Numerical modeling of microchannel reactors with gradient porous surfaces for hydrogen production based on fractal geometry

A microchannel reactor with a porous surface catalyst support has been applied to methanol steam reforming (MSR) for hydrogen production. The fluid flow, heat transfer, and hydrogen production efficiency of the microchannel reactor are significantly affected by the fabricated porous surface support, such as the pore sizes and their distributions. This paper presents a novel microchannel reactor with a gradient porous surface as the reaction substrate to enhance the performance of the microreactor for hydrogen production. Numerical modeling of the gradient porous surface is developed based on fractal geometry, and three different types of porous surfaces as the catalyst supports (two gradient porous surfaces and one uniform pore-size surface) are investigated. The fluid flow and heat transfer characteristics of these three types of microchannel reactors are studied numerically, and the results showed that the microreactor with a positive gradient pore sized surface exhibited relatively better overall performance. Experimental setups and tests were performed and the results validate that the microchannel reactor with a positive gradient porous surface can increase the heat transfer performance by up to 18% and can decrease the pressure drop by up to 8% when compared to a microreactor with a uniform pore sized surface. Hydrogen production experiments demonstrated that the microreactor with positive gradient pore sizes has the highest methanol conversion rate of 56.3%, and this rate is determined to be 6% and 9% higher than that of microreactors with reverse gradient porous surfaces and uniform pore sized surface, respectively.     

Multi-scale dimensionless prediction model of PEMFC sealing interface leakage rate based on fractal geometry

The performance of the sealing is closely related to the working efficiency and safety of PEMFCs and the interfacial leaking behavior is heavily influenced by the heterogeneous rough surfaces. And the BPPs and Gaskets of the fuel cell sealing structure are ultra-thin, which makes obtaining leakage rate by experiments difficult. In this paper, a highly efficient method to calculate the PEMFC sealing interface leakage rate is proposed based on the fractal geometry and numerical simulations. FEM and LBM methods are used to analyze the nonlinear contact and microscopic gas flow behaviors in the numerical simulation processes. Furthermore, a multi-scale dimensionless model of PEMFC interface leaking is established by numerical simulated results. The multi-scale dimensionless model can significantly reduce the negative influence of the single observation scale on interface leakage rate prediction accuracy and solve the problem of the inability to analyze interface leakage rates at different scales simultaneously. By comparison with the Roth's theory, the traditional model that only considers the small elastic deformation has a large deviation in the evaluation of the leakage rate of the rubber seal interface, while the multi-scale model can accurately predict the interface leakage rate under different PEMFC Gasket compression rates.     

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 a zeolite/water adsorption cooling system with non-constant condensing pressure

A new transient two-dimensional model with non-constant condensing pressure for a zeolite/water adsorption cooling cycle is proposed in this paper. This numerical model focuses on the heat and mass transfer behaviors in the adsorber and is solved by the control volume method. Due to the heat transfer limitation in the condenser, the simulated pressure during the isobaric generation phase of the cycle is not constant and will decrease with time. Compared with the model for constant condensing pressure, the cycle duration and cycled adsorbate for the base case are increased. Furthermore, the effect of mass flow rate of condenser cooling water on system performance is also investigated. It is found that both COP and SCP increase with an increase in the mass flow rate of cooling water in the condenser.     

Hydrogen absorption performance of a novel cylindrical MH reactor with combined loop-type finned tube and cooling jacket heat exchanger

A novel cylindrical metal hydride (MH) reactor with loop-type finned tube and jacket heat exchanger was proposed in this work. This MH reactor is expected to possess high performance due to the enhanced heat transfer, compact structure and good gas tightness. A three-dimensional multi-physical model for hydrogen absorption was presented to investigate the evolutions of temperature and concentration in the MH bed, as well as the mean reaction rate of hydrogen absorption process. The effects of different fin configurations on the performance of the proposed MH reactor were also examined. It was indicated that the evolution curve of the mean reaction rate for the whole hydrogen absorption process can be divided into two stages. The reaction rate in the first stage is mainly dependent on the initial conditions (i.e., temperature and gas pressure) of MH bed, whereas the second stage is mainly influenced by the heat dissipation from MH bed to cooling fluid. For the proposed MH reactor, the total charging time for reaching 90% hydrogen saturation can be decreased by 56.8% and 81.9% as compared with that for cylindrical MH reactor with finned double U-shape tube heat exchanger and cylindrical MH reactor with finned single-tube heat exchanger, respectively. Also, it was found that the interlaced layout design of inner and outer fins can improve the uniformity of the temperature distribution inside the MH bed as compared with the parallel layout configuration. Besides, it was showed that increasing the number of fins with keeping the total fin volume constant, the absorption performance of the reactor can be improved.     

Combined heat and mass transfer in natural convection flow from a vertical wavy surface in a power-law fluid saturated porous medium with thermal and mass stratification

This work studies the coupled heat and mass transfer by natural convection near a vertical wavy surface in a non-Newtonian fluid saturated porous medium with thermal and mass stratification. The surface of the vertical wavy plate is kept at constant wall temperature and concentration. A coordinate transformation is employed to transform the complex wavy surface to a smooth surface, and the obtained boundary layer equations are then solved by the cubic spline collocation method. Effects of thermal and concentration stratification parameters, Lewis number, buoyancy ratio, power-law index, and wavy geometry on the important heat and mass transfer characteristics are studied. Results show that an increase in the thermal and concentration stratification parameter decreases the buoyancy force and retards the flow, thus decreasing the heat and mass transfer rates between the fluid and the vertical wavy surface. It is shown that an increase in the power-law index, the thermal stratification parameter, or the concentration stratification parameter leads to a smaller fluctuation of the local Nusselt and Sherwood numbers with the streamwise coordinate. Moreover, the total heat transfer rate and the total mass transfer rate of vertical wavy surfaces are higher than those of the corresponding smooth surfaces.     

丁胞结构强化换热机理的场协同分析

采用数值方法计算了丁胞结构流道内对流换热过程,并运用场协同理论分析了丁胞结构强化换热的机理,分析了丁胞大小、深度以及Re等对换热过程的影响。结果发现,丁胞的前侧是换热弱化区,而后侧才是强化换热区,但总体表现为强化换热效果,在低Re条件下,Nu较普通流道高1．2～1．5倍,是一种较好的强化换热方式。     

Numerical investigation of H2 absorption in an adiabatic high-temperature metal hydride reactor based on thermochemical heat storage: MgH2 and Mg(OH)2 as reference materials

A two-dimensional mathematical model to predict the thermal performance of an adiabatic hydrogen storage system based on the combination of magnesium hydride and magnesium hydroxide materials has been developed. A simple geometry consisting of two coaxial cylinders filled with the hydrogen and thermochemical heat storage materials was considered. The main objective was to gain a better knowledge on the thermal interaction between the two storage media, and to determine the dependence of the hydrogen absorption time on the geometric characteristics of the reactor as well as the operation conditions and the thermophysical properties of the selected materials. The dimensions of the two compartments where the two materials are filled were chosen based on the results of a preliminary analytical study in order to compare the absorption times obtained analytically and numerically. The numerical results have shown that the hydrogen absorption process can be completed in a shorter interval of time than analytically as a result of the larger temperature gradient between the magnesium hydride and magnesium hydroxide beds. This was mainly due to variation of temperature in the thermochemical heat storage material during the more realistic dehydration reaction in the numerical solution. Larger temperature gradients, thus a faster hydrogen absorption process can also be achieved by increasing the hydrogen absorption pressure. Moreover, it was found that the increase of the thermal conductivity of the magnesium hydroxide material is crucial for a further improvement of the performance of the MgH₂–Mg(OH)₂ combination reactor.