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By meticulously considering the hydrogen evolution reaction (HER) on the nickel foam cathode with both uniform and gradient pore diameters, a multi-field three-dimensional model is developed utilizing the open-source computational fluid dynamics (CFD) program OpenFOAM. Leveraging this simulation, we investigate the impacts of different gradient orientations and thickness configurations on critical processes including mass transfer and ion transport. The findings indicate that at a thickness ratio of 1:1 in the positive gradient arrangement, the pressure drop and hydrogen volume fraction are reduced by 10.42% and 18.66% compared to the negative gradient, respectively. However, the negative gradient arrangement with a thickness ratio exceeding 4:1 exhibits superior performance with specific external flow. This work reveals the critical roles of gradient electrodes on HER processes and provides important design principles of the electrode microstructure to promote further developments of alkaline water splitting systems to meet the requirements of their practical applications. [Display omitted] • A multi-field simulation model for hydrogen evolution reaction is constructed. • Electrodes better in gas exclusion and ion transport have the lower overpotential. • Electrodes with gradient porosity schemes foster mass and ion transport. [ABSTRACT FROM AUTHOR]

This paper investigates heat transfer performance and bubble behavior in gradient metal foam in saturated pure deionized water and n -heptanol solutions under pool boiling conditions. The gradient metal foam consists of a 40 PPI nickel foam layer with 4 mm thickness and a 10 PPI copper foam layer with 4 mm thickness; the foam porosity is 0.98. The experimental results show that n -heptanol increases the wettability of the copper surface and adversely affects the boiling heat transfer. At low heat flux, small departure bubbles attach onto the metal skeleton and then decrease in size. As heat flux increases, two common phenomena occur in the gradient foam: bubbles escape from the metal skeletons without cracking and bubbles separate into two smaller bubbles. When heat flux increases to 1 × 10 5 W m −2 , two neighboring bubbles move upwards and are then sucked into the metal foam due to coalescence. [ABSTRACT FROM AUTHOR]

A numerical study on the solid–liquid phase change in open-cell metal foams is carried out by the phase field method originated from the Ginzburg–Landau theory. The Brinkman–Forchheimer extended Darcy equation and the local thermal non-equilibrium model are adopted to take natural convection and the heat transfer between metal foams and phase change materials (PCMs) into consideration, respectively. The result proves that the phase field model is reliable and effective in modeling metal foam enhanced phase change heat transfer in thermal energy storage. The effects of key parameters, such as Rayleigh number, porosity and pore density, on the melting and solidification process are investigated and it is found that they have great influence on the solid–liquid phase change. The phase field, flow field and temperature distributions in the melting and solidification process are obtained. Furthermore, kinetic undercooling in the solidification process is studied. It is concluded that heat conduction through the ligament of metal foams plays a dominant role in the solid–liquid phase change and kinetic undercooling effect is weakened as kinetic coefficient decreases. [ABSTRACT FROM AUTHOR]

Pool boiling heat transfer of metal foams with gradient pore densities has been investigated at atmospheric pressure in saturated deionized water. The gradient foams are made of two foam layers. The foam materials are copper and nickel. Pore densities of foam layers are from 5 PPI to 100 PPI, while the porosity remains the fixed value of 0.98. A parametric study is performed by varying the foam pore size and thickness. The effects of alumina nanoparticles and surfactant (SDS) on pool boiling heat transfer of gradient metal foams are also investigated in the present study. Images of foam fiber deposited by nanoparticles are captured by SEM. Compared to single-layer foams, gradient metal foams significantly enhance pool boiling heat transfer and the enhancement is heavily dependent on foam thickness and pore density gradient. The effects of SDS and nanoparticles on pool boiling heat transfer of gradient foams are dependent on concentrations. [ABSTRACT FROM AUTHOR]

Highlights: [•] V-shaped groove effect on metal foam pool boiling heat transfer is investigated experimentally. [•] Groove effect depends on its width, number, distribution, foam material and pore density. [•] The CHF could be delayed by the V-shaped grooved metal foams. [Copyright &y& Elsevier]

Abstract: Metal Foam-enhanced Cascaded Thermal Energy Storage (MF-CTES) has been proposed to solve the problem of poor heat transfer during heat exchange process, which is caused by unavoidable decrease of temperature differences. This paper conducts a theoretical study examining the overall thermal performance of Single-stage Thermal Energy Storage (STES), Cascaded Thermal Energy Storage (CTES) and MF-CTES, with both heat exchange rate and exergy efficiency being considered. The main findings are: heat exchange rate of STES is improved by CTES (up to 30%), and is further improved by MF-CTES (by 2–7 times); exergy efficiency of STES cannot be significantly improved by CTES (−15% to +30%), nor by MF-CTES; exergy transfer rate of STES is increased by CTES (up to 23%), and is further increased by MF-CTES (by 2–7 times). [Copyright &y& Elsevier]

Abstract: Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches. [Copyright &y& Elsevier]

Abstract: The effects of metal foams on heat transfer enhancement in Phase Change Materials (PCMs) are investigated. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, in which the coupled heat conduction and natural convection are considered at phase transition and liquid zones. The numerical results are validated by experimental data. The main findings of the investigation are that heat conduction rate is increased significantly by using metal foams, due to their high thermal conductivities, and that natural convection is suppressed owing to the large flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance is improved when metal foams are embedded into PCM; this implies that the enhancement of heat conduction offsets or exceeds the natural convection loss. The results indicate that for different metal foam samples, heat transfer rate can be further increased by using metal foams with smaller porosities and bigger pore densities. [Copyright &y& Elsevier]

Abstract: In this paper, the feasibility of using porous materials such as metal foams and expanded graphite to enhance the heat transfer capability of PCMs in high temperature thermal energy storage system is experimentally investigated. Experimental results showed that the heat transfer rate can be enhanced through addition of the porous materials by 2.5times compared to that of pure NaNO3 in the heating process from 250–300°C. However the heat transfer rate could be reduced by half in the liquid region since the natural convection can be severely suppressed by the porous structures. In an attempt to further reveal the natural convection effect in PCMs embedded in porous materials, two heating methods (boundary conditions) are examined. Finally the issue of corrosion process is briefly discussed. [Copyright &y& Elsevier]

Film condensation on vertical plate embedded in metal foams was numerically investigated based on some modification for Nusselt theory. Related numerical model was established by introducing Brinkman-Darcy model. Advection and inertial force in the condensate film were considered, from which the non-linear effect of cross-sectional temperature distribution on condensation heat transfer was also involved. Condensation characteristics were discussed and compared with those on smooth surface. Effects of key parameters on condensate film thickness were examined. Furthermore, with either the increase in porosity or the decrease in pore density, the condensate layer becomes thinner. [ABSTRACT FROM AUTHOR]

Abstract: Latent heat storage (LHS) can theoretically provide large heat storage density and significantly reduce the storage material volume by using the material’s fusion heat, Δh m . Phase change materials (PCMs) commonly suffer from low thermal conductivities, being around 0.4Wm−1 K−1 for inorganic salts, which prolong the charging and discharging period. The problem of low thermal conductivity is a major issue that needs to be addressed for high temperature thermal energy storage systems. Since porous materials have high thermal conductivities and high surface areas, they can be used to form composites with PCMs to significantly enhance heat transfer. In this paper, the feasibility of using metal foams and expanded graphite to enhance the heat transfer capability of PCMs in high temperature thermal energy storage systems is investigated. The results show that heat transfer can be significantly enhanced by both metal foams and expanded graphite, thereby reducing the charging and discharging period. Furthermore, the overall performance of metal foams is superior to that of expanded graphite. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the experimental investigation on the solid/liquid phase change (melting and solidification) processes have been carried out. Paraffin wax RT58 is used as phase change material (PCM), in which metal foams are embedded to enhance the heat transfer. During the melting process, the test samples are electrically heated on the bottom surface with a constant heat flux. The PCM with metal foams has been heated from the solid state to the pure liquid phase. The temperature differences between the heated wall and PCM have been analysed to examine the effects of heat flux and metal foam structure (pore size and relative density). Compared to the results of the pure PCM sample, the effect of metal foam on solid/liquid phase change heat transfer is very significant, particularly at the solid zone of PCMs. When the PCM starts melting, natural convection can improve the heat transfer performance, thereby reducing the temperature difference between the wall and PCM. The addition of metal foam can increase the overall heat transfer rate by 3–10 times (depending on the metal foam structures and materials) during the melting process (two-phase zone) and the pure liquid zone. The tests for investigating the solidification process under different cooling conditions (e.g. natural convection and forced convection) have been carried out. The results show that the use of metal foams can make the sample solidified much faster than pure PCM samples, evidenced by the solidification time being reduced by more than half. In addition, a two-dimensional numerical analysis has been carried out for heat transfer enhancement in PCMs by using metal foams, and the prediction results agree reasonably well with the experimental data. [Copyright &y& Elsevier]

Abstract: An analytical model is developed to characterize the radiative transport process in highly porous, open-celled metal foams having idealized cellular morphologies in terms of fundamental radiative parameters such as emissivity, reflectivity and configuration factors. In comparison with the conventional two-flux approach or the diffusion approximation utilizing the Rosseland mean coefficient, the present model is explicit and yet relatively simple. Overall, the predicted effective radiative conductivity as a function of pore size and relative density (defined as the ratio of foam density to solid density) agrees well with that measured using a guarded-hot-plate apparatus for steel alloy foams. A systematic parametric study is subsequently carried out. The contribution of reflectance to thermal radiation is found to be significant, up to 50%, but the effect of temperature gradient is relatively small. The equivalent radiative conductivity increases linearly with increasing cell size for a fixed relative density, whilst for a given cell size the variation of relative density only has a small effect on radiation due to the mixed effects of increased emission and extinction. [Copyright &y& Elsevier]

This paper presents results from experimental measurements on radiative transfer in FeCrAlY (a steel based high temperature alloy) foams having high porosity (95%) and different cell sizes, manufactured at low cost from the sintering route. The spectral transmittance and reflectance are measured at different infrared wavelengths ranging from 2.5 to 50 μm, which are subsequently used to determine the extinction coefficient and foam emissivity. The results show that the spectral quantities are strongly dependent on the wavelength, particularly in the short wavelength regime (<25 μm). Whilst the extinction coefficient decreases with increasing cell size, the effect of cell size on foam reflectance is not significant. When the temperature is increased, the total extinction coefficient increases but the total reflectance decreases. An analytical model based on geometric optics laws, diffraction theory and metal foam morphology is developed to predict the radiative transfer, with cell size (or cell ligament diameter) and porosity identified as the two key parameters that dictate the foam radiative properties. Close agreement between the predicted effective foam conductivity due to radiation alone and that measured is observed. At fixed porosity, the radiative conductivity of the metal foam increases with increasing cell size and temperature. [Copyright &y& Elsevier]

Phase change material (PCM) as highly efficient temperature control material has been used in terrestrial photovoltaic-thermoelectric (PV-TE) coupling system to achieve higher power generation efficiency. However, the mass increase results from addition of PCM limits its application in space system. In present paper, a two-dimensional model for PV-PCM-TE coupling system was developed to investigate the energy transfer and conversion performance under space conditions. Firstly, the suitable PCM was selected, both the pure PCM and its metal foam composite PCM (MFCPCM) are used. Then, performance comparison for PV-TE, PV-PCM-TE and PV-MFCPCM-TE systems were performed. The results show PCM can significantly enhance the total efficiency, but causes power density dramatically decreasing. The average total efficiencies for PV-TE, PV-PCM-TE and PV-MFCPCM-TE are 29.50%, 30.50% and 30.61%, and the corresponding average power density are 30.34 W/kg, 22.39 W/kg and 21.20 W/kg, respectively. After that, an orthogonal experiment was designed to reveal the effects of structure parameters on power generation efficiency and power density of PV-MFCPCM-TE system. The optimum geometric parameters to achieve the best power generation efficiency and power density were derived. The results show that after optimization, the average total efficiency and power density of PV-MFCPCM-TE are 30.83% and 31.35 W/kg, both of them are higher than those of the traditional PV-TE system (29.50% and 30.34 W/kg). The present work is beneficial for the application of PV-PCM-TE system in space conditions. [ABSTRACT FROM AUTHOR]

As a new-type extending surface, metal foam owns great potential in next generation heat transfer technologies. Convective heat transfer performance in metal foams is numerically investigated based on the local thermal equilibrium (LTE) model and the local thermal non-equilibrium (LTNE) model. The solid–fluid temperature difference and relative deviation are put forward for quantifying LTNE effect. The effects of basic parameters on heat transfer are analysed in depth and the LTNE conditions in metal-foam tube for efficient heat exchangers are summarized. It is indicated that the relative deviation is a more suitable criterion for LTNE effect in metal foam than the solid–fluid temperature difference. The LTNE effect in metal foam is conspicuous for low porosity, large fluid–solid thermal conductivity difference, small duct size, low pore density, and low Reynolds number. Measures lowering proportion of local convective thermal resistance in total thermal resistance, or the ratio of thermal resistance of solid to that of fluid can weaken LTNE effect in metal foam. There is no necessary relationship between thermal performance of metal-foam heat exchangers and corresponding LTNE effect. Clarifying LTNE conditions in porous foams can lay a foundation for the demarcating criterion of LTE/LTNE models. This can also guide quick and accurate thermal design and verification of metal-foam heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract: A phase field model is for the first time employed to solve the phase change problem in a metal foam-embedded Latent Thermal Energy Storage (LTES) system. A phase field model deals with free boundary problems without tracing their positions, and therefore provides potentials of being extended to consider more complicated mechanisms: multi-dimension and volume change. The coupled heat transfer between PCMs and metal foams is solved based on the non-equilibrium heat transfer theory. Two phase fields are introduced to deal with phase change and volume change. Comparisons are made between the model predictions and the experimental data available in existing literatures. Two dimensionless groups of the material and structure parameters are identified to control the effectiveness of the system. An Effectiveness Map is also produced to distinguish the conditions under which incorporating metal foam into the PCMs is sensitive, lowly sensitive or irrelevant. The map provides a useful tool to guide the metal selection and structure design of the metal foams when enhancing the heat transfer of PCMs. Finally case studies are also carried out, in which the minimum thermal conductivity of copper/aluminium foams and the maximum k int can be determined by referring to the obtained Effectiveness Map. [Copyright &y& Elsevier]

Abstract: Pool boiling heat transfer on horizontal metallic foam surface with crossing and single-directional V-shaped groove was experimentally investigated at atmospheric pressure using deionized saturated water as the working liquid. Boiling patterns were captured using a high speed camera. The porosity of studied copper foam was fixed as 0.95. Thirty test samples categorized into three groups were studied. The influences of crossing V-shaped groove array configuration, single-directional groove width, and number on the heat transfer performance were examined respectively for the three groups in turn. The boiling process was governed by combinational interaction of three major factors of bubble escaping resistance, heat transfer surface area, and capillary force. The exhibited heat transfer performance of the metallic foam depended on whichever of the above factors dominated the boiling progress. Almost all V-shaped grooves could enhance the pool boiling heat transfer compared with foam surface without grooves. The optimal groove array configuration corresponding to highest heat transfer coefficient was connected to its groove volume ratio which was the very transition point for the behavior of the dominant factor. The optimal volume ratio for crossing groove array was 7.76% at pore density of 100PPI and 5.84% at pore density of 130PPI. The effect of groove number was heavily dependent on foam thickness. The optimal groove width of 2mm was found for the three groove number values (1, 2, and 3). The boiling characteristics were illustrated with boiling patterns obtained from the high speed camera. The present study could aid enhanced boiling surface design of sintered porous surface. [Copyright &y& Elsevier]

Abstract: The current study investigated the saturated pool boiling heat transfer of deionized water with added surfactant on a horizontal metallic foam surface with V-shape grooves under atmospheric pressure. The influences of the groove configurations, sodium dodecyl sulfate (SDS) concentration, foam thickness, and thermal conductivity of foam material on heat transfer performance and bubble growth patterns were studied. SDS with concentrations of 100, 400, and 800ppm was used as the surfactant. The foam porosity was fixed at 0.95. V-shape grooves with various widths and groove number were manufactured in the foam samples, with three pore densities of 20, 100, and 130PPI. Results showed that the effects of the groove configuration, SDS concentration, and thickness on boiling heat transfer are heavily dependent on foam pore density. The counter-flow between the released bubbles and sucked liquid plays a significant role in heat transfer performance. The existence of sufficient grooves delays the critical heat flux (CHF), whereas SDS can achieve CHF earlier. [Copyright &y& Elsevier]

Abstract: The two equation numerical model has been applied for parallel flow double-pipe heat exchanger filled with open cell metal foams. The model fully considered solid–fluid conjugated heat transfer process coupling heat conduction and convection in open cell metal foam solid matrix, interface wall and fluid in both inner and annular space in heat exchanger. The non-Darcy effect and the wall thickness are also taken into account. The interface wall heat flux distribution along the axial direction is predicted. The numerical model is firstly verified and then the influences of solid heat conductivity, metal foam porosity, pore density, relative heat conductivity and inner tube radius of the heat exchanger on dimensionless temperature distribution and heat transfer performance of heat exchanger are numerically studied. It is revealed that the proposed numerical model can effectively display the real physical heat transfer process in the double pipe heat exchanger. It is expected to provide useful information for the design of metal foam filled heat exchanger. [ABSTRACT FROM AUTHOR]

[Display omitted] • Independent thermal management based on thermochemical heat storage is proven. • The dehydration reaction kinetic model with pressure term is established. • The applicability of analytical models to high hydrogen permeability is discussed. • Numerical models including heat transfer, gas flow, and reaction are developed. • Reaction characteristics of low hydrogen permeability are revealed. Metal hydride is a promising alternative for hydrogen storage. A novel metal hydride reactor coupled with thermochemical heat storage material is proposed to reduce the energy loss. The hydrogen charging and discharging processes are numerically and analytically investigated in this paper. Hydrogen is stored in a compacted metal hydride disk, so the reaction characteristics under low permeability are studied. The dehydration kinetic equation has been re-established to determine the equilibrium temperature at a corresponding pressure. The reactor takes 139 min for the hydrogen charging process and takes 512.8 min for the hydrogen discharging process. The reaction characteristics are found to be affected by heat transfer and gas pressure at the permeability of 5.9 × 1 0 - 16 m 2. The analytical model is found to be only applicable to the prediction of reaction time with high permeability of hydrogen in metal hydride. The thermal management of thermochemical heat storage materials has been proven to be feasible. By increasing the hydration equilibrium temperature from 321.8 °C to 350 °C, the reaction time can be decreased to 43.1 %. By decreasing the dehydration equilibrium temperature from 300 °C to 270 °C, the reaction time can be decreased to 84.5 %. To enhance the heat transfer, meal foam is added to the thermochemical heat storage material. Adding metal foam can effectively shorten the reaction time, but will reduce the hydrogen production. A metal foam with a porosity of 0.96 will reduce the production of hydrogen by 3 %. [ABSTRACT FROM AUTHOR]

The flow boiling heat transfer mechanisms inside open-cell metal foam are investigated by Lattice Boltzmann method. The 2D reconstructed model of open-cell metal foam is proposed based on the scan image and the thermal responses of metal foam are considered. The effects of Reynolds number, porosity and Rayleigh number on bubble motion and heat transfer performance are revealed. The results show that the dominant force shifts from buoyant force to shear force with increasing Re number, thus the "sweeping off" effect is further intensified by the accelerating sliding bubbles. The superposition of enhanced boiling heat transfer and thermal conduction are responsible for the wall superheat reduction at the central heating areas. Bubble departure diameter decreases with increasing Reynolds number due to the high bubble sliding velocity. The failure of "sweeping off" effect results in the formation of vapor film which worsens the flow boiling heat transfer performance of metal foam with low porosity (ε = 0.94). The convection heat transfer is depressed by bubble motion confinement inside the metal foam at low Rayleigh number. [ABSTRACT FROM AUTHOR]