Flow Blurring Research Articles

The high-throughput atomization of polymer solutions for fiber synthesis in a single step aided with corona ionizers

Polymer microfibers are ubiquitous structures across virtually all technological fields. Their applications include, for instance, filter media, tissue regeneration, wound healing and dressing, and reinforcement materials. The most effective methods for fabrication of fibrous micro and nanomaterials rely on electric fields to spin a liquid jet into an ultrafine thread that rapidly dries up forming a fiber. Continuous spinning and collection leads to formation of fiber mats. Here we report a robust yet simple approach for the massive production of liquid threads, which upon acquiring electrical charges in-flight are collected downstream in the form of fibers. The entire process takes place on-line in a single step. The liquid threads are produced through the fragmentation of a polymer solution bulk due to a turbulent interaction of a gas–liquid interface in the interior of an engineered device, a so-called Flow Blurring atomizer. The particularity of this approach consists precisely in such vigorous interaction, at the micrometer scale, which triggers a bubbly motion in the interior of the device, that is a “micro-mixing”. Subsequently, the threads are passed through ionized air currents, at ambient conditions, and then stretched to sub-micrometer dimensions by electric fields. Polyvinylpyrrolidone (PVP) as well as carbon nanotubes (CNTs) or graphene oxide sheets (GOSs)-containing PVP fibers, with diameters in the range 100–900 nm, were synthesized via this approach. In the cases studied herein the method was operated at liquid flow rates (i.e. production rates) of 0.2 mL/min but it could be readily increased up to a few tens of mL/min. The method requires further improvement and optimization, nevertheless it is a promising alternative for mass production of polymer fibers.

REGIMES FOR FLOW-BLURRING AND FLOW-FOCUSING ATOMIZATION OF NEWTONIAN AND NON-NEWTONIAN FLUIDS

This paper presents a detailed mapping of flow-blurring (FB) and flow-focusing (FF) atomization as relevant to Newtonian fluids and non-Newtonian fluids. Two Newtonian fluids with different viscosities are tested, along with a non-Newtonian fluid, where the properties are similar to those of human saliva. Images featuring the fragmentation characteristics are presented with regime diagrams describing the transition from flow-focusing to flow-blurring. Flow-blurring refers to a mode of atomization where the fluid is partially aerated with gas bubbles to assist breakup, while flow-focusing has similarities to air-blast atomization (AB). The regime transition map developed for FF/FB atomizers reveals the parameters, which define transitions from the flow-focusing to the flow-blurring regime. Along with the transition identification, the breakup regime map also details the fragment morphology of the atomizing liquid stream as a function of the governing dimensionless groups.

Integrating a micro-mixing mechanism and on-line thermal processing for the large-scale ejection of polymeric liquid threads for producing ultrafine fibers.

Micro/nanofibers are structures that nowadays have a wide range of cutting-edge applications including energy generation and storage devices, smart textiles, cell growth, and tissue engineering. These fibrous materials are mostly produced from polymer solutions spun, under laminar flow conditions, into nanofibers by external forces. However, the turbulent interaction of gas-liquid interfaces offers an innovative approach for the high-throughput production of nanofibers. Here, we present Flow Blurring (FB), a solely pneumatic approach for the massive production of liquid threads of polymer solutions, which relies on a micro-mixing mechanism that triggers a turbulent motion capable of fragmenting a viscous flow. The as-ejected threads are subsequently processed thermally, on-line in a single-step, thus producing micro/nanofibers that form mats. The method operates with relatively large liquid flow rates, equivalent of a high production rate, and is thus suitable for industrial production of engineered nanomaterials. In this work, we used solutions of poly(vinyl alcohol) (PVA) to study its ejection and fragmentation dynamics through computational fluid dynamics (CFD) simulations. In addition, the physics underlying the regulation of the liquid flow rate in FB atomizers are proposed. Fibers with typical diameters in the range 400-800 nm were produced by online heating of the liquid threads. Liquid ejection experiments were performed under different operating conditions thus verifying the capability of the method for synthesizing submicrometer-sized fibers with high uniformity and production rates suitable for scaling up.

Massive production of fibroin nano-fibrous biomaterial by turbulent co-flow

Among the different polymers (proteins, polysaccharides, etc.) that make up natural fibers, fibroin is a protein produced by silk spinning animals, which have developed an optimized system for the conversion of a highly concentrated solution of this protein into high-performance solid fibers. This protein undergoes a self-assembly process in the silk glands that result from chemical gradients and by the application of mechanical stresses during the last step of the process. In the quest for a process that could mimic natural spinning at massive scales, we have discovered that turbulence offers a novel and promising solution: a turbulent liquid jet can be formed by a chemically green and simple coagulating liquid (a diluted solution of acetic acid in etanol) co-flowing with a concentrated solution of fibroin in water by the use of a Flow Blurring nebulizer. In this system, (a) the co-flowing coagulant liquid extracts water from the original protein solution and, simultaneously, (b) the self-assembled proteins are subjected to mechanical actions, including splitting and stretching. Given the non-negligible produced content with the size and appearance of natural silk, the stochastic distribution of those effects in our process should contain the range of natural ones found in animals. The resulting easily functionalizable and tunable one-step material is 100% biocompatible, and our method a perfect candidate to large-scale, low-cost, green and sustainable processing of fibroin for fibres and textiles.

Review of Flow Blurring Atomization: Advances and Perspectives

Review of Flow Blurring Atomization: Advances and Perspectives

Near-field spray characterization of a high-viscosity alternative jet fuel blend C-3 using a flow blurring injector

Efficient and clean combustion of jet fuels highly relies on their spray characteristics. Prior studies have shown that the flow blurring (FB) injector developed in the recent decade could generate fine sprays at the injector’s immediate exit rather than a jet core of conventional fuel nozzles. The current study experimentally investigates spray characteristics of a viscous renewable jet fuel, C-3, using FB atomization. The effect of Air-to-Liquid Mass Ratio (ALR) across the injector is investigated. Results show the stable and fine FB sprays are obtained at ALRs of 1.0–2.5 for the high viscosity fuel. Cumulative droplet size distribution and Sauter Mean Diameter (SMD) obtained using high-spatial-resolution Shadowgraph imaging technique (SIT) reveal the fine atomization is rapidly accomplished at approximate 1.67–2.67 D downstream the injector exit with the diameter D of 1.5 mm for all the ALRs. The spray velocity measurements using Particle Image Velocimetry (PIV), the SMD by SIT, and Weber number estimate consistently show rigorous secondary droplet disintegration at spray periphery. Estimation of Weber and Ohnesorge numbers suggests that oscillatory deformation is dominant at the spray center across the effective ALRs, and droplets at higher ALR and spray periphery experience more vibrant breakup modes such as bag, bag-stamen or multi-mode. The results quantitatively demonstrate that the FB injector can generate ultra-fine and stable sprays for the viscous bio-jet fuel C-3, suggesting the potential use of this fuel injector for renewable jet fuels regardless of high viscosity to achieve stable, complete, and premixed combustion with ultra-low emissions.

Characterization of Flow-Focusing and Flow-Blurring Modes of Atomization

This paper presents a characterization of flow-blurring (FB) and flow-focusing (FF) modes of atomization using a new atomizer, which enables continuous transition from FB to FF regimes by controlling the recess of a liquid tube. The atomizer has optical access and features flexibility in controlling the Weber number (We) and the air/liquid mass flow ratio (ALR). The Phase Doppler Particle Analyzer (PDPA) technique is employed here along with long-distance microscopic and direct imaging to observe and characterize the atomization regime. Experiments were carried out with water and air as the working fluids at room temperature and in an atmospheric pressure environment. The results show that operation in the FB regime generates smaller droplet sizes in the spray core compared to the FF regime. However, the benefit of FB decays with increasing We, and eventually the difference between both regimes decreases. Among the distinctive features of FB sprays found only in the near-field region are: a more noticeable change in the mean droplet size with the radial direction, more pronounced polydispersity, larger centerline axial velocity and larger axial and radial velocity fluctuations. Liquid tube recess was also found to be an important parameter in controlling the spray behavior in the FB regime. It was found to impact the droplet size, the values and shape of the axial velocity variation curves, as well as the range of radial velocity values. It is also shown that, at a high enough We, effective differentiation between FB and FF atomization regimes became harder and rather limited to the visual confirmation of the presence of a bubble in the liquid tube.

Dynamics of formation of poly(vinyl alcohol) filaments with an energetically efficient micro-mixing mechanism

We present a pneumatic approach for massive production of poly(vinyl alcohol) (PVA) filaments based on a mixing mechanism at the micrometer scale using so-called Flow Blurring (FB) atomizers. This micro-mixing is triggered by a turbulent, bubbly motion generated by implosion of a gas current into a liquid feeding tube. The energy of the gas, the liquid viscosity, and the geometry of the atomizer play an active role in the size and shape of the ejecta. The shear viscosity of aqueous solutions of PVA of various molecular weights was investigated to assess their rheological nature using a dimensionless parameter based on the solutions’ concentration and the polymer’s molecular weight and its entanglement molecular weight. The solutions exhibited a shear thinning behavior at low shear rates and a Newtonian behavior at moderate rates. PVA solution with viscosity above the threshold value is prone to forming filaments during atomization with FB devices. Analyses of the dynamics of the atomization revealed two main types of ejections depending on the liquid flow rate and viscosity: either a bundle of filaments formed from within the atomizer or a more continuous single structure developed in the vicinity of the atomizer exit. Furthermore, based on Kolmogorov’s energy cascade, we propose a scaling law for the mean filament diameter as a function of liquid properties, atomizer geometry, and imposed pressure. The present work may have significant implications in the large-scale processing of liquids leading to useful materials.

Experimental investigation into flow blurring atomization

The twin-fluid atomizer employing the cross-stream mixing arrangement for air and liquid prior to the orifice exit is investigated with a scaled-up size transparent injector. Simplistic optical flow diagnostics techniques (Mie scatter and high-speed LED-backlit imaging) were employed to characterize the effect of operating parameters on internal two-phase flow development and the external spray characteristics of a transparent flow blurring (FB) injector of 6 mm exit orifice diameter. Two successive Mie scattering images of the spray are utilized for the PIV analysis along with mathematical manipulation of Mie scatter intensity profile at any axial location to contrast the spray quality for two different operating conditions. Derived droplet velocity field from the PIV algorithm is used to show the spray development confines to the near field region of five diameters downstream of the nozzle exit. The experiments were carried out for different values of air to liquid ratio (ALR) ranging from 1 to 2.5 with an increment of 0.25 for 5 different Liquid flow rates (LFR). Results indicate that the internal two-phase flow mixing characteristics of FB injector dictates the spray formation and its quality. This internal two-phase flow mixing is responsible for the jet break up in liquid tube itself leading to finely atomized spray. Laplace number defined as a product of Euler and Weber number is proposed as an injector performance number as it uniquely links to the spray quality. The impact of operating parameters such as ALR on relative atomization efficiency and spray quality in the near field is characterized.

A combined experimental and computational study of flow-blurring atomization in a twin-fluid atomizer

A twin-fluid pneumatic injector system with coaxial liquid and air streams mixed slightly upstream of the outlet, is studied in a cross-flow arrangement to understand its working principle. The gap between the orifice and the liquid tube, where the air and liquid cross-flow occurs, is known to determine the modes of working of the atomizer for the given operating flow rates of air and liquid. The two distinct modes, namely the flow focusing (FF) and flow blurring (FB) are observed in the present study, and the transition from FF to FB mode is explored using optical visualization. The characteristics of the internal two-phase flow mixing that occurs due to backflow recirculation in FB mode are also investigated by performing large eddy simulation (LES) using the volume of fluid (VoF) approach. The results of the numerical simulation are validated with experimental observations for the length of backflow recirculation observed in the liquid tube. The present study demonstrates the development of multiple microjets, which explains experimental observations of the lack of core jet and the almost uniform distribution of radial Sauter mean diameter (SMD) reported in the literature. The high-resolution backlit image of a scaled-up transparent injector with a 6 mm orifice diameter validates the numerical results that describe the internal two-phase flow.

Application of Low-Cost Electrochemical Sensors to Aqueous Systems to Allow Automated Determination of NH3 and H2S in Water.

Usage of commercially available electrochemical gas sensors is currently limited by both the working range of the sensor with respect to temperature and humidity and the spikes in sensor response caused by sudden changes in temperature or humidity. Using a thermostatically controlled chamber, the sensor response of ammonia and hydrogen sulfide sensors was studied under extreme, rapidly changing levels of humidity with the aim of analyzing nebulized water samples. To protect the sensors from damage, the gas stream was alternated between a saturated gas stream from a Flow Blurring® nebulizer and a dry air stream. When switching between high and low humidity gas streams, the expected current spike was observed and mathematically described. Using this mathematical model, the signal response due to the change in humidity could be subtracted from the measured signal and the sensor response to the target molecule recorded. As the sensor response is determined by the model while the sensor is acclimatizing to the new humid conditions, a result is calculated faster than that by systems that rely on stable humidity. The use of the proposed mathematical model thus widens the scope of electrochemical gas sensors to include saturated gas streams, for example, from nebulized water samples, and gas streams with variable humidity.

Flow blurring atomization of Poly(ethylene oxide) solutions below the coil overlap concentration

Atomization of polymer solutions has important technological implications across many fields. Here, we investigated the atomization dynamics of diluted, polymer solutions using Flow Blurring (FB) technology. Aqueous solutions of poly(ethylene oxide) [PEO] of viscosity-averaged molecular weight in the range 100000 g/mol – 4000000 g/mol and varying concentrations were sprayed with a FB atomizer having an orifice diameter (D) of 700μm and a liquid feed-tube-to-orifice separation (H) of 100μm. The solutions belong to the dilute regime, where polymer coil overlap does not occur, that is φ=cmMvMe<φcrit (Modesto-López, Pérez-Arjona, & Gañán-Calvo, 2019). Shear viscosity measurements indicated that the solutions had viscosities of the order of that of the solvent and exhibited a Newtonian-like behavior. However, during the atomization, and due to the relatively high shear stress induced in the atomizer, the solutions exhibit extensional rheology, which most likely arises from the stretching of the polymer chains in-flight. Although initially the atomization resulted in formation of filaments, these broke up into droplets at relatively short distances from the atomizer discharge orifice as elucidated by images from ultra-high speed videos. The phenomenon is in contrast with that observed in FB-based atomization of semi-diluted polymer solutions with concentrations larger than the polymer coil overlap concentration, c∗. FB atomization of the diluted solutions resulted in a decrease in droplet size with increasing the gas-to-liquid mass ratio (GLR). The approach herein aims at understanding the droplet formation dynamics of viscoelastic, polymer solutions with FB, for applications in large-scale synthesis of materials.

Effect of injector swirl number on near-field spray characteristics of a novel twin-fluid injector

Compared to liquid jets/films from conventional air-blast injectors, flow blurring (FB) injection has generated fine sprays immediately at the injector exit, resulting in the clean combustion of various fuels including viscous oils. The FB atomization involves primary atomization by bubble bursting of the bubbly internal two-phase flow due to the dramatic pressure drop at the injector exit and the secondary break-up of droplets/ligaments by Rayleigh–Taylor instability. Recently, a novel swirl-burst (SB) injector has been developed to further enhance the secondary atomization, which has resulted in less lifted and cleaner flames by integrating swirling atomizing air with the FB concept. The present study investigates the effect of injector swirl number (ISN) on spray characteristics including droplet velocity and size distributions in the injector near field (8 mm × 4 mm) using particle image velocimetry and shadowgraph imaging for the SB injectors with diameter, D, of 1.5 mm. The study investigates four ISNs from 2.0 to 3.5 with an increment of 0.5, corresponding to various swirl vane angles. Results show that all the ISNs have achieved a short break-up length (~ 1.67D) and fine atomization with final droplet diameter ranging from 12 to 40 µm and SMD around 25 µm. Droplets are evenly distributed at each axial plane for all the ISNs. Increase in the ISN results in enhanced secondary atomization and hence finer droplets within 1.67D downstream of the injector exit. Spray angles are typically narrower at the increased ISN. Regardless of the variations within the injector near field and the different spray angles, the break-up length and final droplet size are nearly independent of the ISN.

Flow Blurring-Enabled Production of Polymer Filaments from Poly(ethylene oxide) Solutions.

Flow blurring (FB) atomizers are relatively simple yet robust devices used for the generation of sprays from solutions of a wide range of viscosities. In this work, we have demonstrated that FB devices may also be applied for massive production of liquid filaments from polymeric solutions. They can later be transformed into solid filaments and fibers, leading to the production of so-called fiber mats. The liquid precursors consisted of poly(ethylene oxide) (PEO) solutions of varying molecular weights (105 [100k] to 4 × 106 g/mol [4M]) and concentrations. The FB device was operated in the gas pressure range of 3–6 bar. Except for solutions of PEO 100k, all solutions exhibited a shear thinning behavior. For massive filament production, a threshold polymer concentration (ct) was identified for each molecular weight. Below such concentration, the atomization resulted in droplets (the classical FB functioning mode). Such a threshold value decreased as the PEO molecular weight increased, and it coincides with the polymer coil overlap concentration, c*. The viscoelastic nature of the solutions was also observed to increase with the molecular weight. A 3.2 dependency of the zero-shear rate viscosity on a so-called Bueche parameter was found for filament production, whereas a nearly linear dependency was found for droplet production. In general, the mean diameter of the filaments decreased as they traveled downstream from the atomization point. Furthermore, at a given distance from the atomizer outlet and gas pressure, the mean filament diameter slightly shifted toward larger sizes with increasing PEO molecular weight. The tendency agrees well with the calculated filaments’ Deborah number, which increases with PEO molecular weight. The approach presented herein describes a high-throughput and efficient method for the massive production of viscous filaments. These may be transformed into fibers by an on-line drying step.

The Design Process for a Closed Combustion Chamber Flow Blurring Nozzle

This research outlines a process whereby a flow blurring nozzle isoptimized for use in a meso-scale combustion chamber. Flow blurringis defined as the generation of small turbulence scales in a liquid from asingular back-flow pattern of a gas. Flow blurring nozzles are beginningto be adapted in many technical applications, from emission spectrom-etry of heavy metals in biodiesel, vaporization of high viscosity fuels tomeso-scale combustion applications. This nozzle can vaporize liquidsat low flow rates efficiently and inexpensively. It uses an air stream tobreak up the liquid but it operates in a novel flow blurring regime dif -ferentiating it from a regular air blast atomizer. There are two issues withusing this nozzle for combustion applications. The first is that the airused to vaporize the hydrocarbon in the flow blurring nozzle is insuf -ficient to burn all the hydrocarbon and it is difficult to increase this airsupply. The second issue is that the vaporized mixture at the exit of theflow blurring nozzle has a relatively high velocity. The mixture velocitymust be decelerated to enable stable combustion without blowoff. Thisarticle outlines the design process for solving both these issues. In total,five design iterations were implemented before a satisfactory final de -sign was achieved.

The Design Process for a Closed Combustion Chamber Flow Blurring Nozzle

This research outlines a process whereby a flow blurring nozzle is optimized for use in a meso-scale combustion chamber. Flow blurring is defined as the generation of small turbulence scales in a l...

Investigation of water spray characteristics in the near field of a novel swirl burst injector

The limit of fossil fuels and more stringent emission standards have prompted the exploration of renewable biofuels and clean combustion technology. Clean and efficient combustion of liquid fuels mainly relies on spray fineness achieved by fuel injectors. Conventional injectors such as the air blast (AB) nozzles are only effective for low-viscosity fuels. Hence, viscous source oils of biofuels have to be converted into “drop-in” fuel such as biodiesel, leading to the high production cost of biofuels. In the past decade, the flow blurring (FB) injection using a two-phase flow concept has proved to generate fine sprays for even highly viscous liquids in the near field of injector exit, rather than a typical fuel jet from AB injectors. Thus, the FB injection has achieved cleaner burning of fossil fuels and enabled clean lean-premixed combustion of viscous oils including biodiesel source oils, signifying cost-effective biofuel utilization without the conversion cost. Recently, our group designed a novel twin-fluid swirl burst (SB) injector that integrates the advantages of FB and further improves secondary atomization caused by Rayleigh-Taylor instabilities. The SB design innovatively introduces swirling atomizing air (AA) to stimulate more vigorous interaction between AA and large droplets or ligaments from the primary FB atomization by bubble-bursting. The present study compares the water spray micro-characteristics of FB and SB injectors in the near field of the injector exit at the same flow conditions using high spatial resolution particle image velocimetry and shadowgraph imaging systems. Results show that, compared to the FB atomization, the novel SB injector has successfully generated a wider spray with finer droplets immediately at the exit orifice and a shorter atomization completion length, indicating the improved secondary atomization. Estimate of Weber number and Ohnesorge number consistently substantiates the improved SB atomization and suggests bag breakup dominating the secondary atomization for both injectors. Radial profiles of Sauter Mean Diameter indicate the SB injection yields uniform droplet size distribution compared to the FB injection that has relatively larger droplets mainly at the spray periphery. Regardless of the variations in the atomizer near region, both SB and FB injector generate fine sprays with similar final droplet size.

Development and combustion characterization of a novel twin-fluid fuel injector in a swirl-stabilized gas turbine burner operating on straight vegetable oil

Prior studies have indicated that the flow blurring (FB) injection generates fine sprays, hence clean and premixed combustion of fuels with a wide range of viscosity, though the increase in fuel viscosity causes lifted flames that might undergo blow off, and offset flame sustainability. The present study aims to achieve improved fuel pre-vaporization and fuel-air mixing hence, less-lifted flames and ultimately yield stable, clean, and premixed combustion of viscous fuels using a novel twin-fluid injector, called swirl burst (SB) injector. The novel injector design integrates the advantages of the FB atomization with swirling atomizing air (SAA). Preliminary spray diagnostics have shown that the novel SB injector effectively generated the SAA and enhanced secondary atomization in terms of shorter breakup length and wider spray angle compared to the FB injector. The current study investigates the global combustion performance of the SB injector for viscous straight vegetable oil (VO) by comparing with that of the FB injection. The work also explores the effect of atomizing air to liquid mass ratio (ALR) on the SB flames. The investigated combustion features include visual flame images, emissions, and thermal characteristics of straight VO sprays in a lab-scale (7 kW) swirl-stabilized gas turbine burner. Results of blue flames, temperature profiles of the combustor outer surface and gas products, ultra-low emissions of carbon monoxide and nitrogen oxides, consistently show that the novel SB concept and FB successfully achieved clean and stable lean premixed combustion for non-preheated straight VO. Compared to the FB injection, the use of the SB injector successfully resulted in a less-lifted VO flame with lower CO emissions due to early atomization completion and longer droplet path for faster fuel pre-vaporization, and better fuel-air mixing by the strengthened aerodynamic interaction between the SAA and primary combustion air. Increase in ALR for the SB injector resulted in cleaner combustion, again due to possible finer droplet evolution and enhanced aerodynamic interaction between reactant flows.

Visualization and size-measurement of droplets generated by Flow Blurring® in a high-pressure environment

Flow Blurring® (FB) atomization is a highly efficient method to produce aerosols. It originates from an unexpected turbulent back flow motion in the interior of the atomizer. The onset for the appe...

Effects of Fluid Properties on Spray Characteristics of a Flow-Blurring Atomizer

In order to understand the reasons for the apparent benefits of using a flow-blurring (FB) atomizer in a combustion system, it is necessary to first examine fundamental spray characteristics under nonreacting conditions. Previous work on FB atomizers, however, has mostly involved only water and a relatively narrow range of parameters. In this study, a phase Doppler anemometry (PDA) instrument was used to characterize FB atomizer sprays and determine the effects of varying surface tension and viscosity of the liquid. Operating at room pressure and temperature (i.e., a “cold spray”), droplet sizes and velocities were measured for water, a water/surfactant mixture (lower surface tension), a water/glycerol mixture (higher viscosity), and glycerol (much higher viscosity). For all of the tested fluids, with the exception of pure glycerol, the FB atomizer produced small droplets (below 50 μm) whose size did not vary significantly in the radial or axial direction, particularly above a characteristic distance from the atomizer exit. Results show that the spray is essentially unaffected by a 4.5× decrease in surface tension or a 7× increase in viscosity, and that Sauter mean diameter (SMD) only increased by approximately a factor of three when substituting glycerol (750× higher viscosity) for water. The results suggest that the FB atomizer can effectively atomize a wide range of liquids, making it a useful fuel-flexible atomizer for combustion applications.