In this paper a numerical and experimental investigation on impinging jets with metal foam is carried out. The channel is partially filled with porous media. The physical model and geometry has been made considering that the lower impinging surface is heated at uniform heat flux, q w , and the upper surface is adiabatic. The flow is 2D, unsteady, laminar, and incompressible. The distance between the upper confining surface and the lower heated surface is H (40 mm). Fully developed fluid dynamic and thermal flow is assumed at the outlet sections and the fluid is air. The porous material is considered as homogeneous and isotropic. All the thermophysical properties of the fluid and the solid matrix of the porous medium are assumed constant except for the variation in density with the temperature giving rise to the buoyancy forces. Metal foam of 10 PPI is considered. It was investigated many configuration taking into account the ratio s/H and D j /H. The values of ratio s/H ranging from 0 to 1 while values of the ratio D j /H raging from 0.3–1.2. Results in terms of wall temperature profiles, local and average Nusselt numbers are presented for different Reynolds values. For the considered parameters, it is obtained that Nusselt number has a weak dependence of D j /H, in fact, for the ratio equal to 0.3, it is noted that Nu is higher than the ratio equal to 0.6.

The present work aims to investigate a honeycomb module with PCM for electronic passive cooling. The system is a honeycomb squared solid matrix filled with Phase Change Material. The PCM is paraffin wax and the honeycomb material is aluminum. Transient regime numerical simulations are created with different configurations, for different pores per unit of length (PPU) and different thermal power. A numerical model with honeycomb structure is compared with a porous medium model. The behavior of the porous medium is modeled with the Darcy-Forchheimer assumption and local Thermal Non-Equilibrium (LTNE) hypothesis is employed to simulate the heat transfer between the fluid zone and the solid zone. The results are presented in term of melting time, melt fraction evolution, temperature fields and stored energy for the charging phase.

Phase change materials (PCMs) are attractive components of thermal energy storage systems due to their high energy densities and the relatively small temperature differences needed for effective charging and discharging. In this study, experimental data were examined in order to better understand energy exchange in packed beds and porous media comprised of solid/liquid phase change capsules. Air was used as the heat transfer fluid; hot air was injected in order to drive the melting process, while cold air was injected to accomplish freezing or solidification of the PCM. Theory was developed to describe the temperature variations throughout the bed. Temperatures in the bed were found to vary exponentially near the phase change fronts. For cold air injection into a bed initially above the phase change temperature, a second wave was observed ahead of the phase change front that can be described as a broadening traveling thermal wave due to diffusion/dispersion. For hot air injection into a packed bed of solidified PCM capsules initially at a temperature below the phase change temperature, the thermal waves in the cold region showed isotherm velocity retardation due to incomplete thawing. A shrinking core model of the melting or solidification of the PCM in the capsules was developed to document the internal heat transfer constraints within the capsules. The results of that study support the conclusion that slower wave velocities occur due to partial melting.

The exponential growth in electronic power has brought amazing technology but with it also comes a burden in high heat production that threatens the safety of the product since electronics’ failure rate increases by its operating temperature. As such, cooling techniques have a key role to keep the temperature of electronics devices, such as processors, memory and graphics chips, below a maximum operating temperature. In this work, a porous filled heat exchanger has been numerically modeled to investigate the cooling effectiveness and temperature distribution on the base of the heat exchanger subjected to high heat flux leaving these devices. The effects of different nanofluid coolants (0.75% double walled carbon nanotube in water (DWCNT), 1% alumina in water, and 1% diamond in 20:80 ethylene glycol/water), porous materials (copper and annealed pyrolytic graphite (APG)), and porosity values are investigated. The coolant enters from an inlet channel normal to the base, moves through the porous field, and then leaves the heat exchanger through two opposite exit channels parallel to the base. The study is performed for two and three dimensional geometries in which two different designs are studied for 3D cases. One of the designs has a rectangular cross sectional inlet channel (along transverse direction) and the other design has square one. The results indicate that utilizing APG porous matrix, for all studied coolants of pure water and water based nano fluidics, improves substantially the cooling of the base of the heat exchanger in 2D and 3D with rectangular inlet. The results also show that utilizing carbon nanofluids (DWCNT) as coolant for high porosity structures, in both copper and APG porous matrices, improves cooling efficiency and temperature uniformity over the base, for all 2D and 3D cases. The effect of inlet channel geometry, square and rectangular, is also investigated for either similar velocity or similar mass flow rate at the inlet channel entrances.

The utilization of porous media can enhance the heat transfer process due to its large heat transfer area within limited space. The natural convection in porous media widely exists in various heat transfer equipment and the related flow and heat transfer in porous spaces is one complicated transport phenomenon, for which the accurate prediction is challenging. Pore-scale models can predict transport phenomena in porous media in pore space, which can be used in the modeling of flow and heat transfer in porous media under local thermal non-equilibrium condition. The pore-scale study includes the reconstruction of porous structure and the direct numerical simulation of transport phenomena in the pore spaces. In this paper, the geometrical reconstruction approach was developed to generate the porous region using the tomographic reconstruction, which is one nondestructive imaging technique. The porous sample was scanned on a micro-CT scanner with micrometer resolution. 2D sliced scan images were obtained and then stacked to reconstruct the 3D porous geometry. A double-population thermal lattice Boltzmann model was established to predict the natural convection in reconstructed porous media at pore scale.

The porous composite system is consists of porous medium and free fluid layer, which has extensive industrial applications. The study method for the flow field in the porous composite system includes the microscopic, mesoscopic and macroscopic approaches. When the two-domain approach is adopted, which is one of the macroscopic methods, the momentum transport boundary conditions at the interface between porous medium and free fluid layer is essential to analyze the flow field in the system. When Darcy equation is adopted to describe the flow in porous region, the Beavers-Joseph (BJ) interface condition can be used. When Darcy-Brinkman equation is adopted to describe the flow in porous region, the stress-jump (Ochoa-Tapia & Whitaker: OTW) interface condition can be used. To utilize these interface conditions, the velocity slip coefficient used in the BJ interface condition and the stress-jump coefficient used in the OTW interface condition should be specified. In this paper, a brush configuration is approximately treated as the equivalent porous media in the composite system. A numerical simulation method is used to obtain the microscopic solution for the flow in the system based on the Navier-Stokes equation applied in whole system, and an analytical method is used to obtain the corresponding macroscopic solution based on the two-domain approach. By comparing the microscopic and macroscopic solutions, the velocity slip coefficient and the stress-jump coefficient are determined since they can be treated as adjustable parameters. The influence of different flow types, including Poiseuille flow, Couette flow, and free boundary flow, are investigated. Also the impact of free fluid layer thickness and porous structure on the velocity slip coefficient and the stress-jump coefficient are discussed. The results indicate that, the velocity slip coefficient and the stress-jump coefficient are not only the parameters which depend on the porous structure, but also depend on the thickness of free fluid layer and flow type. When the thickness of free fluid layer is lower than a certain value, the impact of free fluid layer thickness on the velocity slip coefficient and the stress-jump coefficient is much obvious. In addition, when the thickness of free fluid layer is small, these coefficients are found to be dependent on the flow type. However, when the thickness of free fluid layer is large, the stress jump coefficient is independent of the thickness of free fluid layer and the flow type. Thus the stress jump coefficient obtained for a specific case can be used to predict velocity for different flow types and different thickness of free fluid layers.

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.

The lid-driven flow inside a porous square cavity is numerically simulated. The porous media is modelled on the microscopic scale (heterogeneous porous medium) with a square heat conductive single block representing the solid constituent. Conversely, the fluid relies between the block and the cavity surfaces. A vertical positive thermal gradient, obtained by keeping the sliding-lid temperature T H higher than the base one T C , aligned with the gravity force enables a gravitational stable condition where the buoyant-induced flow does not occurs spontaneously. Instead, the flow comes about as the cavity top surface slides with constant velocity. Conservation equations are applied separately for each constituent and are coupled by boundary conditions at the fluid to solid interface (block surface). The Boussinesq-Oberbeck approximation accounts for the buoyant effects. The equations are solved via the finite volume method with the use of the SIMPLE algorithm for the pressure-velocity coupling and QUICK interpolation scheme for the treatment of the advection terms. The aim of the present work is to investigate how variations on the flow parameters and the block size affect the thermal process throughout the cavity. A top lid velocity based Reynolds number evaluates the intensity of the forced convection process while the Grashof number is associated with the intensity of buoyancy. The flow parameters cover only the laminar regime, such as 10 2 ≤ Re ≤10 3 and 10 3 ≤ Gr ≤10 7 . The Re and the Gr numbers are also analyzed by the means of the Richardson number, Ri , which accounts the relative predominance of buoyancy over the inertia effects. Moreover, a clear fluid cavity and enclosure configurations with three different block dimensions, namely B = 0.3, 0.6 and 0.9, are simulated. The heat transfer across the cavity can be characterized as a competitive effect, since the flow is hindered as the buoyancy effect rises. Results show that an increase in Re , or decrease in Gr , enhances the heat transfer, revealing a convection dominant regime. Alternatively, an increase in Gr , or a decrease in Re , leads the fluid to a stagnant-prone condition where a conduction dominant regime is verified. Thus, the surface-average Nusselt number, Nu av , tends to unity as the flow is confined to the adjacency of the sliding-lid. The placement of the single block in the cavity can enhance or hinder the heat transferred, depending on the flow regime. For instance, if a B = 0.6 block is inserted in the presence of a convection dominant regime, the Nu av is increased. Conversely, if the fluid is quiescent, a B = 0.6 block alters the flow path and the Nu av decreases. Intense blockage effects are observed for larger values of B since the block interferes on the flow more significantly. For a convection dominant regime, for instance, a B = 0.9 block causes the Nu av to drop. However, in the presence of stagnant fluid, the same obstacle forces the flow to circumvent it. Thus, the Nu av number increases, indicating that heat transfer mode returns to a convective pattern.

In this paper, a numerical investigation on Latent Heat Thermal Energy Storage System (LHTESS) based on a phase change material (PCM) in a metal foam is accomplished. A vertical shell and tube LHTESS made with two concentric aluminum tubes is investigated. The internal surface of the hollow cylinder is at a constant temperature above the PCM melting temperature to simulate the heat transfer from a hot fluid. The other external surfaces are assumed adiabatic. The phase change of the PCM is modeled with the enthalpy porosity theory while the metal foam is considered as a porous media that obeys to the Darcy-Forchheimer law. Local thermal non-equilibrium (LTNE) model is assumed to analyze the metal foam and some comparison are accomplished with the local thermal equilibrium model assumption. The governing equations are solved employing the Ansys-Fluent 15 code. Numerical simulations for PCM, PCM in the porous medium in LTE and in LTNE assumptions are obtained. Results as a function of time for the charging phase are carried out for different porosities and assigned pore per inch (PPI). The results show that at high porosity the LTE and LTNE models have the same melting time while at low porosity the LTNE has a larger melting time. Moreover, the presence of metal foam improves significantly the heat transfer in the LHTESS giving a very faster phase change process with respect to pure PCM, reducing the melting time more than one order of magnitude.

Combined natural convection and forced convection gets a great attention for its importance in practical applications in various modern systems such as electronic cooling, chemical vapor deposition and solar energy systems. Buoyancy force due to the heating of the lower cavity wall induces secondary flows hence the local heat transfer increases. The onset point of the secondary flows is important because it delineates the region after which the two-dimensional laminar flow becomes three-dimensional and a transition motion from laminar to turbulent flow is observed. In this paper, mixed convection in a horizontal channel partially filled with a porous medium and the lower wall heated at uniform heat flux is studied experimentally and numerically. The heated wall temperature profiles as a function of the Gr/Re 2 values are presented. Pictures of flow visualization along longitudinal, transversal and horizontal section are given. Average Nusselt numbers are evaluated. Results in terms of wall temperature profiles, local and average Nusselt numbers are presented for different Reynolds and Rayleigh number values. Some comparison between experimental and numerical results are accomplished.

We present numerical results for turbulent heat transfer past a backward-facing-step channel with a porous insert. A non-linear eddy viscosity model was applied to handle turbulence. For a constant Darcy number, the thickness of the porous insert was varied in order to analyze its effects on the flow pattern, particularly the damping of the recirculating bubble past the insert. Further, the reduction of the Nusselt number along the bottom heated surface, when using porous materials inside the channel, was investigated. The numerical technique employed for discretizing the governing equations was the control-volume method. The SIMPLE algorithm was used to correct the pressure field and the classical wall function approach was utilized in order to handle flow calculations near the wall. Comparisons of results simulated with different porous materials were presented.

The catalytic dehydrogenation reaction of isobutane to isobutylene is simulated in a commercial-scale heterogenous fixed bed reactor (FBR). The porous medium method in ANSYS Fluent combined with the reaction model capability was utilized to predict the flow behavior and species transport in a bed of spherical particles. Physical and material properties of a dehydrogenating catalyst of Chromium Oxide (Cr 2 O 3 ) on Aluminum Oxide Support (Al 2 O 3 ) were employed in the model. Several reaction models were implemented using a customized User-defined Function (UDF) subroutine. Simulation results were validated against literature data for a similar process. Good agreement was observed for the conversion of alkanes to alkenes within acceptable accuracy. It is concluded that the power-law model showed the least fit for the feed conversion and product selectivity compared to the other studied reaction models.

Results of a numerical study considering the periodic natural convection inside a fluid saturated porous medium are presented. The porous medium is obtained by placing four, large and uniformly distributed solid obstacles of regular (square) shape inside the enclosure, a structure that hinders the option of seeking a porous-continuum modeling approach. The periodic heating is achieved by imposing a time-periodic and spatially uniform high temperature condition at one of the walls of the enclosure, while the other wall is maintained at a constant, uniform and low temperature; the horizontal surfaces are set as adiabatic. Heat transfer results are obtained then by following a continuum modeling approach, and reported on a parametric form with the Prandtl number fixed equal to 7, and the Rayleigh number inside the enclosure varying from 10 3 to 10 7 . The boundary layer interference phenomenon, observed for the case of constant horizontal heating, is also observed in the case of periodic heating. The visualization of the natural convection process via isotherms and streamlines, together with the periodic (time-varying) Nusselt number, allows the identification of a singular dynamic behavior, including the storage of thermal energy inside the enclosure.

Turbulent natural convection in a two-dimensional horizontal composite square cavity is numerically analyzed using the finite volume method and the thermal non-equilibrium approach. Distinct energy equations for the working fluid and for the porous matrix are proposed reflecting different energy balances for each phase. The composite square cavity is formed by three distinct regions, namely, clear, porous and solid region. It was found that the fluid begins to permeate the porous medium for values of Ra greater than 10^6. Nusselt number values show that for the range of Ra analyzed there are no significant variation between the laminar and turbulent model solution. When comparing the effects of Ra and Da on Nu , results indicate that the solid phase properties have a greater influence in enhancing the overall heat transferred trough the cavity.

Liquid droplet sorption in unsaturated porous media is an important process for a wide range of applications. However, visualizing droplet sorption is difficult due to the opaque nature of porous media used in most of the applications. In this study, microCT is used to characterize the sorption profiles of single/multiple nanofuid droplets in PMMA powder beds. The microCT scan results reveal that the region of the powder bed saturated with nanofluid has a substantially higher gray scale value than that in the unsaturated region, thus enabling the visualization of the sorption profile. It also shows that nanoparticles enhance the visualization of the sorption profiles as they tend to accumulate and stay near the wetting front even after water evaporation.

Heat pipes have been shown to be one of the most efficient passive cooling devices for electronic cooling. Only a handful of studies were capable of solving transient performances of heat pipes based on realistic assumptions. A segregated finite volume base scheme using SIMPLE algorithm is used along with system pressurization and overall mass balance to solve mass transfer at the interface, continuity, momentum and energy equations. The fluid flow and heat transfer are solved throughout the wick and vapor core and no assumptions are made at the locations where evaporation and condensations occur. Water is the working fluid and variable densities are used for both liquid and vapor phases to account for continuity at the interface as well as inside of wick and vapor core. The wick is modeled as a non-homogeneous porous media and the effective thermal conductivities and viscous properties are calculated for each type of structure separately using the available relations from the literature. In this study, an axisymmetric two-dimensional solver for cylindrical heat pipe is developed using FLUENT package with the help of User Defined Functions (UDFs) and User Defined Scalar (UDS). The model is tested for grid and time step independency and the results show the stability and accuracy of the proposed method. The numerical results of the present study were in good agreement with the data from previous numerical and experimental studies available in the literature. Additionally, two different wick structures were studied to determine its effect on the thermal performance of heat pipes.

This paper presents a numerical study aimed at understanding the impact of the mushy zone constant, A mush , on simulated phase change heat transfer with varying geometries (inclination angles, and fin additions). This parameter is found in the Carman-Koseny equation, which is used in the modified heat capacity-porosity formulation for modeling phase change; this approach models fluid flow within the mushy region as flow through a porous medium. The melting of dodecanoic acid inside a rectangular thermal storage unit was simulated in COMSOL Multiphysics 4.4 with A mush being varied for each geometry. The predicted numerical melt front positions were directly compared to published experimental results [18–20]. Results have shown that the influence of A mush on the melting rate of the PCM is reduced with increasing inclination angles of the enclosure; where the melt interface is perpendicular to the direction of gravity. This has been attributed to the reduced velocity magnitudes that appear near the melt interface in such geometries. The addition of the fins showed that near the fin region A mush does not play an important role. However, it was observed that the model over predicts the velocity near the fin in the one fin model, which caused a mismatch between numerical and experimental results.

In the Compressed Air Energy Storage (CAES) approach, air is compressed to high pressure, stored, and expanded to output work when needed. The temperature of air tends to rise during compression, and the rise in the air internal energy is wasted during the later storage period as the compressed air cools back to ambient temperature. The present study focuses on designing an interrupted-plate heat exchanger used in a liquid-piston compression chamber for CAES. The exchanger features layers of thin plates stacked in an interrupted pattern. Twenty-seven exchangers featuring different combinations of shape parameters are analyzed. The exchangers are modeled as porous media. As such, for each exchanger shape, a Representative Elementary Volume (REV), which represents a unit cell of the exchanger, is developed. The flow through the REV is simulated with periodic velocity and thermal boundary conditions, using the commercial CFD software ANSYS FLUENT. Simulations of the REVs for the various exchangers characterize the various shape parameter effects on values of pressure drop and heat transfer coefficient between solid surfaces and fluid. For an experimental validation of the numerical solution, two different exchanger models made by rapid prototyping, are tested for pressure drop and heat transfer. Good agreement is found between numerical and experimental results. Nusselt number vs. Reynolds number relations are developed on the basis of pore size and on hydraulic diameter. To analyze performance of exchangers with different shapes, a simplified zero-dimensional thermodynamic model for the compression chamber with the inserted heat exchange elements is developed. This model, valuable for system optimization and control simulations, is a set of ordinary differential equations. They are solved numerically for each exchanger insert shape to determine the geometries of best compression efficiency.

Following a long-lasting failure in the cooling system of a pressurized water reactor (PWR), the superheated core can be efficiently cooled down by reflooding. The macroscopic model used at the French Institute of Radioprotection and Nuclear Safety (IRSN) to simulate this process is based on strong assumptions on the microscopic flow patterns. This paper describes the experimental setup designed for the study of boiling in porous media with the emphasis on various pore-scale boiling regimes. The final experimental setup is a two-dimensional porous medium made of 392 cylinders randomly placed between two ceramic plates. Each heating cylinder is a RTD probe (Resistance Temperature Detector), that can give thermal measurements in every point of the test section as well as heat generation. This paper presents preliminary results: pool boiling is characterized for a single cylinder mounted in the test section and reflooding of a line of 9 cylinders is observed.

Two-phase flow loop system using a metal porous heat sink is proposed as a cooling system of the future power electronic devices with a heat load exceeding 300W/cm 2 . In this paper, as the first step, the heat transfer performance of the porous heat sink is evaluated under high heat flux conditions and the applicability and some engineering issues are discussed. The porous medium, which is fabricated by sintering copper particles, has a functional structure with several sub-channels inside it to enhance phase-change as well as discharge of generated vapor outside the porous medium. This porous heat sink is attached onto a heating chip and removes the heat by evaporating cooling liquid passing through the porous medium against the heat flow. Experiments using 30 kW of heating system show that the heat transfer performance of a copper-particles-sintered porous medium with the sub-channels exceeds 800W/cm 2 in both high and low subcooling cases and achieves 300W/cm 2 at a wall temperature of 150 °C ( T in = 70 °C) and 130 °C ( T in = 70 °C). These results prove that this porous heat sink is applicable enough for cooling 300 W/cm 2 class of power electronic devices.