Pores Per Inch Research Articles

Thermal Performance Analysis of Biporous Metal Foam Heat Sink

The present study presents a concept of biporous metal foam heat sink applicable to electronic cooling. This heat sink has two metal foam layers arranged in parallel along the primary flow direction, with different metal foam thickness, porosity, and pore density for each layer. The forced convective heat transfer in biporous metal foam heat sink is numerically investigated by employing the Forchheimer–Brinkman extended Darcy momentum equation and local thermal nonequilibrium energy equation. The effects of geometrical and morphological parameters on thermal and hydraulic performance are discussed in detail, and the heat transfer enhancement mechanism of biporous metal foam is analyzed. The thermal performance of biporous metal foam heat sink is compared with that of uniform metal foam heat sink. The results show that the thermal resistance of the biporous metal foam heat sink decreases with decrease of top layer metal foam porosity at a fixed bottom metal foam porosity of 0.9. It is seen that the biporous metal foam heat sink can outperform the uniform metal foam heat sink with a proper selection of foam geometrical and morphological parameters, which is attributed to the presence of high velocity gradient at the boundary layer that can enhance the convective heat transfer. The best observed thermal performance of biporous metal foam heat sink is achieved by employing 30 pores per inch (PPI) metal foam at the bottom layer, with a fixed 50 PPI metal foam at the top layer for the porosities of both layers equal to 0.9, and the optimal thickness of the bottom foam layer is about 1 mm.

Effects of metal foam meshes on premixed methane-air flame propagation in the closed duct

In many practical applications, the comprehensive measures are needed to suppress the gas explosions, and lining the channel walls with porous materials before a special installation for flame résistance is an effective choice. The aim of the present work is to investigate the effect of porous materials on the premixed flame propagation process in a closed combustion tube. Experiments are carried out to examine the flame dynamics for the stoichiometric methane/air mixtures both in the empty tube and in the presence of porous materials. The results demonstrate that the flame propagation can be divided into five typical stages, and a tri-tulip flame is observed during the development of the classic tulip flame. The inversion distance L of the flame front is selected to characterize the tulip flame evolution. It is worth to notice that the flame tip fronts travel faster with smaller porous densities PPI(pores per inch) from the finger shape stages to flat shape stages. The explosion severity parameters, including peak pressure and maximum pressure growth rates, are compared for all configurations, and the porous density plays an important role on the explosion severity. The maximum rate of pressure rise decreases linearly with the porous densities. It is deduced that the metal foam meshes with the larger porous densities PPI have more significant impacts on suppressing the tulip formation. The coupling of the flame front with the pressure wave plays a significant role in the onset and evolution of the tulip flame. As a whole, the metal foam meshes can reduce the overpressures can reduce the overpressures by33.3%–46.6%, while they have little effect on the flame tip speeds and shapes.

Experimental investigation on a solar parabolic trough collector for absorber tube filled with porous media

In the present work the efficiency of a solar parabolic trough has been investigated experimentally. An absorber filled with metal foam is used in order to improve the heat transfer and increase the efficiency of PTC (parabolic trough collector). The porosity of copper foam is 0.9 and the pore density is 30 PPI (pores per inch). The experiments were performed in different volume flow rates from 0.5 to 1.5 Lit/min and the standard of ASHRAE 93 was used to test the solar collector’s performance. Friction factor and Nu number have been investigated for both cases. It was found that by increasing the mass flow rate, the efficiency of the collector was enhanced and the same pattern can be seen when absorber filled with copper foam. When absorber filled with metal foam the overall loss coefficient UL decreases by 45% and it causes to increase efficiency because less energy is lost.

Development of cylindrical laminated methanol steam reforming microreactor with cascading metal foams as catalyst support

In this study, the cascading metal foams were used as catalyst supports for constructing a new type of cylindrical laminated methanol steam reforming microreactor for hydrogen production. The two-layer impregnation method was used to load the Cu/Zn/Al/Zr catalysts, and the ultrasonic vibration method was then employed to investigate the loading performance of metal foams with different types and thicknesses. Furthermore, the effect of the type of catalyst placement, pores per inch (PPI) and foam type on the performance of methanol steam reforming microreactor was studied by varying the gas hourly space velocity (GHSV) and reaction temperature. Compared with two other types of catalyst placement studied, the microreactor containing catalyst-loaded metal foams without clearance cascading (3×2) showed the highest hydrogen production performance. When the PPI of the metal foam was increased from 50 to 100, both the methanol conversion and the H2 flow rate gradually increased. Our results also showed that a microreactor with Cu foam as a catalyst support exhibits increased hydrogen production and higher stability than those of a microreactor with Ni foam.

Monte Carlo simulation of random, porous (foam) structures for neutron detection

Porous media incorporating highly neutron-sensitive materials are of interest for use in the development of neutron detectors. Previous studies have shown experimentally the feasibility of 6LiF-saturated, multi-layered detectors; however, the random geometry of porous materials has limited the effectiveness of simulation efforts. The results of scatterless neutron transport and subsequent charged reaction product ion energy deposition are reported here using a novel Monte Carlo method and compared to results obtained by MCNP6. This new Dynamic Path Generation (DPG) Monte Carlo method was developed in order to overcome the complexities of modeling a random porous geometry in MCNP6. The DPG method is then applied to determine the optimal coating thickness for 10B4C-coated reticulated vitreous-carbon (RVC) foams. The optimal coating thickness for 4.1275cm-thick 10B4C-coated reticulated vitreous carbon foams with porosities of 5, 10, 20, 30, 45, and 80 pores per inch (PPI) were determined for ionizing gas pressures of 1.0 and 2.8atm. A simulated, maximum, intrinsic thermal-neutron detection efficiency of 62.8±0.25% was predicted for an 80 PPI RVC foam with a 0.2µm thick coating of 10B4C, for a lower level discriminator setting of 75keV and an argon pressure of 2.8atm.

Numerical analysis and comparison of the thermal performance enhancement methods for metal foam/phase change material composite

Three methods to further enhance thermal performance of the metal foam/phase change material (PCM) composite are investigated and compared. These three methods include changing the pores per inch (PPI) of metal foam, modifying the shape of the cold wall and using the discrete heat sources. In this study, the composite consists of two materials: aluminum foam with 90% porosity as metal foam and paraffin wax as PCM. The numerical model based on finite volume method is developed, and the non-equilibrium equation is applied to study the melting process of the paraffin embedded in aluminum foam. The heat loss, the liquid average velocity and the efficiency of latent heat storage are analyzed and discussed. The results show that adopting the aluminum foam with high PPI value or modifying the shape of the cold wall could improve the thermal response of composite. Besides, the discrete heat sources could lead to a large average velocity in the liquid region. Combining the advantages of these methods, an optimization method is also proposed, which could improve the efficiency to 83.32% comparing with the pure paraffin.

Analysis on Experimental Investigation and Mathematical Modeling of Incompressible Flow Through Ceramic Foam Filters

This paper presents experimental results of pressure drop measurements on 30, 50, and 80 pores per inch (PPI) commercial alumina ceramic foam filters (CFF) and compares the obtained pressure drop profiles to numerically modeled values. In addition, it is aimed at investigating the adequacy of the mathematical correlations used in the analytical and the computational fluid dynamics (CFD) simulations. It is shown that the widely used correlations for predicting pressure drop in porous media continuously under-predict the experimentally obtained pressure drop profiles. For analytical predictions, the negative deviations from the experimentally obtained pressure drop using the unmodified Ergun and Dietrich equations could be as high as 95 and 74 pct, respectively. For the CFD predictions, the deviation to experimental results is in the range of 84.3 to 88.5 pct depending on filter PPI. Better results can be achieved by applying the Forchheimer second-order drag term instead of the Brinkman–Forchheimer drag term. Thus, the final deviation of the CFD model estimates lie in the range of 0.3 to 5.5 pct compared to the measured values.

Evaluation of polyurethane foam materials as air filters against fungal contamination

Current air filters used in food processing or storage facilities are expensive and disposable. The ability to use polyurethane foam as air filters against fungal spores would be beneficial as they are both cheap and re-usable. The aim of this study was to evaluate the air filtration capabilities, in terms of fungal spores, of a selection of polyurethane foam(s) of differing combinations of pores per inch (PPI) (50 and 90 PPI) and thickness (15 and 20 mm). Environmental air was used as a source of fungal spores and membrane filtration was used to assess the filtration capabilities of the foams. Spores capable of passing through the foams were captured on cellulose nitrate membrane filters and quantified in CFU counts. Apart from the 50 PPI foam of 15 mm thickness, all the foam samples were effective at significantly reducing the number of spores. The PPI was found to be 2 times more influential on the efficiency of the foam material than the foam thickness. This may be explained through the higher number of pores present and the decrease in thickness of the ribs composing the microstructure of the foam as shown through scanning electron microscope (SEM) micrographs. These studies show that reticulated polyurethane foams at the selected PPI and thickness can be used as effective air filters against fungal spores.

The effect of open-cell metal foams strut shape on convection heat transfer and pressure drop

Metal foams are promising materials for applications where enhancement of heat transfer is needed. Their performance depends on morphological parameters, such as the diameters of cells, pores, and struts, the porosity, the shape of struts.The effect of strut shape on convection heat transfer and pressure drop in open-cell metal foams has been investigated numerically in this paper by introducing the foam shape factor, a parameter that characterizes the shape of the strut. The analysis has been carried out both on real and ideal foams. The geometry of three 40 Pores Per Inch (PPI) real foams, with 0.87, 0.94 and 0.96 average measured porosities, was determined with X-ray Computed Microtomography (XCT) and a morphological analysis of the XCT data was carried out. The geometry of the ideal foams, based on the Lord Kelvin foam model, with the same PPI and porosities as those of the real foams, was generated with the free-to-use software Surface Evolver, building up foams with different strut shapes. Governing equations have been solved with a finite element scheme, assuming a uniform heat flux condition at the solid/fluid boundary of the foam.Results are presented in terms of convection heat transfer coefficients and pressure distributions as a function of strut shape parameters. The comparison between predictions for ideal and real foams, for different strut shapes, confirms that, in all cases, the closer the form of the ideal strut to that of the real strut the better the agreement among predictions. It is also pointed out that the convection heat transfer coefficient is maximized when, at equal porosity, the ligament shape is convex, while a concave strut shape maximizes the pressure drop.

Heat transfer and pressure drop characteristics in a circular tube with mesh cylinder inserts

Heat transfer performance and pressure drop tests were conducted on a circular tube with mesh cylinder inserts. These inserts with different open area rate, 30, 60, and 120 pores per inch (PPI) were tested at Reynolds numbers between 4000 and 18,000. Tap water was used as the working fluid. A high heat transfer coefficient was obtained by using mesh cylinder inserts. The friction factor increases with increases in PPI. Optimal results were obtained for PPI=60. Heat transfer rates and friction factors were enhanced by 2.3–2.6 and 3.4–3.7 times, respectively, to those in the plain tube. Performance evaluation criterion (PEC) values were approximately 1.48–1.7. Numerical method is also employed to analyze the enhancement. The results show that the reason for the heat transfer enhancement is attributed to two factors. One is attributed to the velocity increase at the inlet. Another owes to the high turbulent intensity generated by inserting the mesh issue. Two factors work together to achieve a high heat transfer rate. Compared with other inserts, mesh cylinder inserts can transfer more heat for the same pumping power.

Thermal performance of a 10-kW phase-change plate heat exchanger with metal foam filled channels

Compact-sized organic Rankine cycle (ORC) power generators call for small-scale heat exchangers to work with them. Since the heat transfer area plays a direct role in the performance of heat exchangers, micro-cellular structures such as metal foams are proposed to increase the heat duty of heat exchangers by increasing the surface area while maintaining their small size. In this experimental study, the performance of a 10-kW heat exchanger with channels filled with copper metal foam was investigated. A hot water loop was designed for heat input. The cold side of the heat exchanger works with R245fa as the working fluid. Single-phase and two-phase experiments were performed with different mass fluxes ranging from 180 to 600 kg/m2s. The effect of the pores per inch (PPI), working pressure, and different arrangements of metal foams was also investigated. Although the metal foam increases the pressure drop in the channel, it increases the recovered waste heat from the heat source and overall heat transfer coefficient of the heat exchanger by up to 2.3 times compared to a conventional heat exchanger without metal foams.

Passive control of centrifugal fan noise by employing open-cell metal foam

Experimental and numerical studies were conducted to investigate passive control of a forward-curved centrifugal fan noise by employing the open-cell metal foam (OCMF). The results indicated that a reduction of about 5dB in overall A-weighted sound pressure level (OASPL) was successfully achieved with a decline of about 200Pa in total pressure rise (TPR). For the OCMF investigated in the present paper with the pores per inch (PPI) of 20 and the porosity of 97%, the noise control mechanism was to suppress the source strength of tonal noise generated from the periodic impeller–tongue interaction rather than to absorb the broadband acoustic energy. The decline of the aerodynamic performance was mainly caused by reducing the work capacity of the impeller rather than caused by flow loss owing to the flow resistance of the OCMF. An improved design of the OCMF was suggested to achieve more reduction in noise level with less decline of aerodynamic performance.

Heat transfer and pressure drop characteristics of wet air flow in metal foam under dehumidifying conditions

In order to know the possibility of applying metal foam to improve performance of air conditioners, heat transfer and pressure drop characteristics of wet air flow in metal foam under air conditioning conditions were experimentally investigated and compared with those of a louvered fin-and-tube heat exchanger with the same volume. The experimental conditions cover the inlet air velocity from 0.5 to 2 m⋅s−1, Reynolds number from 100 to 500, air temperature from 27 to 32 °C, relative humidity from 50% to 90%, and PPI (numbers of pores per inch) of metal foam from 5 to 15. The research results show that, as the relative humidity of inlet air increases, both the total heat transfer rate and pressure drop in metal foam increase, and the maximal increments are 67%and 62%, respectively; with the increase of metal foam PPI, the heat transfer rate initially decreases slightly and then increases; the heat transfer capacity of copper foam heat exchanger is enhanced by 56% ~ 196% compared to that of the fin-and-tube heat exchanger.

Lattice Boltzmann simulation on phase change heat transfer in metal foams/paraffin composite phase change material

Lattice Boltzmann method (LBM) was used to investigate the latent heat storage (LHS) performance of metal foams/paraffin composite phase change material (CPCM). The effects of metal foams PPI (number of pores per inch) and porosity on PCM melting rate, heat storage capacity and heat storage density were investigated. The results show that the CPCM heat transfer performance is determined by both heat conduction and natural convection. Increasing PPI can enhance heat conduction, but weaken natural convection. When the porosity is small, increasing PPI can enhance LHS performance. When the porosity is large, decreasing PPI can enhance the performance. With porosity decreasing, CPCM heat storage rate is improved and the maximum heat storage capacity almost keeps constant, but the heat storage density dramatically reduces. An optimum metal foams structure with porosity of 0.94 and PPI of 45 is recommended. Further, a performance enhancement scheme with nonuniform metal foams porosity was proposed and numerically validated. The results show that the proposed scheme can improve the uniformity of the heat transfer process and enhance the heat transfer performance.

Forced convection through open cell foams based on homogenization approach: Steady state analysis

To investigate thermal transportation through open cell foams there are various microscopic and macroscopic numerical models along with their limitations available in the literature. The purpose of this study is to investigate the limitations of macroscopic models and to propose some reliable ideas as conclusion that can be used to overcome the existing limitations. Therefore, a combined experimental and numerical study is presented. The experimental study comprises of steady state forced convection experiments which involve three different regimes of heat transfer. Further, in macroscopic models these three regimes of heat transfer namely conduction, thermal dispersion and interstitial convection are governed by stagnant effective thermal conductivity, ke, dispersion conductivity, kd and volumetric heat transfer coefficient, hv respectively. Moreover, the complex structure of the open cell foams is simplified into a rather realistic Kelvin cell model for the determination of ke. The influence of the geometrical parameters such as pore diameter, d and foam porosity, ε is investigated by examining 10, 20 and 30 PPI (pores per inch) alumina foams for a porosity range of 0.79–0.87. The findings of this study reveal that it is important to consider both local thermal non-equilibrium (LTNE) and thermal dispersion together for improved analysis. Further, it is revealed that although with the above consideration, it is possible to exhibit the effect of geometrical parameters on each regime of heat transfer but the accuracy of the results predicted through macroscopic models remain under question as there is no basis to validate the results.

Low-GWP refrigerants flow boiling heat transfer in a 5 PPI copper foam

This paper reports an experimental investigation of the heat transfer performance of the new low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling heat transfer inside a horizontal high porosity copper foam with 5 Pores Per Inch (PPI). Metal foams are a class of cellular structured materials consisting of a stochastic distribution of interconnected pores; these materials have been proposed as effective solutions for heat transfer enhancement during both single and two-phase heat transfer. R1234yf and R1234ze(E) refrigerants are appealing alternatives of the more traditional R134a by virtue of their negligible values of GWP and normal boiling temperatures close to that of R134a, which make them suitable solution in several different applications, such as: refrigeration and air conditioning and electronic thermal management. This work compares the two-phase heat transfer behaviour of these new HFO refrigerants, studying the boiling process inside a porous medium and permitting to understand their effective heat transfer capabilities. The experimental measurements were carried out by imposing three different heat fluxes: 50, 75, and 100kWm−2, at a constant saturation temperature of 30°C; the refrigerant mass velocity was varied between 50 and 200kgm−2s−1, whilst the mean vapour quality varied from 0.2 to 0.95. The two-phase heat transfer and pressure drop performance of the two new HFO refrigerants is compared against that of the more traditional R134a.

Sodium borohydride hydrogen generator using Co–P/Ni foam catalysts for 200 W proton exchange membrane fuel cell system

The response characteristics of electroless-deposited Co–P/Ni foam catalysts for sodium borohydride hydrolysis were investigated. The effect of nickel foam geometry on the properties of the catalysts was evaluated. As the PPI (pores per inch) of the nickel foam increased, the hydrogen generation rate per gram of the deposited catalyst increased due to an increase in surface area. The response characteristics of various catalysts were compared under real operating conditions. When a thin nickel foam with high PPI was used, the response characteristics of the catalyst improved due to an increase in the amount of the deposited catalyst and surface area. Finally, a 200 W PEMFC (proton exchange membrane fuel cell) system using electroless-deposited Co–P/Ni foam (110 PPI) catalyst was investigated. The response time to reach a hydrogen generation rate sufficient for a 200 W PEMFC was 71 s, and the energy density of a 200 W fuel cell system for producing 600 Wh was 252.1 Wh/kg. A fuel cell system using Co–P/Ni foam catalysts can be widely used as a power source for mobile applications due to fast response characteristics and high energy density.

Modulated heat transfer tube with short conical-mesh inserts: A linking from microflow to macroflow

This paper creates new method that uses microflows through mesh pores to modulate flow and temperature fields. Modulated heat transfer tube (MHTT) was constructed by suspending consecutive conical-mesh inserts in a tube. Because there are too many 3D mesh pores (10–100μm size) for a conical mesh insert, numerical simulations in laminar flow regime were performed by an 3D to 2D conversion of mesh pores applying equal equivalent diameter criterion and total flow area criterion of mesh pores. The multiscale grid generation linked micron scale of mesh pores and macroscale of the tube. Covering the present data ranges, MHTT had Nusselt numbers which are 1.4–4.1 times of that in a bare tube, PEC (performance evaluation criterion) was up to 2.2, demonstrating excellent heat transfer enhancement at low flow rate pumping cost. The perfect MHTT performance comes from the distinct flow field: an attached hydraulic boundary layer having large velocity and its gradient near the tube wall, a weak circulating flow region upstream of the mesh insert and a weakly positive flow region downstream of the mesh insert. A thin thermal boundary layer on the tube wall was reached to enhance heat transfer. The periodic unit length S and diameter of the conical mesh insert δ were optimized. The varied slopes of Nu versus Re were found and explained while increasing Re. There are best matches of PPI (pores per inch) and Re (Reynolds number). Low PPI mesh insert was suggested at low Re, high PPI mesh insert deformed streamlines to deviate from the perfect flow and temperature fields. Meanwhile, high PPI mesh insert had better performance at high Re. Low PPI mesh insert had good performance, which can be further improved by raising PPI. Because metallic mesh screen is commercialized and cheap, MHTT has wide potential engineering applications.

Heat Transfer and Pressure Drop Characteristics of Finned Metal Foam Heat Sinks Under Uniform Impinging Flow

A numerical investigation was carried out to characterize the thermal performance of finned metal foam heat sinks subject to an impinging air flow. The main objective of the study was to quantify the effects of all relevant configurational parameters (channel length, channel width, fin thickness, and fin height) of the heat sink upon the thermal performance. Open-cell aluminum foam having fixed porosity of 0.9118 and fixed pore density of five pores per inch (PPI) was used in the study. A previously validated model based on the porous medium approach was employed for the numerical simulation. Various simulation cases for different combinations of channel parameters were carried out to obtain the Nusselt number correlation. Based on the inviscid impinging flow, a pressure drop correlation was derived for impinging flow in finned metal foam heat sinks. By using these correlations, the thermal performance of finned metal foam heat sinks was compared with the conventional plate-fin heat sinks. It was demonstrated that the finned metal foam heat sinks outperformed the plate-fin heat sinks on the basis of given weight or given pumping power.

Performance improvement of high power liquid-cooled heat sink via non-uniform metal foam arrangement

The present study proposes a novel concept by employing non-uniform arrangements of metal foams alongside the heat sink applicable for liquid cooling. A total of 7 arrangements by varying PPI (pores per inch) of metal foam are made and additional 4 multiple channel designs are fabricated for comparison. Test results indicate that non-linearity term in Forchheimer equation rises significantly with increase of PPI (or with the decrease of permeability). The results suggest that the transition from the linear Darcy regime to the quadratic Forchheimer regime depends on the metal foam structure. It is also found that metal foam appreciably outperforms conventional multiple channel heat sink. The thermal resistance can be reduced by more than 62% as compared to that of empty plate design. For a given pumping power, the non-uniform arrangement of metal foam shows the best performance by employing a larger PPI metal foam at the inlet, followed by a smaller PPI metal foam, and the least PPI metal foam should be placed at the rear part of the heat sink.