Porous Ribs Research Articles

Effect of tapered porous ribs on the performance of Proton exchange membrane fuel cells

The conventional trench ridge structure suffers from the difficulty of removing liquid water and poor gas transport performance. The water removal and gas transport performance inside the flow field are enhanced by introducing a tapered porous rib flow field (TPRFF). In order to investigate the performance of TPRFF, a complex three-dimensional, multiphase, non-isothermal agglomeration model was developed using numerical techniques. The numerical results show that the TPRFF exhibits significant improvements in oxygen concentration distribution, oxygen distribution uniformity and water removal compared to a conventional parallel flow field (CPFF). In addition, the dissolved water content remained almost constant while the liquid water saturation was lower. The study also investigated the effect of different cathode exit heights on the performance of the fuel cell using TPRFF. The results showed that the net power density increased by 14.0 %, 11.9 %, 8.7 %, and 1.5 % for outlet heights of 1.3 mm, 1.0 mm, 0.7 mm, and 0.4 mm, respectively, while it decreased by 27 % for an outlet height of 0.1 mm. Finally, the effect of different stoichiometric ratios on cell performance was also investigated in the novel flow field.

Experimental investigation on the thermal performance of a large-size aluminum vapor chamber for multi-point heat sources

Abstract With the development of electronic information technology, the integration and power of electronic equipment continue to increase. As an efficient heat transfer element, vapor chambers are widely used in the field of heat dissipation of electronic devices. However, greater challenges have been posed in terms of higher heat dissipation capacity, larger size, and lighter weight. Therefore, a large-scale aluminum vapor chamber with a size of 340 mm×295 mm ×7.5 mm is designed for the heat dissipation of multi-point array heat sources. Multiple parallel porous ribs are sintered to form capillary wicking channels and vapor diffusion paths, which efficiently transfer the heat from the middle of the vapor chamber to the cold plate on the two sides. The transient working characteristics and heat dissipation performance under different working conditions are experimentally investigated. The results show that there is obvious temperature instability, which can be suppressed by the tilt of the vapor chamber and the increase of the heating power. Under the tilt condition, the temperature rises in the vertical direction due to the influence of gravity, while the inclination angle has basically no effect. The vapor chamber can work stably at the total heating power of 2100 W with the smallest thermal resistance 0.03 °C/W. The single-point heat flux can reach 7.3W/cm2 for the 128 heat sources. Compared to a traditional vapor chamber, the proposed aluminum vapor chamber provides a thermal management solution for large-size electronic devices with multiple heat sources.

An experimental study on flow boiling heat transfer in porous-ribbed micro-channels

Experiments were carried out to investigate flow boiling heat transfer characteristics in a micro-channel heat sink with porous ribs at three rib-heights (i.e. Hrib = 200 μm, 400 μm, and 600 μm), three mass fluxes (i. e. 200 kg/m2s, 300 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (30–1300 kW/m2). The heat transfer coefficients and temperature uniformity in porous-ribbed micro-channels under different conditions were analyzed and compared to the smooth micro-channels (i.e. smooth flat surface and smooth-ribbed surfaces). The flow regimes in both porous and smooth micro-channels were visualized under typical flow boiling conditions. The results showed that micro-channels with higher rib heights experience an earlier transition to the critical heat flux (CHF) region due to the narrower flow path. Compared to the smooth micro-channel, the configuration of porous coating significantly enhances the heat transfer in the micro-channel without ribs or with lower rib height (Hrib = 200 μm) due to the strong hydrophilicity, good liquid storage, and rewetting ability on porous surfaces. At G = 400 kg/m2s, the porous-flat surface achieves a 67.9 % enhancement of heat transfer coefficient in the micro-channel compared to the smooth-flat micro-channel. However, with a higher rib height (Hrib = 400 μm and 600 μm), the thermal resistance is increased which hinders the heat conduction along the rib-height direction. A significant superheat is observed at the bottom of porous surface, thereby deteriorating the heat transfer performance in the porous ribbed micro-channel.

Heat transfer enhancement of nanofluids in concentric cylindrical heat exchanger

Two models of heat transfer channels for concentric cylindrical heat exchangers with porous ribs are developed herein to optimize the transfer efficiency of the heat storage reactor channels. Rings and spiral ribs are added to the smooth circular tube inside the reactor. Spoiler holes are arranged in the ribs to reduce the stagnation area around the ribs. The effects of coolant (SiO2–H2O nanofluids), rib shapes, and the number of holes in the rib on the overall performance of the heat storage system were numerically investigated. Results revealed that the 0.5% mass fraction nanofluids are the most suitable working fluid for channel thermal transport in the simulated Reynolds number range. Both ring ribs and spiral ribs reinforce the heat exchange of the pipe to varying degrees. Spiral perforated ribs have superior heat transfer and drag reduction performance compared with ring ribs. The comprehensive performances of spiral ribs are always higher than those of ring ribs under all conditions.

Thermal analysis and structure optimization of wavy microchannel with combining ribs of porous layer and solid wall in multi objective genetic algorithm

The combining rib of porous layer and solid wall is set in wavy microchannel heat sink to enlarge heat transfer area and convection with smaller increase in flow resistance, in which the consumption of pump power relates to the ratios of wave amplitude to length or to width and the thickness of porous layer due to vortices generation while coolant passing through channel. The local thermal equilibrium occurs in porous layer, and the Forchheimer-Brinkman-Darcy relations were used to describe the flow inside porous ribs. The effects of channel-to-pitch width ratio, width ratio of porous layer to pitch and microchannel height on its cooling performance are investigated, and the parameter of figure of merit (FOM) is defined to evaluate thermo-hydraulic performance in wavy microchannel. Besides, the multi objective genetic algorithm method is employed to optimize structure parameters in wavy microchannel to obtain better performances, in which the increase in thermal resistance from 0.062 K/W to 0.166 K/W together with consumption of pump power decreasing from 0.00416 W to 0.00185 W can be obtained in the Pareto front set, and it relates to the heat dissipation in convection between coolant and bottom surface or vertical side ribs. Compared to wavy microchannel with solid rib, the 37.61% and 16.67% decreases respectively in thermal resistance and consumption of pump power occur in the case with optimal combining rib of porous layer and solid wall.

Direct numerical simulations of turbulent channel flow with a rib-roughened porous wall

To describe the effects of porous roughness on turbulence, we have carried out direct numerical simulations using the lattice Boltzmann method. The simulated flows are fully developed turbulent flows in channels consisting of a solid smooth top wall and a porous bottom wall with transverse porous ribs whose heights are 10 % of the channel height. The considered ratios of the rib spacing to the rib height are $w/k\simeq 1$ and 9. The Kelvin-cell structure is applied to construct faithfully the porous media whose porosities are $\varphi \ge 0.79$ . Three kinds of porous media having different permeabilities are considered. The most permeable one has an approximately one order higher permeability than that of the least permeable one. The higher permeability case is designed to have a pore scale that is the same as the rib height so that it is the most permeable case for the rib roughness with the designed porosity. In the simulations, the bulk Reynolds number is set to $Re_b=5500$ , and the corresponding permeability Reynolds numbers are $Re_K=2.2\unicode{x2013}7.5$ . The simulated field data and the drag coefficient, which includes both the pressure drag by the ribs and the frictional drag over the porous wall, are analysed to understand the characteristics of the permeable roughness in terms of permeability. The decomposition of the drag coefficient into the integrated laminar, rib-drag, dispersion and turbulence parts elucidates the transition mechanism between the typical d-type to k-type roughness depending on $Re_K$ . By the double (time and space) averaged budget equations for the dispersion and Reynolds stresses, we explain how the energy generated by the roughness transfers to turbulence through dispersion resulting in the k-type characteristics. The nominal roughness sublayer thickness and the characteristic roughness height are introduced with the parameters obtained by fitting the velocity data to Best's and Nikuradse's logarithmic velocity formulae. Along with data in the literature, it is suggested that the ratio of the characteristic roughness height to the nominal roughness sublayer thickness becomes constant irrespective of the rib spacing in the full permeable-wall turbulence at $Re_K> 7$ .

Improved flow boiling characteristics in minichannel with open-cell porous ribs

Porous metal has been proven to exhibit significant heat transfer enhancement in minichannel. Focusing on fully utilizing the advantages of porous metal in heat transfer and reducing pressure penalty, a porous ribs minichannel (PRM) heat sink employing open-cell porous copper as rib was designed and fabricated. PRM heat sink has 19 minichannels that are 1 mm in width and 2 mm in depth. Commercial open-cell porous copper with a porosity of 0.85 and 120 PPI (Pore per Inch, PPI) was selected. Flow boiling experiments were carried out with ethanol as the working fluid. The flow patterns of two heat sinks were recorded with a high speed photograph system and comparative analyzed. High-frequency periodic motion of vapor core boundary in width direction of porous ribs minichannel was recorded. This is considered a fluctuation of density wave along the channel width direction. Cooperating with capillary force, movement of vapor core boundary induces mass transfer between adjacent channels, compensating for uneven flow distribution between channels. The results show that PRM heat sink has better heat transfer performances and flow stability with a slight increment in pressure drop compared with solid ribs minichannel (SRM) heat sink. The porous ribs minichannel heat sink provides a new way to relieve flow insatiability for heat exchangers and thermal management systems.

Analysis of performance stability under conditions of high & low humidity of polymer electrolyte fuel cells with interdigitated gas flow channels formed on a gas diffusion layer: An X-ray imaging and modeling study

Recently, a new concept was developed for the design of polymer electrolyte fuel cells, based on a flat, solid separator and a porous GDL with interdigitated gas-flow channels. This newly designed cell has demonstrated the ability to overcome the principal issues of the conventional design, in which the interdigitated flow channels are formed on the separator; the latter can experience low performance under high and low humidity conditions. In the present study, we have sought to reveal the mechanism of the performance stability improvement. The temperature and gas flow distributions are calculated by numerical simulation, and the water distribution is visualized by X-ray imaging. From these results, the porous ribs in the newly designed cell are found to play several important roles as follows: under conditions of excess water, the porous ribs help to alleviate water accumulation in the GDL by acting as a reservoir for excess water and also by increasing the temperature in the GDL; and, under conditions of water shortage, the porous ribs alleviate the dry-out of the GDL by withdrawing water from the reservoir and also by decreasing the rate of gas flow forced through the GDL.

Heat transfer and flow characteristics in symmetric and parallel wavy microchannel heat sinks with porous ribs

Based on the synergistic design concept, double-layered microchannel heat sinks with parallel and symmetric wavy porous fins are developed with the desire to simultaneously attain pressure drop reduction, heat transfer enhancement, and cooling uniformity improvement. Using a 3D fluid-solid conjugate model, the hydrodynamic and thermal details are numerically studied to compare two wavy configurations with the solid- and porous-fin designs. The results demonstrate that for wavy microchannel heat sinks, the porous-fin design can significantly enhance heat transfer performance and reduce pressure drop. The symmetric configuration yields a higher pressure drop reduction, whereas the parallel one provides a higher increment in thermal performance. As a result, using the porous design, two wavy configurations have nearly the same level of pressure drop penalty, but the parallel configuration contributes to higher thermal performance. The decreased flow rate of the channel due to the permeation of coolant fluids into porous ribs and the slip effect on the channel wall contribute to the pressure drop reduction. The permeation effect of coolant greatly restrains secondary flow characteristics induced by wavy walls which are responsible for the enhanced coolant mixing in conventional heat sinks. Consequently, the increased inlet flow velocity at a constant pumping power contributes to the improved thermal performance, namely the reduced thermal resistance and the increased Nusselt number. With a stronger coolant permeation owing to the jet-like impingement flow, the parallel configuration yields a slightly lower thermal performance compared to the symmetric configuration in the new design. Finally, the parametric analysis at a fixed pumping power is further carried out for optimizing the proposed design.

Thermal performance of double-layer porous-microchannel with phase change slurry

Combining porous media paved on sidewall and microencapsulated phase change material (MPCM) suspension as coolant is presented in double-layered microchannel heat sink (MCHS), in which more surface area of porous copper matrix with high thermal conductivity as well as greater temperature difference between the heating wall and working fluid during phase transition of MPCM will enhance heat transfer. The Forchheimer-Brinkman-Darcy model together with energy equation in local thermal equilibrium are employed to deal with fluid flow and heat transfer in porous ribs, and the equivalent heat capacity method is used to describe the phase change process of microcapsules under laminar flow. The influences of coolants, heat sink designs (including the thickness, height, porosity and pore size of porous medium as well as channel number) and working conditions (involving inlet velocity, heat flux and suspension concentration) on thermal and hydraulic characteristics are numerically investigated. The performance evaluation factor (PEF) is introduced to evaluate the comprehensive thermal–hydraulic performance of double-layered MCHS, and the effective performance evaluation factor (PEFeff) is used to compare the overall performance of porous-wall MCHS between conventional arrangements, different channel aspect ratio and channel height ratio configurations. The thickness and height of porous media paved on sidewall should be adjusted reasonably, especially for the arrangement of non-equal porous media, to obtain better overall performance. Different from the variable porosity configuration, the double-layered MCHS cannot rely on losing part of flow performance to obtain better thermal performance in smaller pore size mode, so a larger pore size can be selected to obtain better thermal–hydraulic performance according to actual working conditions. The PEF at larger flow velocity decreases due to higher pressure drop exceeding the benefit of more convection on heat transfer enhancement, in which case the phase change effect in porous-wall mode with MPCM suspension as coolant is suppressed and higher viscosity amplifies the effect of frictional resistance, resulting in a greater drop ratio of PEF than water. The greater PEF in porous-wall MCHS with suspension concentration increasing from 3% to 20% arises than that in non-porous mode, due to the larger surface area of copper matrix with high thermal conductivity and more MPCM particles available for phase transition. The larger aspect ratio configuration can achieve greater PEFeff rise and better overall performance by optimizing fluid velocity in the current mode due to greater thermal resistance drop and less pressure drop rise at lower flow rates, but the variable height ratio structure makes the comprehensive performance of porous-wall MCHS inferior to conventional arrangement due to larger pressure drop rise outweighing the benefits of heat transfer enhancement at the same total volume flux.

A porous-rib flow field for performance enhancement in proton exchange membrane fuel cells

Concentration loss under solid rib induced by oxygen starvation and water flooding in conventional solid-rib flow fields (sr-FFs) limits the cell performance. Herein, a porous-rib flow field (pr-FF) is proposed for proton exchange membrane fuel cells (PEMFCs) to improve the gas transport and water removal under the ribs. The multi-physical processes and cell performance are investigated via a three-dimensional multi-phase PEMFC model. Numerical results show that the limiting current density and peak power density for the pr-FF-based PEMFC are increased by 15% and 9% respectively, while the pressure drop is reduced by 38%. A higher oxygen concentration and a lower liquid saturation under the porous ribs are obtained, in addition to the improved uniformities of oxygen concentration and current density. The pr-FF-based PEMFC is suggested to operate under a fully humidified condition for yielding a higher performance, where the peak power density is increased by 14% corresponding to the increase in cathode inlet humidity from 0.5 to 1.0. More interesting, the porous-rib design can enhance the cell performance regardless of the porosity. The proposed pr-FF provides an alternative solution for performance enhancement of PEMFCs by enhancing under-rib mass transport without increasing pumping power.

Investigation of double-layered wavy microchannel heatsinks utilizing porous ribs with artificial neural networks

Microchannel heatsinks with porous media, wavy and multi-layered configurations introduce compact designs and high efficiencies in cooling applications. Current Computational Fluid Dynamics (CFD) based study bridges the concept by introducing a novel double layered microchannel heatsink with wavy up-down and porous ribs alongside each other. Since Artificial Neural Network (ANN) trained by CFD results reveals designs with better thermal and hydraulic performance with an insignificant computational time, ANN is employed for finding the best performers and effects of waviness on Nusselt number, pressure drop, and maximum temperature difference at the bottom of the microchannel for a Reynold number range of 100 < Re < 800 by defining a thermal efficiency factor. Even though the pressure drop increases in the wavy design, the thermal performance is affected positively. Nusselt number increases more than 55% for Re = 800. The thermal performance increases with the introduction of Dean vortices, especially at higher Re. The best design provides maximum bottom temperature difference of 1.2 °C, with a Thermal Efficiency Factor of 1.34. However, it is observed that better thermal efficiency factors are attained by using a wavy channel at the top layer and keeping the bottom layer straight for the final design.

Improved thermal–hydraulic performance of a microchannel with hierarchical honeycomb porous ribs

AbstractMicrochannel cooling technology has been proven to be extraordinarily efficient for the removal of heat flux in electronic devices. Here, we numerically investigate the thermal and hydraulic characteristics of microchannels with different shapes of ribs, namely, circular ribs, square ribs, regular ribs, and hierarchical honeycomb ribs. Those ribs are composed of solid and porous media. Results indicate that heat transfer can be promoted by adding different shapes of ribs. Especially, microchannels with hierarchical honeycomb ribs possess the greatest heat transfer capability. Pressure drop (Δp) and friction factor (f) display the largest values when microchannel ribs are hierarchical honeycomb and solid, whereas Δp of the microchannel with hierarchical honeycomb porous ribs dramatically decreases, which achieves an 81.1%–81.7% reduction. Results of j/f show that the microchannel with hierarchical honeycomb porous ribs presents the best comprehensive performance, which is ascribed to the superior heat transfer capability and the low Δp induced by the ultra‐hydrophobic effect.

Heat transfer and fluid flow analysis of microchannel heat sinks with periodic vertical porous ribs

In this study, a detailed numerical analysis of the fluid flow and heat transfer of a three-dimensional microchannel is performed to evaluate the effect of using periodic vertical porous and solid ribs with various geometrical shapes, including rectangular, elliptical, isosceles triangular, backward triangular and forward triangular on the walls of this microchannel. Darcy-Brinkman-Forchheimer equations are used to model transport through the porous medium. The results for microchannels with solid ribs and with porous ribs are compared to each other. It is found that at the lowest studied inlet velocity (uin=0.25m/s), for the rib heights of Hr=0.025mm,0.05mmand0.07mm, the average Nusselt number of the porous-rib microchannel is 2.16, 2.1 and 1.3 times greater than the solid case, respectively; and, 2.505, 3.62 and 4.01 times greater than the no-ribs microchannel, respectively. It is found that for the same increase in the Nusselt number, the porous ribs induces much less pressure drop compared to the solid cases. Accounting the thermal and hydraulic performance simultaneously, Figure of Merit (FOM) is calculated. The results show that porous ribs lead in much better FOMs compared to the solid ribs case. Effects of the height, porosity and Darcy number of the ribs are also investigated.

Optimization of Convective Heat Transfer in Microchannels Equipped by Porous Ribs

Optimization of Convective Heat Transfer in Microchannels Equipped by Porous Ribs

Investigation of Double-Layered Wavy Microchannel Heatsinks Utilizing Porous Ribs with Artificial Neural Networks

Investigation of Double-Layered Wavy Microchannel Heatsinks Utilizing Porous Ribs with Artificial Neural Networks

Evaluation of porous rib and flow pulsation on microchannel thermal performance using a novel thermal lattice Boltzmann method

In this study, a new thermal lattice Boltzmann model (TLBM) is developed to simulate conjugate heat transfer in a microchannel heat sink (MCHS) with porous ribs and pulsatile flow inlet. The examined parameters include the rib to channel height ratio (Hr*), Strouhal number (St), and Reynolds number (Re), which are varied from 0.25 to 0.75, 0 to 3.2, and 100 to 300, respectively. The proposed TLBM is based on the double distribution function framework and capable of addressing heat transfer in combined fluid and porous media conditions. The simulation data show that the flow pulsation inside MCHS with porous ribs not only improves the bulk mean temperatures but also eliminates the rib recirculation zones at some phases, which promotes both the local and overall heat transfer. For fixed Re, the overall Nusselt number (Nu‾/Nu0) and Fanning friction coefficient (f‾/f0) increase and then decrease with increasing St. The maximum values appear at St = 2. For fixed Re and St, the thermal performance factor (TPF) displays a single peak trend with Hr* and the optimal TPF of 2.2 occurs at Hr*=0.5. In addition, compared with the previous ribbed, baffled, and porous MCHSs, the proposed porous rib design under pulsating conditions reports the highest Nu‾/Nu0 of 13.4 in the range of f‾/f0≤200. Finally, the correlations of Nu‾/Nu0 and f‾/f0 with Re, St, and Hr* for the present MCHS are established for the first time, with average differences below 6.7% and 12.4%, respectively.

Thermohydraulic performance analysis of graded porous media microchannel with microencapsulated phase change material suspension

The phase change process in microcapsules is described using equivalent heat capacity method, and the Forchheimer-Brinkman-Darcy equation based on the volume-averaging method is employed to account for the fluid flow in porous ribs. The feasibility in the novel design is validated by analyzing the effects of porosity (ε) arrangement, pore size (dp) arrangement as well as geometric parameters of porous-wall microchannel heat sink (MCHS) on the thermal and hydraulic characteristics under different inlet velocity, heating flux and mass fraction of microencapsulated phase change material (MPCM) suspension. The thermal resistance of the porosity gradient in porous media is less than that of linear variation, which is due to the more uneven velocity distribution in porous and fluid regions. The lower thermal resistance and more uniform temperature distribution occur in the graded porous media microchannel with higher height ratio, as well as the smaller and larger porosity in porous media respectively near the bottom and top walls. The size of microchannel limits the size of dp, making the thermo-hydraulic characteristics of gradient-dp mode worse than that of gradient-ε under the same conditions, and the thermal resistance decreases as well as pressure drop increases with the augment thickness of porous media, so it is necessary to select the appropriate structural parameters and coolants according to the specific working conditions to obtain better comprehensive performance. The performance evaluation factor (PEF) is introduced to evaluate the overall thermohydraulic performance of MCHS under different heat sink designs, and the stepwise increasing porosity mode with porosity varying along the y direction has larger velocity difference between the upper and lower layers and more reasonable MPCM particle distribution under the larger mass fraction of suspension, so as to obtain the larger rise ratio of PEF, up to 61%.

Performance evaluation of porous gas channel ribs in a polymer electrolyte fuel cell

Commercialization of hydrogen fuel cells is scaling at an unprecedented rate. As a result, developers are focused on stack manufacturability and the associated challenges to scaling-up an assembly with thousands of individual parts that have strict dimensional tolerances. This paper first proposes a fuel cell architecture designed for continuous manufacturing that could result in all components of a stack being contained in a rolled good. This concept would move the reactant gas distribution channels into the gas diffusion substrate. The performance of the porous ribs that would result from this concept is compared with a conventional solid rib flow distributor. Performance and cell resistance are presented in a series of single factor experiments that investigate a wide PEMFC operating space. The results show that a porous rib cathode flow field is feasible, with performance benefits at near- and over-saturated operating conditions.

Analysis of entropy generation of ferrofluid flow in the microchannel with twisted porous ribs: The two-phase investigation with various porous layers

Heat sinks are always in the center of electronic cooling researchers' attention. Microchannels, due to their increased surface and better thermal performance than that of normal heat sinks were used in electronic cooling devices. In this numerical investigation, the first and second laws of the thermodynamic impact of twisted porous ribs on the microchannel were studied. The effects of the Reynolds number and volume fraction of nanoparticles were investigated. The range of Reynolds number was 250 to 1000. All types of the microchannel in this study consisted of a clear microchannel, a twisted porous ribbed microchannel with two and three layers of twisted porous ribs. The results show that by inserting porous ribs, the local cross-section decreases, and the local Reynolds number increases. Also, twisted porous ribs are the cause of nanofluid flow circulation and make the heat transfer coefficient greater. Increasing the porous layer from one layer to three layers has an increasing effect on the heat transfer coefficient, and the friction factor and increasing the φ is the reason for increasing the friction factor. Finally, the microchannel with triple layers of twisted porous ribs has better performance in total entropy generation, and by inserting twisted porous ribs, the entropy generation decreases.