Numerical modeling of microchannel reactor with porous surface microstructure based on fractal geometry

A microchannel reactor with porous surface for hydrogen production can enhance fluid flow and heat transfer characteristics. To improve the fluid flow and heat transfer characteristics of a microreactor with a porous surface, a numerical model is proposed based on fractal geometry. The porous surface in the microreactor is fabricated using a layered powder sintering and dissolution method with NaCl particles, in which two sizes of NaCl particles (180–280 μm and 280–450 μm) are utilized. For the construction of the porous surface, these two types of fabricated surfaces are measured and the fractal dimensions are characterized as 1.905 and 1.849, respectively. Subsequently, a numerical model based on fractal geometry for a microchannel reactor with porous surface is developed to study the fluid flow and heat transfer characteristics. This is followed by the microchannel reactor fabrication and experimental testing. Both model calculation and experimental results demonstrate that a microreactor with a porous surface can enhance the heat transfer performances compared with that with a non-porous surface, and that a microchannel reactor fabricated with larger NaCl particles (280–450 μm) has better heat transfer characteristics compared with a microreactor with small NaCl particles (180–280 μm). Thus, the developed numerical model based on fractal geometry can be used to accurately predict the fluid flow and heat transfer characteristics of the microreactor for hydrogen production.     

The use of inert porous media based reactors for hydrogen production

The present review paper examines the production of hydrogen in inert porous media based reformer by thermal partial oxidation. Here we consider, specifically, the rich combustion of hydrocarbon fuels and the conversion of H₂S to hydrogen. The different technologies to produce hydrogen beside the experimental and numerical work done in this field are presented. The effect of different operating conditions, such as the equivalence ratio, the mass flow rate and the reactant feed temperature are explained. Additionally, design parameters, including the reactor geometry and porous material specifications, are discussed.     

A numerical study of laminar convective heat transfer in microchannel with non-circular cross-section

Three-dimensional numerical simulations of the laminar flow and heat transfer of water in silicon microchannels with non-circular cross-sections (trapezoidal and triangular) were performed. The finite volume method was used to discretize the governing equations. Numerical results were compared with experimental data available in the literature, and good agreements were achieved. The effects of the geometric parameters of the microchannels were investigated, and the variations of Nusselt number with Reynolds number were discussed from the field synergy principle. The simulation results indicate that when the Reynolds numbers are less than 100, the synergy between velocity and temperature gradient is much better than the case with Reynolds number larger than 100. There is an abrupt change in the intersection angle between velocity and temperature gradient around Re=100. In the low Reynolds number region the Nusselt number is almost proportional to the Reynolds number, while in the high Reynolds number region, the increasing trend of Nusselt number with Reynolds number is much more mildly, which showed the applicability of the field synergy principle. In addition, for the cases studied the fully developed Nusselt number for the microchannels simulated increases with the increasing Reynolds number, rather than a constant.     

A modification of a Nusselt number correlation for forced convection in porous media

We report numerical simulations of forced convection heat transfer rates of a steady laminar flow in a two-dimensional model of porous media to elucidate the differences observed between the numerical predictions of Kuwahara et al. (2001) [Int. J Heat Mass Trans. 44, 1153–1159] and Gamrat et al. (2008) [Int. J Heat Mass Trans. 51, 853–864]. A modification in the correlation given by Kuwahara et al. (2001) is proposed to make the results of the three numerical studies comparable and in agreement with the experimental data.     

Minimization of hot spot in a microchannel reactor for steam reforming of methane with the stripe combustion catalyst layer

Hot spot formation is inevitable in a heat exchanger microchannel reactor used for steam reforming of methane because of the local imbalance between the generated and absorbed heat. A stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 130 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.68 and 2.51, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 7.64% less than that of the uniform stripe catalyst layer.     

Numerical modeling of methane pyrolysis in a bubble column of molten catalysts for clean hydrogen production

Methane pyrolysis using molten catalysts in a bubble column reactor (BCR) has recently been proposed to produce hydrogen with separable carbon particles as byproducts. In this study, a numerical model of the BCR of molten catalysts for methane pyrolysis was developed and validated using experimental data. Based on a non-isothermal 1-D simplification, continuous liquid and discrete bubble phases were considered by incorporating submodels for bubble behaviors, catalytic and homogeneous reactions, heat/mass transfer, and a submerged orifice for methane supply. The initial bubble diameter was predicted using the correlation derived from measurements. When applied to experiments with Ni₍₂₇₎Bi₍₇₃₎ and mixtures of KCl–MnCl₂, the model accurately reproduced the methane conversion at different temperatures and column heights. Furthermore, detailed information on the key phenomena was acquired, including the profiles of the bubble diameter, rise velocity, reaction rates, temperature, and gas composition. A sensitivity analysis confirmed that the uncertainties regarding the physical properties of molten catalysts had a negligible impact. A comparison of the performances of Ni₍₂₇₎Bi₍₇₃₎ and KCl₍₅₀₎MnCl₂₍₅₀₎ under the same reaction conditions revealed a favorable influence of the catalyst density on methane conversion because of the increased pressure. The proposed model would be useful in reactor optimization and scale-up with high hydrogen productivity.     

散热管内插入扰流丝强化传热的数值模拟

本文利用CFD软件Fluent对某种通过插入扰流丝强化传热的散热器进行了数值模拟。分别以水和P-32透平油为研究对象,计算得出了不同流速时无扰流丝和含扰流丝两种情况下散热管的传热效果。通过分析得出,此扰流丝能够通过打破速度边界层,形成湍流,增强换热。     

Numerical investigation on subcooled pool film boiling of liquid hydrogen in different gravities

A clear understanding of bubble dynamics and heat transfer characteristics of hydrogen boiling in microgravity is significant for achieving safe and high-efficiency utilization of liquid hydrogen in space. In the present paper, a numerical simulation model is developed to predict the subcooled pool film boiling for liquid hydrogen in different gravities. The computations are based on the volume of fluid method combined with Lee's phase change model. The results show that the bubble released from the wavy gas-liquid interface might grow to a larger size before departure with the decrease of gravity, and poor heat transfer performance is observed in reduced gravity. However, once the gravity level is low enough or the subcooling of liquid is sufficiently large, instead of bubble formation and release at the vapor-liquid interface, a thin gas film layer is almost observed and maintained in the surface of horizontal flat or wire heater.     

Computational fluid dynamics modeling of a solid oxide electrolyzer cell for hydrogen production

A 2D computational fluid dynamics (CFD) model was developed to study the performance of a planar solid oxide electrolyzer cell (SOEC) for hydrogen production. The governing equations for mass continuity, momentum conservation, energy conservation and species conservation were discretized with the finite volume method (FVM). The coupling of velocity and pressure was treated with the SIMPLEC (Semi-Implicit Method for Pressure Linked Equations – Consistent) algorithm. Simulations were performed to investigate the effects of operating/structural parameters on heat/mass transfer and the electric characteristics of a planar SOEC. It is found that the gas velocity at the cathode increases significantly along the main flow channel, as the increase in H₂ molar fraction decreases the density and viscosity of the gas mixture at the cathode. It is also found that increasing the inlet gas velocity can enhance the SOEC performance. Another important finding is that the electrode porosity has small effect on SOEC performance. The results of this paper provide better understanding on the coupled heat/mass transfer and electrochemical reaction phenomena in an SOEC. The model developed can serve as a useful tool for SOEC design optimization.     

Experimental studies and modeling of advanced alkaline water electrolyser with porous nickel electrodes for hydrogen production

The commercial hydrogen production by water electrolysis is limited by the high cost of electricity. The production cost can be minimized, if the cell module is operated with the minimum voltage at maximum current density. In the present study, porous nickel electrodes were developed indigenously on an engineering scale and used in an advanced zero gap filter press type bipolar electrolyser to minimize the cell voltage. As the cell voltage–current density characteristic of the cell module is unique feature of its design and the operating parameters, the polarization experiments were carried out using this cell module and the cell voltage–current density characteristics were generated at different operating temperatures. Further, the system is modelled for its electrochemical performance and the parameters accounting for different losses such as Ohmic and activation over potential, were estimated at different temperatures. These different parameters were compared with the data existing in literature and based on the analysis, the present cell module is found to be superior to the existing commercial electrolyzers in terms of energy efficiency.     

Height effect on heat transfer characteristics of sintered porous blocks with a confined slot jet

This work numerically investigates the heat transfer of a sintered porous block under a confined slot air jet. The width of the jet nozzle (W) is 5 mm; the ratio of the porous block length to the jet nozzle width (L/W) is 12, and the Prandtl number (Pr) is 0.7. Variable parameters are the ratio of the porous block height to the jet nozzle width (H/W) and the Reynolds number (Re). The findings reveal that the cooling performance with the sintered porous block was better than that with an aluminum foam block, and was at least 5.8 times as large as that without it. The Nusselt number increased as the H/W fell. The effect of Reynolds number on the heat transfer was negligible at Re ≤ 1000 but considerable at Re > 1000.     

Numerical modeling of a bayonet heat exchanger-based reactor for sulfuric acid decomposition in thermochemical hydrogen production processes

Sulfur-based thermochemical hydrogen production cycles represent one of the most appealing options to produce hydrogen from water on a large scale. The Hybrid Sulfur is one of the most advanced thermochemical cycles. The high temperature section of the process, common to all sulfur-based cycles, operates the sulfuric acid thermal decomposition reaction at temperatures on the order of 800 °C. The paper shows and discusses the modeling results obtained for a bayonet heat exchanger based high temperature reactor that decomposes the sulfur compounds into sulfur dioxide and oxygen. A detailed transport phenomena model, including suitable decomposition kinetics, has been set up using a finite volume numerical approach. A preliminary configuration of the reactor, established based on process simulation results and on the initial reactor prototype developed at Sandia National Laboratory, has been examined and simulated. Results, obtained for a reactor driven by thermal power provided by helium flow, demonstrate the effective decomposition performance at maximum temperatures on the order of 800 °C and pressures of 14 bar. For a laminar flow configuration a sulfur dioxide production yield of about 28 wt% (with sulfur trioxide reduction from 69 wt% to approximately 33 wt%) has been achieved, representing decomposition rates practically equal to the corresponding equilibrium values. Limited pressure drops of approximately 2500 Pa have also been achieved in the sulfur mixture region.     

微界面扩散过程对二元合金凝固过程数值模拟的影响

针对反向凝固工艺实验研究的传热传质现象，进行了数值模拟，讨论分析了微界面质扩散过程对二元合金凝固过程数值模拟的影响。结合实验数据，认为微观偏析模型的选择对新生相生长的影响不可忽略。     

Effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat

This paper reports the effects of particle sizes on methanol steam reforming for hydrogen production in a reactor heated by waste heat. The unsteady model was set up, which has been applied to investigate the effects of particle sizes (1.77 mm–14.60 mm) on particle temperature, heat transfer quantity, overall coefficient of heat-transfer, etc. The heat transfer performance of waste heat recovery heat exchanger is improved when the particle size increases, which is conducive to increase hydrogen production. The particle temperature change rate, the specific enthalpy change rate, the moving velocity of the maximum heat release rate particle, the contribution rate of solid phases, the heat release rate and the overall coefficient of heat-transfer increase, but the effective time of heat transfer decreases. When the particle size increases from 1.77 mm to 14.60 mm, the solid phase average contribution rate increases from 89.43% to 94.03%, the overall coefficient of heat-transfer increases from 1.39 W m⁻² K⁻¹ to 13.41 W m⁻² K⁻¹, the heat release rate increases from 48.9% to 99.9% and the effective time of heat transfer reduces from 48 h to 6.7 h.     

Numerical study of flow distribution uniformity for the optimization of gradient porosity configuration of porous copper fiber sintered felt for hydrogen production through methanol steam reforming micro-reactor

A macroscopic numerical method is proposed to study the flow distribution uniformity of a novel porous copper fiber sintered felt (PCFSF), which has gradient porosities and was developed as the methanol steam reforming micro-reactor catalyst support for hydrogen production for fuel cell applications. The macroscopic porous media developed by the ANSYS/FLUENT software is used to represent the PCFSF. Our results indicate that the gradient porosity can reshape the flow distribution of PCFSFs greatly, thus producing significant influence on their performance. It is further revealed that, for a PCFSF with a determined gradient porosity configuration but different reactant feed directions, the velocity uniformity can be used as a quantitative criterion to evaluate the performance of hydrogen production. Furthermore, new gradient PCFSFs are produced according to the flow distribution of original gradient PCFSFs. The preliminary experimental results of the new gradient PCFSFs of 0.8-0.9-0.7 and 0.7-0.9-0.8 exhibit better methanol conversion and H₂ flow rate. This indicates that the numerical method can be used for the optimization of PCFSFs' gradient porosity configuration, which consists of the shape and position of the interfaces between different porosity portions, the number of interfaces and the porosity distribution in different portions.     

Feasibility of preparing additive manufactured porous stainless steel felts with mathematical micro pore structure as novel catalyst support for hydrogen production via methanol steam reforming

In this paper, an additive manufacturing prepared porous stainless steel felt (AM-PSSF) is proposed as a novel catalyst support for hydrogen production via methanol steam reforming (MSR). In the method, 316 L stainless steel powder with diameter of 15–63 μm is processed by the additive manufacturing technology of selective laser melting (SLM). To accomplish the preparation, the reforming chamber where the AM-PSSF is embedded is firstly divided into an all-hexahedron mesh. Then, the triply periodic minimal surface (TPMS) unit with mathematical form, high interconnectivity and large specific surface area is mapped into the hexahedrons based on shape function, forming the fully connected three-dimensional (3D) micro pore structure of the AM-PSSF. By correlating the mathematical parameter and the porosity of the TPMS unit, and taking into account the SLM process, the porosity of the AM-PSSF is well controlled. Based on the designed 3D pore structure model, the AM-PSSF is produced using standard SLM process. The application of the AM-PSSF as catalyst support for hydrogen production through MSR indicates that: 1) both the naked and catalyst-coated AM-PSSF have the characteristics of high porosity, large specific surface area and high connectivity; 2) the MSR hydrogen production performance of the AM-PSSF is better than that of the commercial stainless steel fiber sintered felt. The feasibility of AM-PSSF as catalyst support for MSR hydrogen production may pave a better way to balance different requirements for catalyst support, thanks to the excellent controllability provided by AM on both the external shape and the internal pore structure, and to the produced rough surface morphology that benefits the catalyst adhesion strength. In addition, catalyst support with pore structures that are more accommodated with the flow field and the reaction rate of MSR reaction may be prepared in future, since the entire catalyst support structure, from macro scale to micro scale, is under control.     

Assessment methodology of promising porous materials for near ambient temperature hydrogen storage applications

With the rapid increasing of the available number of novel porous materials, a straightforward and low-cost testing methodology to assess those suitable for near ambient temperature hydrogen storage applications is needed. In this work, we developed a new assessment methodology to quickly identify those porous materials potentially suitable for near ambient temperature hydrogen storage applications. We introduced the usable capacity map showing why the absolute adsorption capacity at the temperature of 77 K is not a good indicator to compare the material's storage performance. In fact, some porous material that shows low usable capacity at 77 K appear to be better adsorbent at a higher temperature. Moreover, we demonstrated that using quick cyclic adsorption isotherm or TDS is possible to easily individuate those materials that are the most suitable for near ambient temperature applications. Therefore, as a general result, we showed that among the three commercial activated carbon, used here as case study, the one with the higher content of ultramicroporosity is the most promising because the optimum operating temperature shifts towards ambient temperature.