Effects of microreactor wall heat conduction on the reforming process of methane

In this paper, the effect of wall conduction of an autothermal tubular methane microreformer is investigated numerically. It is found that the axial wall conduction can strongly influence the performance of the microreactor and should not be neglected without a careful a priori investigation of its impact. By increasing the wall thermal conductivity, the maximum wall surface temperature is decreased. Due to the complex exothermic–endothermic nature of the chemistry of reforming, the axial variation of the wall temperature is not monotonic. Methane conversion and hydrogen yield are strongly dependent on the wall inner surface temperature, hence the heat conduction through the channel wall. The equivalence ratio and the wall thickness also significantly affect the reforming effectiveness and must be carefully considered in reactor optimization. Furthermore, it is found that exothermic oxidation reaction mechanisms, especially partial oxidation, are responsible for syngas (hydrogen and carbon monoxide) production near the inlet. Farther downstream, in the oxygen deficient region, endothermic steam reforming is the main hydrogen producing mechanism. By increasing the thermal conductivity, steam reforming becomes stronger and partial oxidation becomes weaker. For all investigated inlet conditions, the highest hydrogen yield is obtained for no or very low conductive walls.     

FTIR spectral analysis of bituminous binders: reproducibility and impact of ageing temperature

This RILEM round robin study with nine participating laboratories investigated bitumen ageing, its effect on chemical properties and its reproducibility. The impact of temperature used for short-term (RTFOT) binder ageing on the combined short- and long-term (PAV) aged samples was investigated; thereby the effect of reduced mixing temperature such as those relevant for warm mix asphalt technologies on long term ageing was examined. Four 70/100 penetration graded bituminous binders from different sources were selected. In addition to the standard RTFOT temperature of 163 °C, two additional temperatures, 143 and 123 °C were used. The Fourier transform infrared spectroscopy (FTIR) analysis was carried out using an integration method which considers the area below the absorbance spectrum around a band maximum using baseline and tangential approaches. A statistical investigation into the reproducibility of FTIR spectra analysis based on the accumulated data was done. To assess the reproducibility, the coefficient of variation (CV) was taken as a benchmark parameter. Carbonyl and sulfoxide indices were calculated using different baseline correction methods and tangential and baseline integration, respectively. It was shown that the tangential method was not influenced by the applied baseline correction. However, in all considered cases, the tangential method led to significantly worse reproducibility (CVs ranging from 20 to 120%) compared to the baseline method. The sulfoxide indices calculated by both methods were not affected by the baseline correction method used. Impacts of changes in the short-term ageing temperature on short- or long-term aged samples could not be found whereas differences between different binder sources could be detected. RTFOT temperature and therefore mix production temperature had a stronger impact on the formation of sulfoxide structures than for carbonyl structures. The findings from this study show the most reproducible of all considered methods when more than one laboratory is providing FTIR data.     

Stability analysis for multi-agent systems using the incidence matrix: Quantized communication and formation control

The spectral properties of the incidence matrix of the communication graph are exploited to provide solutions to two multi-agent control problems. In particular, we consider the problem of state agreement with quantized communication and the problem of distance-based formation control. In both cases, stabilizing control laws are provided when the communication graph is a tree. It is shown how the relation between tree graphs and the null space of the corresponding incidence matrix encode fundamental properties for these two multi-agent control problems.     

Decentralized Navigation Functions for Multiple Robotic Agents with Limited Sensing Capabilities

The decentralized navigation function methodology, established in our previous work for navigation of multiple holonomic agents with global sensing capabilities is extended to the case of local sensing capabilities. Each agent plans its actions without knowing the destinations of the others and the positions of those agents lying outside its sensing neighborhood. The stability properties of the closed loop system are checked via Lyapunov stability techniques for nonsmooth systems. The collision avoidance and global convergence properties are verified through simulations. This work was partially presented in [5].     

Fast and exergy efficient start-up of micro-solid oxide fuel cell systems by using the reformer or the post-combustor for start-up heating

The start-up process of a micro-solid oxide fuel cell system strongly influences its overall efficiency, especially for portable applications where a frequent switch-on and switch-off is required. We present herein a novel start-up process for such systems that exploits existing units, such as the post-combustor or the reformer, as a heat source to reach the operation temperature of the cell at 600 °C. Our experimental results show that the employment of platinum catalysts in the post-combustor or rhodium catalysts in the reformer for total oxidation of butane by air combined with an electrically heated wire led to a faster and more efficient start-up than conventional start-up methods using only electrical energy. By using the post-combustor as heat source, the start-up time could be reduced by 79% and the exergy cost by 86%. The latter includes the cost of the stand-alone fuel cell system to produce electrical energy for the joule heating of the wire (i.e. the system efficiency is accounted for). There are several advantages to use the reformer as heat source during start-up, such as prevention of coking of the fuel cell or improved heat transfer by internal heating of the other components. The start-up performance, however, was lower than that of the post-combustor: the start-up time could be reduced by 65% and the exergy cost by 68% compared to a conventional start-up.     

Magnetic properties and critical currents of epitaxial YBa2Cu3O7-x films

The authors report results of magnetic and transport measurements on thin epitaxial films of YBa₂Cu₃O_7-x which show critical current densities of 10⁷ A/cm² at 4.2 K. They exhibit well-formed symmetrical hysteresis loops and flux-trapping effects and linear susceptibilities at low fields. Magnetic and transport critical currents are in good agreement at low temperatures. The above properties are attributed to strong pinning from point defects which are suggested to be more numerous in films than in bulk single crystals. Diamagnetic shielding effects can be very large and are proportional to the critical current at zero field; however, there is a large penetration of H_a at all field values. Field-cooled magnetization is always very small, being only a few percent of the diamagnetic shielding. This small value is attributed to a balance between trapped flux and expelled flux in the cooling process. The strong pinning in attributed to a high density of defects in the film     

The bulk photovoltaic effect in ferroelectric Pb(Zr,Ti)O3 thin films

Abstract

The bulk photovoltaic effect (BPE) has been investigated in lead zirconate titanate (PZT) thin films. Measurements of the kinetics, spectral distribution and photocurrent hysteresis loops have been made. In the extrinsic spectral region, the steady-state photocurrent is primarily due to the BPE, where the photovoltaic tensor component has been determined to be G₃₁ = 10_?9 cm/V. However, in the intrinsic region, the BPE has not been determined due to the strong contribution from photoinjection currents. Finally, it is shown that the BPE may be the driving force for photoinduced hysteresis changes in PZT thin films, particularly in the extrinsic spectral region.     

Electronic and Ionic Trapping at Domain Walls in BaTiO3

We have coupled electron paramagnetic resonance (EPR) and polarization–voltage measurements to understand the effects of reducing ambients on the remanent polarization and density of paramagnetic centers in BaTiO₃, single crystals. Two types of reducing ambients were explored; one was done under vacuum (slightly reducing) and the second was performed in forming gas (very reducing). It is found that the vacuum anneal caused a reduction in the remanent polarization and a concomitant decrease in the isolated Fe³⁺ EPR resonance. The Fe³⁺–V_o complex EPR signal was relatively unaffected by this vacuum anneal. By injecting charge using an ultraviolet (UV) light and an applied bias combination, the polarization and the isolated Fe³⁺ signal intensity were restored, thereby suggesting that the suppression of the remanent polarization is due to trapping of electronic charge at the domain walls. For the forming gas anneal, we observe a much larger decrease in remanent polarization with an accompanying decrease in both the isolated Fe³⁺ and Fe³⁺–V_o complex EPR signals. For this anneal, charge injection by the UV light/bias combination did not restore the polarization nor the EPR densities. The remanent polarization, the isolated Fe³⁺, and the Fe³⁺–V_oV_o complex could be restored only by a reoxidizing anneal, suggesting that ionic defects {oxygen vacancies) are now responsible for pinning the domain walls. Collectively, these results suggest reducing anneals can suppress the amount of switchable polarization in BaTiO₃ by either electronic or ionic trapping mechanisms.     

Remelting phenomena in the process of splat solidification

A combined theoretical and experimental study is reported which investigates remelting phenomena during the splat cooling of two liquid-metal droplets impacting sequentially on a substrate. Under conditions of sufficiently high superheat it was proposed theoretically and demonstrated experimentally that an initial deposit is remelted by the subsequent impact of molten material. It is shown that the amount of superheat as well as the variation of thermophysical properties, particularly the latent heat and the melting temperature, influence the degree of remelting. Experimental findings supported to a certain extent the theoretical model assumptions that the splats could be represented by thin discs and that the heat transfer and solidification within the splat propagates in the axial direction only. However, the experiments showed that these assumptions are better suited for the central region of the splat. The occurrence of remelting often depended on the radial location for a given amount of superheat. For most part, the splat exhibited globular microstructure. Lamellar structures were observed near the top and the periphery of the splat, indicating slower cooling rates at these locations. The theoretical model constituted a good compromise between accuracy and simplicity and predicted the correct trends of the remelting phenomenon.