Helical Static Mixer Research Articles

Comparing flows of FENE-P, sPTT, and Giesekus model fluids in a helical static mixer

Helical static mixers are used widely for mixing of non-Newtonian fluid flows in the laminar regime. We study flows of three viscoelastic constitutive models (sPTT, FENE-P, and Giesekus) in the helical static mixer using computational fluid dynamics. These three models have similarities in steady viscometric flows in that they all exhibit shear thinning and their planar extensional viscosities can be matched, but their responses can differ in complex geometries. We observe flow distribution asymmetries at the element intersections for all three models, which hinders the mixing performance of the device. These have previously been observed with the constant shear viscosity FENE-CR model. The asymmetry behaves similarly between the sPTT and Giesekus models, however the FENE-P model behaves in a distinct manner; beyond a critical degree of elasticity, the asymmetry sharply changes direction. This was also observed previously with the FENE-CR model. These results suggest that shear thinning and second-normal stress differences (present in the Giesekus model) do not significantly influence mixing performance in the range of conditions studied. We show that increasing the aspect (length/diameter) ratio of the mixer elements mitigates the poor mixing caused by elasticity. Overall, this study provides insight into the behaviour of these well-used constitutive models in complex, industrially-relevant flows.

3D concrete printing for tunnel linings: Opportunities and challenges

The use of shotcrete for hard rock tunnel linings has drawbacks such as irregular surfaces and rebounds. By contrast, extrusion-based 3D concrete printing presents a promising solution with precise deposition and reduced rebound. This study discusses the potential of 3D printing for tunnel linings, focusing on mitigating shear failure and addressing interfacial detachment. Achieving high strength early for stable adhesion to rock surfaces is paramount for countering shear failure. A twin-pipe pumping system was introduced utilizing a helical static mixer to blend concrete involving chemical triggers, effectively managing the stiffness and fluidity during pumping. In addition, the adhesion to the substrate must be addressed. Based on the above discussion, this study offers insights into the fundamental challenges, paving the way for advancing extrusion-based 3D concrete printing for tunnel linings.

On the geometry and material stability of tubular PVDF-TiO2 static mixer membrane contactors for photocatalytic ozonation of micropollutants

The prevention of ozone hotspots in ozonation reactors for wastewater treatment is essential for effective degradation of micropollutants without forming harmful by-products. Membrane contactors can provide a bubble-free ozonation process that is able to decrease by-product formation, e.g., bromate formation in bromide-charged wastewater. Furthermore, the geometry of the reactor influences the distribution of dissolved ozone in a membrane contactor. Turbulence promoters can significantly decrease ozone hotspots and increase ozone mass transfer into wastewater. This work presents a fabrication methodology to produce PVDF-TiO2 helical static mixer membranes using the rotation-in-a-spinneret technology. Static mixer membranes combine the functionality of a membrane contactor and a static mixer in a single component. Furthermore, the PVDF matrix is doped with TiO2 particles as photocatalyst. Thus, we present a membrane-based photocatalytic ozonation process to increase the concentration of hydroxyl radicals for degradation of micropollutants under UV radiation. Static mixer membranes increase the ozone mass transfer coefficient by 70% compared to conventional hollow fiber membranes. Furthermore, the specific photocatalytic degradation rate of methylene blue increases by 50% using static mixer membranes. However, this work also reveals a lack of long-term stability of PVDF-TiO2 membranes in the corrosive environment of photocatalytic ozonation. The molecular weight of PVDF decreases by 11% after use in photocatalytic ozonation resulting in a loss of mechanical stability of the membranes. Similar material concepts are required based on ceramic materials which avoid wetting of the porosity.

Optimization of the Ex Situ Biomethanation of Hydrogen and Carbon Dioxide in a Novel Meandering Plug Flow Reactor: Start-Up Phase and Flexible Operation.

With the increasing use of renewable energy resources for the power grid, the need for long-term storage technologies, such as power-to-gas systems, is growing. Biomethanation provides the opportunity to store energy in the form of the natural gas-equivalent biomethane. This study investigates a novel plug flow reactor that employs a helical static mixer for the biological methanation of hydrogen and carbon dioxide. In tests, the reactor achieved an average methane production rate of 2.5 LCH4LR∗d (methane production [LCH4] per liter of reactor volume [LR] per day [d]) with a maximum methane content of 94%. It demonstrated good flexibilization properties, as repeated 12 h downtimes did not negatively impact the process. The genera Methanothermobacter and Methanobacterium were predominant during the initial phase, along with volatile organic acid-producing, hydrogenotrophic, and proteolytic bacteria. The average ratio of volatile organic acid to total inorganic carbon increased to 0.52 ± 0.04, while the pH remained stable at an average of pH 8.1 ± 0.25 from day 32 to 98, spanning stable and flexible operation modes. This study contributes to the development of efficient flexible biological methanation systems for sustainable energy storage and management.

Viscoelastic flow asymmetries in a helical static mixer and their impact on mixing performance

Helical static mixers are often used during the processing of formulated products with complex rheological properties, such as viscoelasticity. Previous experimental studies have highlighted that increasing the viscoelasticity of the flow hinders the mixing performance in the laminar flow regime. In this study, we use computational fluid dynamics to investigate the flow of a FENE-CR model fluid in a helical static mixer. The numerical results show clearly that the reduced mixing performance is caused by flow distribution asymmetries which develop at the mixer element intersections. The results allow us to quantify the degree of asymmetry for the range of conditions studied, which is correlated with the quantified mixing performance for each simulation. The mixing is quantified using a Lagrangian particle tracking technique, and a new mixing index is defined based on the mean nearest distance between the two sets of tracked particles. The results show that the asymmetry parameter does not follow a pitchfork bifurcation, as it typically does for elastic instabilities in symmetrical geometries such as the cross-slot. For low values of the extensibility parameter, L2, the flow remained (Eulerian) steady for all Reynolds Re and Weissenberg Wi numbers studied. At fixed Re and Wi, increasing L2 causes the flow to become transient and greatly increases the magnitude of the asymmetry. The results presented in this study help us to understand the effects that viscoelasticity can cause in mixing processes.

Helical Static Mixer Simulations for Its Integration in the Pour Plate Method: Mixing Agar and a Nutrient Solution

In microbiology laboratories, it is essential to obtain high-quality samples where the culture media are completely homogeneous. The pour plate method includes having to melt the culture media—a mixture of agar and a nutrient solution—before seeding. A static mixer is designed for this purpose, employing CFD (Computational Fluid Dynamics) with the software OpenFOAM to simulate the fluid’s behaviour in a helical static mixer with both internal and external configurations. The objective is to validate the CFD model by comparing it with the literature and provide a first approach to the mixer design. After satisfactory validation of the model, the results of the initial designs for Reynolds number 14 reveal the notably different behaviour of the fluids during mixing due to their differing properties, since agar and the nutrient solution present a high viscosity ratio. While the mixing efficiency is similar for the internal and external mixers, improved performance is demonstrated in the internal mixer, even for a shorter version. The external version of the mixer reaches a value for a mixing efficiency of 0.89 whereas the internal version performs more homogeneous mixing for the same number of mixing elements. This evaluation is based on a simplified internal design for computational simulations, whereas the external mixer is easier to manufacture but more complex to implement computationally. Finally, homogeneous mixing is achieved for the internal mixer configuration when adjusting its dimensions to those available on the market.

Analysis of heat transfer enhancement due to helical static mixing elements inside cooling channels in machine tools

In this contribution, the effectiveness of helical static mixers in different arrangements and flow configurations/regimes is explored. By means of a thorough numerical analysis, the application limits of helical static mixers for the heat transfer enhancement inside cooling channels of machine tools are provided. The numerical simulations were processed with the commercial finite volume Computational Fluid Dynamics (CFD) code, ANSYS Fluent 2020 R2. This study shows that there exists an optimal range of application for static mixers as heat exchange intensifier depending on the flow speed, the transmitted heat flow and the thermal conductivity of the tool. The investigations of this contribution are restricted to single-phase flow in circular cross-sections and straight channel geometries. As a representative application example for a machine tooling, the cooling of a simple injection mold is investigated. The research carried out reveals that the application of static mixing elements for enhancement of heat transfer is very effective, particularly for fluid flow with low to medium Reynolds numbers, close-contour cooling, high values of heat fluxes, and high thermal conductivity of the tooling material.

Blending performance of helical static mixer used for twin-pipe 3D concrete printing

A twin-pipe pumping system has been developed to overcome the conflicting requirements in 3D concrete printing between pumping and deposition. In the twin-pipe pumping system, a helical static mixer, consisting of a series of mixing elements, is used to blend a cement-based mixture and a limestone-based mixture right before extrusion. As these two mixtures go through the static mixer, non-moving mixing elements continuously blend the materials in a flow-division pattern. However, the blending performance of the helical static mixer used for twin-pipe 3D concrete printing has not yet been reported. This paper presents the results of an experimental study on the relation between the mechanical behavior and the blending efficiency of the helical static mixer. Based on the binary images of the polished specimens printed with a different number of mixing elements, the blending performance was characterized by the coefficient of variation of the row-wise intensity distribution. A reasonable linear relationship was established between the mechanical strength (flexural strength and tensile strength) and the mixing homogeneity of the two mixtures. In addition, a higher number of mixing elements was observed to lead to a more dense pore structure, most probably due to the better compaction of the material by the higher pumping pressure.

Stiffening controllable concrete modified with redispersible polymer powder for twin-pipe printing

To achieve stiffening control in 3D concrete printing, a twin-pipe pumping (TPP) system has been developed, where two streams of cementitious materials are pumped separately via two pumps and blended over a helical static mixer right before extrusion. However, in addition to the interlayer region, the presence of striations consisting of unmixed limestone-based regions reduces the mechanical integrity of printed elements in earlier research on TPP strategy making use of a cement-based mixture and a limestone-based mixture. In this work, we examined the use of redispersible polymer powder (RDP) as a means to improve the mechanical performance of such 3D printed elements. Limestone powder in the limestone-based mixture was partially substituted by different dosages of redispersible polymer powder (5, 10, and 15 %). Rheological behavior of the cement-based mixture and the limestone-based mixture was evaluated. Further, we studied the effect of RDP addition on the hydration and the early age mechanical behavior of the combined mixtures. In the hardened state, specimens extracted from 3D printed wall elements were tested for flexural, compressive, and tensile strength. Its microstructure was examined using scanning electron microscopy. From the results of various studies conducted, although adding redispersible polymer powder would slightly reduce the very early age mechanical strength, it enhances the mechanical integrity in the hardened state due to film formation in the limestone-based mixture.

An Experimental Analysis of Gas Reduction in Multiphase Flow with a KMS Helical Static Mixer

The separation dynamics of a multiphase fluid mixture is studied with the aim to validate the gas reduction from the mixture by means of a KMS helical static mixer device through a laboratory prototype based on the jet-pump artificial lift system for oil extraction. The study is focused on the cyclone phenomenon, produced by the KMS, to experimentally verify the reduction of foaming in a reservoir behaving as a horizontal gravity separator of an oil–gas–water mixture under specific dynamics conditions of velocity and pressure of an incoming streamline, as well as some physical mounting configurations of the device. The results were numerical and experimentally validated, projecting the real performance in three-phase separators in an oil extraction field to improve efficiency in pumping machines regarding the negative effects of the gas. Concerning the gas reduction, a KMS of two elements located at 100 mm from the entrance of the horizontal reservoir and 20 mm from a gas casing tube successfully improves the gas isolation from the mixture, from 29.2% to practically 0% over 200 min in the reservoir, to let oil and water become separated by sedimentation. This gas reduction is indirectly assessed by the emulsion that is generated, which affects the time of oil separation in the counterbalance.

Mechanical and microstructural properties of 3D printable concrete in the context of the twin-pipe pumping strategy

To tackle the conflicts between the pumping and deposition process in 3D concrete printing, a twin-pipe pumping (TPP) system with a helical static mixer can be used. In this way, a cement-based mixture (without accelerator) and a limestone powder-based mixture (with a high dosage of accelerator) are transported separately and mixed within the helical static mixer just before extrusion. However, the helical static mixer will introduce striation patterns in the 3D printed concrete due to flow division and the effect of such a heterogeneous phenomenon on the mechanical behavior has not been studied yet. In this paper, the mechanical (compressive, flexural, and tensile bond strength) and the microstructural behavior of 3D printed specimens made using the TPP system was explored. During the mechanical tests, the cracks initiated within the limestone powder striation. In addition, the difference in the mechanical strength for the specimens under difference loading directions was observed. Microstructural investigations based on mercury intrusion porosimetry revealed that the pore size distribution of 3D printed specimens (critical pore size of 210.0 nm and 197.5 nm for the bulk and interface region, respectively) was distinct from that of the mould cast specimens (critical pore size of 62.7 nm). Although the mould cast specimens had higher porosity (19.75%), the volume of macropores was found to be higher in the 3D printed specimens, which was more detrimental to the compressive strength. Further, porosity analysis of micrographs showed that the macropores in the 3D printed specimens were primarily presented in the limestone powder-based striations. In addition, a small quantity of ettringite was also observed in the bulk of the limestone powder-based striation, which may be caused by ion migration between the two regions after the inline mixing process by the static mixer.

Design and operation of an enhanced pervaporation device with static mixers

AbstractPervaporation has a high potential for separating miscible solutions, particularly azeotropic mixtures. However, mass transfer limitations have long been a common concern in pervaporation device design. Therefore, in this work, we design a static mixer‐based pervaporation device using water–ethanol separation as a model system and further develop computational fluid dynamics tools to investigate systematically all the influencing parameters. In the experiments, we use three‐dimensional printed helical static mixers in the feed liquid channel to enhance mass transfer and implement a Sulzer pervaporation membrane for fast removal of water from ethanol. Using flow and mass‐transfer simulations, we fit the membrane mass transfer coefficient and provide predictive models for optimal process design. Our pervaporation assembly exhibits promising performance and potential toward pervaporation processes for the removal of water from organics and is preferably scaled out by using stackable designs.

Investigation of Mixing Behavior of Hydrogen Blended to Natural Gas in Gas Network

Hydrogen is of great significance for replacing fossil fuels and reducing carbon dioxide emissions. The application of hydrogen mixing with natural gas in gas network transportation not only improves the utilization rate of hydrogen energy, but also reduces the cost of large-scale updating household or commercial appliance. This paper investigates the necessity of a gas mixing device for adding hydrogen to existing natural gas pipelines in the industrial gas network. A three-dimensional helical static mixer model is developed to simulate the mixing behavior of the gas mixture. In addition, the model is validated with experimental results. Parametric studies are performed to investigate the effect of mixer on the mixing performance including the coefficient of variation (COV) and pressure loss. The research results show that, based on the, the optimum number of mixing units is three. The arrangement of the torsion angle of the mixing unit has a greater impact on the COV. When the torsion angle θ = 120°, the COV has a minimum value of 0.66%, and when the torsion angle θ = 60°, the COV has a maximum value of 8.54%. The distance of the mixing unit has little effect on the pressure loss of the mixed gas but has a greater impact on the COV. Consecutive arrangement of the mixing units (Case A) is the best solution. Increasing the distance of the mixing unit is not effective for the gas mixing effect. Last but not least, the gas mixer is optimized to improve the mixing performance.

Mixing Characteristics Of Gas-Liquid Flow In A Static Mixer: A Numerical Study

Mixing characteristics of gas-liquid co-current upward flow inside a vertical pipe equipped with a helical static mixer element were numerically investigated. The results from computational fluid dynamics (CFD) simulations with Euler-Euler model of three different length to diameter ratio (L/D) of the static mixer elements were compared. All simulated static mixers provide a better mixing condition in the comparison with the one without a static element. The sudden increase of rotational strength indicated by the liquid velocity curl was observed once the gas-liquid flows enter the static-mixer element zone. The smallest L/D static mixer provides the highest liquid velocity curl in the smallest axial distance providing the most effective mixing process among the tested elements. The best mixing characteristics shown by radial gas distribution was achieved with the static mixer with a smallest L/D.

Experimental and Numerical Study on Flow Resistance and Bubble Transport in a Helical Static Mixer

Flow resistance and bubble transport in a helical static mixer were studied experimentally and numerically. The inline mixer increases the volume fraction of gas in liquids by breaking bubbles into smaller sizes with a micrometer size in the flow experiments. The gas–liquid flow was simulated by a combination of computational fluid dynamics and Taylor expansion methods of moments. The friction factor of the helical static mixer is much smaller than that of the Kenics static mixers. The pressure drop increases with the Reynolds number, and the increment is larger when the Reynolds number is higher. The equidistant pressure drop increases with the argument of Reynolds number, and increases when the pitch decreases from upstream to downstream. The energy expenditure increases significantly when the variable-pitch coefficient is too small. The bubble geometric mean diameter decreases and the geometric standard deviation increases when the gas–liquid fluid flows through the mixer. The variable pitch structure enhances the bubble breakup effectively. The change of the bubble size decreases with the argument of the Reynolds number. The effect of the mixer has a limitation on breaking the bubbles.

Comparison between Helical and Elliptical Static Mixers Performance in Column Flotation of Sungun Copper Plant

The flotation circuit in Sungun copper plant consists of two column flotation cells as cleaner, having fixed-spargers system. To achieve the expected aims in flotation step, there are serious operational challenges such as: fast choking of the static mixers, boiling problem, burping phenomena and pulp overflow to concentrate lander, maintenance and control problems. An attempt was exerted by implementing new helical static mixer in one of cleaner cells instead of old elliptical type to overcome the challenges. The changes resulted in proper performance of the column whereas burping phenomena due to choking was eliminated, finer bubbles were produced, and the boiling and overflow problems were solved. Also, the static mixers life time increased to 7 months in helical column cells from one month in elliptical column cells. In addition to 40% air consumption reduction and 20% solid percent increase in final product, the grade of Cu and Mo increased by helical static mixer replacement up to about 18.7% from 16.8% (11%) and to 511.1 ppm from 263 ppm (94%) in the cleaner step, respectively. Recovery of Cu and Mo were increased about 1.5% and 0.2%, respectively. Finally, the results proved the effectiveness of finer bubble generation on grade improvement is depend on minerals hydrophobicity as Mo grade increased more than Cu.

Continuous process for biodiesel production from palm fatty acid distillate (PFAD) using helical static mixers as reactors

Abstract Three-step continuous process for producing biodiesel from palm fatty acid distillate (PFAD) was optimized by response surface methodology (RSM). PFAD has high content of free fatty acids (FFA) and is not suited for human consumption: normally it is used in soap or in animal feed. The key parts of the three-step continuous process took place in helical static mixers (HSMs) used as continuous reactors. The three-step process was optimized by RSM with 5 levels for each of three factors in central composite design (CCD). First step was esterification, then second step was esterification, and third was transesterification. Methyl ester purities of 71.01 wt.% from first step, 95.94 wt.% from second step, and 99.96 wt.% from third step were achieved with total chemical consumption of (115.1 wt.% MeOH, 13.5 wt.% H2SO4, and 5.0 g/L KOH), and total residence time 147 s in the 3 HSMs. In continuous processing the maximum yields were 109.5 wt.% first-esterified oil, 117.0 wt.% second-esterified oil, and 129.0 wt.% crude biodiesel, and purified biodiesel 86.4 wt.%, in the separated phases from first, second, and third steps, and after purification, respectively. Ester purity from three-step process meets the standard specifications for commercial biodiesel in Thailand, US, and Europe.

A Theoretical-Experimental Comparison of an Improved Ammonia-Water Bubble Absorber by Means of a Helical Static Mixer

The heat transfer in double pipe heat exchangers is very poor. This complicates its application in absorption cooling systems, however, the implementation of simple passive techniques should help to increase the heat and mass transfer mainly in the absorber. This paper carried out a simulation and its experimental comparison of a NH3-H2O bubble absorption process using a double tube heat exchanger with a helical screw static mixer in both central and annular sides. The experimental results showed that the absorption heat load per area is 31.61% higher with the helical screw mixer than the smooth tube. The theoretical and experimental comparison showed that the absorption heat load difference values were 28.0 and 21.9% for smooth tube and the helical mixer, respectively. These difference values were caused by the calculation of the log mean temperature difference in equilibrium conditions to avoid the overlap of solution temperatures. Therefore, the theoretical and experimental results should be improved when the absorption heat is included in the heat transfer equation or avoiding the operation condition when output is lower than input solution temperature.

Optical measurements in evolving dispersed pipe flows

Optical laser-based techniques and an extensive data analysis methodology have been developed to acquire flow and separation characteristics of concentrated liquid–liquid dispersions. A helical static mixer was used at the inlet of an acrylic 4 m long horizontal pipe to actuate the dispersed flows at low mixture velocities. The organic (913 kg m^{-3}, 0.0046 Pa s) and aqueous phases (1146 kg m^{-3}, 0.0084 Pa s) were chosen to have matched refractive indices. Measurements were conducted at 15 and 135 equivalent pipe diameters downstream the inlet. Planar laser induced fluorescence (PLIF) measurements illustrated the flow structures and provided the local in-situ holdup profiles. It was found that along the pipe the drops segregate and in some cases coalesce either with other drops or with the corresponding continuous phase. A multi-level threshold algorithm was developed to measure the drop sizes from the PLIF images. The velocity profiles in the aqueous phase were measured with particle image velocimetry (PIV), while the settling velocities of the organic dispersed drops were acquired with particle tracking velocimetry (PTV). It was also possible to capture coalescence events of a drop with an interface over time and to acquire the instantaneous velocity and vorticity fields in the coalescing drop.

Design and Development of Mathematical Model for Static Mixer

A numerical model for simulating Residence Time Distribution (RTD) of turbulent flows in helical static mixers is proposed and developed to improve the understanding of static mixers. The results of this model is presented in terms of different volumetric flow rate to illustrate the complicated flow patterns that drive the mixing process in helical static mixers. The computed results are also used to predict the amount of mixing that occurs within a mixing device. Such theoretical estimates need, however, always to be thoroughly checked against observations in static mixer. To check the reliability of the theoretically estimated RTD from the simulation by the application of the model equation, a comparison of the same with those obtained from observed data experiments in static mixer using statistical characteristics is done. Comparison between RTD curves shows that motionless mixture can improve the performance of reactor.