in relation to heat transfer on a film-cooled blade. The flow domain consists of the coolant flow through the plenum and hole-pipes for the three staggered rows of shower-head holes on the VKI rotor, and the main flow over the blade. A multi-block grid is generated that is nearly orthogonal to the various surfaces. It may be noted that for the VKI rotor the shower-head holes are inclined at 30° to the spanwise direction, and are normal to the streamwise direction on the blade. Wilcox’s k-ω turbulence model is used. The present study provides a much better comparison for the span-averaged heat transfer coefficient on the blade surface with the experimental data than an earlier analysis wherein coolant velocity and temperature distributions were specified at the hole exits rather than extending the computational domain into the hole-pipe and plenum. Details of the distributions of coolant velocity, temperature, k and ω at the hole exits are also presented.

Effect of Nanofluid on Heat Transfer Enhancement for Mixed Convection Flow Over a Corrugated Surface

2020 ◽  
Vol 45 (4) ◽  
pp. 373-383
Author(s):  
Nepal Chandra Roy ◽  
Sadia Siddiqa
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Nusselt Number ◽  
Mixed Convection ◽  
Local Nusselt Number ◽  
Richardson Number ◽  
Thermal Boundary Layer ◽  
Volume Fraction ◽  
Convection Flow ◽  
Mixed Convection Flow

AbstractA mathematical model for mixed convection flow of a nanofluid along a vertical wavy surface has been studied. Numerical results reveal the effects of the volume fraction of nanoparticles, the axial distribution, the Richardson number, and the amplitude/wavelength ratio on the heat transfer of Al2O3-water nanofluid. By increasing the volume fraction of nanoparticles, the local Nusselt number and the thermal boundary layer increases significantly. In case of \mathrm{Ri}=1.0, the inclusion of 2 % and 5 % nanoparticles in the pure fluid augments the local Nusselt number, measured at the axial position 6.0, by 6.6 % and 16.3 % for a flat plate and by 5.9 % and 14.5 %, and 5.4 % and 13.3 % for the wavy surfaces with an amplitude/wavelength ratio of 0.1 and 0.2, respectively. However, when the Richardson number is increased, the local Nusselt number is found to increase but the thermal boundary layer decreases. For small values of the amplitude/wavelength ratio, the two harmonics pattern of the energy field cannot be detected by the local Nusselt number curve, however the isotherms clearly demonstrate this characteristic. The pressure leads to the first harmonic, and the buoyancy, diffusion, and inertia forces produce the second harmonic.

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