International Journal of Numerical Methods for Heat & Fluid Flow

Fast transient solution of a two-layered counter-flow microchannel heat sink : Table of Contents

S.L. Beh, K.-K. Tio, G.A. Quadir, K.N. Seetharamu — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to apply asymptotic waveform evaluation (AWE) to the transient analysis of a two-layered counter-flow microchannel heat sink. Design/methodology/approach – A two-layered counter-flow microchannel heat sink in both steady state and transient conditions is analysed. Finite element analysis is used in the steady state analysis whereas AWE is used in the transient analysis. Findings – A two-layered microchannel produces different temperature distribution compared to that obtained for a single-layered microchannel. The maximum temperature occurs at the middle of the bottom wall whereas the maximum temperature of a single-layered microchannel is at the outlet of the bottom wall. The time taken to reach steady state is also investigated for different coolant flow rate and heat flux boundary conditions. It is observed that when fluid velocity increases, the time taken to reach steady state decreases, however, when the heat flux increases, the time taken to reach steady state does not change. Research limitations/implications – The fluid is incompressible and does not undergo phase change. The use of AWE provides an alternative method in solving heat transfer problem. Practical implications – New and additional data will be useful in the design of a microchannel heat sink for the purpose of cooling of electronic components. Originality/value – AWE is widely used in analyses of signal delays in electronic circuits, and rarely applied to mechanical systems. The present study applies AWE to heat transfer problems, and reveals that it reduces the computational time considerably. The results obtained are compared with conventional methods available in the literature, and they show good agreement. Hence the computational time is reduced, and the accuracy of results is verified.

Pressure drop caused by two-phase flow of oil/water emulsions through sudden expansions and contractions: a computational approach : Table of Contents

Manmatha K. Roul, Sukanta K. Dash — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to compute the pressure drop through sudden expansions and contractions for two-phase flow of oil/water emulsions. Design/methodology/approach – Two-phase computational fluid dynamics (CFD) calculations, using Eulerian–Eulerian model, are employed to calculate the velocity profiles and pressure drops across sudden expansions and contractions. The pressure losses are determined by extrapolating the computed pressure profiles upstream and downstream of the expansion/contraction. The oil concentration is varied over a wide range of 0-97.3 percent by volume. The flow field is assumed to be axisymmetric and solved in two dimensions. The two-dimensional equations of mass, momentum, volume fraction and turbulent quantities along with the boundary conditions have been integrated over a control volume and the subsequent equations have been discretized over the control volume using a finite volume technique to yield algebraic equations which are solved in an iterative manner for each time step. The realizable per phase k- e turbulent model is considered to update the fluid viscosity with iterations and capture the individual turbulence in both the phases. Findings – The contraction and expansion loss coefficients are obtained from the pressure loss and velocity data for different concentrations of oil–water emulsions. The loss coefficients for the emulsions are found to be independent of the concentration and type of emulsions. The numerical results are validated against experimental data from the literature and are found to be in good agreement. Research limitations/implications – The present computation could not use the surface tension forces and the energy equation due to huge computing time requirement. Practical implications – The present computation could compute realistically the two-phase pressure drop through sudden expansions and contractions by using a two-phase Eulerian model and hence this model can be effectively used for industrial applications where two-phase flow comes into picture. Originality/value – The original contribution of the paper is in the use of the state-of-the-art Eulerian two-phase flow model to predict the velocity profile and pressure drop through industrial piping systems.

Flow over solid blocks in open ended cavity: Effects of blocks' orientations and aspect ratios on the heat transfer rates : Table of Contents

S.Z. Shuja, B.S. Yilbas, S.M.A. Khan — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to consider flow over heat generating bodies in an open-ends cavity, which finds applications in electronics cooling and industrial processing. Heat transfer rates depend on the flow situation in the cavity, which is influenced by the cavity inlet and exit port locations, heat transferring body size and its orientation in the cavity, and the cavity size. Consequently, modeling of flow over heat transferring bodies in an open-ends cavity and examination of the effect of the aspect ratio and orientation of the heat transferring bodies on the flow field and heat transfer rates becomes essential. Design/methodology/approach – The flow over heat generating solid blocks situated in an open-ends cavity is considered and the effects of blocks' orientations and aspect ratios on flow field as well as heat transfer rates are examined. A numerical scheme using a control volume approach is introduced to predict flow field in the cavity and heat transfer rates from the blocks. Findings – It is found that complex flow structure is generated in the cavity due to the aspect ratios and orientations of the blocks. This, in turn, influences significantly heat transfer rates from the blocks in the cavity. Research limitations/implications – Surface areas of blocks are kept the same and aspect ratio is varied such that the surface area of each block remains the same in the simulations. In addition, Steady flow situation is considered for governing equations of flow and heat transfer in the cavity. However, for the future study transient heating and flow situations can be considered while varying the surface araes of the blocks. This will provide useful information on the circulations in the cavity and the enhancement of heat transfer due to the complex flow structure. Practical implications – In practice, cooling effectiveness can be improved through changing the aspects ratio of the heat generating bodies in the cavity. Originality/value – The findings are original and will be useful for the scientists and the design engineers working the specific area of heat transfer and fluid flow.

Numerical simulation of pulsatile turbulent flow in tapering stenosed arteries : Table of Contents

Bin Xiao and Yuwen Zhang — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to investigate the geometric effects and pulsatile characteristics during the stenotic flows in tapering arteries. Design/methodology/approach – The low Reynolds number k - ? turbulence model is applied to describe the stenotic flows in the tapering arteries in this paper. The results are divided into two sections. The first section characterizes the geometric effects on the turbulent flow under steady condition. The second section illustrates the key physiological parameters including the pressure drop and wall stress during the periodic cycle of the pulsatile flow in the arteries. Findings – The tapering and stenoses severity intensify the turbulent flow and stretch the recirculation zones in the turbulent arterial flow. The wall shear stress, pressure drop and velocity vary most intensively at the peak phase during the periodic cycle of the pulsatile turbulent flow. Originality/value – This paper provides a comprehensive understanding of the spatial-temporal fluid dynamics involved in turbulent and transitional arterial flow with stenoses. The low Reynolds number k - ? turbulence model method is applied for the analyses of the geometric effects on the arterial flow and fluid feature during the periodic cycle.

Non-equilibrium viscous shock-layer technique for computing hypersonic flow around blunt-nosed slender bodies : Table of Contents

S. Ghasemloo, M. Mani — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to present a non-equilibrium viscous shock layer (VSL) solution procedure that considerably improves computational efficiency, especially for long slender bodies. Design/methodology/approach – The VSL equations are solved in a shock oriented coordinate system. The method of solution is spatial marching, implicit, finite-difference technique, which includes coupling of the normal momentum and continuity equations. In the nose region, the shock shape is specified from an algebraic expression and corrected through global passes through that region. The shock shape is computed as part of the solution beyond the nose region and requires only a single global pass. For this study, a seven-species (O2, N2, O, N, NO, NO+, e-) air model is used. Findings – The present approach eliminates the need for initial shock shape, which was required by previous method of solution. This method generates its own shock shape as a part of solution and the input shock shape obtained from a different solution is not required. Therefore, in comparison with the other VSL methods, the present approach dramatically reduces the CPU time of calculations. Moreover, by using the shock oriented coordinate systems the junction point problem in sphere-cone configurations is solved. Practical implications – This method is an excellent tool for parametric study and preliminary design of hypersonic vehicles. Originality/value – The present method provides a computational capability which reduces the CPU time, and expands the range of application for the prediction of hypersonic heating rates.

Conjugate heat transfer in porous triangular enclosures with thick bottom wall : Table of Contents

Yasin Varol, Hakan F. Oztop, Ioan Pop — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to study the conjugate heat transfer via natural convection and conduction in a triangular enclosure filled with a porous medium. Design/methodology/approach – Darcy flow model was used to write governing equations with Boussinesq approximation. The transformed governing equations are solved numerically using a finite difference technique. It is assumed that the enclosure consists of a conducting bottom wall of finite thickness, an adiabatic (insulated) vertical wall and a cooled inclined wall. Findings – Flow patterns, temperature and heat transfer were presented at different dimensionless thickness of the bottom wall, h, from 0.05 to 0.3, different thermal conductivity ratio between solid material and fluid, k, from 0.44 to 283 and Rayleigh numbers, Ra, from 100 to 1000. It is found that both thermal conductivity ratio and thickness of the bottom wall can be used as control parameters for heat transport and flow field. Originality/value – It is believed that this is the first paper on conduction-natural convection in porous media filled triangular enclosures with thick wall. In the last years, most of the researchers focused on regular geometries such as rectangular or square cavity bounded by thick wall.

Geometrical and Rayleigh number effects in the transient laminar free convection between two vertically eccentric spheres : Table of Contents

Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to study the transient natural convection of a Newtonian fluid which develops in a closed spherical annulus delimited by two vertically eccentric spheres by using a bispherical coordinates system. The inner sphere is heated by a heat flux of constant density and the outer one is maintained isothermal. Design/methodology/approach – The transfer equations are written by using a bispherical coordinates system. The Navier-Stokes equations are solved and coupled with the energy equation by using the alternating direction implicit (ADI) and the successive over relaxation (SOR) methods. Findings – The study of the stream function and the Nusselt number shows that the convection motion is reinforced for the geometries characterized by positive values of the eccentricity with heat exchange increasing. The Nusselt number increases with the modified Rayleigh number. The heat exchange increases with the radius ratio. The results show that the steady state is reached faster when the modified Rayleigh number increases and the influence of the eccentricity is very low on the establishment of the steady state. The fluids flow depends strongly on the eccentricity and the modified Rayleigh number. Research limitations/implications – Simulations are performed for modified Rayleigh numbers ranging from 103 to 106, for eccentricities varying between –0.6 and +0.6 and for radius ratio between 1.5 and 2. Originality/value – The results of eccentricity and modified Rayleigh number effects in transient natural convection between vertically eccentric spheres have been displayed.

Thermophoresis particle deposition – thermal radiation interaction on natural convection heat and mass transfer from vertical permeable surfaces : Table of Contents

H.M. Duwairi, Rebhi. A. Damseh — Sun Jun 14 14:15:06 BST 2009

Abstract:
Purpose – The purpose of this paper is to study thermophoresis particle deposition and thermal radiation interaction on natural convection heat and mass transfer by steady boundary layer flow over an isothermal vertical flat plate embedded in a fluid saturated porous medium. Design/methodology/approach – The governing partial differential equations are transformed into non-similar form by using special transformation and then the resulting partial differential equations are solved numerically by using an implicit finite difference method. Findings – Different results are obtained and displaced graphically to explain the effect of various physical parameters on the wall thermophoresis deposition velocity and concentration profiles. It is found that the increasing of thermal radiation parameter or dimensionless temperature ratio heats the fluid and decreases temperature gradients near permeable wall, which increases local Nusselt numbers and decreases wall thermophoresis velocities. It is also found that the effect of power indices of either temperatures or concentration enhances both local Nusselt numbers and wall thermophoresis velocities. Comparison with previously published work in the limits shows excellent agreement. Originality/value – The paper presents useful conclusions based on graphical results obtained from studying numerical solutions for thermophoresis-thermal radiation heat and mass transfer interaction by steady, laminar boundary layer over a vertical flat plate embedded in a porous medium.