•The flow uniformity of large-sized PEMFC was improved with an optimized distribution zone.•Partially dotted matrix can achieve both improved uniformity and reduced manufacturing cost.•Positional and ...size effects of partially dotted matrix were clarified.
Proton exchange membrane fuel cell (PEMFC) is an important device to convert clean-fuel hydrogen into electricity, which has the potential to reduce pollutant emission and our dependence on fossil fuels. Of the many development and manufacturing challenges that need to overcome before large-scale commercialization can be realized, one of the most pivotal ones is flow distribution uniformity. To this end, a distribution zone with partially dotted matrix was proposed, and showed much more effectiveness when compared with the un-optimized distribution zone with an empty chamber. The positional and size effect of the dotted matrix was investigated, and results showed that flow uniformity was maximized when the dotted matrix was placed near the gas inlet. Interestingly, less dotted matrix size could achieve better performance, indicating the potential of this approach to reduce manufacturing cost. The optimized system could achieve more uniform flow with standard deviation of 1.808 × 10-8, maldistribution factor of 3.967 × 10-3 and variable coefficient of 0.4143%, more superior to other similar models reported. Furthermore, a full cell-level simulation was also conducted, affirming the superiority of the proposed model with increased power densities and water distribution uniformities.
Grate-fired boiler is of considerable interest for burning biomass owing to its reduced sensitivity to variations in fuel composition and size. However, grate-fired technology exhibits unsatisfactory ...performance in terms of combustion efficiency, contaminant emissions, and flame stability. CFD modelling can provide reasonable optimizations to overcome these drawbacks and ultimately achieve clean and efficient energy production. Conventional approach simulates the fuel-bed and freeboard combustion separately using an in- and over-bed coupling procedure, where data of the gases leaving from the packed-bed top and the radiative heat flux emitted by the high-temperature flame onto the bed cannot be interacted in real-time. This may cause distinct deviations in the calculations. In this paper, a three-dimensional (3D) full-scale integrated model is developed for the simulation of grate-fired boiler, in which the intensive combustion of both the freeboard and fuel-bed is solved in one scheme using the Eulerian-Lagrangian method. The gas phase is considered as a continuous medium and calculated by the Eulerian model, while the solid particles are considered as a discrete phase and tracked via the Lagrangian method. The reliability of the model is well verified through various field measurements and observations. Results shows that the key parameters are non-uniformly distributed along the grate width, i.e., the aerodynamic and combustion environment is poor at the vicinity of water-cooled walls compared with that at the furnace center. This results in a significant lag in volatile gases release and char oxidation near the water-cooled wall on both sides, which finally causes obvious differences in the distribution of temperature and species. For instance, the peak of CO concentration at the center is 12.6 % at 3.3m from the feed entrance, while that near the water-cooled wall is 8.6 % around 3.5m. The char burnout ratio in the bottom ash can be accurately calculated by this model, with a negligible relative error between the simulation of 81.97 % and the measurement of 82.50 %. It also provides a novel method for the calculation of fly ash particles by using size-grouped and non-spherical particles. Small-size particles in the vicinity of the feed inlet, as well as a part of the particles burning at the end of the grate are entrained into the freeboard by the primary air supplied beneath the grate and the high-momentum secondary air on the bed top. This model can provide valuable theoretical guidance for optimizing the refined distribution of fuel and air in grate-fired power plants and mitigating irregular deposition (corrosion) on heat exchangers and water-cooled walls caused by fly ash.
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•An integrated model is developed for accurate modeling of travelling grate boiler.•Real-time mass and heat transfer between the fuel bed and freeboard is achieved.•Key parameters are found to be non-uniformly distributed along the grate width.•Non-spherical and size-grouped particles are used to better represent agricultural straw.•A novel method for calculating fly ash in grate-fired boiler has been developed.
For a real-valued sequence (xn)n=1∞, denote by SN(ℓ) the number of its first N fractional parts lying in a random interval of size ℓ:=L/N, where L=o(N) as N→∞. We study the variance of SN(ℓ) (the ...number variance) for sequences of the form xn=αan, where (an)n=1∞ is a sequence of distinct integers. We show that if the additive energy of the sequence (an)n=1∞ is bounded from above by N3−ε/L2 for some ε>0, then for almost all α, the number variance is asymptotic to L (Poissonian number variance). This holds in particular for the sequence xn=αnd,d≥2 whenever L=Nβ with 0≤β<1/2.
Among the many consequences for production processes, new technologies, including 3D‐printing and web‐based co‐design, provide end‐users with increasingly effective means to locally reshape and ...co‐produce products to fit their needs, decentralizing part of the production process. But do firms have an incentive to design goods that are self‐customizable or rather to retain centralized production? Is users' surplus increased when self‐customization is an available option? We analyze a duopoly where firms can offer a standard version only, or a menu with two product/price options: a standard and a customizable version. Different from firm‐based customization, customizing consumers pay the same price to the firm, who cannot price discriminate among them. We find that both firms' introducing a self‐customizable product can be an equilibrium, with an increase in profits, consumers' surplus, and welfare. However, there also arise situations in which one firm only produces standardized goods. The equilibrium configuration depends upon the cost of self‐customization and the distribution of consumers.
A novel optimisation method is developed for optimum strengthening design of 2D multi-story steel moment resisting frames using buckling restrained brace (BRB) dampers and externally welded steel ...plates in compliance with ASCE 41-06. The proposed method is based on the concept of Uniform Distribution of Deformation (UDD), in which structural materials are redistributed from strong to weak parts of a structure until the material capacity is fully exploited. For the first time, an adaptive power function is introduced to improve the computational efficiency and convergence speed of the UDD optimisation method. The results of the proposed optimisation method are compared with metaheuristic optimisation methods using Genetic Algorithm (GA) and Particle Swarm Optimisation (PSO) for a three story and a nine story frame. The demand to capacity ratios of deformation-controlled and force-controlled structural members based on ASCE 41-06 are considered as design constraints in the optimisation process. The results demonstrate the efficiency of the proposed optimisation method in finding the optimum design solution with significantly less computational costs (up to 300 times less number of analyses) compared to both GA and PSO methods. It is shown that, in general, a more suitable distribution of dampers is accompanied by a more uniform distribution of demand to capacity ratios, which confirms the concept of UDD optimisation method.
•A new algorithm developed for optimal rehabilitation of steel frames with BRB dampers.•Adaptive power function is introduced to improve computational efficiency and convergence speed.•Efficiency of the method is demonstrated for optimum performance-based design of 3 and 9-story frames.•The proposed method leads to optimum design with up to 450 times less number of analyses compared to GA and PSO.
•A proper shape for a 14.6 Ah lithium-ion battery is found.•Performance of the optimized case is compared with the conventional LIB.•Using counter and l-shape tabs leads to reducing the maximum ...temperature.•The LIB performance increases by optimizing the cell aspect ratio.
This study is focused on finding a proper shape for a 14.6 Ah lithium-ion battery cell to control the temperature at an optimal range and improve the performance of LIB. To this end, the impacts of twelve tab locations, cell aspect ratios, and C-Rates on the temperature deviation and performance of LIBs are examined by means of 3D numerical simulations. A comprehensive D factor as the non-dimensional parameter is used to evaluate the impacts of effective parameters on the uniformity analysis of the temperature, potential, and current. Compared to the conventional LIB with nominal lab (case 1), changing the tab locations to the counter and l-shape tabs (cases 3 and 8) reduces the maximum temperature and increases the potential discharge. Results indicate that optimizing the cell aspect ratio in the LIBs with the nominal, counter, and l-shape tabs (cases 1, 3, and 8) can effectively decrease the temperature difference and enhance the life and performance of LIBs. Results illustrate that the effect of optimizing the cell aspect ratio in the LIB with the counter tab (case 3) is more than in LIBs with the nominal and l-shape tabs (cases 1 and 8).
A celebrated unit distance conjecture due to Erdős says that the unit distances cannot arise more than Cϵn1+ϵ times (for any ϵ>0) among n points in the Euclidean plane (see e.g. 8 and the references ...contained therein). In three dimensions, the conjectured bound is Cn43 (see e.g. 6 and 11). In dimensions four and higher, this problem, in its general formulation, loses meaning because the Lens example shows that one can construct a set of n points in dimension 4 and higher where the unit distance arises ≈n2 times (see e.g. 1). However, the Lens example is one-dimension in nature, which raises the possibility that the unit distance conjecture is still quite interesting in higher dimensions under additional structural assumptions on the point set. This point of view was explored in 4, 5, 7 and has led to some interesting connections between the unit distance problem and its continuous counterparts, especially the Falconer distance conjecture (3). In this paper, we study the unit distance problem and its variants under the assumption that the underlying family of point sets is uniformly distributed. We prove several incidence bounds in this setting and clarify some key properties of uniformly distributed sequences in the context of incidence problems in combinatorial geometry.
•A gear dynamics model considering the non-uniform crack is established by using the ‘slice method’.•The roll and yaw motions generated by gear slices with uneven crack are considered.•Vibration ...feature of the gear in presence of uneven crack is revealed.•The mapping relationship between the vibrations and the severity of crack is established.
Gear tooth root crack is a typical fault of gear transmissions and it threatens the operation safety of gear systems. Therefore, precise detection of the tooth root crack is of a great significance for the prevention of serious safety accidents. As one of the commonly used methods, vibration-based detection has drawn great attentions. However, the mapping relationship between the vibrations and the severity of tooth root crack has not been established well, especially for the crack with non-uniform distribution due to the complexity of its dynamic modelling. To address this problem, an improved gear dynamics model considering non-uniform distribution of the gear tooth root crack fault is established in this paper by using the ‘slice method’. Compared with the traditional methods, the advantage of the proposed model is that it enables the accurate modelling of the non-uniform dynamic mesh forces along tooth width that will further excite the support bearings. Then, the dynamic simulation of the gear transmission with non-uniform tooth root crack is performed. The results indicate that the improved model can reveal the vibration characteristics of the gear transmission due to the extra roll and yaw motions caused by the non-uniform tooth root crack. In addition, the correlation between the vibration and the crack size and location is quantified. This study could provide theoretical reference for quantitative detection and diagnosis of gear tooth root crack.
In any dimension d≥2, there is no known example of a low-discrepancy sequence which possesses Poisssonian pair correlations. This is in some sense rather surprising, because low-discrepancy sequences ...always have β-Poissonian pair correlations for all 0<β<1d and are therefore arbitrarily close to having Poissonian pair correlations (which corresponds to the case β=1d). In this paper, we further elaborate on the closeness of the two notions. We show that d-dimensional Kronecker sequences for badly approximable vectors α→ with an arbitrary small uniformly distributed stochastic error term generically have β=1d-Poissonian pair correlations.
•A bottom-to-up strategy is adopted to design, fabricate and test the electrode.•Increasing O-functional groups enhances the numbers and activity of active sites.•All the experimental results are ...consistent with the computational predictions.•The battery delivers an EE of 80.1% and EU of 69.8% at 320 mA cm−2.•The high-performance is related to the good transport properties of carbon cloth.
In this work, a bottom-to-up strategy is adopted to design, fabricate and test a uniformly distributed bismuth nanoparticle-modified carbon cloth electrode for vanadium redox flow batteries (VRFBs). The first-principles study reveals that increasing the number of oxygen-functional groups on the surface of carbon fibers can promote the uniform distribution of electrodeposited bismuth nanoparticles, which increases the effective surface areas and active sites. Results also show that the oxygen-functional groups and bismuth exhibit a synergistically catalytic effect, which enhances the kinetics of redox reactions. Therefore, carbon cloth substrate with a high content of oxygen-functional groups is fabricated and tested. The material and electrochemical characterizations, including scanning electron microscope (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), verify the predictions of the first-principles study. Battery tests show that the VRFBs with the prepared electrode enables an energy efficiency of 88.4% at 160 mA cm−2, 19.6% higher than that with the original electrode. Additionally, the battery is capable of delivering an energy efficiency of 80.1% at a high current density of 320 mA cm−2, which are among the highest performances in the open literature. Finally, it is also proved that the prepared bismuth nanoparticle-modified carbon cloth electrode outperforms the bismuth nanoparticle-modified carbon paper electrode, ascribed to the excellent ion/mass transport properties of carbon cloth.
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