Flow Manifold Optimization for a Uniform Velocity Distribution in a Laminated Microreactor with Micro‐Pin‐Fin Arrays

Velocity uniformity in reaction channels has a significant effect on the performance of laminated microreactors with micro‐pin‐fin arrays (MPFAR) for hydrogen production. The hydrogen production efficiency can be improved by optimizing the structure of flow manifolds. The relationship between the flow manifold structure and the velocity distribution in the reaction channel with MPFAR is established by an equivalent electrical resistance network model validated via simulation. The effects of the flow manifold structure on the velocity distribution are investigated. The results show that the velocity distribution can be improved by increasing the y‐direction coordinate Y_pi of the flow manifold inlet tube. The flow manifold structure is optimized for better velocity distribution in reaction channels with different MPFAR widths.     

Effects of Distribution Channel Dimensions on Flow Distribution and Pressure Drop in a Plate‐Fin Heat Exchanger

A numerical investigation on flow distribution and pressure drop characteristics in a plate‐fin heat exchanger is presented. Influences from length and width of the distribution channel were particularly investigated. Flow distribution in the studied model was improved by increasing the length or reducing the width of the distribution channel, but at the cost of an increased pressure drop. The relationship of flow distribution and pressure drop was analyzed by the porous medium approach. A dynamic balance phenomenon was observed and further studied. Based on the results, a novel strategy for the attainment of flow uniformity, which causes negligible pressure drop variation, was proposed. Finally, a performance effectiveness factor was developed for predicting the effect of the proposed strategy on the performance of plate‐fin heat exchangers.     

Experimental Characterization of a Flow‐through Pulsation Damper Regarding Pressure Pulsations and Vibrations

Pressure attenuators, especially side branch bladder‐type accumulators, for suppression of pressure pulsations in industrial fluid applications are not suitable for higher‐pressure pulsation frequencies, e.g., with centrifugal pumps. Accumulators of the flow‐through‐type promise a faster response time and thus a wider application range. The damping performance of a flow‐through pressure accumulator with respect to fluid pressure pulsations and the resulting structure‐borne noise was investigated. The influence of varying pulsation frequencies, damper precharge pressure, and damping effect throughout the piping system was evaluated. Experiments were performed in an industrial‐scale closed‐loop test facility, with both pressure pulsations and piping vibrations in the focus. The damper, characterized by means of high‐speed video analysis, proved to be capable of damping effectively the pressure pulsations and piping vibrations induced by a centrifugal pump.     

Flow‐Field Designs of Bipolar Plates in PEM Fuel Cells: Theory and Applications

A generalized model developed by Wang was modified for flow field designs of the most common layout configurations with U‐type arrangement, including single serpentine, multiple serpentine, straight parallel, and interdigitated configurations. A direct and quantitative relationship was established among flow distribution, pressure drop, configurations, structures, and flow conditions. The model was used for a direct, systematic, and quantitative comparison of flow distributions and pressure drops among the most common layout configurations of interest. The straight parallel configuration had the lowest pressure drops but suffered the most possibility of the uneven flow distribution across the channels. The single serpentine had the best flow distribution but had the highest pressure drops. The flow distribution and the pressure drop in the multiple serpentine was between the straight parallel and the single serpentine. Finally, we suggested basic criteria of the flow field designs of bipolar plates for the industrial applications. This provides a practical guideline to evaluate how far a fuel cell is from design operating conditions, and measures how to improve flow distribution and pressure drop.     

Effect of PTFE Content in Gas Diffusion Media and Microlayer on the Performance of PEMFC Tested under Ambient Pressure

Polytetrafluoroethylene (PTFE) content in the fuel cell electrode plays an important role on the performance of polymer electrolyte membrane fuel cell (PEMFC) when the cell is tested under low temperature and under ambient pressure. PTFE is added to the PEM fuel cell electrode to improve the mechanical strength and to help in removing the product water formed on the cathode; however, higher PTFE loading increases the resistance and thus decreases the performance of the cell and very low PTFE content has the disadvantage of water flooding in long‐term operation. We have investigated the effect of the PTFE content in the gas diffusion media (carbon paper) and in the microlayer on the performance of PEMFC operating at ambient pressure. The PTFE contents in these two layers have to be finely matched to get the best performance of the cells. The polarisation behaviour, electrochemical surface area and the electrochemical impedance spectra have been analysed. The results are presented in this paper.     

Statistical Analysis of the Effect of the Temperature and Inlet Humidities on the Parameters of a PEMFC Model

An individual PEM fuel cell 6‐parameter mechanistic model was developed. In parallel, experimental polarization curves were obtained at different temperature and inlet gas humidities conditions. The 6 model parameters were determined by fitting the semi empirical model to the experimental curve using a non linear regression method. Finally, a statistical analysis was carried out in order to determine which operating conditions (temperature and inlet humidities) have a significant effect on which model parameters. A black box model was built in order to relate the model parameter values to the significant operating conditions for each one of them. The obtained model was able to satisfactory reproduce the experimental behaviour of the system at low current densities.     

Effect of Nonionic Surfactant on the Micro‐Emulsifying Water in Silicone Media

Reverse micro‐emulsion dyeing technology shows a high dye uptake and fixation rate, and effectively decreases the amount of waste water in the textile industry. However, the influence of surfactants and co‐surfactants on micro‐emulsifying water in reverse micro‐emulsions has not been investigated. In this study, the micro‐emulsifying mechanism in silicone media and the influence of nonionic surfactants and co‐surfactants on micro‐emulsifying water have been investigated. The results show that a large amount of water can be emulsified using alcohol‐polyoxyethylene ethers, especially under 3–5 ethylene oxide groups in molecular structure. As co‐surfactant, alcohol which contains 4–9 carbon chains could spontaneously form reverse micro‐emulsions in silicone media. The effects of solution pH and ionic strength on the amount of emulsified water were also evaluated. The results show that a large amount of water could be emulsified in silicone media at solution pH 2–9. Meanwhile, the amount of emulsified water decreased with increasing ionic strength. Thus, the optimum conditions for water emulsification are low ionic strength and pH between 2 and 9.     

Organic Additives for the Enhancement of Laminar Flow in a Brain‐Vessels‐Like Microchannel Assembly

Organic polymers were extracted from okra, aloe vera, and hibiscus leaves and used as drag‐reducing additives (DRAs) to enhance the laminar flow in custom‐made microchannels that simulate the human brain vessels. The experiment was conducted using an open‐loop microfluidic system. The flow enhancement performance was evaluated as the function of percentage of flow increment of mucilage additives at different concentrations. Okra mucilage showed greater flow enhancement performance at higher mucilage concentration while both aloe vera and hibiscus mucilage performed better at lower additive concentration. The findings prove the potential of these organic polymers as DRAs to enhance the blood flow.     

CFD Simulation of Flow Distribution in the Header of Plate‐Fin Heat Exchangers

The flow distribution through a plate‐fin heat exchanger is studied by using a computational fluid dynamics (CFD) code, FLUENT. The flow distribution through any heat exchanger affects its performance. In designing a heat exchanger, it is assumed that the fluid is uniformly distributed through the heat exchanger core. In practice, however, it is impossible to distribute fluid uniformly, because of an improper inlet configuration, imperfect design, and a complex heat transfer process. The CFD simulation of the flow distribution in the header of a conventional plate‐fin heat exchanger is presented. It is found that the flow maldistribution is very serious in the y‐direction of the header. A modified header is proposed and simulated using CFD. The modified header configuration has a more uniform flow distribution than the conventional header configuration. Hence, the efficiency of the modified heat exchanger is seen to be higher than that of the conventional heat exchanger.     

Effect of the Deformation Temperature on the Shape‐Memory Behavior of Epoxy Networks

The impact of the deformation conditions, specifically the temperature, on the shape‐memory behavior and characteristics of epoxy SMPs is studied. By simply varying the temperature during deformation (i.e., the programming step of the SM effect), the ultimate strain of the formulated epoxy was improved three‐ to five‐fold, thereby providing for an increased range of reachable deformation strains during SM thermo‐mechanical cycling. This research unveils newly developed epoxy‐based SMPs with improved deformability range and high strength with intrinsically good thermal and chemical stability.

     

The Influence of Calcium‐Stearate‐Coated Calcium Carbonate and Talc on the Quiescent and Flow‐Induced Crystallization of Isotactic Poly(propylene)

The influence of Ca‐stearate‐coated CaCO₃ and talc on the quiescent and flow‐induced crystallization of iPP is studied using different methods. Comparison of rheometry and DSC shows that rheometry is an interesting tool to monitor crystallization kinetics. It is observed that the Ca‐stearate coating degrades at commonly used annealing temperatures, influencing the crystallization behavior of the CaCO₃‐containing polymer. WAXD indicates that the CaCO₃ does not significantly influence the degree of crystallinity. As shear intensifies, both the pure and particle‐containing polymers crystallize faster; however, their behavior also becomes increasingly similar. There are indications that shear influences the organization of the CaCO₃ aggregates.

     

Effect of Serpentine Multi‐pass Flow Field Channel Orientation in the Liquid Water Distributions and Cell Performance

A commercial 50 cm² polymer electrolyte membrane (PEM) fuel cell with serpentine flow fields was operated at 2.0 bar and 60 °C with two orientations of the flow field channels with respect to gravity, i.e. horizontal and vertical channels. A 3 × 3 test matrix of anode and cathode reactants relative humidity was used for the performance assessment of the cell in both orientations. The cell performance and operating data, including cell voltage and resistance, were measured, and neutron radiographs were recorded during the entire operation in order to gain knowledge of the liquid water distributions within the cell for both orientations. A quantitative analysis of the results is presented in this work, comparing the cell operation for both flow field orientations. It is observed that the configuration with horizontal cathode flow field channels presents a better cell performance, and less amount of liquid water blocking the flow field channels. Thus, the results show that the selection of the cell orientation has an influence on the final performance, and it is therefore, a design parameter to be considered for a real application. The differences in the cell water content are quantitatively analyzed and discussed.     

Prediction of Two‐Phase Frictional Pressure Drop for the Concurrent Gas and Newtonian Liquids Upflow through Packed Beds

Correlations were developed to predict frictional pressure drop for concurrent gas‐liquid upflow through packed beds covering all the three identified flow regimes, i.e. bubble flow, pulse flow and spray flow. The observation that the gas and liquid flow rates have different influences on the two‐phase pressure drop in different flow regimes, was taken into consideration in the development of these correlations. More than 600 experimental pressure drop data from the present study and literature covering a wide range in gas‐liquid systems, flow rates and column packing were used.     

Effect of the ZIF‐8 Distribution in Mixed‐Matrix Membranes Based on Matrimid® 5218‐PEG on CO2 Separation

In theory, the combination of inorganic materials and polymers may provide a synergistic performance for mixed‐matrix membranes (MMMs); however, the filler dispersion into the MMMs is a crucial technical parameter for obtaining compelling MMMs. The effect of the filler distribution on the gas separation performance of the MMMs based on Matrimid®‐PEG 200 and ZIF‐8 nanoparticles is demonstrated. The MMMs were prepared by two different membrane preparation procedures, namely, the traditional method and non‐dried metal‐organic framework (MOF) method. In CO₂/CH₄ binary mixtures, the MMMs were tested under fixed conditions and characterized by various methods. Finally, regardless of the MMM preparation procedure, the incorporation of 30 wt % ZIF‐8 nanoparticles allowed to increase the CO₂ permeability in MMMs. The ZIF‐8 dispersion influenced significantly the separation factor.     

A Thermal Decomposition Model of Aminoguanidium 5,5′‐Azobis‐1H‐Tetrazolate and the Effect of Pressure and Particle Size on the Rate of Decomposition

The temperature histories of aminoguanidinium 5,5′‐azobis‐1H‐tetrazolate (C₄H₁₄N₁₈, AGAT) were measured in order to construct a thermal decomposition model of the compound. The effects of chamber pressure and AGAT particle size were also examined. The results of the study suggest that the thermal decomposition of AGAT occurs in three phases: solid phase, condensed phase, and residue. It was found that the condensed phase consists of decomposition zone I where dramatic temperature rise occurs, decomposition zone II where gradual temperature rise occurs, and the cooling zone where decomposition and temperature stop rising. It was also suggested that the temperature of the decomposition surface which is the interface of the solid phase and the condensed phase was ∼500 K, and that N₂ and NH₃ were suggested to occur in the vicinity of the decomposition surface of decomposition zone I. In addition, it was suggested that the thickness of the decomposition zones I and II decreases and that the maximum‐temperature‐reached increases with an increase in atmospheric pressure. The rate of decomposition of AGAT was found to follow Vieille's equation and the rate of decomposition increases with an increase in pressure. The rate of decomposition increased slightly with an increase in particle size.     

Relay Redox and Lewis Acid Catalysis in the Titanocene‐ Catalyzed Multicomponent Assembly of 1,5‐Enynes

Herein we describe a direct, multicomponent assembly of 1,5‐enynes. The titanocene‐catalyzed coupling of an aryl aldehyde, iodoalkyne, and allylsilane enables the convergent and rapid synthesis of this versatile architectural motif in good to excellent yields.     

Effect of cross‐linked polycarboxylate‐type superplasticizers on the properties in cementitious system

Fluidity and slump loss are main technical indicators for the quality of concrete. They are related to the dispersion of cement and hydration process, and are greatly affected by the structure of superplasticizers (SPs). For the purpose of obtaining SPs with excellent fluidity and slump retention, water‐soluble cross‐linked polycarboxylate ether copolymers (CLPCs) of acrylic acid and alkenyl alcohol type polyoxyethylene ether were synthesized. In order to gain full understanding of the effect of cross‐linked SPs on the properties in cementitious system, properties of the new SPs in cement paste were compared to those of traditional ones without cross‐linking agents. Mortar tests showed that CLPC performed obvious slow‐release function and kept about 10 mm higher than its initial paste flow after 150 min, while PC incurred loss of 15 mm after 150 min. Adsorption of CLPC on cement resulted in zeta potential being roughly the same as that in the case of PC, and enabled more effective slump retention to cement suspensions by rheological property tests. Ettringite peaks disappeared for CLPC on 1‐day curve and reappeared after 35 days, and the intensity of CH peaks for CLPC appeared weaker than Blank and PC after 1 day. The microstructure of CLPC after 1 day showed a very dense structure of smaller and thinner crystallites, which indicated that the early hydration was greatly delayed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40856.     

Visualization of the Drop Deformation and Break‐Up Process in a High Pressure Homogenizer

For the creation of sub‐micron emulsions in fluids of low viscosity the high pressure homogenizer (HPH) is usually chosen. One way of obtaining deeper knowledge of exactly what happens in the active region is to visualize it. In this work, a drop deformation and break‐up visualization system based on a modified Particle Image Velocimetry (PIV) system is described. The system reproduces the gap in a HPH and has been used with pressures up to 18 MPa and drops as small as 5 μm. The optics of the system are analyzed taking into account limiting factors such as the lens resolving power, the focal depth, and the duration of the laser pulses. It is shown that it is possible to resolve drops down to a few μm moving in excess of 100 m/s, and that the main limitations are the resolving power and in the focal depth of the objectives. Examples are shown from capillary drop creation and from the deformation and break‐up of drops in a HPH. It can be concluded that in a HPH, the drops are only deformed to a limited extent in the inlet of the gap, and that all drop break‐up occurs far downstream of the gap.     

Effect of the Air‐Jet and the False‐Twist texturing processes on the stress‐relaxation of polyamide 6.6 yarns

Polyamide 6.6 multifilament yarns were converted to crimped fibers by texturing to simulate the properties of natural staple fiber yarns for textile applications. Texturing is carried out by mechanical stresses (turbulences or twisting) under thermal or hydrothermal conditions which affect the fine structure of the fiber. Two polyamide yarns with the same linear density but composed of filaments of different fineness were textured by the False‐Twist (thermal) and the Air‐Jet (hydrothermal) procedures. The influence of texturing and filament fineness on the relaxation behavior of the yarns was studied. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007