Suhil Kiwan

Using Porous Fins for Heat Transfer Enhancement

This work introduces a novel method that enhances the heat transfer from a given surface by using porous fins. The thermal performance of porous fins is estimated and compared with that of the conventional solid fins. It is found that using porous fin of porosity ∈ may enhance the performance of an equal size conventional solid fin and, as a result, save 100 e percent of the fin material. The effect of different design and operating parameters on the porous fin thermal performance is investigated. Examples of these parameters are Ra number, Da number, and thermal conductivity ratio. It is found that more enhancement in the porous fin performance may be achieved as Ra increases especially at large Da numbers. Also, it is found that there is an optimum limit for the thermal conductivity ratio beyond which there is no further improvement in the fin performance.

Investigation Into the Similarity Solution for Boundary Layer Flows in Microsystems

An investigation toward the existence of a complete similarity solution for boundary layer flows under the velocity slip and temperature jump conditions is carried out. The study is limited to the boundary layer flows resulting from an arbitrary freestream velocity U(x)=U o x m and wall temperature given by T w ―T ∞ =Cx n . It is found that a similar solution exists only for m = 1 and n = 0, which represents stagnation flow on isothermal surface. This case has been thoroughly investigated. The analysis showed that three parameters control the flow and heat transfer characteristics of the problem. These parameters are the velocity slip parameter K 1 , the temperature jump parameter K 2 , and Prandtl number. The effect of these parameters on the flow and heat transfer of the problem has been studied and presented. It is found that the slip velocity parameter affects both the flow and heat transfer characteristics of the problem. It is found that the skin friction coefficient decreases with increasing K 1 and most of changes in the skin friction takes place in the range 0 < K 1 < 1. The skin friction coefficient is found to be related to K 1 and Re x according to the relation: C f =3.38Re ―0.5 x (K 1 + 1.279) ―0.8 for 0 <K 1 <5 with an error of ±4%. On the other hand, the correlation between Nu, K 1 , K 2 , and Pr has been found by the equation Nu=[(0.449+1.142K 1.06 1 )/(0.515+K 1.06 1 )](K 2 +1.489Pr ―0.44 ) ―1 , for 0 <K 1 , K 2 <5, 0.7≤Pr≤5 within a maximum error of ±3%.

Flow and Heat Transfer Over a Stretched Microsurface

The convection heat transfer induced by a stretching flat plate has been studied. Similarity conditions are obtained for the boundary layer equations for a flat plate subjected to a power law temperature and velocity variations. It is found that a similarity solution exists only for a linearly stretching plate and only when the plate is isothermal. The analysis shows that three parameters control the flow and heat transfer characteristics of the problem. These parameters are the velocity slip parameter K 1 , the temperature slip parameter K 2 , and the Prandtl number. The effect of these parameters on the flow and heat transfer of the problem has been studied and presented. It is found that the slip velocity parameter affect both the flow and heat transfer characteristics of the problem. It is found that the skin friction coefficient decreases with increasing K 1 and most of the changes in the skin friction takes place in the range 0 <K 1 <1. A correlation between the skin friction coefficient and K 1 and Re x has been found and presented. It is found that c f = 23Re ―0.5 x (K 1 +0.64) ―0.884 for 0 <K 1 <10 with an error of ±0.8%. Other correlations between Nu and K 1 and K 2 has been found and presented in Eq. (28).

Size optimization of conventional solar collectors

An expression for the optimum length of a flat-plate solar collector that maximizes the life-cycle savings of the collector is derived. An expression has been obtained also for the optimal distribution of a finite amount of thermal insulation that minimizes the energy loss from the back side of a flat-plate solar collector.

Effect of thermal losses on the microscopic two-step heat conduction model

AbstractThe e•ects of radiative and convective thermal losses on the thermal behavior of thin metal films, as described by themicroscopic two-step heat conduction model, are investigated. It is found that radiative losses from the electron gas aresignificant in thin films having –L=T 3e ƒ<10 y22 , while radiative losses from the solid lattice are significant when–L=T 3e ƒ–T 4e =T 4l ƒ<10 y22 . Also, it is found that convective losses from the thin metal film are insignificant in mostpractical operating conditions. O 2001 Elsevier Science Ltd. All rights reserved.1. IntroductionHigh-rate heating of thin metal films is a rapidlyemerging area in heat transfer [1–13]. When a thin film isexposed to a very rapid heating process, such that in-duced by a short-pulse laser, the typical response timefor the film is an order of picoseconds which is com-parable to the phonon–electron thermal relaxation time.Under these situations, thermal equilibrium betweensolid lattice and electron gas cannot be assumed andheat transfer in the electron gas and the metal latticeneeds to be considered separately. Models describing thenon-equilibrium thermal behavior in such cases arecalled the microscopic two-step models. Two micro-scopic heat conduction models are available in theliterature. The first one is the parabolic two-step model[1–5,8–10] and the second one is the hyperbolic two-stepmodel [1,3,7,11].Ultrafast heating of metals consists of two majorsteps of energy transfer which occur simultaneously. Inthe first step, electrons absorb most of the incidentradiation energy and the excited electron gas transmitsits energy to the lattice through inelastic electron–phonon scattering process [1,3]. In the second step, theincident radiation absorbed by the metal film di•usesspatially within the film mainly by the electron gas. Fortypical metals, depending on the degree of electron–phonon coupling, it takes about 0.1–1 ps for electronsand lattice to reach thermal equilibrium. When the ul-trafast heating pulse duration is comparable with or lessthan this thermalization time, electrons and lattice arenot in thermal equilibrium. As a result, the thermalbehavior of the thin film under the e•ect of the micro-scopic parabolic heat conduction model is described by[1,3]C

Natural Convection Heat Transfer in an Open-Ended Inclined Channel-Partially Filled with Porous Media

A numerical simulation of the steady-state, laminar, two-dimensional, natural convection heat transfer in an open-ended channel partially filled with an isotropic porous medium is presented. The Darcy-Brinkman-Forchheimer model along with Boussinesq approximation is used to describe the fluid flow in the porous region. Meanwhile, the Navier-Stokes equation along with Boussinesq approximation is used to describe the flow in the clear flow region. The dependence of the average Nusselt number on Rayleigh number, inclination angle, Darcy number, inertia coefficient, Prandtl number, porous width to channel width ratio, the ratio of the porous effective conductivity to fluid conductivity, and channel width to length ratio is investigated. The numerical results obtained indicate that air gap presence may reduce the average flow in the porous substrate to zero. This leads to the presence of an optimum average Nusselt number at low and high values of the effective thermal conductivity ratios.

Trial solution methods to solve the hyperbolic heat conduction equation

Abstract Trial solution methods combined with Laplace transformation technique are used to present an analytic approximate solution for the hyperbolic heat conduction (HHC) equation. The trial solution methods used in this work are weighted residual methods and Ritz variational method. The weighted residual methods involves the application of different optimizing criteria, which are the collocation, subdomain, least square and the Galrekin optimizing methods. Trial solution procedures are carried out after transforming the HHC equation from the time domain into the Laplace domain. The solution of the transformed equation is expanded in the form of a shape function. The shape function is a function of space and undetermined coefficients. In this work, two shape functions are used: polynomial and hyperbolic. Applying the trial solution methods yields a system of algebraic equations that is solved symbolically using a commercial computerized symbolic code. Finally, the solution in time domain is obtained by inverting the solution of the transformed equation. It is found that the trial solution methods using polynomial approximate functions up to fourth order are not able, to capture the sharp gradient in the vicinity of the heat wave. Whereas, the hyperbolic shape function mimic the exact solution for all methods.

Using localized wall discharge to control the fluid flow and heat transfer for the flow over a backward facing step

Purpose – Studying the effect of localized wall discharge on the fluid flow and heat transfer for a flow over backward facing step is the main purpose of this paper. Jet is used to control the reattachment length which controls the fluid flow and heat transfer downstream the step. Several parameters are to be investigated: geometric; expansion ratio, location of the jet, and jet angle flow; Reynolds number, jet velocity.Design/methodology/approach – Numerical simulation using both the standard K−e and renormalized group turbulence theory (RNG) K−e models are used to model flow in the computational domain. The energy equation is also used to model the heat transfer characteristics of the flow. The model equations are solved numerically using a finite volume code.Findings – It is found that the presence of the wall jet at a proper location can significantly influence the flow and heat characteristics of the problem. Furthermore, varying the ratio of the jet velocity to the main stream velocity could play an...

Rarefied flow and heat transfer characteristics over a vertical stretched surface

Similarity solution for the steady-state two-dimensional laminar natural convection heat transfer for a rarefied flow over a linearly vertical stretched surface is being proposed. Similarity conditions are obtained for the boundary layer equations for the vertical flat plate subjected to power law for the temperature variations. It is found that the similarity solution exists for linear temperature variation and linear stretching surface. The study shows that there are three different parameters affecting the flow and heat transfer characteristics for the rarefied flow over a vertical linearly stretched surface. These parameters represent the effects of the velocity slip (K1), temperature jump (K2), and the Prandtl number (Pr). The effects of these parameters are presented. It is found that the velocity slip parameter affects both the hydrodynamic and thermal behaviors of such flows. Correlations for the skin friction as well as Nusselt number are being proposed in terms of Grashof number (Grx), the slip velocity parameter (K1), and the temperature jump parameter (K2).

Temperatures of circular rods due to rotational and oscillatory axial sliding motion

Using Greens function method, surface temperature rise due to frictional heating in oscillatory sliding and rotating rods is studied. The problem under consideration consists two rods, one of them is stationary and the other exhibits both oscillatory axial sliding and rotational motion. In terms of the obtained closed form solutions for the temperature distribution in both rods, the effect of different parameters on the rod thermal behavior may be investigated.