Porous Medium Interconnector Effects on the Thermohydraulics of Near-Compact Heat Exchangers Treated as Porous Media

2006 ◽  
Vol 129 (3) ◽  
pp. 273-281 ◽  
Author(s):  
K. Sumithra Raju ◽  
Arunn Narasimhan
Keyword(s):  
Porous Medium ◽  
Porous Media ◽  
Heat Exchangers ◽  
Fluid Transport ◽  
Energy Transport ◽  
Metal Foam ◽  
Volume Ratio ◽  
Numerical Data ◽  
Darcy Number ◽  
Compact Heat Exchangers

A novel approach of treating near-compact heat exchangers (NCHX) (surface to volume ratio, α=100-300m2∕m3 with hydraulic diameter DM∼6mm) as a “global” porous media, whose thermohydraulic performance is being influenced by the presence of “local” tube-to-tube porous medium interconnectors, connecting the in-line arrangement of tubes (D=2mm) having square pitch of XT=XL=2.25, is investigated in this study using numerical methods. The thermohydraulics of the global porous media (NCHX) are characterized by studying the effect of transverse thickness (δ) and permeability (represented by Dai) of the local metal foam type porous medium interconnectors on the global heat transfer coefficient (Nu) and nondimensional pressure drop (ξ). The fluid transport in the porous medium interconnectors is governed by the Brinkman–Darcy flow model while the volume averaged energy equation is used to model energy transport, with the tube walls kept at constant temperature and exchanging heat with the cooling fluid having Pr=0.7 under laminar flow (10<Re<100). For the chosen NCHX configuration, ξ and Nu increases for an increase in Re and also with an increase in the thickness (δ) of the interconnecting porous medium. However, as the local Darcy number (Dai) of the interconnecting porous medium increases, the ξ decreases but the Nu increases. Treating the heat exchanger as a global porous media this result translates to an increase in the ξ and Nu as the global permeability (represented by Dag) decreases, where the decrease in Dag is because of either an increase in δ or a decrease in Dai. Separate correlations predicting ξ and Nu as a function of Re and Dag (which in turn is correlated to δ and Dai) have been developed for the chosen NCHX configuration, both of which predict the numerical data with ±20% accuracy.

Numerical Study on the Performance of a Tube With Inserted Porous Media of Various Thermal Conductivities

2016 ◽  
Author(s):  
Tariq Amin Khan ◽  
Wei Li
Keyword(s):  
Thermal Conductivity ◽  
Porous Medium ◽  
Porous Media ◽  
Velocity Profile ◽  
Fluid Transport ◽  
Energy Transport ◽  
Numerical Study ◽  
Darcy Number ◽  
Local Thermal Equilibrium ◽  
Working Fluid

Numerical study is performed on the effect of thermal conductivity of porous media (k) on the Nusselt number (Nu) and performance evaluation criteria (PEC) of a tube. Two-dimensional axisymmetric forced laminar and fully developed flow is assumed. Porous medium partially inserted in the core of a tube is investigated under varied Darcy number (Da), i.e., 10−6 ≤ Da ≤ 10−2. The range of Re number used is 100 to 2000 and the conductivity of porous medium is 1.4 to 202.4 W/(m.K) with air as the working fluid. The momentum equations are used to describe the fluid flow in the clear region. The Darcy-Forchheimer-Brinkman model is adopted for the fluid transport in the porous region. The mathematical model for energy transport is based on the one equation model which assumes a local thermal equilibrium between the fluid and the solid phases. Results are different from the conventional thoughts that porous media of higher thermal conductivity can enhance the performance (PEC) of a tube. Due to partial porous media insertion, the upstream parabolic velocity profile is destroyed and the flow is redistributed to create a new fully develop velocity profile downstream. The length of this flow redistribution to a new developed laminar flow depends on the Da number and the hydrodynamic developing length increases with increasing Da number. Moreover, the temperature profile is also readjusted within the tube. The Nu and PEC numbers have a nonlinear trend with varying k. At very low Da number and at a lower k, the Nu number decreases with increasing Re number while at higher k, the Nu number first increases to reach its peak value and then decreases. At higher Re number, the results are independent of k. However, at a higher Da number, the Nu and PEC numbers significantly increases at low Re number while slightly increases at higher Re number. Hence, the change in Nu and PEC numbers neither increases monotonically with k, nor with Re number. Investigation of PEC number shows that at very low Da number (Da = 10−6), inserting porous media of a low k is effective at low Re number (Re ≤ 500) while at high Re number, using porous material is not effective for the overall performance of a tube. However, at a relatively higher Da number (Da = 10−2), high k can be effective at higher Re number. Moreover, it is found that the results are not very sensitive to the inertia term at lower Da number.

Numerical Analysis of Wooden Porous Media Effects on Heat Transfer From a Staggered Tube Bundle

2008 ◽  
Vol 130 (1) ◽  
Author(s):  
Mohammad Layeghi
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Porous Medium ◽  
Porous Media ◽  
Numerical Analysis ◽  
Pressure Drop ◽  
Tube Bundle ◽  
Numerical Data ◽  
Darcy Number ◽  
Finite Volume Approach

A numerical analysis of forced convective heat transfer from a staggered tube bundle with various low conductivity wooden porous media inserts at maximum Reynolds numbers 100 and 300, Prandtl number 0.7, and Darcy number 0.25 is presented. The tubes are at constant temperature. The extended Darcy–Brinkman–Forchheimer equations and corresponding energy equation are solved numerically using finite volume approach. Parametric studies are done for the analysis of porous medium thermal conductivity and Reynolds number on the local Nusselt number distribution. Three different porous media with various solid to fluid thermal conductivity ratios 2.5, 5, and 7.5 are used in the numerical analysis. The results are compared with the numerical data for tube bundles without porous media insert and show that the presence of wooden porous media can increase the heat transfer from a tube bundle significantly (more than 50% in some cases). It is shown that high conductivity porous media are more effective than the others for the heat transfer enhancement from a staggered tube bundle. However, the presence of a porous medium increases the pressure drop. Therefore, careful attention is needed for the selection of a porous material with good heat transfer characteristics and acceptable pressure drop.

A Survey of Important Auxiliary Equations for Selected Compact Heat Exchanger

2005 ◽  
Author(s):  
Richard G. Carranza
Keyword(s):  
Heat Exchanger ◽  
Surface Area ◽  
Heat Exchangers ◽  
Volume Ratio ◽  
Compact Heat Exchanger ◽  
Fin Efficiency ◽  
Compact Heat Exchangers ◽  
Central Location

Important auxiliary equations are presented that are typically used in compact heat exchanger research. These relationships are presented only for selected compact heat exchangers — bare pipe, helically finned pipe, plate finned pipe, spined pipe, and plate exchangers. The equations primarily address issues relating to heat exchanger geometry, surface area to volume ratio, and fin efficiency. Furthermore, they are organized in a systematic manner and consolidated in one central location for easy reference.

Analysis of Darcy Flow in Confined Porous Media Including Wall Effect

2011 ◽  
Author(s):  
Nihad Dukhan
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Porous Media ◽  
Heat Exchange ◽  
Velocity Field ◽  
Darcy Number ◽  
Momentum Transport ◽  
Mean Velocity ◽  
Viscous Shear ◽  
The Mean

Effective cooling lies at the heart of reactor design and safe operation. Materials for cooling systems include solid porous media (e.g. metal foam). This is due to the large surface area per unit volume and the random internal structure of such porous medium. The former promotes heat exchange rates by providing large surface area, while the latter enhances it by providing vigorous mixing of the working fluid, which gives rise to what is called dispersion (an added mechanism of heat transfer). Hence, momentum transport in porous media is critical for heat transfer analysis, computation and design. Porous media are also used as storage of nuclear waste. In such applications, the porous medium is confined by solid boundaries. These impermeable boundaries give rise to shear stress and boundary layers, which strongly influence the velocity field and the pressure drop inside the porous medium. The velocity field directly influence the heat transfer rate, while the pressure drop determines the required pumping power. The Brinkman-extended Darcy equation describes the momentum transport due to fully developed Newtonian fluid flow in confined porous media. This equation is an extension of the famous Darcy equation, and it contains the viscous shear at the boundaries as well as the viscous shear on the internal surface of the porous medium. The equation is solved analytically inside and outside the boundary layer in a cylindrical porous-media system. As, expected, the volume-averaged velocity decays as the distance from the boundary increases. The mean and maximum velocities are obtained and their behavior is investigated in terms of the Darcy number and the ratio of the effective to the actual fluid viscosity. The friction factor is defined based on the mean velocity and is found to be inversely proportional to the Reynolds number, the Darcy number and the mean velocity. The analytical velocity can be directly substituted in the governing convection heat transfer equation to assess the heat transfer performance of confined cylindrical heat exchange systems.

Analysis of Brinkman-Extended Darcy Flow in Porous Media and Experimental Verification Using Metal Foam

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Nihad Dukhan
Keyword(s):  
Porous Media ◽  
Metal Foam ◽  
Waste Heat ◽  
Darcy Number ◽  
Momentum Transport ◽  
Flow In Porous Media ◽  
Mean Velocity ◽  
Ground Water Pollution ◽  
The Mean ◽  
Media Flows

Momentum transport in porous media exists in numerous engineering and process applications, e.g., ground water pollution, storage of nuclear waste, heat exchangers, and chemical reactors. In many of such applications, the porous medium is confined by solid boundaries. These impermeable boundaries give rise to shear stress and boundary layers. The Brinkman-extended Darcy equation describes the momentum transport due to Newtonian fluid flow in confined porous media. This equation is solved analytically in a cylindrical system, employing an existing fully-developed boundary-layer concept particular to porous media flows. The volume-averaged velocity increases as the distance from the boundary increases reaching a maximum at the center. The mean and maximum velocities are obtained and their behavior is investigated in terms of pertinent flow parameters. The friction factor is defined based on the mean velocity and is found to be inversely proportional to the Reynolds number, the Darcy number, and the mean velocity. The analytical results are verified by experiments using two types of metal foam. In the Darcy regime, reasonably good agreement is found between the analytical and the experimental friction factors for the 20-pore-per-inch foam, while a poor agreement is found for the 10-pore-per-inch foam.

Convective Heat Transfer of Al2O3 Nanofluids in Porous Media

2016 ◽  
Vol 139 (3) ◽  
Author(s):  
Navid O. Ghaziani ◽  
Fatemeh Hassanipour
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Medium ◽  
Porous Media ◽  
Heat Transfer Rate ◽  
Transfer Rate ◽  
Particle Concentration ◽  
Metal Foam ◽  
Volume Fraction ◽  
Constant Heat Flux

In this study, the performance of a heat sink embedded with a porous medium and nanofluids as coolants is analyzed experimentally. The nanofluid is a mixture of de-ionized water and nanoscale Al2O3 particles with three different volumetric concentrations: ζ = 0.41%, 0.58%, and 0.83%. The experimental test section is a rectangular minichannel filled with metal foam, which is electrically heated to provide a constant heat flux. The porous medium is assumed to be homogeneous and the flow regime is laminar. The result of heat transfer enhancement by slurry of Al2O3 nanofluid in porous media is studied under various flow velocities, heat flux, porous media structure, and particle concentration of nanofluid. The effect of particles volume fraction on heat transfer coefficient is also studied. This experimental study discovers and/or confirms the following hypotheses: (1) nanoparticle slurry in conjunction with metal foam has a significant effect on heat transfer rate; (2) there is an optimum permeability for the foam resulting in maximal heat transfer rate; (3) for a fixed particle concentration, smaller particles are more effective in enhancing heat transfer; and (4) increasing particle concentration results in some gains, but this trend weakens after a threshold.

Natural Convection in Enclosed Porous Media With Rectangular Boundaries

1970 ◽  
Vol 92 (1) ◽  
pp. 21-27 ◽  
Author(s):  
B. K. C. Chan ◽  
C. M. Ivey ◽  
J. M. Barry
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Porous Media ◽  
Natural Convection ◽  
Transfer Rate ◽  
Darcy Number ◽  
Field Equations ◽  
Geometric Aspect ◽  
Lumped Parameter ◽  
Different Temperatures

Numerical methods are used to solve the field equations for heat transfer in a porous medium filled with gas and bounded by plane rectangular surfaces at different temperatures. The results are presented in terms of theoretical streamlines and isotherms. From these the relative increases in heat transfer rate, corresponding to natural convection, are obtained as functions of three-dimensionless parameters: the Darcy number Da, the Rayleigh number Ra, and a geometric aspect ratio L/D. A possible correlation using the lumped parameter Da Ra is proposed for Da Ra greater than about 40.

Local Thermal Equilibrium in Forced Convection Through Porous Media

2010 ◽  
Author(s):  
A. Nouri-Borujerdi
Keyword(s):  
Porous Medium ◽  
Pressure Gradient ◽  
Forced Convection ◽  
Thermal Equilibrium ◽  
Fluid Transport ◽  
Convection Heat Transfer ◽  
Darcy Number ◽  
Equation Model ◽  
Temperature Fields ◽  
Local Thermal Equilibrium

Forced convection heat transfer through a channel filled with a porous medium is investigated using perturbation method. Two-energy equation model is utilized to represent the assumption of local thermal non-equilibrium which exists between the solid and fluid phases. The Brinkman-Forchheimer extension of the Darcy model is used to represent the fluid transport within the porous medium. Analytical solution is obtained for both fluid and solid temperature fields incorporating the effects of various pertinent parameters such as the Darcy number, the Biot number, the thermal conductivity and the pressure gradient. It is found that the Darcy number and the pressure gradient have significant effects on the local thermal equilibrium assumption.

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