Flow Resistor Research Articles

Numerical Investigation of Flow Distribution in PEMFC and Pemwe Stacks Considering Two-Phase Flow in the Stack Headers

PEM fuel cell (PEMFC) and PEM water electrolyzer (PEMWE) stacks are assembled by connecting a series of unit cells. The stack headers are conduits formed by the ports of these unit cells; see Fig. 1(a). The flow field in the headers affects the flow uniformity of the stack, which significantly impacts the stack’s performance and lifespan. This study aims to investigate the fluid flows in the stack headers of PEMFC and PEMWE. A specific objective of this study is to understand how two-phase flow (liquid water and gases) in the headers would impact the flow distribution inside the stacks.The present study investigates the flow inside a stack by two-dimensional (2D) and three-dimensional (3D) models, as shown in Figure 1(b) and (c) respectively. The 3D model was constructed with an inlet header and an outlet header with flow resistors in between to represent the unit cells.1,2 CFD simulations using ANSYS Fluent were performed over a 100-cell PEMFC stack based on the k-ε RNG turbulence model. The two-dimensional 100-cell PEMFC stack model was built with COMSOL Multiphysics and considers the diffusion and transport, proton and electron conduction, electrochemical reactions on the catalytic layer, and heat transfer in the unit cells. The flow distribution obtained from the 3D model was used as the boundary conditions for the 2D model to calculate the effects of flow distribution on the overall performance of the stack. It should be pointed out that in the 3D models of PEMFC and PEMWE headers, simulations using the volume of fluid (VOF) model to track the liquid water droplets in a PEMFC and bubbles in a PEMWE were carried out to gain insights into the impact of two-phase flow on stack header flow sharing.The simulation results show the existence of unstable and highly transient turbulence within the PEMFC outlet header, as seen in Figs. 1(d) and 1(e). Jet flows from individual cells cause vortices inside the header, reducing the effective flow area. Liquid water accumulates on the inner wall of the header and is swept out by the cross-flow. In the dead-end region, liquid water undergoes helical motion under the influence of vortices, resulting in a longer residence time in the header and making it less prone to be expelled. This study parametrically investigated the effects of header size and air stoichiometry. As the cross-sectional area of the inlet header decreases from A=1,225 mm2 to A=857.5 mm2, the coefficient of mass flow variation within the stack increases, indicating a decrease in the uniformity of flow distribution inside the stack, as shown in Fig. 1(f). The air stoichiometry also affects the flow distribution inside the stack; as the air stoichiometry λ decreases from 2.2 to 1.4, the uniformity of gas distribution in the stack deteriorates significantly.The methodology used in the present study provides a reference for the parameter design and operating conditions of PEMFC and PEMWE stacks. It helps to gain insights into the optimization of stack header design and energy management.

A gravity-driven droplet fluidic point-of-care test

We report a simple droplet fluidic point-of-care test (POCT) that uses gravity to manipulate the sequence, timing, and motion of droplets on a surface. To fabricate this POCT, we first developed a surface coating toolbox of nine different coatings with three levels of wettability and three levels of slipperiness that can be independently tailored. We then fabricated a device that has interconnected fluidic elements-pumps, flow resistors and flow guides-on a highly slippery solid surface to precisely control the timing and sequence of motion of multiple droplets and their interactions on the surface. We then used this device to carry out a multi-step enzymatic assay of a clinically relevant analyte-lactate dehydrogenase (LDH)-to demonstrate the application of this technology for point-of-care diagnosis.

Pulsatile cerebral paraarterial flow by peristalsis, pressure and directional resistance

BackgroundA glymphatic system has been proposed that comprises flow that enters along cerebral paraarterial channels between the artery wall and the surrounding glial layer, continues through the parenchyma, and then exits along similar paravenous channels. The mechanism driving flow through this system is unclear. The pulsatile (oscillatory plus mean) flow measured in the space surrounding the middle cerebral artery (MCA) suggests that peristalsis created by intravascular blood pressure pulses is a candidate for the paraarterial flow in the subarachnoid spaces. However, peristalsis is ineffective in driving significant mean flow when the amplitude of channel wall motion is small, as has been observed in the MCA artery wall. In this paper, peristalsis in combination with two additional mechanisms, a longitudinal pressure gradient and directional flow resistance, is evaluated to match the measured MCA paraarterial oscillatory and mean flows.MethodsTwo analytical models are used that simplify the paraarterial branched network to a long continuous channel with a traveling wave in order to maximize the potential effect of peristalsis on the mean flow. The two models have parallel-plate and annulus geometries, respectively, with and without an added longitudinal pressure gradient. The effect of directional flow resistors was also evaluated for the parallel-plate geometry.ResultsFor these models, the measured amplitude of arterial wall motion is too large to cause the small measured amplitude of oscillatory velocity, indicating that the outer wall must also move. At a combined motion matching the measured oscillatory velocity, peristalsis is incapable of driving enough mean flow. Directional flow resistance elements augment the mean flow, but not enough to provide a match. With a steady longitudinal pressure gradient, both oscillatory and mean flows can be matched to the measurements.ConclusionsThese results suggest that peristalsis drives the oscillatory flow in the subarachnoid paraarterial space, but is incapable of driving the mean flow. The effect of directional flow resistors is insufficient to produce a match, but a small longitudinal pressure gradient is capable of creating the mean flow. Additional experiments are needed to confirm whether the outer wall also moves, as well as to validate the pressure gradient.

Portable comprehensive two-dimensional micro-gas chromatography using an integrated flow-restricted pneumatic modulator

Two-dimensional (2D) gas chromatography (GC) provides enhanced vapor separation capabilities in contrast to conventional one-dimensional GC and is useful for the analysis of highly complex chemical samples. We developed a microfabricated flow-restricted pneumatic modulator (FRPM) for portable comprehensive 2D micro-GC (μGC), which enables rapid 2D injection and separation without compromising the 1D separation speed and eluent peak profiles. 2D injection characteristics such as injection peak width and peak height were fully characterized by using flow-through micro-photoionization detectors (μPIDs) at the FRPM inlet and outlet. A 2D injection peak width of ~25 ms could be achieved with a 2D/1D flow rate ratio over 10. The FRPM was further integrated with a 0.5-m long 2D μcolumn on the same chip, and its performance was characterized. Finally, we developed an automated portable comprehensive 2D μGC consisting of a 10 m OV-1 1D μcolumn, an integrated FRPM with a built-in 0.5 m polyethylene glycol 2D μcolumn, and two μPIDs. Rapid separation of 40 volatile organic compounds in ~5 min was demonstrated. A hybrid 2D contour plot was constructed by using both 1D and 2D chromatograms obtained with the two μPIDs at the end of the 1D and 2D μcolumns, which was enabled by the presence of the flow resistor in the FRPM.

Contactless entropy measurement with infrared sensors for degradation monitoring

Entropy is a valuable magnitude to monitor degradation processes and energy efficiency. Since no commercial equipment is available to measure entropy, this paper proposes a new contactless entropy measurement system based on infrared sensors. Infrared radiation of a black body, such an electric resistor, is measured with four different types of commercial sensors. Transport thermodynamics in terms of entropy is experimentally inferred from temperature and radiated power. Entropy flow rate from the black body, entropy flow rate received by the sensor and entropy generation rate during the transport process are evaluated. A simple and inexpensive electronic set-up is used to measure degradation in the resistor, which was powered above its power rating. A relationship between entropy flow rate and resistor degradation is found. The degradation modes, i.e., fatigue and failure, in the resistor can be distinguished using contactless entropy flow measurements.

A self-contained acoustofluidic platform for biomarker detection.

Self-contained microfluidic platforms with on-chip integration of flow control units, microreactors, (bio)sensors, etc. are ideal systems for point-of-care (POC) testing. However, current approaches such as micropumps and microvalves, increase the cost and the control system, and it is rather difficult to integrate into a single chip. Herein, we demonstrated a versatile acoustofluidic platform actuated by a Lamb wave resonator (LWR) array, in which pumping, mixing, fluidic switching, and particle trapping are all achieved on a single chip. The high-speed microscale acoustic streaming triggered by the LWR in the confined microchannel can be utilized to realize a flow resistor and switch. Variable unidirectional pumping was realized by regulating the relative position of the LWR in various custom-designed microfluidic structures and adoption of different geometric parameters for the microchannel. In addition, to realize quantitative biomarker detection, the on-chip flow resistor, micropump, micromixer and particle trapper were also integrated with a CMOS photo sensor and electronic driver circuit, resulting in an automated handheld microfluidic system with no moving parts. Finally, the acoustofluidic platform was tested for prostate-specific antigen (PSA) sensing, which demonstrates the biocompatibility and applied potency of this proposed self-contained system in POC biomedical applications.

Lab-on-a-Contact Lens Platforms Fabricated by Multi-Axis Femtosecond Laser Ablation.

Contact lens sensing platforms have drawn interest in the last decade for the possibility of providing a sterile, fully integrated ocular screening technology. However, designing scalable and rapid contact lens processing methods while keeping a high resolution is still an unsolved challenge. In this article, femtosecond laser writing is employed as a rapid and precise procedure to engrave microfluidic networks into commercial contact lenses. Functional microfluidic components such as flow valves, resistors, multi-inlet geometries, and splitters are produced using a bespoke seven-axis femtosecond laser system, yielding a resolution of 80µm. The ablation process and the tear flow within microfluidic structures is evaluated both experimentally and computationally using finite element modeling. Flow velocity drops of the 8.3%, 20.8%, and 29% were observed in valves with enlargements of the 100%, 200%, and 300%, respectively. Resistors yielded flow rate drops of 20.8%, 33%, and 50% in the small, medium, and large configurations, respectively. Two applications were introduced, namely a tear volume sensor and a tear uric acid sensor (sensitivity 16 mg L-1 ), which are both painless alternatives to current methods and provide reduced contamination risks of tear samples.

Biomechanics of urinary bladder: slow-filling and slow-emptying cystometry and accommodation.

OBJECTIVEIn a concept of accommodation of detrusor pressure to volume as an autonomous potency of the bladder, a crucial physiological biomechanical role has been attributed to spontaneous contraction activity. This concept is experimentally investigated on pig bladder in vitro.METHODSSlowly emptying of not-stimulated pig bladders via a flow resistor has been recorded and the effect of spontaneous contractions on the tonic pressure during emptying by expulsion has been studied.RESULTSThe expulsed volume can be separated in a reduction of elastic volume and of rest volume. Tonic pressure is determined by the elastic volume in combination with elastic compliance. In an accommodated state completely transient superimposed pressure waves affect rest volume not elastic volume. Accommodation of tonic detrusor pressure to bladder volume is based on equilibration between passive elongations and active transient contractions distributed in bladder wall.CONCLUSIONMaintenance of a tonic accommodated detrusor pressure to a constant or slowly varying volume, obtained by a process of equilibration between passive elongations and active contractions, can be understood as an autonomous potency of a bladder. The earlier presented concept of active accommodation has been validated by the experiments. The pressure-volume relation of the bladder is fundamentally revised. Total volume V can be virtually separated in an elastic volume VE and a plastic or rest volume VR. Both parts change with V and in changing ratio. Tonic pressure marks a border between VE and VR.

A profile shaping and surface finishing process of micro electrochemical machining for microstructures on microfluidic chip molds

In the lithography-based fabrication process of microstructures on microfluidic chip molds, the shortcoming of limited substrate materials seriously shortens the service lifetime of the mold. This research focuses on a profile shaping and surface finishing process of micro electrochemical machining (ECM) for microstructures with targeted dimensional and geometrical characteristics on the mold steel S136H. The effects of parameters on the material removal region and single-layer depth are firstly investigated. The sidewall taper of the machined groove increases with the machining voltage and decreases with the scanning velocity. Besides, the bottom surface quality is improved with increasing path spans. Then, the mathematical models of the material removal region and single-layer depth are analyzed, which are regarded as guidance for path planning and parameter optimization. In ECM experiments, a relatively higher machining voltage (20 V) is utilized to rapidly machine basic profiles of the designed flow resistor in the profiles shaping procedure. Sloping sidewalls and rough surfaces are finished by using a lower machining voltage (14 V) and a higher scanning velocity (500 μm s−1) in the surface finishing procedure. As a result, the specially designed flow resistor with a dimensional deviation < 10 μm, and surface roughness Ra < 500 nm is obtained. The proposed ECM process is proved to machine microstructures with high precision and curve profiles on the mold steel.

Passive Flow Control for Series Inflatable Actuators: Application on a Wearable Soft-Robot for Posture Assistance

This paper presents a passive control method for multiple degrees of freedom in a soft pneumatic robot through the combination of flow resistor tubes with series inflatable actuators. We designed and developed these 3D printed resistors based on the pressure drop principle of multiple capillary orifices, which allows a passive control of its sequential activation from a single source of pressure. Our design fits in standard tube connectors, making it easy to adopt it on any other type of actuator with pneumatic inlets. We present its characterization of pressure drop and evaluation of the activation sequence for series and parallel circuits of actuators. Moreover, we present an application for the assistance of postural transition from lying to sitting. We embedded it in a wearable garment robot-suit designed for infants with cerebral palsy. Then, we performed the test with a dummy baby for emulating the upper-body motion control. The results show a sequential motion control of the sitting and lying transitions validating the proposed system for flow control and its application on the robot-suit.

Reducing the hydrogen content in liquid helium

Helium has the lowest boiling point of any element in nature at normal atmospheric pressure. Therefore, any unwanted substance like impurities present in liquid helium will be frozen and will be in solid form. Even if these solid impurities can be easily eliminated by filtering, liquid helium may contain a non negligible quantity of molecular hydrogen. These traces of molecular hydrogen are the cause of a known problem worldwide: the blocking of fine capillary tubes used as flow resistors in helium evaporation cryostats to achieve temperatures below 4.2 K. This problem seriously affects a wide range of cryogenic equipment used in low temperature physics research and leads to a dramatic loss of time and costs due to the high price of helium. Here, we present first the measurement of molecular hydrogen content in helium gas. Three measures to decrease this molecular hydrogen are afterward proposed; (i) improving the helium quality, (ii) release of helium gas in the atmosphere during purge time for the regeneration cycle of the helium liquefier’s internal purifier, and (iii) installation of two catalytic converters in a closed helium circuit. These actions have eliminated all blockages of capillaries at low temperatures now for more than two years.

Flow modeling and experimental verification of flow resistors used in microfluidic chips driven by capillary force

A microfluidic chip driven by capillary force has the advantage of low cost and ease of manufacturing in batches, and its flow behavior is dominated by the geometry and surface characteristics of the microchannel. The design of mirochannel structures is very important for the microfluidic chips. This study presents a semi analytical method for the design of capillary microchannel. In this method, the quasi-steady state approximate solution method of the Young–Laplace equation is used to derive the capillary surface tension, and the parallel flow assumption based on the Reynolds equation is used to derive the resistance of the channel wall. A computational fluid dynamics simulation is used to provide the inlet effect coefficient and channel shape effect coefficient of this model. The availability of the semi analytical model is verified by the experiment. This model realizes the flow analysis of two-dimensional capillary flow channel with a continuous shape of the flow channel wall, providing a fast and accurate method for the structural design of the microfluidic chip driven by capillary force.

Performance Characteristics of Positive Expiratory Pressure Devices.

Positive expiratory pressure (PEP) therapy imposes expiratory flow resistance to increase airway diameter and enhance mucus clearance. PEP is achieved several ways. Oscillatory PEP devices (OPEP) generate repeated occlusions that are known to reduce mucus viscosity. There are many marketed devices, but comparative performance is mostly unreported. The purpose of this study was to evaluate performance characteristics of many PEP/OPEP devices. For OPEP devices, we defined an optimal performance metric by creating an oscillation index that combines the OPEP performance characteristics. PEP devices (TheraPEP, EzPAP, VersaPAP, Resistex, AccuPEP, AccuPAP, and Threshold PEP) and OPEP devices (Acapella DH, Acapella DM, Acapella Choice, ShurClear, Aerobika, VibraPEP, vPEP, and PocketPEP with and without the Oxyjet attachment) were tested by adjusting simulated expiratory flow from 5 L/min to 30 L/min in increments of 5 L/min using a standard flow meter. All devices showed varying performance characteristics. As expiratory flow increased, mean PEP increased for most devices. The TheraPEP showed a mean PEP of 13 cm H2O across all settings. For OPEP devices, there was a major difference between pressure and flow waveforms. The Acapella DH, ShurClear, and Aerobika showed the highest flow amplitude, flow frequency, and oscillation index. PEP devices behaved similarly and as expected, with increased pressure with increased flow (flow resistors) or flow independence (threshold resistors). There was much greater variation in the performance of the OPEP devices. A higher oscillation index indicates better mechanical performance characteristics. Many devices have similar characteristics. However, the devices with the highest oscillation index have the highest flow amplitude and frequency, which may indicate better clinical performance.

Modelling compressors, resistors and valves in finite element simulation of gas transmission networks

In previous papers, the authors introduced a new formulation of the flow equations, involving friction and gravity effects, in gas transportation networks. They also proposed finite element methods combined with Newton-like algorithms for numerical solution. The main goal of the present paper is to include into the above formulation the usual devices existing in a gas transmission network other than pipelines, namely, compressors, pressure control valves (PCV), flow control valves (FCV) and resistors. One of the main novelties of this paper is that, unlike many related ones, all of the above devices are modelled as nodes instead of edges of the graph which makes numerical solution simpler and computationally cheaper. This is the case, for instance, in the frequent situation where there are many valves in the network but only a few of them are working in each specific simulation scenario. Instead of introducing one edge per valve in the graph, we only consider that, at specific time intervals, some valves are acting which leads to simple modifications in the discrete system which are made dynamically and locally. An interesting feature of the full discrete model is that mass conservation is exactly preserved which is a very important issue, particularly for long time simulations.

Hydrogen contamination in liquid helium

For some years now, massive problems are often occurring when operating standard flow cryostats with pumped liquid helium. There are a large number of reports from various parts of the world, describing blockage problems arising typically within a few hours. Standard equipment for measuring material properties at temperatures below 4.2 K is massively affected. Hydrogen contamination within the used liquid helium could be identified as direct cause. With helium evaporating in the narrow throttling passages, hydrogen in solid state accumulates and is forming blockages. Usually internal capillaries or inlet valves are concerned. In consequence, the helium flow is reduced or ceasing completely, the operating temperature of the cryostat can’t be upheld, and the measurement run must be interrupted. Extremely low concentrations in the sub-ppm range are sufficient to block the helium flow within a few hours of operation. This contribution contains first quantifications regarding these contamination. A semi-quantitative analysis method using a narrow flow resistor, as well as gas chromatographic investigations led to new findings. Effects within the liquid helium supply that give rise to the problem were scrutinized. The collected results should lead to an understanding and to feasible solutions of the problem.

A pump-free tricellular blood-brain barrier on-a-chip model to understand barrier property and evaluate drug response.

Disruption of the blood-brain barrier (BBB) leads to various neurovascular diseases. Development of therapeutics required to cross the BBB is difficult due to a lack of relevant in vitro models. We have developed a three-dimensional (3D) microfluidic BBB chip (BBBC) to study cell interactions in the brain microvasculature and to test drug candidates of neurovascular diseases. We isolated primary brain microvascular endothelial cells (ECs), pericytes, and astrocytes from neonatal rats and cocultured them in the BBBC. To mimic the 3D in vivo BBB structure, we used type I collagen hydrogel to pattern the microchannel via viscous finger patterning technique to create a matrix. ECs, astrocytes, and pericytes were cocultured in the collagen matrix. The fluid flow in the BBBC was controlled by a pump-free strategy utilizing gravity as driving force and resistance in a paper-based flow resistor. The primary cells cultured in the BBBC expressed high levels of junction proteins and formed a tight endothelial barrier layer. Addition of tumor necrosis factor alpha to recapitulate neuroinflammatory conditions compromised the BBB functionality. To mitigate the neuroinflammatory stimulus, we treated the BBB model with the glucocorticoid drug dexamethasone, and observed protection of the BBB. This BBBC represents a new simple, cost-effective, and scalable in vitro platform for validating therapeutic drugs targeting neuroinflammatory conditions.

High-speed and high-resolution cell manipulation by utilizing asymmetric flow resistors

High-speed and high-resolution cell manipulation by utilizing asymmetric flow resistors

Study of flow and control of gas mixture for the Resistive Plate Chamber (RPC)

The upcoming underground INO-ICAL experiment will be instrumented with 28,800 of RPCs (glass based electrode of 3 mm thick) of size (1.85× 1.74) m2, which are the active elements and the key goal of the ICAL is to precisely measure the neutrino(s) mass. The RPCs are operated in the avalanche mode, using a gas mixture of R134a (95.2%), I-Butane (4.5%) and SF6 (0.3%). The number of RPCs that would be used is large with a total volume of gas of ∼ 200 m3, a Closed Loop gas mixing System (CLS) is mandatory. The performance of the RPCs depends on various parameters namely, the environmental conditions such as atmospheric pressure, ambient temperature, humidity etc., flow rate of gas mixture into the RPCs, concentration of the gas mixture (should be maintained constant throughout the operation), quality of gas, the RPC input gas pressure, uniformity of conductive coating on the surface, uniform gas gap thickness, nozzle positions, electrode thickness, ageing etc. We have tried to achieve the optimum flow rate of gas mixture (few SCCM) into the RPCs in the CLS without deteriorating the performance of the RPC. A modest low flow rate is ideal due to the usage of glass as electrodes and low back ground radiation (low cosmic rays only) unlike CERN-CMS experiments where the electrodes used are of Bakelite and are operated under high radiations with flow rates of few litres per hour. In this paper we present the flow resistors that are fabricated (capillary) and are suitable for final ICAL RPCs, the primitive simulation studies done to understand the distribution of the flow of gas mixture inside an RPC and also the behaviour of gas flow with different position of the input gas nozzles of an RPC. We also conclude here with results of the safe operating pressure parameters required for the operation of ICAL RPC in the CLS.

Fabrication of integrated microfluidic devices by direct ink writing (DIW) 3D printing

This paper describes a simple method to apply 3D printing to fabricate microfluidic devices integrated with fluid handling and functional components. We used a direct ink writing (DIW) 3D printer to dispense a fast-curing flexible silicone resin on various substrates to form microchannels. The dispensed silicone was interfaced with another flat substrate to form microchannels with tunable dimensions. Using this method, we fabricated channels with dimensions as small as 32 μm in width and 30 μm height. We fabricated basic microfluidic modalities (e.g. straight and branched channels, mixers, chambers and droplet generators) as well as functional modalities (e.g. valves, variable flow resistors and gradient generators) on an optically transparent substrate. The method can be readily extended to fabricate microchannels on a diverse range of functional substrates. We showcased this capability by fabricating microchannels on an unmodified printed-circuit board (PCB) to form the interface between the fluid and the electric circuits, and microporous membranes to perform air-liquid human keratinocyte cell culture. Our approach has enabled rapid prototyping of microfluidic devices integrated with functional components required for lab-on-a-chip applications, complementing current approaches in 3D printed microfluidics that are restricted in attainable dimensions and available materials.

Tailoring porous media for controllable capillary flow

HypothesisControl of capillary flow through porous media has broad practical implications. However, achieving accurate and reliable control of such processes by tuning the pore size or by modification of interface wettability remains challenging. Here we propose that the liquid flow by capillary penetration can be accurately adjusted by tuning the geometry of porous media. MethodologiesOn the basis of Darcy’s law, a general framework is proposed to facilitate the control of capillary flow in porous systems by tailoring the geometric shape of porous structures. A numerical simulation approach based on finite element method is also employed to validate the theoretical prediction. FindingsA basic capillary component with a tunable velocity gradient is designed according to the proposed framework. By using the basic component, two functional capillary elements, namely, (i) flow accelerator and (ii) flow resistor, are demonstrated. Then, multi-functional fluidic devices with controllable capillary flow are realized by assembling the designed capillary elements. All the theoretical designs are validated by numerical simulations. Finally, it is shown that the proposed concept can be extended to three-dimensional design of porous media.