Straight Parallel Channels Research Articles

A novel multi-channel applicator with a U-shaped channel for vaginal intracavity brachytherapy.

Endometrial cancer is one of the most common gynecological malignancies in the world. Vaginal brachytherapy is an important postoperative adjuvant treatment for endometrial cancer. However, a common problem with existing applicators is insufficient dose at the vaginal apex. This study describes the Hangzhou (HZ) cylinder, a novel 3D printed vaginal intracavity brachytherapy applicator, detailing its characteristics, dose distribution, and clinical applications. The HZ cylinder is distinguished by its unique structure: a U-shaped channel with a 2mm diameter, a straight central axis channel of the same diameter, and 10 parallel straight channels. For comparison, standard plans were employed, designed to ensure that a minimum of 95% of the prescribed dose reached 5mm beneath the mucosal surface. We conducted comparative analyses of mucosal surface doses and doses at a 5mm depth below the mucosa between the HZ cylinder and a conventional single-channel cylinder across various treatment schemes. Additionally, the study examined dose differences in target volume and organs at risk (OARs) between actual HZ cylinder plans and hypothetical single-channel plans. In the standard plans, mucosal surface doses at the apex of the vagina were 209.32% and 200.61% of the prescribed dose with the HZ and single-channel cylinders, respectively. The doses on the left and right wall mucosal surfaces varied from 149.26% to 178.13% and 142.98% to 180.75% of the prescribed dose, and on the anterior and posterior wall mucosal surfaces varied from 128.87% to 138.50% and 142.98% to 180.75% of the prescribed dose. Analysis of 24 actual treatment plans revealed that when the vaginal tissue volume dose covering 98% (vaginal D98%) was comparable between the HZ cylinder and virtual single-channel plans (6.74±0.07Gy vs. 6.69±0.10Gy, p=0.24), rectum doses of HZ cylinder plans were significantly lower than those of single-channel plans (D1cc, 5.96±0.56Gy vs. 6.26±0.71Gy, p=0.02 and D2cc, 5.26±0.52Gy vs. 5.56±0.62Gy, p=0.02). The HZ cylinder demonstrates a reduction in dose to the rectum and bladder while maintaining adequate target volume coverage. Its mucosal surface dose is comparable to that of the traditional single-channel cylinder. These findings suggest that the HZ cylinder is a viable and potentially safer alternative for vaginal brachytherapy, warranting further investigation with larger sample sizes.

Manifold Design in a PEM Fuel Cell Stack to Improve Flow Distribution Uniformity

In this paper, a numerical study was performed to investigate the flow distribution in a 52-cell proton exchange membrane (PEM) fuel cell stack. The non-uniformity factor and standard deviation parameters were used to determine the flow distribution uniformity. Flow channels of each bipolar plate were replaced with straight parallel channels filled with porous media to reduce computational costs. The effect of external and integrated humidifiers on the gas distribution among the channels was investigated. Using integrated humidifiers improved the non-uniformity factor and standard deviation by 35% and 19%, respectively. Two methods were employed to improve the flow distribution: gradual reduction of the manifold height, and installing a bump at the bottom wall of the inlet manifold. Reducing the height of the inlet manifold in the stack with integrated and external humidifiers decreased the non-uniformity factor by 62% and 44%, respectively. The installation of the bump on the manifold wall enhanced flow distribution in the stack with the external humidifier. The results show that by using an integrated humidifier in this method, the flow distribution became more non-uniform. The best flow distribution in the stack was obtained with an integrated humidifier and a 90% reduction in manifold height. In this case, the flow rate passing through each channel was more than 99% of the average mass flow rate passing through the entire channel.

Enhancing Heat Removal and H2O Retention Capability of Passive Air-Cooled Polymer Electrolyte Membrane Fuel Cells by Tailoring Cathode Flow-Field Design

This paper reports novel cathode flow-field designs for passive typed air-cooled polymer electrolyte membrane fuel cells (PEMFCs) to help alleviate electrolyte dehydration and performance degradation issues under excess dry air supply conditions. The proposed flow-field designs include 7 three-dimensional (3D) patterned designs in addition to a parallel channel configuration equipped with rectangular baffles to control the airflow for more efficient heat removal. The designs were evaluated numerically using 3D, two-phase PEMFC simulations. Compared to a typical parallel flow channel configuration, the proposed flow-field designs show better heat removal and water retention capability. The improvement in single cell voltage was around 13–75 mV at an operating current density of 0.5 A cm−2, whereas the larger pressure drops around = 6.9–317.6 Pa cm−1 were required because of the more complex flow-field configurations compared to the simple straight parallel channel geometry ( = 7.4 Pa cm−1). This work presents a comprehensive understanding of air-cooled PEMFC operating characteristics under excessive dry air supply conditions and a new design strategy for cathode flow-fields.

Block structure optimization in PEMFC flow channels using a data-driven surrogate model based on random forest

ABSTRACT Rational design of flow channel can promote the removal of liquid water and effectively improve the performance of proton exchange membrane fuel cell (PEMFC). Compared with the conventional parallel straight channels, the block flow channel has a better water removal effect and is more beneficial to the fuel cell performance. In this study, a random forest-trained surrogate model is combined with a genetic algorithm to achieve optimization of the block structure applied in a novel two-block flow channel. A three-dimensional multiphase model based on the volume of fluid (VOF) method is employed to obtain the data sets by varying the block structure (length, height, and width) which are used to train a random forest-based data-driven surrogate model for predicting the water removal time (T) and the maximum pressure drop (∆P) of the channel. Then, the data-driven surrogate model is integrated into the genetic algorithm to optimize the design of the block. The results show that the output values of the surrogate model trained by random forest fit well with the actual values of the physical model, indicated by small mean square error (MSE) of T and ∆P which are 0.1191 and 0.0209, respectively. After optimization by the genetic algorithm, the block parameters of 0.6 mm in length, 0.375 mm in width, and 0.75 mm in height are found to be optimal, and the results are validated by the physical model.

Effect of gradually expanding flow passages on flow boiling of micro pin fin heat sinks

In this paper, effect of gradually expanding flow passages on saturated flow boiling characteristics of micro pin fin heat sinks is experimentally investigated. In this regard, a comparative analysis is performed based on three different heat sinks having straight parallel channels, uniformly distributed micro pin fins, and decreasing number of micro pin fins in flow direction. For these three geometries, visualization supported experiments are conducted, approximately, at the mass flux of 212 kg m–2 s–1, inlet temperature of 75 ± 1 ᵒC and effective heat fluxes of 132 – 272 kW m–2. Working fluid was deionized water. It is concluded that gradually expanding flow passages towards the outlet significantly improves the two-phase heat transfer coefficient up to nearly 501% and 365% compared to straight parallel channels and uniformly distributed micro pin fins, respectively. The structure having decreasing number of micro pin fins obviously suppresses flow boiling instabilities, and also presents lower pressure drop (lower up to 1.45 times) than uniformly distributed micro pin fins. Gradually enlarging flow passages provide escape-ways in the highly saturated region; thus, accelerate the flow and shorten the waiting period of vapor on heat transfer surface. Important conclusions are obtained regarding flow mechanism via flow images.

Three-dimensional simulation of different flow fields of proton exchange membrane fuel cell using a multi-phase coupled model with cooling channel

A suitable cooling flow field design for proton exchange membrane fuel cell (PEMFC) improves the cell's net generated power, besides achieving steady cell performance and a longer lifespan. The innovation in this work lies in the simultaneous simulation of electrochemical and cooling models while accounting for both thermal and electrical contact resistance between the gas diffusion layer and bipolar plates. In this study, flow field designs including straight parallel channels (Case A), straight parallel channels filled with metal foam (Case B), multi-channel serpentine (Case C), novel serpentine channels (Case D), and integrated metal foam (Case E) used for both gas channels and cooling channels are numerically simulated. Results show that the highest uniformity of temperature in the catalyst layer-gas diffusion layer interface is obtained in Case D, which has the largest pressure drop compared to Cases B, C, and E. However, due to the uniform distribution of reactant flows, the maximum temperature observed in the catalyst layer of this flow field was the lowest compared to the rest of the cases. Furthermore, the maximum power density of 0.75 Wcm−2 was observed in Case D at a corresponding voltage of 0.6 V, which reduced when the effect of high pressure drop was taken into account. Following the conclusion of the simulation and analysis, Case D displayed the best cooling performance while Case E produced the maximum net power output.

Hydrodynamic analysis of flow in raceway ponds for algae cultivation under versatile conditions

Open looped channels are an energy-efficient and low-cost solution often adopted for the cultivation of algae. These channels are commonly composed by two parallel straight channels interconnected by two curving sections and, as such, are referred as raceway ponds. The scaling up of algae production systems, usually associated with novel usages of algal products, has created new challenges for raceway-based cultivation systems. In particular, the uncertainties of cultivating novel algae cultures, e.g. macroalgae for which optimal inland cultivation conditions are still under-researched, impose further demands for raceway design and operation. Versatile operational conditions for raceways may be thus desirable for pioneering inland cultivation systems for certain species, applications and climate conditions. In this work, raceways of standard design are evaluated in terms of geometry, specifically raceway length, width and operational conditions, i.e. flow depth and operational velocity, with the use of a CFD tool. The hydrodynamic performance of the studied cases is analysed based on velocity uniformity and power consumption.

Anode Monitoring By Electrochemical Impedance Spectroscopy in Polymer Electrolyte Membrane Fuel Cells

Although widely used to study the operation of Polymer Exchange Membrane Fuel Cells (PEMFC), Electrochemical Impedance Spectroscopy (EIS) remains a complex tool: experimental data are somewhat delicate to interpret because of the wide range of physical phenomena occurring in Membrane Electrode Assemblies (MEA) and in the flow field plates. Therefore, impedance models are often based on oversimplified equations or conversely, include too many correlated parameters. A typical example of the complexity of the phenomena governing fuel cell electrical behavior are the oscillations of oxygen concentration resulting from the measuring signal, that propagate along the gas channels and makes more difficult the interpretation of the low frequency loop [1, 2, 3]. In addition, most of the impedance models assume Fickian oxygen diffusion although, at least in fuel cells fed with air, Stefan-Maxwell equations should be used as in most stationary models [4, 5]. In this regard, oxygen diffusion and thus the low frequency impedance are strongly dependent on the overall convective flux and as a result, on water management [6].However, due to the characteristic times of oxygen diffusion, these limitations do not apply to the high frequency region of the impedance spectra. The high frequency impedance of a PEMFC remains mostly governed by the cathode [7] but at least in some cases, considering the anode in the Electrical Equivalent Circuit (EEC) is necessary to improve the quality of the fit of the model to the experimental data (Figure). Since the anode has an impact on fuel cell impedance, this means that it becomes possible to monitor its ageing while performing Accelerated Stress Tests (AST). Since most of FC lifetime issues presumably originate from the cathode [8, 9] anode degradation has been rarely studied in the literature. Nevertheless, Schwämmlein et al. showed recently that it may be significant in the case of repeated start-up and shut-down - because of potential cycling - and have a measurable impact on FC performance [10].In this work, we performed AST combining potential and humidity cycles to be as representative as possible of the stressful conditions encountered by the FC in real operating conditions. Commercially available MEA with an initial voltage of the order of 0.7 V at 0.5 A/cm² showed a drop in performance comprised between 400 and 900 µV/h, and a significant degradation of the anode operation was observed by EIS (Figure), confirmed by an increase of its local potentials, and linked to Pt ElectroChemical Surface Area (ECSA) losses. Post-mortem physico-chemical analysis of MEA will be carried out to bridge the decrease of performance with the degradation mechanisms occurring at the anode catalytic layer. Note that these tests were carried out using a 30 cm² segmented cell with 5 straight parallel gas channels [11] (30×1 cm² active area) and that its overall performances may be lower than with regular cells.

Numerical Investigation of Incompressible Forced Convection in a Channel with a Rectangular Prism

The aim of the present study is to investigate the effects of the rectangular prism on forced convection in a channel with the Lattice Boltzmann Method (LBM). In this context, numerical analysis of steady and unsteady incompressible flow and heat transfer has been done in a two-dimensional straight parallel channel. Momentum and energy transport are modelled with LBM. This study is used as a single relaxation time rule with a uniform square lattice structure. Different Reynolds numbers (50 and 1000) and constant Prandtl number value (0.7) have been investigated. In this study Nusselt number has been calculated for channel flow with rectangular prism and it was compared with an empty channel. Streamlines and isotherms were presented for the above-mentioned cases. LBM results were validated by commercial CFD code with the same conditions. It is found from results that Nusselt number with a rectangular prism in a channel was increased and the flow goes to transient form at Re=1000. Also, LBM code results are similar accuracy with commercial CFD code.

Pre-deposition layers for alleviating ultrafiltration membrane fouling by organic matter: Role of hexagonally and cubically ordered mesoporous carbons

In this study, ordered mesoporous carbons (OMCs), i.e., CMK-3 and CMK-8 were pre-deposited on ultrafiltration membrane surface to alleviate membrane fouling by organic matter in the water. CMK-3 shows a typical 2D hexagonal symmetrical structure with long straight parallel channels, while CMK-8 exhibits a 3D porous structure with cubic channels. The pre-deposited CMK-3/CMK-8 layers exhibited significantly superior performance for both flux enhancement and fouling mitigation, whereas powdered activated carbon (PAC) showed limited influence. The fouling mechanism was apparently altered by the OMCs pre-deposition layers, and pore blocking always dominated throughout the filtration process without the formation of cake filtration. In comparison with PAC, CMK-3 and CMK-8 were more effective for the rejection of tryptophan-like protein substances, which were the major foulants causing membrane fouling. The effects of pollutant fraction and deposited amount (i.e., 6, 12, 25 and 50 g/m2) were systematically explored, and the results indicated that this technology was more efficient for alleviating the fouling induced by humic acid and bovine serum albumin, whereas ineffective for sodium alginate fouling, and the optimal deposited amount varied with the pollutant fraction. CMK-3 exhibited significantly superior performance than CMK-8, likely due to its long straight parallel channels with lower internal resistance for pollutants adsorption. Both adsorption and size exclusion of the pre-deposited layers contributed to fouling mitigation. These results demonstrate that pre-depositing with OMCs as alternatives to conventional PAC shows promising potential for mitigating membrane fouling in water treatment.

Flow Analysis Based on Cathodic Current Using Different Designs of Channel Distribution In PEM Fuel Cells

In this work, a physical and numerical simulation of cathodic current for different designs of the channel distribution in PEM fuel cells was carried out. The first design consisted serpentine-type channels with abrupt changes in flow direction. On the other hand, Designs 2 and 3 were made of serpentine channels with a more gradual change in flow direction. The fourth design was a crisscross-type channel, which was based on continually redirecting the flow, while Design 5 was made with straight parallel channels. Designs 1–3 had one intake, while Designs 4 and 5 had three. The latter two produced more uniform electrical current distributions than Designs 1–3. It can be concluded that the intakes situated effectively within each design were as important as the shape of the channel configuration. Finally, the parallel channel flow field (Design 5) was the best alternative for current collectors due to its better performance.

Between Physical Complexity and Simplicity: Nailing Down the Cathodic Activation Overpotential in a Small-Scale PEM Fuel Cell

Understanding the kinetics of polymer electrolyte fuel cells (PEFC) remains a key issue for the optimization of the catalyst layer structure with respect to fuel cell performance and ageing. In this study, firstly the impact of hydrogen crossover from anode to cathode through the membrane on the measurable open circuit voltage (OCV) is analyzed extensively. For this purpose, all the other parasitic reactions like platinum oxide formation or carbon corrosion are neglected in a first approach and the influences of the operating conditions on the crossover flux by means of voltammetry measurements and also mass spectrometry in the cathode exhaust gas are examined. These experimental results yield an efficient parameterization of the crossover rate. Incorporated into a physical 1D through-plane cell simulation model, it allows for consistent predictions at and close to OCV conditions. Subsequently, the activation overpotential is parameterized experimentally thanks to electrochemical impedance spectroscopy on a 25 cm2 cell with a straight parallel channel flowfield under differential conditions. Therefore, spectra are recorded for a large range of conditions (Fig. 1a) and the distribution of relaxation times (DRT) is evaluated (Fig. 1b). As a result, an equivalent circuit is set up. Hence, it is possible to fit the experimental data and to extract the kinetic parameters for our model, as functions of electrical load, oxygen partial pressure and temperature. We finally use this extensive dataset for comparing different approaches for modelling the oxygen reduction reaction (ORR) on platinum, ranging from single-step Butler-Volmer kinetics to advanced microkinetic models that take into account the intermediates of the ORR and thereby also the potential dependent surface coverage of the electrode [1-2].

Inertial instabilities in a microfluidic mixing-separating device

Combining and separating fluid streams at the microscale has many scientific, industrial, and medical applications. This numerical and experimental study explores inertial instabilities in so-called mixing-separating micro-geometries. The geometry consists of two straight square parallel channels with flow from opposite directions and a central gap that allows the streams to interact, mix, or remain separate (often also referred to as the H-geometry). Under creeping-flow conditions (the Reynolds number tending to zero), the flow is steady, two-dimensional, and produces a sharp interface between fluid streams entering the geometry from opposite directions. When Re exceeds a critical value, one of two different supercritical, inertial instabilities appears which leads to significant changes in the flow pattern and an increased level of interaction between the two streams, although the flow remains steady. The exact form of the instability is dependent on the gap size and the Reynolds number, and we identify two distinct instabilities, one of which appears in devices with large gaps and another which appears in devices with small gaps. At intermediate gap sizes, both instabilities can occur in the same device (at different onset Re). The experimental results for one gap size are used to validate our numerical method, which is then applied to a wider range of gap sizes. The results suggest that the gap size is of primary importance in determining the type of instability that occurs. With a judicious choice of gap size, the instabilities can be exploited (or avoided) in scientific, medical, or other microfluidic applications.

Performance of electrochemical cell with various flow channels for Bunsen reaction in the sulfur–iodine hydrogen production process

Renewable hydrogen production through sulfur–iodine (SI) cycle is energy efficient and environmentally friendly. Electrochemical cell (EC) has recently been proposed to carry out the Bunsen reaction in the cycle. Existing studies on the EC are limited only to time-consuming experiments whereas cell performance with respect to flow channel is very few. In this study, numerical study is conducted for determining the impact of flow channel on the EC performance of electrochemical Bunsen reaction. Five different configurations of flow channel are physically modelled. A three dimensional, steady state and laminar flow through EC is simulated by solving the governing equations, and show good agreement with experimental data at low current density. It is found that the flow field, local current and potential of serpentine, straight parallel and spiral tubular flow channels distribute almost uniformly, except the significant vortex regions and high resistance at the square bends of straight parallel channel due to the sudden geometric changes. The flow maldistribution in Z-type and U-type parallel flow channels makes them unsuitable for EC. The straight parallel flow channel is found to be optimal due to its low pressure drop as well as low electric power input, followed by serpentine and spiral tubular patterns. The simulation concerning flow channels design favors improving EC performance and the thermal efficiency of SI cycle.

Tracking the Evolution of Mechanical Degradation in Fuel Cell Membranes Using 4D in Situ Visualization

During automotive operation of polymer electrolyte fuel cells, dynamic duty cycles can gradually introduce various damage features within the ionomer membrane, viz. cracks, tears, pinholes, thinning, delamination etc., collectively leading to ultimate operational failure of the fuel cell. There are two main degradation mechanisms that drive damage development within the membrane: (i) chemical degradation due to radical attack; and (ii) mechanical degradation due to hygrothermal variations under mechanically constrained conditions. Although both of these mechanisms are simultaneously active during fuel cell operation, analyzing them in an uncoupled manner could aid in assessing the role of individual damage features in overall membrane failure which could potentially result in the development of effective failure mitigation strategies. Failure analysis studies of fuel cell membranes typically employ scanning electron microscopy (SEM) which is two-dimensional (2D) in nature and is inherently destructive, thereby inhibiting any possibility to track degradation-induced structural changes over time. Consequently, membrane degradation evolution studies are limited to ex situ analysis of different samples at various stages of degradation. In our recent work, laboratory-based X-ray computed tomography (XCT) was utilized to perform three-dimensional (3D) ex situ failure analysis of fuel cell membranes and the 3D nature of this imaging technique revealed novel insights into membrane failure that had thus far eluded the traditional 2D investigations [1]. Additionally, XCT imaging is also non-destructive and non-invasive [2], and the present work leverages these attributes with the objective to extend the 3D failure analysis approach to an in situ investigation of pure mechanical membrane degradation , following a unique 4D approach wherein the fourth dimension represents time or degradation state. This is achieved by utilizing a custom designed X-ray transparent fuel cell fixture [3] with which 3D visualization of identical membrane locations can be performed periodically, thereby offering a novel approach to track the structural/morphological evolution of the membrane in its true sense as a function of degradation. The custom designed X-ray transparent fixture was made with a 9 mm (length) x 4 mm (width) active area and consisted of two co-flow parallel straight channels each having 1 mm width and separated by a 250 μm wide central land region with additional land regions at the two peripheral sides. After assembling a single fuel cell within it, pure mechanical degradation in form of in situ hygrothermal fatigue was generated within the membrane by subjecting the assembled fuel cell held at 80°C to successive cycles of 2 min wet and 2 min dry states with nitrogen gas on both anode and cathode sides to eliminate chemical degradation. A laboratory-based XCT system, ZEISS Xradia 520 Versa® , was used to obtain 3D tomographic images at two different length scales: (i) low resolution (2.1 μm voxel size) large field of view (FOV) scans for inspecting the overall membrane damage; and (ii) high resolution (1.1 μm voxel size) zoomed scans of selected regions of interest for a detailed structural investigation. Tomographic data of identical locations were acquired periodically at every 500 wet/dry cycles to track membrane damage development over time. No cracks had appeared within the membrane up to 1500 wet/dry cycles, whereas a significant number of through-thickness membrane cracks had developed at 2000 cycles. This result suggests that fatigue-driven mechanical degradation progresses non-linearly over time via distinct crack initiation/propagation events. The majority of the membrane crack development occurred under the channel regions which is consistent with higher tensile stresses predicted in these regions by simulation studies [4]. A strong correlation was observed between the presence of beginning-of-life (BOL) MEA defects, mainly cathode catalyst layer cracks and membrane—catalyst layer delamination, and eventual formation of membrane cracks at those locations (cf. Figure 1). In many cases, the shape of newly developed membrane cracks resembled that of the BOL catalyst layer cracks suggesting that localized stress concentration effects may influence both crack initiation and propagation within the membrane. Overall, the novel approach for 4D same-location tracking of membrane degradation reported in this work shows significant potential for improved fundamental understanding of the membrane crack development process during mechanical degradation in fuel cells. Acknowledgements Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.

Mass transport enhancement in redox flow batteries with corrugated fluidic networks

Abstract We propose a facile, novel concept of mass transfer enhancement in flow batteries based on electrolyte guidance in rationally designed corrugated channel systems. The proposed fluidic networks employ periodic throttling of the flow to optimally deflect the electrolytes into the porous electrode, targeting enhancement of the electrolyte-electrode interaction. Theoretical analysis is conducted with channels in the form of trapezoidal waves, confirming and detailing the mass transport enhancement mechanism. In dilute concentration experiments with an alkaline quinone redox chemistry, a scaling of the limiting current with Re0.74 is identified, which compares favourably against the Re0.33 scaling typical of diffusion-limited laminar processes. Experimental IR-corrected polarization curves are presented for high concentration conditions, and a significant performance improvement is observed with the narrowing of the nozzles. The adverse effects of periodic throttling on the pumping power are compared with the benefits in terms of power density, and an improvement of up to 102% in net power density is obtained in comparison with the flow-by case employing straight parallel channels. The proposed novel concept of corrugated fluidic networks comes with facile fabrication and contributes to the improvement of the transport characteristics and overall performance of redox flow battery systems.

Process intensification characteristics of a microreactor absorber for enhanced CO2 capture

Gas separation processes, including post-combustion carbon capture (PCCC) by chemical absorption using liquid solvents can be substantially enhanced using high performance micro-structured surfaces to enhance the surface area available for reaction. The present paper studies the hydrodynamics and mass transfer performance of gas–liquid absorption of CO2 into aqueous diethanolamine in a micro-structured reactor. The system was designed to comprise 15 straight parallel channels in a cross flow inlet configuration. The hydraulic diameter of each channel was 456μm. The performance of the reactor was studied with respect to the absorption efficiency, mass transfer coefficient, acid gas loading ratio, and pressure drop. A flow pattern map was developed using available regime transition criteria. Parametric studies varying the gas and liquid flow rates, as well as their respective concentrations at the reactor inlet, were conducted. The two-phase pressure drop was compared against the predictions of a piecewise model and a reasonably good agreement was obtained. Absorption efficiencies close to 100% were observed under certain operating conditions. The presently achieved values of liquid-side volumetric mass transfer coefficients were between 1–3 orders of magnitude higher than those reported for most conventional gas–liquid absorption systems, which can be attributed to the inherent high specific interfacial area provided through micro-structured surfaces. The results reported here indicate the substantial levels of process intensification that can be achieved using microreactors.

Influence of void space on microscopic behavior of fluid flow in rock joints

Advanced microfluidic technology was used to examine the microscopic viscous and inertial effects evolution of water flow in rock joints. The influence of void space on fluid flow behaviour in rock joints under different flow velocities was experimentally investigated at the micro scale. Using advanced fabrication technology of microfluidic device, micro flow channels of semicircular, triangular, rectangular and pentagonal cavities were fabricated to simulate different void space of rock joints, respectively. Using the fluorescence labelling approach, the trajectory of water flow was captured by the microscope digital camera when it passed over the cavity under different flow velocities. The flow tests show that the flow trajectory deviated towards the inside of the cavity at low flow velocities. With the increase in flow velocity, this degree of flow trajectory deviation decreased until there was no trajectory deviation for flow in the straight parallel channel. The flow trajectory deviation initially reduced from the void corner near the entrance. At the same time, a small eddy appeared near the void corner of the entrance. The size and intensity of the eddy increased with the flow velocity until it occupied the whole cavity domain. The gradual reduction of flow trajectory near the straight parallel channel and the growth of eddy inside the cavity reflect the evolution of microscopic viscous and inertial forces under different flow velocities. The eddy formed inside the cavity does not contribute to the total flow flux, but the running of the eddy consumes flow energy. This amount of pressure loss due to voids could contribute to the nonlinear deviation of fracture fluid flow from Darcy’s law. This study contributes to the fundamental understanding of non-Darcy’s flow occurrence in rock joints at the micro scale.

Performance and Reliability Analysis of Hybrid Concentrating Photovoltaic/Thermal Collectors With Tree-Shaped Channel Nets' Cooling System

Excess temperatures on concentrating photovoltaic (PV) modules can lead to a decrease in electrical efficiency and irreversible structural damage. Therefore, designing an appropriate cooling system becomes necessary for the lifetime and performance of concentrating PV (CPV) modules. The basic design considerations for cooling systems include low and uniform cell temperatures, minimal pumping power, high PV efficiencies, and system reliability. In this paper, a 3-D multiphysics computational model for a hybrid concentrating photovoltaic/thermal (HCPV/T) water collector is developed. The collector consists of a solar concentrator, 40 silicon cells connected in series, and a multichannel liquid cooling system with heat-recovery capability. A conjugate heat transfer model is used, assuming laminar flow through either parallel or tree-shaped branching cooling channels. The temperature distributions within the PV cells are determined for different cooling strategies. Comparisons are made by considering the thermal and electrical performances, such as PV cell temperature, electrical efficiency, and outlet water temperature, between a system incorporating tree-shaped channel networks and one having straight parallel channel cooling arrays. For identical convective surface area and pumping pressures in both configurations, the tree-shaped branched channel cooling networks yield lower PV cell temperatures and more uniform temperature distributions within the PV cells. Additionally, a finite-element mechanical analysis is used to estimate the fatigue life of the PV modules based on the temperature profiles obtained from both cooling channel configurations under a specified pumping pressure. The model results predict that the fatigue life of the module with the branched channels is almost twice that of the module with straight channels.

Fabrication and Characterization of Highly Ordered Porous Alumina Templates by a Two-Step Anodization Process

Highly ordered through-hole anodic porous alumina membranes were fabricated by electrochemical oxidation of aluminum in a controlled two-step process. A teflon dispositive was used to ensure single side anodization. Under the most appropriate condition for the fabrication of ideally ordered anodic aluminum oxide (AAO), the voltage used was 15 V during 24 h in a 15 % w/v sulfuric acid solution. SEM, TEM and FESEM characterization shows that the as-fabricated AAO film has a defect-free array of straight parallel channels perpendicular to the surface. The thickness of the porous membrane is 20 microns, approximately. The ordered channels are formed in a honey comb arrange with a pore diameter in the range 20-30 nm, wall thickness of 10-20 nm, interpore distance of 40 nm, and high aspect ratio of 850. The pore density, quantified by image analysis, is 5.4×1010 pore/cm2; perfect ordering was maintained in the full depth of the membrane. Dimensions of this porous structure provide a convenient way to precision engineer the nanoscale morphology.