Influence of CuO/ZnO/Al2O3 wash-coating slurries on the steam reforming reaction of methanol

A micro-channel reactor for methanol steam reforming is a candidate to supply hydrogen on-site to fuel cells. Micro-channel beds wash-coated with poor quality slurry formulations lead to poorer performance than packed beds using pellet catalysts. This study explored the morphology, X-ray Diffraction (XRD) spectrum, BET surface area and activity of wash-coating catalyst layers from a series of catalyst slurries. All catalyst slurries were prepared from the commercial MDC-3 catalyst. Hydrogen production using wash-coating catalyst layers was performed under packed bed conditions. The results reveal that the solubility level of the MDC-3 catalyst during the slurry preparation process affected the activity of methanol steam reforming. It is difficult to reconstruct the original fine structure, as the MDC-3 catalyst had a higher solubility status after slurry preparation. The volume of the micro-channel catalyst bed was approximately 0.3 cm³. It can supply hydrogen to fuel cells that can produce approximately 8 W with 80% H₂ utilization and 60% fuel cell efficiency.     

Effect of inlet configuration on the refrigerant distribution in a parallel flow minichannel heat exchanger

The refrigerant R-134a flow distribution was experimentally studied for a round header/ten flat tube test section simulating a brazed aluminum heat exchanger. Three different inlet configurations (parallel, normal, vertical) were investigated. Tests were conducted with downward flow for mass flux from 70 to 130 kg m⁻² s⁻¹ and quality from 0.2 to 0.6. Tubes were flush-mounted in the test section, with no protrusion into the header. It is shown that normal and vertical inlet yielded similar flow distribution. As mass flux or quality increased, however, better results were obtained for normal inlet configuration. The flow distribution was worst for the parallel inlet configuration. Possible explanation is provided based on flow visualization results. Correlations were developed to predict the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream.     

Investigations of the unsteady diffusion process in microchannels

This paper is concerned with the investigations and modeling of the unsteady diffusion process along a straight micro-channel with a cross section of 380 μm×360 μm. The studied process is characterized by small Reynolds numbers (Re<10) and high Péclet number (Pe>1000). The 3D computations of the coupled momentum and diffusion equations for isochoric motions are performed with the FLUENT code using the unsteady solvers for both equations. In the limit of stationary solutions, the numerical results are validated by direct flow visualizations and experimental data using confocal microscopy. The computed distributions of concentration provide qualitative and quantitative information on the transitory diffusion process and the rate of solute spreading within the investigated geometry. In particular, the pattern of the “butterfly effect” is represented and analyzed during the non-stationary dynamical process. The work is relevant for the design of novel microfluidics applications where the control of diffusion processes at the walls are important (absorption, extraction, capture of molecules or nano-particles).     

Evaporative heat transfer and pressure drop of R410A in microchannels

Convective boiling heat transfer coefficients and two-phase pressure drops of R410A are investigated in rectangular microchannels whose hydraulic diameters are 1.36 and 1.44 mm. The mass flux was varied from 200 to 400 kg/m²s, heat flux from 10 to 20 kW/m², as the saturation temperatures were maintained at 0, 5 and 10 °C. A direct heating method was used to provide heat flux into the fluid. The boiling heat transfer coefficients of R410A in the microchannels were much different with those in single tubes, and the test conditions only slightly affected the heat transfer coefficients before dryout vapor quality. The present heat transfer correlation for microchannels, which was developed by introducing non-dimensional parameters of Bo, We_l, and Re_l used in the existing heat transfer correlations for large diameter tubes, yielded satisfactory predictions of the present data with a mean deviation of 18%. The pressure drops of R410A in the microchannels showed very similar trends with those in large diameter tubes. The existing two-phase pressure drop correlations for R410A in microchannels satisfactorily predicted the present data.     

Experimental validation of glazed hybrid micro-channel solar cell thermal tile

In this communication, an attempt has been made to evaluate the theoretical performance of a glazed hybrid micro-channel solar cell thermal (MCSCT) tile. Experiment has been performed in indoor condition and it has been observed that there is good agreement between theoretical and experimental values with correlation coefficient and root mean square percentage deviation in range of 0.995–0.998 and 3.21–4.50 respectively. Effect of design parameters on different combination (series and parallel) of glazed hybrid MCSCT tile for Srinagar climatic condition, India has also been evaluated. The theoretical results of glazed hybrid micro-channel photovoltaic thermal (MCPVT) module for 75 W_p have been compared with the result of single channel photovoltaic thermal (SCPVT) module. The average value of electrical and thermal efficiency of glazed hybrid MCPVT module are 14.7% and 10.8% respectively which is significantly higher than SCPVT module. The overall annual exergy efficiency based on second law of thermodynamics has also been evaluated at different mass flow rate for glazed hybrid MCPVT module for Srinagar climatic condition. It has been observed that maximum overall exergy efficiency is 20.28% at 0.000108 kg/s mass flow rate.     

Comparison of candidate secondary electron emission materials

Secondary electron emission (SEE) yields obtained with empirical models deviate significantly from experiment. Therefore they cannot be used to predict the SEE data for various materials. The angular dependencies of SEE in empirical models are also drastically different and inconvenient for comparison. SEE coefficients were calculated by a theoretical method that uses Monte Carlo simulation, empirical theories, and close comparison to experiment, in order to parameterize the SEE yields of highly emissive materials. We have successfully applied this method to bulk Al₂O₃, a highly emissive material for micro-channel plates, as well as to thinly deposited films of Al₂O₃. The simulation results will be used in the selection of an emissive material, and if the emission yield of the material is small, as a resistive material for the deposition and characterization experiments that will be conducted by a large-area fast detector project at Argonne National Laboratory.     

Study of the plasma and heating effect on hydrogen permeation through Pd0.60-Cu0.40 membrane in a micro-channel plate reactor

The diffusion of hydrogen through palladium and palladium-copper alloys membrane have been provided the highest hydrogen selectivity and permeance. In this study the composite Pd_0.60-Cu_0.40 wt% membrane foil with thickness 20 μm was measured in the micro-channel plate reactor (MPR) with gap length 4.5 mm. The hydrogen permeation flux was measured at atmospheric feeding pressure for 100% H₂ concentration in the temperatures range of 423–573 K under heating only and plasma-heating experiments. The plasma firing high voltage source ranges of 10–18 kV are tested. The hydrogen permeation flux and hydrogen permeability have been calculated according to Fick's and Sieverts combining laws with power exponent n-value 0.5. It was found that the maximum hydrogen flux, hydrogen permeability and Permeation rate percent of the heating only experiment at MPR heating temperature of 573 K and flow rate 0.1 l/min. In the plasma heating experiment, it was observed that the maximum hydrogen flux, hydrogen permeability, and permeation rate percent at MPR heating temperature of 573 K and plasma firing voltage of 14 kV. Also, the hydrogen permeation rate percent decreased due to the hydrogen reverse reaction even though the plasma firing voltage increased to 16 kV and 18 kV. The results also reveal that the activation energy and Pre-exponential constant factor decreased with increasing the feeding H₂ flow rate while the linear regression R² decreased with increasing H₂ feeding flow rate that in the heating only experiment, in contrast, the plasma-heating experiment showed non-linearity values. A comparison between both experiments showed the hydrogen permeation flux of the plasma-heating experiment is higher than that obtained from the heating only experiment, additionally; the plasma effect increased at low hydrogen flow rates. In contrast, the energy efficiency of heating only experiment was higher than that obtained from the plasma-heating experiment due to the total energy consumption of plasma experiment is high.     

Multi-objective optimization of a rectangular micro-channel heat sink using the augmented ε-constraint method

ABSTRACT

This article proposes the use of an improved version of the ?-constraint method for the design and optimization of a rectangular micro-channel heat sink. This study aims to optimize the geometric parameters describing the micro-channel, namely its width, its fin width and its depth. The thermal resistance and the pumping power, considered as indicators of the thermal performance, have been approximated by surrogated models based on response surface approximation. The optimization task is formulated as a nonlinear programming problem. This approach has been implemented in GAMS. Sufficient details on both the single-criterion and multi-criteria formulation of the problem have been provided. The implications of the mathematical modelling formulation and the interrelationship between criteria and estimated quantities have been clarified. The use of the augmented ε-constraint method for the multi-objective optimization of a rectangular micro-channel heat sink constitutes the main contribution of this work.     

微通道中合成环氧脂肪酸甲酯

采用盐酸H2O2/HCOOH法,在微通道反应器内对不饱和脂肪酸甲酯进行环氧化反应。考察了双氧水用量、甲酸用量、反应温度及催化剂用量对反应的影响,得到最优的反应条件为:m(脂肪酸甲酯):m(甲酸):m(双氧水)=1:1.5:2,反应温度40℃,催化剂浓盐酸质量分数为3%(即浓盐酸质量占原料脂肪酸甲酯质量的百分数,下同),反应时间为110 s。在该条件下,产品环氧值为4.32%。     

Fabrication by vacuum die casting and simulation of aluminum bipolar plates with micro-channels on both sides for proton exchange membrane (PEM) fuel cells

Among manufacturing methods for bipolar plates, vacuum die casting is an ideal process because arbitrarily complicated shapes and mass production is possible with a high production rate. We report on the fabrication of bipolar plates with micro-channel arrays on both sides by vacuum die casting for use in proton exchange membrane (PEM) fuel cells. The formability, mechanical properties, and microstructures of samples fabricated under various experimental conditions—molten metal temperature, injection velocity, and vacuum assistance—are investigated, and the experimental and simulation results are compared. The die cavity, which is equal to the bipolar plate area, is 200 mm long, 200 mm wide, and 0.8 mm thick. The active area of the channel is 110 mm × 150 mm, and the total plate thickness is 1.1 mm (the width and depth of the channel are 1 and 0.3 mm, respectively). The cast material used in this study is Silafont-36 alloy (Al–Si–Mg–Mn). Good quality samples with very few casting defects are obtained under the following conditions: molten metal temperature of 700 °C; injection velocities for slow and fast shots of 0.3 and 2.5 m s⁻¹, respectively; and vacuum pressure of 30 kPa.