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Article
Publication date: 27 March 2009

Maged A.I. El‐Shaarawia and Ali A. Al‐Ugla

The paper seeks to focus on obtaining the transient torque required to rotate the inner cylinder in open ended vertical concentric annuli for a fluid of Pr = 0.7 in the laminar…

384

Abstract

Purpose

The paper seeks to focus on obtaining the transient torque required to rotate the inner cylinder in open ended vertical concentric annuli for a fluid of Pr = 0.7 in the laminar natural convection flow regime over a wide range of the controlling parameter Gr2/Ta. The inner wall is heated and subjected to an impulsive rotation while the outer one is stationary and maintained adiabatic.

Design/methodology/approach

The governing transient boundary‐layer equations are numerically solved using an iterative linearized finite‐difference scheme.

Findings

The transient induced flow rate and absorbed heat for different annulus heights are presented. High rotational speed (i.e. low values of Gr2/Ta) increases the flow rate and heat absorbed in short annuli. However, for considerably tall annuli, Gr2/Ta has slight effect on the flow and heat absorbed. The steady‐state time is tangibly influenced by Gr2/Ta in considerably short annuli and very slightly affected for considerably tall annuli.

Practical implications

The investigated problem can simulate the start‐up period of naturally cooled small vertical electric motors.

Originality/value

The paper presents results not available in the literature for the effect of Gr2/Ta on the developing velocities, pressure, flow‐rate induced, absorbed heat by fluid and required torque in vertical concentric annuli with impulsively rotated inner walls under the transient free‐convection heat transfer mode.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 19 no. 2
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 19 June 2007

Maged A.I. El‐Shaarawi, Esmail M.A. Mokheimer and Ahmad Jamal

To explore the effect of the annulus geometrical parameters on the induced flow rate and the heat transfer under the conjugate (combined conduction and free convection) thermal…

659

Abstract

Purpose

To explore the effect of the annulus geometrical parameters on the induced flow rate and the heat transfer under the conjugate (combined conduction and free convection) thermal boundary conditions with one cylinder heated isothermally while the other cylinder is kept at the inlet fluid temperature.

Design/methodology/approach

A finite‐difference algorithm has been developed to solve the bipolar boundary‐layer equations for the conjugate laminar free convection heat transfer in vertical eccentric annuli.

Findings

Numerical results are presented for a fluid of Prandtl number, Pr=0.7 in eccentric annuli. The geometry parameters of NR2 and E (the fluid‐annulus radius ratio and the eccentricity, respectively) have considerable effects on the results.

Practical implications

Applications of the obtained results can be of value in the heat‐exchanger industry, in cooling of underground electric cables, and in cooling small vertical electric motors and generators.

Originality/value

The paper presents results that are not available in the literature for the problem of conjugate laminar free convection in open‐ended vertical eccentric annular channels. Geometry effects having been investigated by considering fluid annuli having radii ratios NR2=0.1 and 0.3, 0.5 and 0.7 and four values of the eccentricity E=0.1, 0.3, 0.5 and 0.7. Moreover, practical ranges of the solid‐fluid conductivity ratio (KR) and the wall thicknesses that are commonly available in pipe standards have been investigated. Such results are very much needed for design purposes of heat transfer equipment.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 17 no. 5
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 15 May 2009

Mohamed A. Antar and Maged A.I. El‐Shaarawi

The purpose of this paper is to investigate the problem of entropy generation around a spinning/non‐spinning solid sphere subjected to uniform heat flux boundary condition in the…

370

Abstract

Purpose

The purpose of this paper is to investigate the problem of entropy generation around a spinning/non‐spinning solid sphere subjected to uniform heat flux boundary condition in the forced‐convection regime.

Design/methodology/approach

The governing continuity, momentum, energy and entropy generation equations are numerically solved for a wide range of the controlling parameters; Reynolds number and the dimensionless spin number.

Findings

The dimensionless overall total entropy generation increases with the dimensionless spin number. The effect of increasing the spin number on the fluid‐friction component of entropy generation is more significant compared to its effect on heat transfer entropy generation.

Research limitations/implications

Since the boundary‐layer analysis is used, the flow is presented up to only the point of external flow separation.

Practical implications

Entropy generation analysis can be used to evaluate the design of many heat transfer systems and suggest design improvements.

Originality/value

A review in the open literature indicated that no study is available for the entropy generation in the unconfined flow case about a spinning sphere.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 19 no. 3/4
Type: Research Article
ISSN: 0961-5539

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Article
Publication date: 1 August 1998

Maged A.I. El‐Shaarawi and Esmail M. A. Mokheimer

The paper utilizes a boundary‐layer model in bipolar coordinates to study the developing laminar free convection in vertical open‐ended eccentric annuli with one of the boundaries…

503

Abstract

The paper utilizes a boundary‐layer model in bipolar coordinates to study the developing laminar free convection in vertical open‐ended eccentric annuli with one of the boundaries uniformly heated while the other boundary is cooled and kept isothermal at the ambient temperature. This model has been solved numerically using finite‐difference techniques. Results not available in the literature are presented for a fluid of Prandtl number 0.7 in an annulus of radius ratio 0.5 for three values of the dimensionless eccentricity, namely, 0.1, 0.5 and 0.7. These results include the developing velocity profiles and the pressure along the annulus, the channel heights required to naturally induce different flow rates and the variation of the total heat absorbed by the fluid with the channel height.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 8 no. 5
Type: Research Article
ISSN: 0961-5539

Keywords

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Article
Publication date: 30 March 2010

Fahad G. Al‐Amri and Maged A.I. El‐Shaarawi

This paper's aim is to investigate the effect of surface radiation on the developing laminar forced convection flow of a transparent gas between two vertical parallel plates. The…

594

Abstract

Purpose

This paper's aim is to investigate the effect of surface radiation on the developing laminar forced convection flow of a transparent gas between two vertical parallel plates. The walls are heated asymmetrically, this enhances the effect of radiation even with the two walls having low values of emissivity.

Design/methodology/approach

Numerical techniques were used to study the effect of the controlling parameters on wall temperatures, fluid temperature profiles, and Nusslet number.

Findings

The values of the radiation number at which surface radiation can engender symmetric heating (and hence maximum average Nusslet number on the heated wall and maximum reduction in the maximum heated wall temperature are achieved) are obtained. Threshold values of the radiation number at which radiation effects can be neglected are obtained.

Research limitations/implications

Boundary‐layer flow model is used.

Practical implications

The implications include design of high‐temperature gas‐cooled heat exchangers, advanced energy conversion devices, advanced types of power plants, and many others.

Originality/value

Though a number of analyses of internal flows including radiation effect have been made, most have been directed at the simplest case of the prescribed uniform (isothermal) temperature boundary condition. The available literature that deals with the problem with prescribed heat flux at the walls is limited to fully developed flow or specifying the convection coefficient a priori. The lack of both theoretical and experimental data concerning combined forced convection and surface radiation developing flows between two parallel and its practical importance motivated the present work.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 20 no. 2
Type: Research Article
ISSN: 0961-5539

Keywords

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Article
Publication date: 1 March 2005

Esmail M.A. Mokheimer and Maged El‐Shaarawi

Obtaining the maximum possible flow rates that can be induced by free convection in open‐ended vertical eccentric annuli under fundamental thermal boundary conditions of the…

386

Abstract

Purpose

Obtaining the maximum possible flow rates that can be induced by free convection in open‐ended vertical eccentric annuli under fundamental thermal boundary conditions of the fourth kind (heating or cooling one of the annulus walls with a uniform heat flux while keeping the other wall at ambient temperature). Obtaining the maximum possible flow rates that can be induced by free convection in open‐ended vertical eccentric annuli under fundamental thermal boundary conditions of the fourth kind (heating or cooling one of the annulus walls with a uniform heat flux while keeping the other wall at ambient temperature).

Design/methodology/approach

The fully‐developed laminar free convection momentum equation has been solved numerically using an analytical solution of the governing energy equation.

Findings

Results are presented to show the effect of the annulus radius ratio and the dimensionless eccentricity on the induced flow rate, the total heat absorbed by the fluid, and the fully developed Nusselt numbers on the two boundaries of the annulus for a fluid of Prandtl number 0.7.

Practical implications

Applications of the obtained results can be of value in the heat‐exchanger industry, in cooling of underground electric cables, and in cooling small vertical electric motors and generators.

Originality/value

The paper presents a solution that is not available in the literature for the problem of fully developed free convection in open‐ended vertical eccentric annular channels under thermal boundary conditions of the fourth kind. Also presents the maximum possible induced flow rates, the total heat absorbed by the fluid, and the Nusselt numbers on the two boundaries of the annulus. The effects of N and E (the radius ratio and eccentricity, respectively) on these results are presented. Such results are very much needed for design purposes of heat transfer equipment.

Details

International Journal of Numerical Methods for Heat & Fluid Flow, vol. 15 no. 2
Type: Research Article
ISSN: 0961-5539

Keywords

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