scholarly journals Numerical Analysis of the Improving Thermal Energy Efficiency of Taylor-couette Flow

2021 ◽  
Vol 15 ◽  
pp. 236-247
Author(s):  
Khaoula Ben Abdelmlek ◽  
Fayçal Ben Nejma
Keyword(s):  
Energy Efficiency ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Numerical Study ◽  
Rotation Velocity ◽  
Radius Ratio ◽  
Nusselt Numbers ◽  
Cylindrical Annulus ◽  
The Mean ◽  
Rayleigh Numbers

This paper deals with adimensionnal analysis of natural convection in a horizontal cylindrical annulus. The inner cylinder is isothermally heated and rotates with an angular velocity Ω, however the outer one is kept cold and motionless. The gap between cylinders is defined by an adimensional radius ratio f. The numerical study was carried out using COMSOL Multiphysics. The effects of Rayleigh number ranging from 102 to 106, radius ratio and rotation velocity on the flow pattern and the thermal behavior in the annulus are then elaborated. Particular attention is paid to the effect of different parameters on the local Nusselt numbers on the inner and outer cylinders, the mean Nusselt number and the energy efficiency of the process. Results show that the mean Nusselt number increases with the increase of Rayleigh number. However, it decreases with the increase of the radius ratio f because of the narrowing of the annulus. The results prove also that the heat transfer rate drops with the rise of rotation velocity. Finally, it was found that the energy efficiency achieved its maximum for lower Rayleigh numbers Ra=103, and lower rotation velocities.

A Numerical Study of Natural Convection in a Cavity With Two Fins Attached to Its Vertical Walls

2007 ◽  
Author(s):  
G. A. Sheikhzadeh ◽  
M. Pirmohammadi ◽  
M. Ghassemi
Keyword(s):  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Square Cavity ◽  
Vertical Walls ◽  
The Mean ◽  
Energy Equations ◽  
Rayleigh Numbers

Numerical study natural convection heat transfer inside a differentially heated square cavity with adiabatic horizontal walls and vertical isothermal walls is investigated. Two perfectly conductive thin fins are attached to the isothermal walls. To solve the governing differential mass, momentum and energy equations a finite volume code based on Pantenkar’s simpler method is developed and utilized. The results are presented in form of streamlines, isotherms as well as Nusselt number for Rayleigh number ranging from 104 up to 107. It is shown that the mean Nusselt number is affected by the position of the fins and length of the fins as well as the Rayleigh number. It is also observed that maximum Nusselt number occurs about the middle of the enclosure where Lf is grater the 0.5. In addition the Nusselt number stays constant and does not varies with width of the cavity (lf) when Lf is equal to 0.5 and Rayleigh number is equal to 104 and 107 as well as when Lf is equal to 0.6 and low Rayleigh numbers.

Convective Flow in Annuli of Stationary and Rotating Horizontal Cylinders

1997 ◽  
Author(s):  
T. S. Lee
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Rayleigh Number ◽  
Prandtl Number ◽  
Cooling System ◽  
Cylindrical Annulus ◽  
Receiver System ◽  
The Mean ◽  
Effects Of Rotation

Heat transfer and fluid flow processes in enclosed spaces have been extensively studied due to their importance in energy conversion, storage and transmission systems. Concentric and eccentric annular geometries are most commonly encountered in solar collector-receiver system, cooling system in nuclear reactors etc. For mixed flow in the annulus between concentric or eccentric cylinders in a rotating system, most work were performed for the cases of vertical cylindrical annulus. More recently, the effects of recirculation on the natural convection between the annular region in horizontal rotating cylinders have become a topic of interest to researchers. The applications of these studies include food processing and the interest in seeking improved methods for crystallographic perfection in industrial processes, above studies are for air with Pr≅1.0. However, other effects of rotation on heat transfer characteristics for low Prandtl number fluids are encountered in high power electric machines with heated shafts, such as a mercury slip ring assembly. For the present study, natural convection is driven by vertical temperature gradient and vertical gravity force. The interaction with the effect of rotation of the inner cylinder is expected to lead to complicated flows. Studies show that the mean Nusselt number increases with Rayleigh number. At a Prandtl number of order 1.0 with a fixed Rayleigh number, when the inner cylinder is made to rotate, the mean Nusselt number decreases through out the flow. At lower Prandtl number of the order 0.1 to 0.01, the mean Nusselt number remained fairly constant with respect to the rotational Reynolds number.

Effect of departures from the Oberbeck-Boussinesq approximation on the heat transport of horizontal convecting fluid layers

1980 ◽  
Vol 98 (1) ◽  
pp. 137-148 ◽  
Author(s):  
Guenter Ahlers
Keyword(s):  
Nusselt Number ◽  
Rayleigh Number ◽  
Critical Rayleigh Number ◽  
Boussinesq Approximation ◽  
Initial Slope ◽  
Nusselt Numbers ◽  
Fluid Layers ◽  
Different Temperatures ◽  
Rayleigh Numbers ◽  
Experimental Scatter

Measurements are presented of the Nusselt numbers N and Rayleigh numbers R for shallow layers of 4He gas heated from below. By choosing different temperatures between 2·3 K and 5·1 K and different pressures between 0·07 bar and 1 bar, the extent Q of departures from the Oberbeck-Boussinesq approximation was varied. When R was evaluated at the static temperature at the midplane of the cell, both the critical Rayleigh number Rc and the initial slope N1 of the Nusselt number were found to be independent of Q within experimental scatter. This result agrees with the prediction of Busse (1967). When R was evaluated at the cold end temperature of the cell, both Rc and N1 depended strongly upon Q.

Incipient Buoyant Thermal Convection in a Vertical Cylindrical Annulus

1990 ◽  
Vol 112 (4) ◽  
pp. 959-964 ◽  
Author(s):  
D. L. Littlefield ◽  
P. V. Desai
Keyword(s):  
Rayleigh Number ◽  
Boundary Conditions ◽  
Thermal Convection ◽  
Fluid Motion ◽  
Radius Ratio ◽  
Cellular Behavior ◽  
Cylindrical Annulus ◽  
Rayleigh Numbers ◽  
Number Of Cells

The incipient buoyant thermal convection in a vertical cylindrical annulus when heated from below is examined. The ends are assumed to be free, and the sidewalls perfectly conducting. The temperature needed to initiate fluid motion is expressed nondimensionally in terms of the Rayleigh number. The analytical conflict that arises for annuli of infinite aspects ratios due to insufficient independent boundary conditions is resolved. Calculations for the critical Rayleigh numbers are presented for a variety of geometries, and the corresponding velocity and temperature perturbations are also shown. The number of cells increases as the aspect and radius ratio decrease with a strong bias towards the development of azimuthally varying cells. These changes in cellular behavior are expected based on physical justifications and comparisons with previous studies.

ANALYSIS OF NATURAL CONVECTION HEAT TRANSFER BETWEEN TWO HORIZONTAL CYLINDERS AND THEIR ENCLOSURE

1992 ◽  
Vol 16 (1) ◽  
pp. 17-32 ◽  
Author(s):  
M. Lacroix
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Natural Convection ◽  
Numerical Study ◽  
Convection Heat Transfer ◽  
Natural Convection Heat Transfer ◽  
Convection Heat ◽  
Nusselt Numbers ◽  
Horizontal Cylinders ◽  
Rayleigh Numbers

A numerical study has been conducted for natural convection heat transfer for air around two horizontal heated cylinders placed inside a rectangular enclosure cooled from the side. Three cylinder spacings were investigated. The local and overall Nusselt numbers were determined over the range of Rayleigh numbers from 104 to 106. It is found that the thermal performance of the unit is strongly influenced by the Rayleigh number and, to a lesser extent, by the cylinder spacing. A correlation is suggested for the overall Nusselt number.

scholarly journals Free convection in a vertical cylindrical annulus filled with anisotropic porous medium

2009 ◽  
Vol 13 (1) ◽  
pp. 37-45 ◽  
Author(s):  
Rathinam Thansekhar ◽  
Babu Mahesh ◽  
Sekhar Chandra
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Temperature Distribution ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Free Convection ◽  
Numerical Study ◽  
Radius Ratio ◽  
Permeability Ratio ◽  
Cylindrical Annulus

A numerical study has been carried out for free convection in a vertical cylindrical annulus filled with a porous medium and whose inner wall is isothermally heated and the outer wall is isothermally cooled, the horizontal walls being insulated. The porous medium is assumed to be both hydrodynamically and thermally anisotropic. Numerical results are reported for 0.1 ? K*? 10, 0.1 ? ? ? 10,1 ? A ? 20,2 ? Rr ? 20, and Ra*? 10000. Anisotropy of the porous medium is found to affect fluid flow, temperature distribution and heat transfer significantly. Higher permeability in the vertical direction enhances convective flow intensity and heat transfer inside the annulus. Average Nusselt number on the inner hot wall increases with increase in Rayleigh number or radius ratio, while it decreases with increase in aspect ratio or permeability ratio. The influence of thermal anisotropy is not so significant as that of hydrodynamic anisotropy. The numerically predicted temperature distribution at various locations inside the annulus shows reasonable agreement with experimental results available for isotropic porous medium. Based on a parametric study, correlation for heat transfer is presented in terms of Rayleigh number, aspect ratio, radius ratio, and permeability ratio.

Heat transfer characteristics of nanofluids from a sinusoidal corrugated cylinder placed in a square cavity

2021 ◽  
pp. 095440622110272
Author(s):  
Salaika Parvin ◽  
Nepal Chandra Roy ◽  
Litan Kumar Saha ◽  
Sadia Siddiqa
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Aspect Ratio ◽  
Average Nusselt Number ◽  
Numerical Study ◽  
Heat Transfer Characteristics ◽  
Number Range ◽  
Transfer Characteristics ◽  
Wide Range

A numerical study is performed to investigate nanofluids' flow field and heat transfer characteristics between the domain bounded by a square and a wavy cylinder. The left and right walls of the cavity are at constant low temperature while its other adjacent walls are insulated. The convective phenomena take place due to the higher temperature of the inner corrugated surface. Super elliptic functions are used to transform the governing equations of the classical rectangular enclosure into a system of equations valid for concentric cylinders. The resulting equations are solved iteratively with the implicit finite difference method. Parametric results are presented in terms of streamlines, isotherms, local and average Nusselt numbers for a wide range of scaled parameters such as nanoparticles concentration, Rayleigh number, and aspect ratio. Several correlations have been deduced at the inner and outer surface of the cylinders for the average Nusselt number, which gives a good agreement when compared against the numerical results. The strength of the streamlines increases significantly due to an increase in the aspect ratio of the inner cylinder and the Rayleigh number. As the concentration of nanoparticles increases, the average Nusselt number at the internal and external cylinders becomes stronger. In addition, the average Nusselt number for the entire Rayleigh number range gets enhanced when plotted against the volume fraction of the nanofluid.

scholarly journals Improved Energy Efficiency of Mixed Convection Heating Process in Eccentric Annulus

2021 ◽  
Vol 13 (8) ◽  
pp. 168781402110391
Author(s):  
Ben Abdelmlek Khaoula ◽  
Ben Nejma Fayçal
Keyword(s):  
Heat Transfer ◽  
Energy Efficiency ◽  
Rayleigh Number ◽  
Mixed Convection ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Rotation Speed ◽  
Numerical Study ◽  
Heating Process ◽  
Eccentric Annulus

This paper deals with a numerical study of mixed convection heat transfer in horizontal eccentric annulus. The inner cylinder is supposed hot and rotating, however the outer one is kept cold and motionless. The numerical problem was solved using COMSOL Multiphysics® which is based on finite element method. The resolution of the partial differential equations was conducted through an implicit scheme with the use of the damped Newton’s method. The present numerical analysis concerns the effect of eccentricity, rotation speed and Rayleigh number on the flow patterns, heat transfer rate, and energy efficiency of the process. It was found that the heat transfer rate increases with the increase of Rayleigh number. In addition, the heat transfer rate drops with the increase of rotation speed. Finally, we have demonstrated that maximum energy efficiency is achieved not only with higher Rayleigh number but also it is maximum with small eccentricity.

scholarly journals Effect of Rayleigh Number on Internal Eccentricity in a Heated Horizontal Elliptical Cylinder to its Coaxial Square Enclosure

2020 ◽  
Vol 25 (3) ◽  
pp. 17-29
Author(s):  
Abdelkrim Bouras ◽  
Djedid Taloub ◽  
Zied Driss
Keyword(s):  
Heat Transfer ◽  
Rayleigh Number ◽  
Outer Cylinder ◽  
Boussinesq Approximation ◽  
Square Enclosure ◽  
Nusselt Numbers ◽  
Flow And Heat Transfer ◽  
Commercial Code ◽  
Volume Method ◽  
Rayleigh Numbers

AbstractThis paper deals with numerical investigation of a natural convective flow in a horizontal annular space between a heated square inner cylinder and a cold elliptical outer cylinder with a Newtonian fluid. Uniform temperatures are imposed along walls of the enclosure. The governing equations of the problem were solved numerically by the commercial code Fluent, based on the finite volume method and the Boussinesq approximation. The effects of Geometry Ratio GR and Rayleigh numbers on fluid flow and heat transfer performance are investigated. The Rayleigh number is varied from 103 to 106. Throughout the study the relevant results are presented in terms of isotherms, and streamlines. From the results, we found that the increase in the Geometry Ratio B leads to an increase of the heat transfer coefficient. The heat transfer rate in the annulus is translated in terms of the average Nusselt numbers along the enclosure’s sides. Tecplot 7 program was used to plot the curves which cleared these relations and isotherms and streamlines which illustrate the behavior of air through the channel and its variation with other parameters. The results for the streamlines, isotherms, local and average Nusselt numbers average Nusselt numbers are compared with previous works and show good agreement.

Experimental and Numerical Investigation of Natural Convection Heat Transfer in Horizontal Elliptic Annuli

2008 ◽  
Vol 6 (1) ◽  
Author(s):  
Ramadan Y. Sakr ◽  
Nabil S. Berbish ◽  
Ali A. Abd-Aziz ◽  
Abdalla Said Hanafi
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Rayleigh Number ◽  
Average Nusselt Number ◽  
Orientation Angle ◽  
Elliptic Cylinder ◽  
Major Axis ◽  
Radius Ratio ◽  
Hydraulic Radius ◽  
Axis Ratio

Experimental and numerical studies for natural convection in two dimensional regions formed by a constant flux heat horizontal elliptic tube concentrically located in a larger, isothermally cooled horizontal cylinder were investigated. Both ends of the annulus are closed. Experiments were carried out for the Rayleigh number based on the equivalent annulus gap length ranges from 1.12x107 up to 4.92x107; the elliptic tube orientation angle varies from 0o to 90o and the hydraulic radius ratio, HRR, was 6.4. These experiments were carried out for the axis ratio of an elliptic tube (minor/major=b/c) of 1:3. The numerical simulation for the problem is carried out by using commercial CFD code. The effects of the orientation angle as well as other parameters such as elliptic cylinder axis ratio and hydraulic radius ratio on the flow and heat transfer characteristics are investigated numerically. The numerical simulations covered a range of elliptic tube axis ratios from 0.1 to 0.98 and for the hydraulic radius ratios from 1.5 to 6.4. The results showed that the average Nusselt number increases as the orientation angle of the elliptic cylinder increases from 0o (the major axis is horizontal) to 90o (the major axis is vertical) and with the Rayleigh number as well. Also, the average Nusselt number decreases with the increase of the hydraulic radius ratio. An increase up to 1.75 and further increases in the hydraulic radius ratio leads to an increase in the average Nusselt number. The axis ratio of the elliptic cylinder has an insignificant effect on the average Nusselt number. Both the average and local Nusselt number from the experimental results are compared with those obtained from the CFD code.Both the fluid flow and heat transfer characteristics for different operating and geometric conditions are illustrated velocity vectors and isotherm contours that were obtained from the CFD code. Also, two correlation equations that relate the average Nusslet number with the Rayleigh number, orientation angle, and hydraulic radius ratio and axis ratio are obtained.

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