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Abstract: In the present study, a hydrogen polymer electrolyte fuel cell (PEFC) micropowerplant in combination with a steam reformer fed by methanol and a direct methanol fuel cell (DMFC) micropowerplant are analyzed numerically regarding their exergetic efficiency. The effects of concentration and activation overpotentials, and ohmic resistance on the efficiency are considered in quasi-two-dimensional fuel cell models. The influence of significant operational parameters on the exergetic efficiency is examined numerically. Experimental results are conducted for the steam reformer. This work shows the importance of an exergy analysis of the fuel cell as part of an entire thermodynamic system generating electric power and compares PEFC and DMFC micropowerplants exergetically. [Copyright &y& Elsevier]

• Investigated bubble dynamics in laser induced reaction process of liquid precursor. • Analyzed liquid-vapor phase change to determine the reaction rate from bubble growth. • Obtained the transient triple contact point temperature by experiment and simulation. • Developed a semi-empirical model for reaction rate vs triple contact point temperature. • Validated the transport and reaction model experimentally with an excellent accuracy. Laser assisted low-temperature nano/microfabrication of functional materials on a flexible or rigid substrate using water-based liquid precursor have gained significant attention owing to its high process efficiency and low environment impact. Both of which are essential for the green and low-carbon manufacturing technologies needed for the next generation of electronic devices. Laser induced photo-hydrothermal synthesis (LIPHS) utilizes thermally induced reductive reaction in a liquid precursor to synthesize and deposit nano/micro-patterns on temperature sensitive substrates. During LIPHS, microbubble generation is inevitable as a result of byproduct gas formation and liquid-vapor phase change of the precursor. Transport phenomena of the laser-induced reduction reaction coupled with microbubble dynamics on the liquid-vapor-solid triple contact interface significantly affects the quality of fabricated nano/microstructures. Herein, we investigated microbubble formation and growth on the substrate surface during a LIPHS process for silver on a polyimide substrate surface. Framewise growth of microbubble was captured for different laser powers and solute concentrations to reveal the time dependent bubble diameter variation. It was found that the bubble size and growth rate decreased with increasing solute concentration owing to increased boiling point of the precursor. Bubble temperature and reduction reaction rate were obtained by theoretical analyses on the microbubble growth rate. Transient solid-liquid-vapor triple contact point temperature of the microbubble during the LIPHS process was obtained from a numerical simulation to develop a comprehensive model the first time for predicting the chemical reaction rate. The proposed model was validated experimentally with a relative error of 6.2% at the highest reaction rate. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Based on our previous study, experiments were conducted to explore frictional pressure drop and heat transfer performance of de-ionized water flowing through rectangular microchannels having longitudinal vortex generators (LVGs). The experimental investigation was conducted under three-sided constant wall temperature boundary condition for the Reynolds numbers ranging from 350 to 1500 for LVGs with different number of pairs and dimensions. The aspect ratios of rectangular microchannels were 0.25 and 0.0667, and the corresponding hydraulic diameters were 160μm and 187.5μm, respectively. The heights of associated LVGs were 1/4H, 3/4H and H, respectively, where H was the height of microchannel. It was found that heat transfer performance was enhanced by 12.3–73.8% and 3.4–45.4% for microchannels with aspect ratios of 0.0667 and 0.25, respectively, while the pressure losses were increased by 40.3–158.6% and 6.5–47.7%, respectively; and the overall heat transfer performances of some specific microchannels were more than 1 in our study. With the help of LVGs, the critical Reynolds numbers were lower than 1000, which were smaller than the generally accepted value of 2300 for regular channel flow. [Copyright &y& Elsevier]

• Laser direct synthesis and patterning (LDSP) is applicable for flexible electronics. • Thermocapillary yields concave surface profile of microline fabricated from LDSP. • A model is derived and validated to analyze the marangoni effect in LDSP. Laser direct synthesis and patterning technology (LDSP) is a promising way to realize flexible electronic circuitry. In LDSP, functional materials microstructure can be fabricated in-situ on flexible substrates using laser induced photo thermal reduction of ions from precursor that is free of nanoparticles. However, previous studies showed that concave curvature was found on the surface of developed LDSP micro-lines. This occurrence of concave curvature is contradictory to the convex structure expected from the temperature distribution of the precursor and on the substrate. Controlled surface morphology is crucially important for the fabrication of electronic circuitry with ultra-fine structure. Therefore, an analysis of the transport phenomena prevailing near the laser processing spot by both numerical simulation and experiment was carried out. The underlying physics behind the surface reconstruction of silver micro-lines fabricated by LDSP was investigated. The analysis included the opto-thermo-fluidic transport phenomena as well as thermo-capillary flow, i.e. the Marangoni effect. Results showed that the Marangoni effect dominated the movement kinetics on the interfacial surface between the ultra-thin silver nanocluster film and the surrounding fluid during the LDSP process. The surface morphology of the synthesized and deposited silver micro-line is therefore affected by the Marangoni effect. An empirical model was developed to determine the values of characteristic parameters for the Marangoni effect in the present case. The model was applied successfully to determine the micro-line surface morphology evolved during multiple laser scans in LDSP process with three different scanning speeds. Theoretical solution on the silver line thickness was compared with that obtained from the experiments, and the discrepancy was less than 12%. The results and analysis techniques developed provide insights to the coupled heat and mass transport phenomena with thermo-capillary effects, and can be used to improve the quality of microstructures fabricated by LDSP and similar technologies. [ABSTRACT FROM AUTHOR]