Kamel Hooman

Numerical simulation of laminar to turbulent nanofluid flow and heat transfer over a backward-facing step

This paper presents a numerical study of heat transfer to turbulent and laminar Cu/water flow over a backward-facing step. Mathematical model based on finite volume method with a FORTRAN code is used to solve the continuity, momentum, energy and turbulence equations. Turbulence was modeled by the shear stress transport (SST) K-ω Model. In this simulation, three volume fractions of nanofluid (0%, 2% and 4%), a varying Reynolds number from 50 to 200 for the laminar range and 5000 to 20,000 for the turbulent range, an expansion ratio of 2 and constant heat flux of 4000 W/m2 were considered. The results show the effect of nanofluid volume fraction on enhancing the Nusselt number in the laminar and turbulent ranges. The effect of expansion ratio was clearly observed at the downstream inlet region where the peak of the Nusselt number profile was referred to as enhanced heat transfer due to the generated recirculation flow. An increase of pressure drop was evident with an increasing Reynolds number and decreasing nanofluid volume fraction, while the maximum pressure drop was detected in the downstream inlet region. A rising Reynolds number caused an increasing Nusselt number, and the highest heat transfer augmentation in the present investigation was about 26% and 36% for turbulent and laminar range, respectively compared with pure water.

Entropy generation for forced convection in a porous channel with isoflux or isothermal walls

A numerical study is reported to investigate the entropy generation due to forced convection in a parallel plate channel filled by a saturated porous medium. Two different thermal boundary conditions are considered being isoflux and isothermal walls. Effects of the Peclet number, the porous medium shape factor, the dimensionless temperature difference for isothermal walls, the dimensionless heat flux for isoflux walls, and the Brinkman number on the Bejan number as well as the local and average dimensionless entropy generation rate are examined.

Analytical Solution of Forced Convection in a Duct of Rectangular Cross Section Saturated by a Porous Medium

A theoretical analysis is presented to investigate fully developed (both thermally and hydrodynamically) forced convection in a duct of rectangular cross-section filled with a hyper-porous medium. The Darcy-Brinkman model for flow through porous media was adopted in the present analysis. A Fourier series type solution is applied to obtain the exact velocity and temperature distribution within the duct. The case of uniform heat flux on the walls, i.e. the H boundary condition in the terminology of Kays and Crawford [1], is treated. Values of the Nusselt number and the friction factor as a function of the aspect ratio, the Darcy number, and the viscosity ratio are reported.

Natural Convection in a Nanofluids-Filled Portioned Cavity: The Lattice-Boltzmann Method

Numerical simulation of natural convection in a nanofluids-filled partitioned square cavity is presented. Two independent solvers, an in-house LBE-BGK code and the commercially available software CFD-ACE, are used to achieve this goal. While the partitioning plates are generating heat at a uniform temperature, the vertical walls are isothermally cooled allowing for the removal of the internally generated heat with the horizontal walls being adiabatic. While the particle volume fraction is kept constant at 5%, the effective Rayleigh number, the length, and the orientation of the partition have been parametrically varied from 103–107, 0.25H–0.75H, and horizontal to vertical, respectively.

Comparison of the Finite Volume and Lattice Boltzmann Methods for Solving Natural Convection Heat Transfer Problems inside Cavities and Enclosures

Different numerical methods have been implemented to simulate internal natural convection heat transfer and also to identify the most accurate and efficient one. A laterally heated square enclosure, filled with air, was studied. A FORTRAN code based on the lattice Boltzmann method (LBM) was developed for this purpose. The finite difference method was applied to discretize the LBM equations. Furthermore, for comparison purpose, the commercially available CFD package FLUENT, which uses finite volume Method (FVM), was also used to simulate the same problem. Different discretization schemes, being the first order upwind, second order upwind, power law, and QUICK, were used with the finite volume solver where the SIMPLE and SIMPLEC algorithms linked the velocity-pressure terms. The results were also compared with existing experimental and numerical data. It was observed that the finite volume method requires less CPU usage time and yields more accurate results compared to the LBM. It has been noted that the 1st order upwind/SIMPLEC combination converges comparatively quickly with a very high accuracy especially at the boundaries. Interestingly, all variants of FVM discretization/pressure-velocity linking methods lead to almost the same number of iterations to converge but higher-order schemes ask for longer iterations.

Thermal Assessment of Forced Convection Through Metal Foam Heat Exchangers

Using a thermal resistance approach, forced convection heat transfer through metal foam heat exchangers is studied theoretically. The complex microstructure of metal foams is modeled as a matrix of interconnected solid ligaments forming simple cubic arrays of cylinders. The geometrical parameters are evaluated from existing correlations in the literature with the exception of ligament diameter which is calculated from a compact relationship offered in the present study. The proposed, simple but accurate, thermal resistance model considers: the conduction inside the solid ligaments, the interfacial convection heat transfer, and convection heat transfer to (or from) the solid bounding walls. The present model makes it possible to conduct a parametric study. Based on the generated results, it is observed that the heat transfer rate from the heated plate has a direct relationship with the foam pore per inch (PPI) and solidity. Furthermore, it is noted that increasing the height of the metal foam layer augments the overall heat transfer rate; however, the increment is not linear. Results obtained from the proposed model were successfully compared with experimental data found in the literature for rectangular and tubular metal foam heat exchangers.

Heatline Visualization of Natural Convection in a Porous Cavity Occupied by a Fluid With Temperature-Dependent Viscosity

Temperature-dependent viscosity effect in buoyancy driven flow of a gas or a liquid in an enclosure filled with a porous medium is studied numerically based on the general model of momentum transfer in a porous medium. The exponential form of viscosity-temperature relation is applied to examine three cases of viscosity-temperature relation: constant, decreasing, and increasing. Application of arithmetic and harmonic mean values of the viscosity is also investigated for their ability to represent the Nusselt number versus the effective Rayleigh number. Heat lines are illustrated for a more comprehensive investigation of the problem.

VISCOUS DISSIPATION EFFECTS ON THERMALLY DEVELOPING FORCED CONVECTION IN A POROUS MEDIUM: CIRCULAR DUCT WITH ISOTHERMAL WALL

A numerical study is reported to investigate the thermal development of forced convection in a circular tube filled by a saturated porous medium, with uniform wall temperature, and with the effects of viscous dissipation included. A theoretical analysis is also presented to find expressions for the temperature profile and the Nusselt number for the fully developed region. The Brinkman model is employed. It is seen that while the developing Nusselt number is significantly dependent on the Brinkman number, the fully developed Nusselt number is independent of the Brinkman number, but both the developing and the fully developed Nusselt numbers, depend on the Darcy number.

Enhanced double-pass solar air heater with and without porous medium

An energy and exergy study has been done on a double-pass flat-plate solar air collector with and without porous medium embedded inside the lower channel of the collector. Energy conservation equations are used to derive the energy-governing equations for all components of the collector. A second law analysis (the second law of thermodynamics) was carried out to optimize the mass flow rate, which leads to optimization of the energy efficiency. Theoretical results for energy and exergy efficiencies are plotted for the cases of with and without embedded porous medium. Results of this research show that the porous medium embedded inside the lower channel leads to an increase in the thermal efficiency of the collector of more than 30% compared with the case without porous medium, hence showing the importance of employing porous medium in thermal solar collectors. On the other hand, the pressure drop in the air caused by friction with porous medium is not negligible and is also studied here, using the second law analysis.

Energy flux vectors as a new tool for convection visualization

Purpose – The aim of this paper is to introduce a new technique for convection visualization. This is similar to Bejans heatlines and is even an exact match to Landau and Lifshitzs energy streamlines for two‐dimensional geometries.Design/methodology/approach – The work benefits from a combination of numerical and analytical tools to show that, in two‐dimensional space, heatlines and energy streamlines are effectively the same. More importantly, the energy flux vectors are tracing both of them accurately; as verified for some cases of free and forced convection problems in this paper.Findings – The new technique is easier to implement compared to the existing counterparts which are available in the literature. More specifically, the advantage of this new technique is that, contrary to heatlines and energy streamlines, it does not require further numerical analysis in addition to solving momentum and energy equations.Originality/value – Energy flux vectors offer higher resolution compared to existing visu...