Stability of buoyancy-driven flow in a vertical channel with one heated wall

2021 ◽  
Vol 33 (8) ◽  
pp. 084103
Author(s):  
S. Zeraati Dizjeh ◽  
J. Brinkerhoff
Keyword(s):  
Vertical Channel ◽  
Heated Wall

Incipient Flow Boiling in a Vertical Channel With a Wavy Wall

2010 ◽  
Author(s):  
T. Netz ◽  
R. Shalem ◽  
J. Aharon ◽  
G. Ziskind ◽  
R. Letan
Keyword(s):  
Heat Transfer ◽  
Temperature Field ◽  
High Speed ◽  
Flow Boiling ◽  
Heated Surface ◽  
Vertical Channel ◽  
Infrared Camera ◽  
Heat Fluxes ◽  
Heated Wall ◽  
Boiling Incipience

In the present study, incipient flow boiling of water is studied experimentally in a square-cross-section vertical channel. Water, preheated to 60–80 degrees Celsius, flows upwards. The channel has an electrically heated wall, where the heat fluxes can be as high as above one megawatt per square meter. The experiment is repeated for different water flow rates, and the maximum Reynolds number reached in the present study is 27,300. Boiling is observed and recorded using a high-speed digital video camera. The temperature field on the heated surface is measured with an infrared camera and a software is used to obtain quantitative temperature data. Thus, the recorded boiling images are analyzed in conjunction with the detailed temperature field. The dependence of incipient boiling on the flow and heat transfer parameters is established. For a flat wall, the results for various velocities and subcooling conditions agree well with the existing literature. Furthermore, three different wavy heated surfaces are explored, having the same pitch of 4mm but different amplitudes of 0.25mm, 0.5mm and 0.75mm. The effect of surface waviness on single-phase heat transfer and boiling incipience is shown. The differences in boiling incipience on various surfaces are elucidated, and the effect of wave amplitude on the results is discussed.

Channel Flow Along a Wavy Heated Wall

2011 ◽  
Author(s):  
S. Barboy ◽  
A. Rashkovan ◽  
G. Ziskind
Keyword(s):  
Vertical Channel ◽  
Turbulence Models ◽  
Momentum Equation ◽  
Constant Heat Flux ◽  
Heated Wall ◽  
Coupling Scheme ◽  
Wall Region ◽  
Pressure Gradients ◽  
Wavy Wall ◽  
Flow And Heat Transfer

The present study deals with the effects of wall geometry on the fluid flow and heat transfer in a vertical channel with a wavy wall. The waviness is characterized by wave amplitude and period. The wavy wall is heated with a constant heat flux. A detailed parametric investigation of the effect of waviness is performed for different flow conditions. An enhanced version of the turbulence models is required in order to resolve the near-wall region. In particular, a single wall law for the entire wall region can be achieved by blending linear (viscous) and logarithmic (turbulent) laws-of-the-wall. This approach allows the fully turbulent law to be easily modified and extended to take into account other effects such as pressure gradients or variable properties. Second order discretization scheme for momentum equation and turbulence scalar equations was used. SIMPLE pressure-velocity coupling scheme was employed. The results show how the flow and geometry parameters, namely, the Reynolds number and the amplitude and period of waviness, affect such features as the existence of flow separation, its location and size of the recirculation zones. These features determine the temperature distribution on the wavy wall. An attempt is done to assess the effect of flow and geometry parameters quantitatively.

Viscous Dissipation Effect on a Steady Generalised Couette Flow of Heat-Generating/Absorbing Fluid in a Vertical Channel

2019 ◽  
Vol 74 (7) ◽  
pp. 605-616
Author(s):  
Abiodun O. Ajibade ◽  
Tafida M. Kabir
Keyword(s):  
Mixed Convection ◽  
Couette Flow ◽  
Viscous Dissipation ◽  
Reverse Flow ◽  
Vertical Channel ◽  
Heated Wall ◽  
Fluid Temperature ◽  
Dissipation Effect ◽  
Mixed Convection Parameter ◽  
Convection Parameter

AbstractThis article investigates the viscous dissipation effect on steady generalised Couette flow of heat-generating/absorbing fluid in a vertical channel. Equations of energy and momentum are obtained and solved using the homotopy perturbation method. The influences of the dimensionless flow parameter have been plotted graphically and discussed for varying values of the controlling parameters. During the course of computation, it is found that fluid temperature and velocity increase with an increase in viscous dissipation and also seen that growing mixed convection parameter Gre leads to a corresponding rise in temperature and velocity. It is further discovered that heat absorption leads to increase in the heat transfer on the heated wall. Finally, it is concluded that heat generation contributes to increase the mixed convection, hence, it requires decrease in mixed convection parameter to bring about a reverse flow near the stationary plate.

High Rayleigh Number Laminar Natural Convection in an Asymmetrically Heated Vertical Channel

1989 ◽  
Vol 111 (3) ◽  
pp. 649-656 ◽  
Author(s):  
B. W. Webb ◽  
D. P. Hill
Keyword(s):  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Vertical Channel ◽  
Local Heat Transfer ◽  
Heated Wall ◽  
Uniform Heat Flux ◽  
Parallel Plates ◽  
Heating Rates ◽  
Wide Range

Experiments have been performed to determine local heat transfer data for the natural convective flow of air between vertical parallel plates heated asymmetrically. A uniform heat flux was imposed along one heated wall, with the opposing wall of the channel being thermally insulated. Local temperature data along both walls were collected for a wide range of heating rates and channel wall spacings corresponding to the high modified Rayleigh number natural convection regime. Laminar flow prevailed in all experiments. Correlations are presented for the local Nusselt number as a function of local Grashof number along the channel. The dependence of both average Nusselt number and the maximum heated wall temperature on the modified Rayleigh number is also explored. Results are compared to previous analytical and experimental work with good agreement.

Subcooled Flow Boiling in a Rectangular Channel With Added Turbulence and Longitudinal Vortices

2012 ◽  
Author(s):  
Gregor Bloch ◽  
Christina Jochum ◽  
Tobias Schechtl ◽  
Thomas Sattelmayer
Keyword(s):  
Isotropic Turbulence ◽  
Flow Boiling ◽  
Flow Direction ◽  
Rectangular Channel ◽  
Vertical Channel ◽  
Nucleate Boiling ◽  
Secondary Flows ◽  
Heated Wall ◽  
Working Fluid ◽  
Flow Velocities

Experiments are conducted to analyze the influence of turbulence and secondary flows on heat transfer and CHF in sub-cooled flow boiling. Inserts creating turbulence and stationary vortices are placed below a vertical channel with a heated wall and upward flow direction with flow velocities up to 1.2 m/s. The boiling chamber is of square shape with inner dimensions of 40 × 40 mm2. Boiling regimes range from onset of nucleate boiling up to fully developed film boiling. Influence of the inserts is measured for varying flow velocities and subcooling from 4 K to 27 K. Flow parameters are measured with Particle Image Velocimetry (PIV). A decay of nearly isotropic turbulence within only few diameters is observed, while stationary swirls exhibit longer penetration depths. Boiling experiments are conducted with unsteady heating with a low boiling hydrocarbon (dodekafluoromethylpentanone) as working fluid. Results from boiling experiments show a positive influence of the inserts on the boiling process, increasing with higher subcooling and flow velocities.

Effects of Clearance Between Heating Walls on Natural Cooling in a Channel Model of Electronic Equipment

2007 ◽  
Author(s):  
Yasushi Nishino ◽  
Ryoji Imai ◽  
Shinji Nakagawa ◽  
Masaru Ishizuka
Keyword(s):  
Temperature Rise ◽  
Printed Circuit Boards ◽  
Channel Model ◽  
Maximum Velocity ◽  
Vertical Channel ◽  
Cooling Capacity ◽  
Electronic Equipment ◽  
Heated Wall ◽  
Parallel Plates ◽  
The Relationship

Making electronic products smaller in size requires air passages in the products to be narrow. For effective thermal management with natural convection, the relationship between cooling performance and a space for the air passages must be clarified. In this study, the natural cooling capacity and flow field in relatively small electronic equipment have been investigated. A channel model was used as an experimental model of electronic equipments. The channel model has two vertical copper walls modeling the printed circuit boards and two transparent walls modeling the casing walls. The walls constitute a vertical channel with the height of 120mm, the depth of 56mm, and the variable width. The width of the channel is called “a wall clearance” here and it is varied from 5mm to 15mm. The copper walls were heated using electric heaters. Temperatures in the model were measured with thermo-couples. In addition, velocity distributions in the channel were quantitatively measured using a particle image velocimetry (PIV). The natural cooling capacity was obtained as functions of the wall clearance and heating power. Temperature rise of the heated wall showed small differences with the clearances of 10 mm and 15mm. However, when the clearance was decreased to 5mm, temperature rise increased. The relationship between Nusselt number and Rayleigh number obtained in this study agrees with those obtained for the parallel plates without side walls. The results of the velocity measurement revealed that the velocity showed a 33% decrease when the wall clearance decreased from 10mm to 5mm. On the other hand, the maximum velocity in the channel showed a 10% increase when the clearance decreased from 15mm to 10mm. The changes in the velocity profiles depending on the heating conditions are clarified.

Advances in Modeling Critical Heat Flux in LWR Boiling Flows With the NEK-2P CFD Code

2018 ◽  
Author(s):  
Adrian Tentner ◽  
Prasad Vegendla ◽  
Ananias Tomboulides ◽  
Aleks Obabko ◽  
Elia Merzari ◽  
...  
Keyword(s):  
Heat Flux ◽  
Critical Heat Flux ◽  
Vertical Channel ◽  
Nucleate Boiling ◽  
Heated Wall ◽  
Transfer Model ◽  
Axial Distribution ◽  
Two Phase ◽  
Wide Range ◽  
Boiling Flow

The paper focuses on the extension of the NEK-2P Wall Heat Transfer model, which was initially developed for the analysis of Critical Heat Flux (CHF) under Dryout (DO) conditions to the simulation of CHF under Departure from Nucleate Boiling (DNB) conditions. The paper presents results of recent NEK-2P analyses of several CHF experiments including both DO and DNB conditions. The CHF experiments analyzed have measured the axial distribution of wall temperatures in two-phase boiling flow in a vertical channel with a heated wall. The axial distribution of the calculated wall temperatures is compared with the corresponding experimental data. Reasonably good agreement with measured data is obtained in predicting the CHF location and post CHF wall temperature magnitudes illustrating the ability of the NEK-2P code and Extended Boiling Framework (EBF) models to simulate the CHF phenomena for a wide range of thermal-hydraulic conditions.

Observed Flow Reversals and Measured-Predicted Nusselt Numbers for Natural Convection in a One-Sided Heated Vertical Channel

1984 ◽  
Vol 106 (2) ◽  
pp. 325-332 ◽  
Author(s):  
E. M. Sparrow ◽  
G. M. Chrysler ◽  
L. F. Azevedo
Keyword(s):  
Natural Convection ◽  
Numerical Solutions ◽  
Vertical Channel ◽  
Threshold Value ◽  
Channel Length ◽  
Heated Wall ◽  
Number Range ◽  
Channel One ◽  
Recirculating Flow ◽  
Nusselt Numbers

A three-part study encompassing both experiment and analysis has been performed for natural convection in an open-ended vertical channel. One of the principal walls of the channel—the heated wall—was maintained at a uniform temperature, while the other principal wall was unheated. The experiments, which included flow visualization and Nusselt number measurements, were carried out with water in the channel and in the ambient which surrounds the channel. At Rayleigh numbers which exceeded a threshold value, the visualization revealed a pocket of recirculating flow situated adjacent to the unheated wall in the upper part of the channel. The recirculation was fed by fluid drawn into the top of the channel, adjacent to the unheated wall. Average Nusselt numbers for the heated wall were measured over a three orders of magnitude range of a single correlating parameter, which includes the Rayleigh number and the ratio of the channel length to the interwall spacing. The Nusselt numbers were found to be unaffected by the presence of the recirculation zone. Numerical solutions obtained via a parabolic finite difference scheme yielded Nusselt numbers in good agreement with those of experiment. The numerical results covered the Prandtl number range from 0.7 to 10.

Natural Convection of Air through an Asymmetrically Heated Vertical Channel with a Baffle

2016 ◽  
Vol 846 ◽  
pp. 7-11
Author(s):  
Cheng Wang Lei ◽  
Tuo Tian Wang
Keyword(s):  
Natural Convection ◽  
Reverse Flow ◽  
Air Flow ◽  
Vertical Channel ◽  
The Other ◽  
Heated Wall ◽  
Two Dimensional ◽  
Flow Through ◽  
Vertical Ventilation ◽  
Ventilation Performance

The buoyancy-induced air flow through a two-dimensional vertical ventilation channel is calculated. One of the channel walls is heated uniformly, and the other wall is adiabatic. A thin baffle is placed on the heated wall to manipulate the air flow through the channel. Numerical results are obtained for baffles of different lengths and placed at various heights along the heated wall. It is found that the baffle is effective in weakening a reverse flow at the exit of the channel, and significant enhancement of ventilation performance may be achieved with the presence of the baffle.

Hysteresis Phenomena at the Onset of Subcooled Nucleate Flow Boiling in Microchannels

2003 ◽  
Author(s):  
Magdalena Piasecka ◽  
Mieczyslaw E. Poniewski
Keyword(s):  
Liquid Crystal ◽  
Flow Boiling ◽  
Saturation Temperature ◽  
Vertical Channel ◽  
Heated Wall ◽  
Liquid Crystal Thermography ◽  
Boiling Incipience ◽  
The Impact ◽  
Hysteresis Phenomena ◽  
Wide Wall

In this paper, attempts were made to experimentally investigate the boiling incipience in a narrow rectangular vertical channel of 1 mm depth with an external 40 mm wide wall heated uniformly and others assumed quasiadiabatic. The “boiling front” location was determined from the temperature distribution of the heated wall obtained from liquid crystal thermography. Boiling incipience occurs when considerable rise in wall temperature above the saturation temperature takes place. Thus, boiling incipience is accompanied by “nucleation hysteresis”. The impact of various factors on the boiling incipience in microchannels, such as: pressure, the inlet liquid subcooling and flow velocity were investigated.

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