Search

Installing blocks in cathode flow field can effectively enhance the transfer of oxygen from channel to the reaction sites of catalyst layer, thus boosting the performance of the fuel cell. In this work, an optimization methodology combined with genetic algorithm and three-dimensional fuel cell modeling is developed to optimize the design of partially blocked channel for a proton exchange membrane fuel cell (PEMFC) with parallel flow field. In the optimization, the heights of the blocks are assumed to be linearly increased and two parameters (i.e., height of the first block and the height increase between adjacent blocks) are considered. The impact of the optimized design of the blocked channel on cell performance is analyzed, and the effects of the optimized blocked channel designs with increasing-height and uniform-height block height distributions were also compared in detail. With this optimization methodology, the optimal height distribution of the blocks in the channel can be obtained for various block numbers. With varying the block numbers, the cell voltage and net cell power are firstly improved until the maximal values reached and then lowered. The maximal net cell power is reached for the block number of 16. As compared with the flow channel without adding blocks, the net power of the PEMFC can be enhanced by about 10.9%. For pressure drop behavior, with the optimized block height distribution, the total pressure drop in cathode flow field can be maintained in similar level with varying block numbers from 4 to 20. Considering both the net power and pressure drop, the optimized blocked channels with adding 8 to 16 blocks are recommended in this study. Besides, it is indicated that the performance of the optimized block design with increasing-height is higher than that of the optimized block design with uniform-height.

The applications of vibration and oscillation in heat transfer process have attracted much attention for their positive impacts on heat transfer enhancement. In this research, the flow behaviors and heat transfer in stationary and periodically cosine vibrating tubes were numerically investigated in detail. The effects of wall vibration with the amplitude ranging from 1 to 5 mm and the low frequency ranging from 1 to 10 Hz on the fluid flow resistance and heat transfer enhancement were studied for various Reynolds numbers from 5971 to 13933. The numerical results demonstrated that the heat transfer performance in the vibrating tube could be significantly enhanced by the wall vibration. The averaged Nusselt numbers with the periodical wall vibration can be as high as 2.854 times of that for static tube. However, the friction factor f of fluid flow with wall vibration is also increased by 83.3% as compared with the case for static tube. By completely evaluating j/f1/3-factor, all vibrating tubes exhibited better overall heat transfer efficiency than that of static tube. The j/f1/3-factor is increased with the increase of vibration amplitude and frequency, and the maximum enhancement ratio of 176.9% can be achieved in this study. Besides, a correlation of the heat transfer enhancement ratio for vibrating tube was also derived based on the simulation results with a maximum error of 14.77%.

This paper presents a 3D macro-segment network model for a vanadium redox flow battery with serpentine flow field. The proposed network model is coupled of electrolyte flow module, species transfer module, charge transfer module. In flow resistance network module, the characteristics of electrolyte flow in the serpentine flow channel and under-rib convection in the porous electrode are all considered. In addition, the electrode intrusion and the electrode non-uniform structure caused by compression are taken into account. In species transfer network module, the convection, diffusion and migration between adjacent segments and reversible electrochemical reactions and self-discharge reactions inside the segment are all analyzed in the ion conservation equation. In charge transfer network module, each battery segment, which is composed of the channel, electrode and membrane segment in series, is connected in parallel and has the same output voltage. In this paper, the battery performance including charge-discharge voltages and cell pressure drop under different electrode compression ratios are validated. In the following, the battery performance under single and multiple charge-discharge cycle are investigated using the proposed network model. Besides, the field distribution of key parameters in terms of velocity, pressure, ions concentration and current density are analyzed and validated with finite element method model data. The proposed 3D macro-segment network model is not only able to effectively consider the distribution difference inside the battery caused by the flow field, but also capable of reducing the computational resources, which renders the network model is suitable for the fast prediction of battery performance.

In this study, asymmetric porous electrode compression and asymmetric blocked serpentine flow field designs are proposed. With a well-developed 3-D VRFB model incorporating electrode compression effect, the compression ratio for each half-cell and the block factor of each flow field are delicately optimized, and their impacts on battery performance as well as power loss mechanism were analyzed in detail. Results indicate that the optimal asymmetric structure with four design variables are negative electrode compression rate of 0.38, positive electrode compression rate of 0.5, negative block factor of 0.9 and positive block factor of 0.6 at flow rate of 80 ml mi n − 1 and current density of 200 mA c m − 2 . As compared with the optimal symmetric structure design (i.e., electrode compression rate of 0.41 and block factor of 0.8 for both electrodes), the optimal asymmetric structure shows 3.3%-5.4% increment in the net discharge power for various state of charge from 0.1 to 0.9, which is mainly due to the decreased reaction loss and pump loss in negative side as well as the reduced ohmic loss in positive side. Also, it was shown that the same optimal structure with four design parameters can also be achieved by using genetic algorithm, which indicates the reliability of the present optimization.

A novel cylindrical metal hydride (MH) reactor embedded rectangle heat exchange channels (RHEC) was proposed in this work. The hydrogen absorption of RHEC was studied and compared with the longitudinal finned single-tube reactor (LFST) and multilayer finned single-tube reactor (MFST). The results indicated that the charging time for 90% saturation for RHEC decreases by nearly 40% and 38% compared with LFST and MFST, respectively. Then, a performance improvement method from heat transfer to structure is applied to further enhance the hydrogen absorption of RHEC. According to the bed temperature distributions, the MH bed in RHEC can be separated as three areas along radial direction. The central region and extended region are in poor heat transfer, while the near channel region possesses better heat transfer. Therefore, the heat transportation in central region and extended region need to be improved. It was found that the decrease of slant angle of heat exchange channels has nearly no effect on mean absorption performance, but can significantly accelerate the heat transportation in central region. Besides, interlaced fin layout can further accelerate the heat transportation in reaction bed compared with parallel fin layout. Moreover, adding metal foam in heat exchange channels can enhance hydrogen absorption remarkably.

Non-uniform solar flux distribution in a parabolic trough concentrator (PTC) causes large temperature gradients on absorber tube surface, making the PTC inefficient and damaged. To solve the problem, a novel method is proposed for designing the additional secondary reflector (SR) in a PTC to improve uniform heat flux distribution on absorber tube surface. In the design, the heat flux between upper and lower surfaces of absorber tube is well balanced by optimizing the location of absorber. Then, the SR with segmented broken-line type composed of multiple plane mirrors is delicately designed with local heat flux compensation strategy. Two case studies are conducted to compare the performances of the newly designed SRs with other existing SR designs. It is shown that the uniformity of heat flux distribution can be improved to over 90% for present SR design, much higher than those for other existing SR designs. Also, the optical efficiency of the PTC with present SR design is also increased as compared with other designs. The results indicate that this proposed method is competent for designing SR in a PTC with uniform heat flux distribution.

Tree-shaped fins were introduced to improve the heat transfer of metal hydride reactor. A two-dimensional steady-state model was first applied to obtain the optimal geometric structures of tree-shaped fins using genetic algorithm with the constraint of fixed fin mass. Then, the hydrogen absorption performance of MH reactors with embedded radial fins and tree-shaped fins were numerically studied and compared using a multi-physical model. It was indicated that the hydrogen absorption time for 90% saturation for the reactor with optimized tree-shaped fins nearly decreases by 20.7% as compared with that for radial fins. The heat transfer and absorption performance of MH reactor were much sensitive to the length ratio of tree-shaped fins, which can be improved with increasing the length ratio. Besides, the performance of optimized tree-shaped fins reactor with variable angle ratio was only slightly better than that for fixing angle ratio at 1, indicating that the angle ratio of tree-shaped fins can be kept at 1 for simplifying the design. Moreover, the absorption performance of the reactor can be enhanced with increasing the maximum branch level of tree-shaped fins. Furthermore, it was shown that the optimized geometric structures of tree-shaped fins under different MHs thermal conductivities are nearly identical.

A blocked serpentine flow field is proposed for vanadium redox flow battery (VRFB) in order to improve electrolyte penetration and convection in the porous electrode such that the overall battery performance can be boosted. A three-dimensional mathematical model with considering non-uniform electrode structure under compression and electrode intrusion into channels was applied for simulating and comparing the VRFB performance with conventional serpentine flow field and blocked serpentine flow field. It was indicated that for blocked serpentine flow field, with increasing the block height from 0 mm to 1.5 mm, the discharge voltage of the battery is obviously boosted due to the enhanced species transport and improved flow distribution in the porous electrode. Also, it was shown that the overall battery performance in terms of net discharge power and system efficiency reaches maximum for the blocked serpentine flow field with 1.4 mm block height among different flow field designs, which is attributed to the tradeoff between the effects of increased pump loss and the improved charge-discharge performance. This study can provide beneficial advice for designing the suitable flow field for VRFB with improved mass transport performance.

A novel rotary serpentine flow field with increased electrolyte penetration and improved species distribution is proposed for ameliorating performance of vanadium redox flow battery (VRFB). Three-dimensional multi-physical VRFB model was employed to analyze the flow and transport behavior as well as the overall performance of the VRFB with different flow field designs. It was indicated that the VRFB with this new rotary serpentine flow field design exhibits higher discharge voltage and lower over-potential at different states of charge as compared with conventional serpentine flow field and interdigitated flow field, which is mainly attributed to the enhanced mass transport of species from channel region to the porous electrode and improved distribution uniformity of active species inside electrode. Overall, the battery with the rotary serpentine flow field can achieves the highest power-based efficiency during whole discharge process, which is about 4.2% -12.6% higher than that for conventional serpentine flow field and about 1% - 4% higher than that for interdigitated flow field. Besides, it was shown that the battery with the rotary serpentine flow field can result in higher net discharge power at various electrolyte flow rates as compared with conventional serpentine and interdigitated flow fields due to the improved discharge voltage and reasonable pump loss, indicating the superiority of the proposed rotary serpentine flow field.