Effect of inlet conditions on taylor bubble length in microchannels

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.The effect of inlet conditions on the frequency and size of the bubbles that form during gas-liquid Taylor flow in microchannels is investigated in this paper. Three different inlet configurations, T-, Y- and Mjunction as well as three test channels with hydraulic diameters 0.345mm, 0.577mm and 0.816mm were used. The test fluids were nitrogen and water or octane, that have different surface tension. It was found that bubble length increased with increasing gas flowrate, gas inlet size and liquid surface tension and decreasing liquid flowrate. From the different inlet configurations, the M-junction resulted in the largest bubbles and the Y-junction in the smallest ones particularly at low liquid flowrates. The experimental bubble sizes were tested against a number of literature correlations but the agreement was not very good. Two new correlations were developed for the T- and the Y-junctions to calculate the unit cell (one bubble and one slug) frequency from which the bubble length can be found. Bubble lengths predicted from these correlations were in good agreement with experimental ones obtained from video recordings

Investigation of liquid phase characteristics in an inclined open microchannel

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.An important variable in the designing of gas-liquid reactors is the interfacial area available for the conduction of the two phases. Falling film microreactors (FFMR) are devices which can offer extended specific surfaces (up to 20,000m2/m3) and for this reason they are used in many multiphase processes. The aim of this work is to assess the effect of the microchannel width as well as the flow rate and the physical properties of the liquid phase on the geometrical characteristics (i.e. thickness and surface shape) of the liquid film, which were measured using Micro Particle Image Velocimetry (μ-PIV). The experiments were conducted in single microchannels with widths of 1200, 600 and 300μm and for Reynolds numbers between 0.9 and 39.7, while water and aqueous solutions of glycerol and butanol were used as working fluids. It was also verified that a common expression for predicting film thickness in macroscale is not applicable in microscale

Hydrodynamic characterisation of layered herringbone microchannels

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.The performance of a layered herringbone microstructured channel is compared with the staggered herringbone micromixer (SHM) originally proposed by Stroock et al. (2002). The layered configuration uses a single set of herringbone structures for two adjacent channels. Mixing and residence time distributions (RTDs) are studied both theoretically, via computational fluid dynamics and particle tracking algorithms, and experimentally. Experimental RTD measurements were performed by monitoring the concentration of a tracer dye by means of a LED-photodiode system. The proposed layered design gives similar results in terms of mixing and RTD as the standard SHM and it outperforms the behaviour of a rectangular channel

A Hydrodynamic Study of Benzyl Alcohol Oxidation in a Micro-Packed Bed Reactor

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.The various flow regimes prevalent during gold-palladium catalyzed benzyl alcohol oxidation in a micro-packed bed reactor and their influence on reaction performance are identified. The reaction is studied in a 300μm deep x 600μm wide silicon-glass micro-structured reactor packed with 65μm catalyst particles at a temperature of 120°C, pressure of 1 bar (g), using pure oxygen and neat benzyl alcohol as the feed. Significant improvement in the conversion and selectivity to the main product, benzaldehyde, is observed with increasing gas flowrate and decreasing liquid flowrate, which coincides with a change in the flow pattern from “liquid-dominated slug” (segregated regions of liquid and gas slugs) to “gas-continuous trickle” (thin film coated catalyst particles with gas flowing through the voids). The latter flow regime results in enhanced external mass transfer due to an increase in the available interfacial area and shorter diffusional distances. Results show selectivity up to 81% at a catalyst space time of 76 gcatgalc(-1).s, outperforming a conventional batch laboratory reactor

Residence time distributions in laminated microstructured plate reactors

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Residence time distributions (RTDs) have been investigated experimentally for systems with straight and zig-zag channels. The channels are formed by microstructured plates placed on top of each other and containing obstacles and holes to allow flow in 3 dimensions. Experimental RTD measurements were performed by monitoring the concentration of a tracer dye by means of a LED-photodiode system. The RTD was obtained for five different flowrates for both geometries. It was found that the zig-zag channel configuration gives a narrower distribution as compared to the straight channel one. Furthermore, as the flowrate increased the standard deviation of the distribution increased

Synthesis of silver nanoparticles using a microfluidic impinging jet reactor

Synthesis of silver nanoparticles (NPs) in an impinging jet reactor (IJR) was investigated due to its unique properties of efficient mixing and lack of channel walls which avoid fouling. Silver NPs were formed at room temperature by reducing silver nitrate with sodium borohydride in the presence of sodium hydroxide. Two types of ligand were used to stabilize the NPs, trisodium citrate and polyvinyl alcohol (PVA). Weber number, the ratio between inertial forces and surface tension forces, is used to characterise flow in impinging jets. Flow regimes were investigated for Weber numbers in the range of 13-176. A liquid sheet/chain regime was identified at lower Weber numbers ( 90). Mixing time was found to be in the range 1-7 ms, using the Villermaux-Dushmann reaction system and Interaction by Exchange with the Mean mixing (IEM) model. Fastest mixing occurred at Weber number ca. 90. Using trisodium citrate as a ligand, NP size decreased from 7.9±5.8 nm to 3.4±1.4 nm when flow rate was increased from 32 ml/min to 72 ml/min using 0.5 mm jets; and from 6.4±3.4 nm to 5.1±4.6 nm when flow rate was increased from 20 ml/min to 32 ml/min using 0.25 mm jets. Using PVA as a ligand, NP size decreased from 5.4±1.6 nm to 4.2±1.1 nm using 0.5 mm jets and stayed relatively constant between 4.3±1 nm to 4.7±1.3 nm using 0.25 mm jets. In general the size of the NPs decreased when mixing was faster

Synthesis of Silver Nanoparticles using Non-Fouling Microfluidic Devices with Fast Mixing

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Silver nanoparticles were synthesized in an impinging jet reactor using silver nitrate as a precursor, trisodium citrate as a stabilizer and sodium borohydride as a reducing agent. The effect of mixing time on the nanoparticle morphology was investigated by means of UV-Vis spectroscopy, used as a characterization tool. It was observed that as the mixing time became shorter the nanoparticles retained a similar average diameter but produced more aggregates. The mixing time was characterized using the ‘Villermaux-Dushmann’ reaction system together with the Interaction by Exchange with the Mean mixing model. The mixing time achieved was of the order of a few ms for flowrates in the range 18-28 ml/min. The synthesis of silver nanoparticles was carried out at the same flowrates to link mixing time to silver nanoparticle morphology

An engineering approach to synthesis of gold and silver nanoparticles by controlling hydrodynamics and mixing based on a coaxial flow reactor

In this work we present a detailed study of flow technology approaches that could open up new possibilities for nanoparticle synthesis. The synthesis of gold and silver nanoparticles (NPs) in a flow device based on a coaxial flow reactor (CFR) was investigated. The CFR comprised of an outer glass tube of 2 mm inner diameter (I.D.) and an inner glass tube whose I.D. varied between 0.142 and 0.798 mm. A split and recombine (SAR) mixer and coiled flow inverter (CFI) were further employed to alter the mixing conditions after the CFR. The ‘Turkevich’ method was used to synthesize gold NPs, with a CFR followed by a CFI. This assembly allows control over nucleation and growth through variation of residence time. Increasing the total flow rate from 0.25 ml/min to 3 ml/min resulted initially in a constant Au NP size, and beyond 1 ml/min to a size increase of Au NPs from 17.9 ± 2.1 nm to 23.9 ± 4.7 nm. The temperature was varied between 60 – 100 °C and a minimum Au NP size of 17.9 ± 2.1 nm was observed at 80 °C. Silver NPs were synthesized in a CFR followed by a SAR mixer, using sodium borohydride to reduce silver nitrate in the presence of trisodium citrate. The SAR mixer provided an enhancement of the well‐controlled laminar mixing in the CFR. Increasing silver nitrate concentration resulted in a decrease in Ag NP size from 5.5 ± 2.4 nm to 3.4 ± 1.4 nm. Different hydrodynamic conditions were studied in the CFR operated in isolation for silver NP synthesis. Increasing the Reynolds number from 132 to 530 in the inner tube created a vortex flow resulting in Ag NPs in the size range between 5.9 ± 1.5 nm to 7.7 ± 3.4 nm.. Decreasing the inner tube I.D. from 0.798 mm to 0.142 mm resulted in a decrease in Ag NP size from 10.5 ± 4.0 nm to 4.7 ± 1.4 nm. Thus, changing the thickness of the inner stream enabled control over size of the Ag NPs

In-Silico Conceptualisation of Continuous Millifluidic Separators for Magnetic Nanoparticles

Magnetic nanoparticles are researched intensively not only for biomedical applications, but also for industrial applications including wastewater treatment and catalytic processes. Although these particles have been shown to have interesting surface properties in their bare form, their magnetisation remains a key feature, as it allows for magnetic separation. This makes them a promising carrier for precious materials and enables recovery via magnetic fields that can be turned on and off on demand, rather than using complex (nano)filtration strategies. However, designing a magnetic separator is by no means trivial, as the magnetic field and its gradient, the separator dimensions, the particle properties (such as size and susceptibility), and the throughput must be coordinated. This is showcased here for a simple continuous electromagnetic separator design requiring no expensive materials or equipment and facilitating continuous operation. The continuous electromagnetic separator chosen was based on a current-carrying wire in the centre of a capillary, which generated a radially symmetric magnetic field that could be described using cylindrical coordinates. The electromagnetic separator design was tested in-silico using a Lagrangian particle-tracking model accounting for hydrodynamics, magnetophoresis, as well as particle diffusion. This computational approach enabled the determination of separation efficiencies for varying particle sizes, magnetic field strengths, separator geometries, and flow rates, which provided insights into the complex interplay between these design parameters. In addition, the model identified the separator design allowing for the highest separation efficiency and determined the retention potential in both single and multiple separators in series. The work demonstrated that throughputs of ~1/4 L/h could be achieved for 250–500 nm iron oxide nanoparticle solutions, using less than 10 separator units in series