M.O. Hamdan, M.A. Al‐Nimr and M.K. Alkam
Investigates numerically the mechanism of enhancing heat transfer by using porous substrate. The numerical investigation is carried out for transient forced convection in the…
Abstract
Investigates numerically the mechanism of enhancing heat transfer by using porous substrate. The numerical investigation is carried out for transient forced convection in the developing region of a parallel‐plate channel partially filled with a porous medium. A porous substrate is inserted in the channel core in order to reduce the boundary layer thickness and hence, enhance heat transfer. Darcy‐Brinkman‐Forchheimer model is used to simulate the physical problem. Results of the current model show that the existence of the porous substrate may improve the Nusselt number at the fully developed region by a factor of four and even higher depending on the value of Darcy number. It is found that the maximum Nusselt number is achieved at an optimum thickness. Also, the study shows that partially filled channels have better thermal performance than the totally filled ones. However, there is an optimum thickness of porous substrate, beyond it the Nusselt number starts to decline.
Details
Keywords
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…
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
Keywords
T.K. Aldoss, M.A. Al‐Nimr and A.F. Khadrawi
The transient hydrodynamics and thermal behavior of free convection flow over an isothermal vertical flat plate is investigated.
Abstract
Purpose
The transient hydrodynamics and thermal behavior of free convection flow over an isothermal vertical flat plate is investigated.
Design/methodology/approach
The study focuses on the role of the local acceleration term in the magnetohydrodynamic (MHD) momentum equation. A finite difference method based on a second‐order differential equation is used to solve the differential equations.
Findings
It is found that the local acceleration term has insignificant effect on the flow behavior especially at large values of magnetic forces. Also, it is found that the effect of the magnetic forces on the flow hydrodynamics behavior is significant but its effect on the thermal behavior is insignificant. It has been realized that the local acceleration term is usually small compared to the magnetic retarding force, and hence can be neglected.
Research limitations/implications
A quantitative description of the operating and geometrical parameters within which the local acceleration term may be significant is not available in the literature yet. Also, the authors' intention is to improve physical understanding of the hydrodynamic and thermal behaviors of the present problem.
Originality/value
The study provides results concerning the thermal behavior of free convection flow.
Details
Keywords
M.A.I. El‐Shaarawi, M.A. Al‐Nimr and M.A. Hader
The paper presents a finite‐difference scheme to solve thetransient conjugated heat transfer problem in a concentricannulus with simultaneously developing hydrodynamic and…
Abstract
The paper presents a finite‐difference scheme to solve the transient conjugated heat transfer problem in a concentric annulus with simultaneously developing hydrodynamic and thermal boundary layers. The annular forced flow is laminar with constant physical properties. Thermal transient is initiated by a step change in the prescribed isothermal temperature of the inner surface of the inside tube wall while the outer surface of the external tube is kept adiabatic. The effects of solid‐fluid conductivity ratio and diffusivity ratio on the thermal behaviour of the flow have been investigated. Numerical results are presented for a fluid of Pr = 0.7 flowing in an annulus of radius ratio 0.5 with various values of inner and outer solid wall thicknesses.
Details
Keywords
M. Al‐Odat, M.A. Al‐Nimr and M. Hamdan
The thermal stability of superconductor is numerically investigated under the effect of a two‐dimensional hyperbolic heat conduction model. Two types of superconductor wires are…
Abstract
The thermal stability of superconductor is numerically investigated under the effect of a two‐dimensional hyperbolic heat conduction model. Two types of superconductor wires are considered, Types II and I. The thermal stability of superconductor wires under the effect of different design, geometrical and operating conditions is studied. The Effect of the time rate of change of the disturbance and the disturbance duration time is investigated. Generally, it is found that wave model predicts a wider stability region as compared to the predictions of the classical diffusion model.
Details
Keywords
The purpose of this study is to target the solution of nonlinear porous fin problem. In contrast to the various complicated numerical or analytical approximate procedures existing…
Abstract
Purpose
The purpose of this study is to target the solution of nonlinear porous fin problem. In contrast to the various complicated numerical or analytical approximate procedures existing in the literature used to approximate the temperature field over a porous fin, this study outlines a direct method based on series expansion of the temperature in the vicinity of the mounted surface, eventually requiring no numerical treatment at all to resolve the temperature field.
Design/methodology/approach
This study uses a direct method based on series expansion of the temperature in the vicinity of the mounted surface, eventually requiring no numerical treatment at all to resolve the temperature field.
Findings
Explicit closed-form formulae for the fin tip temperature as well as for the heat transfer rate, hence for the fin efficiency, which are functions of the porosity parameter and Biot number, are provided. The thresholds and the convergence regions regarding the physical parameters of the resulting approximations are easy to determine from the residual formula.
Originality/value
The novelty of the method is that the accuracy of the solution is controllable and can be gained up to any significant digit of desire by increasing the number of terms in the series solution.
Details
Keywords
The aim is to study numerically the heat transfer enhancement in a double pipe heat exchanger by using porous fins attached at the external wall of the inner cylinder.
Abstract
Purpose
The aim is to study numerically the heat transfer enhancement in a double pipe heat exchanger by using porous fins attached at the external wall of the inner cylinder.
Design/methodology/approach
The Brinkman‐Forchheimer extended Darcy model is used in the porous regions. The differential equations subjected to the boundary conditions are solved numerically using the finite volume method. Numerical calculations are performed for a wide range of Darcy number (10−6≤Da≤10−1), porous fins height (0≤Hp≤1) and spacing (0≤Lf≤39) and thermal conductivity ratio (1≤Rk≤100). The effects of these parameters are considered in order to look for the most appropriate properties of the porous fins that allow optimal heat transfer enhancement.
Findings
The results obtained show that the insertion of porous fins may alter substantially the flow pattern depending on their permeability, height and spacing. Concerning the heat transfer effect, it is found that the use of porous fins may enhance the heat transfer in comparison to the fluid case and that the rate of improvement depends on their geometrical and thermo‐physical properties. Performance analysis indicated that more net energy gain may be achieved as the thermal conductivity ratio increases especially at high Darcy numbers and heights.
Research limitations/implications
The results obtained in this work are valid for double pipe heat exchangers with the same fluid flowing at the same flow rate in the two ducts and for the case of an inner cylinder of negligible thermal resistance.
Practical implications
The results obtained in this study can be used in the design of heat exchangers.
Originality/value
This study provides an interesting method to improve heat transfer in a double pipe heat exchanger by use of porous fins.
Details
Keywords
M.A.I. El‐Shaarawi, M.A. Al‐Nimr and M.M.K. Al Yah
Transient conjugated forced convection in the thermal entry region of a thick‐walled annulus, filled with a homogeneous and isotropic porous medium, has been numerically…
Abstract
Transient conjugated forced convection in the thermal entry region of a thick‐walled annulus, filled with a homogeneous and isotropic porous medium, has been numerically investigated using finite‐difference techniques. Non‐Darcian effects as well as axial conduction of heat have been considered. The flow is assumed to be hydrodynamically fully developed and steady but thermally developing and transient. The thermal transient is initiated by a step change in the prescribed isothermal temperature on the outer surface of the external tube of the annulus while the inner surface of the internal tube is kept adiabatic. A parametric study is carried out to explore the effects of the Darcy number, the inertia term, the Peclet number and the porous medium heat capacity ratio on the transient thermal behavior in a given annulus.
Details
Keywords
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…
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
Keywords
M.K. Alkam, M.A. Al‐Nimr and Z. Mousa
This study aims to numerically investigate the transient forced convection of non‐Newtonian fluid in the entrance region of porous concentric annuli. The hydrodynamic behavior of…
Abstract
This study aims to numerically investigate the transient forced convection of non‐Newtonian fluid in the entrance region of porous concentric annuli. The hydrodynamic behavior of the flow is assumed to be steady and it is modeled using the non‐Darcian flow and the power law models. The transients in the thermal behaviors result from sudden changes in the boundary temperatures. The effects of different fluid flow and solid matrix parameters on the thermal behavior of the annular are investigated.