Flow Distributor Design Research Articles

Numerical evaluation of an additively manufactured uniform fractal flow mixer

The uniform distribution and mixing of fluids is a common functional requirement in a range of process intensified fluid handling applications. Traditional manufacturing processes have limited the design and performance of flow distributors and mixers. However, the maturing of additive manufacturing (AM) has increased the range of manufacturable geometries, enabling the construction of novel high-performance flow distribution devices. An additively manufactured fractal flow mixer was designed using the concept of self-similar bifurcating channels that symmetrically bifurcates a flow into multiple channels, achieving flow uniformity through a series of sequential stages. After four stages of bifurcation, a y-junction on each channel was introduced before the outlet, mixing the flow from two modules. Computational fluid dynamics (CFD) simulations were carried out to characterise the hydrodynamics and mixing performance of the newly designed mixer over a range of operating conditions and for mixing two gases. Effect of inlet Reynolds numbers (Re) and flow ratio was studied, the maximum relative standard deviation (RSD) was found to be ∼8%, and majority of the flow was uniformly mixed with a distribution index close to unity. The fractal flow mixer was successfully manufactured using vat photopolymerisation (VP) and laser powder bed fusion (L-PBF) techniques without any supports.

Clogging sensitivity of flow distributors designed for radially elongated hexagonal pillar array columns: a computational modelling

Flow distributor located at the beginning of the micromachined pillar array column (PAC) has significant roles in uniform distribution of flow through separation channels and thus separation efficiency. Chip manufacturing artifacts, contaminated solvents, and complex matrix of samples may contribute to clogging of the microfabricated channels, affect the distribution of the sample, and alter the performance of both natural and engineered systems. An even fluid distribution must be achieved cross-sectionally through careful design of flow distributors and minimizing the sensitivity to clogging in order to reach satisfactory separation efficiency. Given the difficulty to investigate experimentally a high number of clogging conditions and geometries, this work exploits a computational fluid dynamic model to investigate the effect of various design parameters on the performance of flow distributors in equally spreading the flow along the separation channels in the presence of different degrees of clogging. An array of radially elongated hexagonal pillars was selected for the separation channel (column). The design parameters include channel width, distributor width, aspect ratio of the pillars, and number of contact zone rows. The performance of known flow distributors, including bifurcating (BF), radially interconnected (RI), and recently introduced mixed-mode (MMI) in addition to two new distributors designed in this work (MMII and MMIII) were investigated in terms of mean elution time, volumetric variance, asymmetry factors, and pressure drop between the inlet and the monitor line for each design. The results show that except for pressure drop, the channel width and aspect ratio of the pillars has no significant influence on flow distribution pattern in non-clogged distributors. However, the behavior of flow distributors in response to clogging was found to be dependent on width of the channels. Also increasing the distributor width and number of contact zone rows after the first splitting stage showed no improvement in the ability to alleviate the clogging. MMI distributor with the channel width of 3 µm, aspect ratio of the pillars equal to 20, number of exits of 8, and number of contact zones of 3 exhibited the highest stability and minimum sensitivity to different degrees of clogging.

Performance analysis of a capacitive deionization stack for brackish water desalination

Capacitive deionization (CDI), a promising ion removal technology, has shown encouraging results in the last decade. In this work a comprehensive characterization of a CDI prototype equipped with 3D graphite felt electrodes is shown. Initially, a study of different flow distributor designs, analyzed by Computational Flow Dynamic (CFD), determined the importance of elements such as small inlet cavities or perforated channels to achieve a uniform fluid flow distribution. Then, an evaluation under different operational parameters and modes (batch and single pass, SP) demonstrated the strong flexibility in terms of performance that the CDI system is able to offer. Thus, in general terms, SP experiments were able to produce larger quantities of water (14–47 L h−1 m−2) than batch experiments (2–24 L h−1 m−2) although obtaining lower salt concentration reductions (1–2 mM vs 6–10 mM, respectively). The CDI stack was tested for 350 galvanostatic cycles (700 h of operation) showing an exceptional robustness in performance. Energy consumption values below 200 Wh m−3 were obtained for SP experiments, whereas that value increased to 600 Wh m−3 for batch tests. Nevertheless, by implementing an energy recovery system these values could be reduced by an average of 40–50%.

Improvement of transport phenomena and COD removal through the design of manifolds in membrane aerated biofilm reactors for sewage treatment

The membrane aerated biofilm reactor (MABR) has demonstrated a huge potential in wastewater treatment plants (WWTPs) due to biofilm stratification and high oxygen transfer efficiency. However, an inappropriate design can negatively impact hydrodynamic and mass transfer, hampering the reactor performance. This work is intended to enhance these phenomena through a rational design of inlet flow distributors (IFD) using computational fluid dynamics (CFD) and mass-transfer simulations. Different IFDs are analyzed at macro and micromixing levels using the residence time distribution (RTD) test and CFD, respectively. It is found that the computed RTD curves agree with experimental data with an error <5 %. The conical wall distributor generates the most homogeneous velocity in the entire reaction zone (membrane zone). The maximum velocity ratio between the channeling and stagnant zones presents a diminution of ∼70 %, and an increment of ∼11.7 % on the chemical oxygen demand (COD) removal if compared against the MABR without IFD, during the sewage treatment. Results show an acceptable agreement of simulations and experimental data, indicating that this approach is satisfactory to conduct a rational design of this type of reactor.

Impacts of reactant flow nonuniformity on fuel cell performance and scaling-up: Comprehensive review, critical analysis and potential recommendations

Fuel Cells (FCs) have witnessed phenomenal improvements in operation and utilization for different stationary and mobile applications. Scaling up limitations of FCs and their integration restrictions into high energy applications because of maldistribution of the reactants’ (fuel and oxidant) flow become among the major drawbacks. The relevant non-uniform electrochemical reactions may result in the FC performance degradation and decrease in lifetime. Therefore, this paper aims at introducing a comprehensive review on the advances in the methods of flow distribution in FCs. The levels in FC design either individual cell or FC stack that introduce non-uniformities are highlighted. The negative impacts on the FC performance, mainly the thermal and water management aspects, have been explicitly introduced. The consequent phenomena the FC experiences such as non-uniform heating, water flooding and membrane drying are presented. Based on the literature review, the critical insights and recommendations are provided alongside a suggestion of a cellular level flow distributor design to be investigated in future studies.

Engineering solutions for flow control in microfluidic devices for spatial multi-dimensional liquid chromatography

Spatial multi-dimensional liquid chromatography can provide unprecedent resolving power by utilizing subsequent orthogonal separation mechanisms, while the analysis time is minimized thanks to parallel development of the analyzed fractions in each dimension. In this study, different engineering solutions have been realized aiming at achieving flow confinement and control during subsequent 1D and 2D chromatographic developments in a microfluidic device for spatial two-dimensional separations. First, the flow distributor design was optimized and physical barriers, i.e., reducing the cross section of microchannels as well as locally integrated monolithic substrates in chip segments, were assessed as a means of flow confinement. Furthermore, an on-chip active-valving, high-pressure approach was successfully developed. The flow in first-dimension separation is confined within a channel situated in a rotating axis containing through-holes. These are either closed during the 1st development or opened when aligned with the 2D flow distributor and the 2D channels after rotation of the axis by 90°, allowing for sample transfer and executing the subsequent 2D analysis. The on-chip active-valving concept has been successfully demonstrated in combination with stationary-phase analyte focusing prior to the 2D development using a microfluidic spatial 2D-LC device containing polymer monolithic stationary phases locally synthesized in the parallel 2D separation channels. Finally, this workflow was successfully employed to separate a mixture of dyes applying gradient RPLC during the 2D development.

CFD analysis of flow distributor designs for numbering-up of biodiesel synthesis

Micro-chemical plants increased production by the parallelization of microdevices. The uniform distribution of flow through the microdevices are made by the flow distributor. Inadequate distributor design reduces the micro-chemical plant performance. The present research aims to perform a numerical analysis of distributor designs. Three designs were evaluated varying the distributor height and the presence or absence of internal obstacle. Analyses of flow rate and phase distribution uniformity and the prediction of biodiesel synthesis occurrence inside the distributors were performed. Different numerical approaches were used for the specific systems, including single-phase flow, multi-component flow and Volume of Fluid (VOF) simulations. The VOF-based model provided good agreement with experimental data of water flow. The viscous dominated oil flow, presented good distribution in all proposed designs. Water and ethanol presented bulk recirculation inside the distributor, depreciating the flow uniformity. The ethanol-oil mixture flow resulted in high flow nonuniformity and in the occurrence of biodiesel synthesis for all operating conditions. Based on the numerical predictions, the micro-chemical plants require two distributors in order to perform the transesterification reaction only inside the microdevices. The simulations were employed as a design tool providing flow details and substantiating the project optimization for further development of micro-chemical plants.

The flow downstream of a bifurcation of a flow channel for uniform flow distribution via cascade flow channel bifurcations

Uniform flow distribution is an important and fundamental issue to miscellaneous devices. The present work studied the details of flow in a channel with a fundamental structure—a symmetric bifurcation of one flow channel into two sub-channels. The structure is designated to split one flow into two streams of even flow rate. Key geometric design factors of the flow channel are investigated regarding the effect to the development of a symmetric velocity profile after the flow channel bifurcation, as the symmetric velocity profile is critical to the even split of the flow into two streams in the next stage of flow channel bifurcation. The study is based on both two-dimensional and three-dimensional CFD analyses. Both laminar and turbulent flows are investigated. Commercial CFD software ANSYS FLUENT® was used. The numerical treatment of convection terms in governing equations was based on the QUICK scheme, and the SIMPLE algorithm was used to treat the coupling of pressure and velocity fields. The obtained results are of great significance to the design of flow distributors using cascade flow channel bifurcations.

CO2 Adsorption: Experimental Investigation and CFD Reactor Model Validation

The National Energy Technology Laboratory is investigating a new process for CO2 capture from large sources such as utility power generation facilities as an alternative to liquid amine based adsorption processes. Many of these advanced dry processes are based upon sorbents composed of supported polyamines. In this analysis, experiments have been conducted in a laboratory-scale fluidized bed reactor and compared to CFD reactor predictions using kinetics obtained from TGA tests. Batch experiments were conducted by flowing a mixture of CO2, H2O, and N2 (simulated flue gas) through a fluidized bed of sorbent material. The exit gas composition time series data is compared to CFD simulations using a 3-dimensional nonisothermal reacting multiphase flow model. The effects of the gas flow rate, distributor design, and particle size are explored through the CFD simulations. It is shown that the time duration for CO2 adsorption decreased for an increase in the gas flow. Fluid bed hydrodynamics indicated that there were regions in the reactor where the inert FCC particles segregated and defluidized; without adversely affecting the capacity of the sorbent to adsorb CO2. The details of the experimental facility and the model as well as the comparative analysis between the data and the simulation results are discussed.

Investigation of a new helical flow distributor design to extract thermal energy from hot water storage tanks

Unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor. The primary purpose of the study is to improve the temperature stratification in hot water storage tanks operated with coiled-tube heat exchangers. The investigated operating mode is the extraction of thermal energy from hot water storage tanks. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the colder water from the significantly hotter bulk fluid of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the cooling tank fluid to a helical flow path along the tube axis which induces additional secondary flow pattern in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different inlet temperatures have been investigated to describe the impact for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers.

3D printed porous media columns with fine control of column packing morphology

In this paper we demonstrate, for the first time, the use of 3D printing (also known as additive manufacturing or rapid prototyping) to create porous media with precisely defined packing morphologies, directly from computer aided design (CAD) models. We used CAD to design perfectly ordered beds with octahedral beads (115μm apothem) packed in a simple cubic configuration and monoliths with hexagonal channels (150μm apothem) in parallel and herringbone arrangements. The models were then printed by UV curing of acrylonitrile-butadiene-styrene powder layers. Each porous bed was printed at 1.0, 1.5 and 2.0mL volumes, within a complete column, including internal flow distributors and threaded 10–32 flow connectors. Close replication of CAD models was achieved. The resultant individual octahedral beads were highly uniform in size, with apothems of 113.6±1.9μm, while the monolith hexagonal cross-section channels had apothems of 148.2±2.0μm. Residence time distribution measurements show that the beds largely behaved as expected from their design void volumes. Radial and fractal flow distributor designs were also tested. The former displayed poor flow distribution in parallel and herringbone pore columns, while the fractal distributors provided uniform flow distribution over the entire cross section. The results show that 3D printing is a feasible method for producing precisely controlled porous media. We expect our approach to revolutionize not only fundamental studies of flow in porous media but methods of chromatography column production.

Performance analysis of a rotary active magnetic refrigerator

Performance results for a novel rotary active magnetic regenerator (AMR) and detailed numerical model of it are presented. The experimental device consists of 24 regenerators packed with gadolinium (Gd) spheres rotating inside a four-pole permanent magnet with magnetic field of 1.24T. A parametric study of the temperature span, cooling power, coefficient of performance (COP) and efficiency of the system was carried out over a range of different hot reservoir temperatures, volumetric flow rates and cooling powers. Detailed modeling of the AMR using a 1D model was performed and compared with the experimental results. An overall mapping of the thermal losses of the system was performed, and good agreement between the experimental and numerical results was found when parasitic heat losses were subtracted from the modeling results. The performance of the system was evaluated via the COP, the exergetic-equivalent cooling power (ExQ), and the overall second law efficiency, η2nd. Losses mapping indicated that friction and thermal leakage to the ambient are the most important contributors to the reduction of the system performance. Based on modeling results, improvements on the flow distributor design and reduction of the cold end thermal parasitic losses are expected to enhance the efficiency of the system. For an operating frequency of 1.5Hz, a volumetric flow rate of 400L/h, a hot reservoir temperature of 297.7K, and thermal loads of 200 and 400W, the obtained temperature spans, ΔTS, were 16.8K and 7.1K, which correspond to COPs of 0.69 and 1.51, respectively. The maximum overall second-law efficiency was 5.6% for a ΔTS of 12.9K at 500L/h and 400W.

Investigation of a new tube-in-tube helical flow distributor design to improve temperature stratification inside hot water storage tanks operated with coiled-tube heat exchangers

Steady and unsteady heat transfer process and the developed flow and temperature field inside hot water storage tanks have been studied with a helical tube-in-tube flow distributor design. The studied flow distributor consists of two helically coiled pipes. The inner pipe is a common helically coiled tube heat exchanger and the outer pipe is a helically coiled flow distributor which separates the hot water from the significantly colder bulk water of the tank to decrease the entrainment effect induced by natural convection. The flow distributor drives the heated water to a helical flow path along the tube axis which induces additional secondary flow in planes normal to the main flow direction over the outer surface of the inner coiled tube. Different heat exchanger side flow rates, inlet temperatures and two gap size between the outer wall of the helical tube and the flow distributor have been investigated to describe the impact of the modification of these parameters for the developed velocity and temperature field. Comparison of the flow and temperature fields of hot water storage tank with common helical tube and the tube-in-tube flow distributor configuration are discussed. Numerical calculations have been carried out to examine different flow and temperature fields in laminar flow regimes. Calculated results of the inner side heat transfer intensity (Nu) have been compared to existing empirical formulas to test the validity of the numerical results in case of common helical tube heat exchanger. The proposed helical flow distributor ensures a highly stratified temperature field of storage tanks operating with helically coiled heat exchangers. In addition to this the suggested flow distributor geometry makes it possible to apply different heat transfer enhancement tools like spiral tape inserts, different kind of corrugations further improve if it is possible the poor outer side heat transfer rate of common helically coiled tube heat exchangers.

Even distribution/dividing of single-phase fluids by symmetric bifurcation of flow channels

This study addresses a fundamental issue of distributing a single-phase fluid flow into a number of flow channels uniformly. A basic mechanism of flow distribution is accomplished through bifurcation of channels that symmetrically split one flow channel into two downstream channels. Applying the basic mechanism, cascades flow distributions are designed to split one flow into a large number of downstream flows uniformly. Some key parameters decisive to the flow distribution uniformity in such a system have been identified, and the flow distribution uniformity of air was studied for several versions of flow distributor designs using CFD analysis. The effect of the key parameters of the flow channel designs to the flow distribution uniformity was investigated. As an example of industrial application, a novel fluid packaging device of high efficiency was proposed and some CFD analysis results for the device were provided. The optimized flow distributor makes a very good uniform flow distribution which will significantly improve the efficiency of fluid packaging. The technology is expected to be of great significance to many industrial devices that require high uniformity of flow distribution.

Experimental study of the flow distribution uniformity in flow distributors having novel flow channel bifurcation structures

Symmetric flow channel bifurcations were proposed in this article for the purpose of creating better flow uniformity in flow distribution. Two categories of flow channel bifurcation structures were considered. Characteristic parameters that can identify and standardize the structural design of the flow channel bifurcations were designated. Six flow distributors, divided into two groups, were fabricated for experimental testing. Each group of distributors is distinguished by one basic channel bifurcation structure category, but distributors within a group differ in the characteristic parameters exhibited in their designs. The axial velocities of the airflow at the exit of the distributor flow channels were measured experimentally, and the uniformity of the flow distribution was investigated for each distributor. Comparison and evaluation of the figure-of-merit of flow distribution uniformity is made for the six studied flow distributors. The novel concept of symmetrically bifurcated flow distributor designs and the experimental results and conclusions obtained in this study are of great significance to the development of high performance industrial devices, such as heat exchangers, reactors, fuel cells, heat sinks, and fluid product packaging machines.

Experimental Optimization of Flow Distributors for Pressure-Driven Separations and Reactions in Flat-Rectangular Microchannels

We report on the results of an experimental study established to optimize the design of microfabricated flow distributors for use in pressure-driven separations and reactions in flat-rectangular channels. For this purpose, the performance of a wide variety of possible flow distributor designs etched in glass/silicon wafers was compared, using CCD camera detection to study the shape and variance of the bands eluting from them. The best performance was obtained with radially interconnected distributors with a diverging inlet section and filled with diamond-shaped pillars, oriented perpendicular to the main flow direction and with a high transversal over axial aspect ratio. It was found that the best distributor designs start with a diverging section containing some 10-12 subsequent rows of high aspect ratio pillars (with a transversal width making up 10-15% of the final channel width) and with a divergence angle selected such that the sloped side-walls run parallel with the sides of the diamond-shaped pillars. After this zone, one or more regions with pillars with a smaller aspect ratio should be provided to increase the number of exit points. To prevent the formation of dead zones in these subsequent zones, so-called distributor wedges can be used to prevent the formation of any dead zones in the wake of the large aspect ratio pillars of the preceding section.

A New Complex Design for Air-Breathing Polymer Electrolyte Membrane Fuel Cells Aided by Rapid Prototyping

One of the most difficult issues to fabricate a fuel cell with a complex design is the manufacturing method. To solve this difficulty, the authors applied an innovative method of fuel cell fabrication, i.e., rapid prototyping technology. The rapid prototyping technology can both fabricate the complex design and shorten the fabrication time. In this paper, the authors used a 3D software (CATIA) on the fuel cell design and utilized the rapid prototyping to accelerate the prototype development of complex stack designs and to verify the practicability of the new fabrication for fuel cells. The honeycomb shape methanol reservoir and cathode structure design of a direct methanol fuel cell (DMFC) and the complex flow distributor design of a monopolar air-breathing proton exchange membrane fuel cell (PEMFC) stack, which were almost impossibly manufactured by traditional manufacturing, were made in this study. The performance of the traditional air-pumping DMFC and that of an air-breathing DMFC were compared in this study. The feasibility of a complex pseudobipolar design DMFC stack was also verified. For the miniature air-breathing PEMFC made by rapid prototyping with ABS material, its performance is close to the state-of-the-art compared to previous published literatures (Hsieh et al. 2006, “Study of Operational Parameters on the Performance of Micro PEMFCs With Different Flow Fields,” Energy Convers. Manage., 47, pp. 1868–1878; Schmitz, A., Wagner, S., Hahn, R., Uzun, H., and Hebling, C., 2004, “Stability of Planar PEMFC in Printed Circuit Board Technology,” J. Power Sources, 127, pp. 197–205; Hottinen, T., Mikkola, M., and Lund, P., 2004, “Evaluation of Planar Free-Breathing Polymer Electrolyte Membrane Fuel Cell Design,” J. Power Sources, 129, pp. 68–72). A new solution to manufacture complex fuel cell design, rapid prototyping, has been first applied to the fabrication of complicated flow channels in ABS materials and directly used in both DMFC and PEMFC in this paper. Its feasibility was verified and its promising performance was also proved.

CFD study on flow distribution uniformity in fuel distributors having multiple structural bifurcations of flow channels

This work studied the issues of uniform flow distribution for general application in fuel cells, fuel processing chemical reactors, and other industrial devices. A novel method for uniform flow distribution was proposed, in which multiple levels of flow channel bifurcations were considered to uniformly distribute a flow into 2 n flow channels at the final stage, after n levels of bifurcation. To study the effect of the flow channel bifurcation structure and dimensions on the flow distribution uniformity, numerical analysis was conducted. Parameters such as the flow channel length and width at each level of bifurcation as well as the curvature of the turning area of flow channels were particularly investigated. Important results concerning the geometrical design of flow distributors for better flow distribution and uniformity are presented. The best structure of a flow distributor was selected based on the criterion of flow distribution uniformity and low pressure loss. Since the studied novel flow distributor distributes a flow into a number of parallel channels in a remarkable uniformity, the flow distribution structure is expected to be widely used in fuel cells, fuel cell systems, and variety of industrial reactors and heat exchangers to significantly improve the performance of these devices. The studied flow regime is limited to laminar flow. A CFD tool FLUENT @ was used for the simulation. The numerical treatment of convection terms in governing equations was based on the QUICK scheme, and the coupled computation solving for pressure and velocity fields was based on the SIMPLE algorithm.

Design and Control Techniques for the Numbering-up of Capillary Microreactors with Uniform Multiphase Flow Distribution

The results of experimental studies on the parallelization of both single-phase and biphasic liquid−liquid slug flow in capillary microreactors are presented. Various flow distributor designs were investigated with respect to the uniformity of liquid−liquid flow in eight parallel capillaries and the two most promising concepts were identified. A novel multichannel phase splitting unit was installed at the downstream end of the capillaries to ensure well-defined phase separation. Further investigations on more complex reaction systems, in which the hydrodynamics depend on conversion and/or selectivity (e.g., polymerization), suggest that an active flow control for individual capillaries will be necessary in such cases. The uniformity of flow rate and flow structure could be achieved using a noninvasive capacitative measurement to characterize the flow in conjunction with specially developed microvalves and actuators for the regulation of flow rates and the manipulation of slug flow structure in each capillary.

The Influence of Differences Between Microchannels on Micro Reactor Performance

AbstractMicrostructured reactors most often contain a large number of micrometer‐sized, parallel channels, instead of a large undivided reaction volume. Individual microchannels behave as plug‐flow reactors without significant axial dispersion and with excellent heat and mass transfer properties. However, since the reaction takes place in a large number of parallel channels, it is important that all channels provide equal residence time and amount of catalyst volume. These issues depend not only on the flow distributor design, but also, for example, on the manufacturing tolerances. Correlations are derived to express the conversion of a multichannel microreactor explicitly as a function of the variance of a number of reactor parameters, viz. the channel flow rate, the channel diameter, the amount of catalyst in a channel, and the channel temperature. It is shown that the influence of flow maldistribution on the overall reactor conversion is relatively small, while the influences of variations in the channel diameter and the amount of catalyst coating are more pronounced. The model outcomes are also compared to experimental results of two microreactors with different catalyst distributions, which show that the presented method is able to provide a quick, though rough estimation of the influence of differences between channels on microreactor performance.