Visualization study of emulsion droplet formation in a coflowing microchannel

An experimental visualization study is conducted to investigate the hydrodynamic characteristics of emulsion droplet formation in a coflowing microchannel. Both monodisperse and polydisperse patterns of drop formation are observed, including dripping regime, jetting regime (widening jetting and narrowing jetting). Especially, two dripping-to-jetting transition regimes and wavy regime with no individual droplet produced are captured and analyzed. A corresponding phase diagram is provided to characterize the transitions between different emulsification patterns through the control of flow rate of continuous phase. In addition, the dependence of generated droplet size on the Capillary number of the continuous phase (Ca) and the Weber number of the dispersed phase (We) is presented. It is indicated that, when Ca is below 3, the generated droplet size is sensitive to the viscous force and the drop formation regime is widening jetting and dripping. However, when Ca exceeds 3, the generated droplet size is approximately independent of Ca, and the droplet formation regime is thinning jetting.     

Numerical study on gas and liquid slugs for Taylor flow in a T-junction microchannel

The rapid development of microfabrication techniques creates new opportunities for applications of microchannel reactor technology in chemical reaction engineering. The extremely large surface-to-volume ratio and the short transport path in microchannels enhance heat and mass transfer dramatically, and hence provide many potential opportunities in chemical process development and intensification. Multiphase reactions involving gas/liquid reactants with a solid as a catalyst are ubiquitous in chemical and pharmaceutical industries. The hydrodynamics of the flow affects the reactor performance significantly; therefore it plays a prominent role in reactor design. For gas/liquid two-phase flow in a microchannel, the Taylor slug flow regime is the most commonly encountered flow pattern. The present study deals with the numerical simulation of the Taylor flow in a microchannel, particularly on gas and liquid slugs. A T-junction empty microchannel with varying cross-sectional width (0.25, 0.5, 0.75, 1, 2 and 3 mm) served as the model micro-reactor, and a finite volume based commercial computational fluid dynamics (CFD) package, FLUENT, was adopted for the numerical simulation. The gas and liquid slug lengths at various operating and fluid conditions were obtained and found to be in good agreement with the literature data. Several correlations in the T-junction microchannel were developed based on the simulation results. The slug flows for other geometries and inlet conditions were also studied.     

Droplets generation under different flow rates in T-junction microchannel with a neck

The 2D top view of the droplet formation in microfluidic T-junction devices with a neck is recorded to estimate the droplet volume under different flow conditions. The channel with a neck at the T-junction, to provide a narrow structure, is proposed but has not been analyzed as the normal T-junction. The droplet generation process is separated into two stages, including the filling stage and the squeezing stage, to develop a droplet size predictive model based on the continuity of both liquids. In a wide range of flow conditions for multiphase microfluidics, this study validates and physically explains the model by analyzing the generation of droplets and coefficients of the model. Results of this study can help to design droplet microfluidic devices, where it is requisite to know the relation between flow conditions and the droplet size.     

Effects of viscosity on coalescence of a bubble upon impact with a free surface

Effects of liquid viscosity on coalescence of a bubble upon impact with a free surface were studied both experimentally and numerically. Experiments were conducted in a pool filled with silicone oil by generating a nitrogen gas bubble and observing it with a high-speed video camera. Full Navier-Stokes equations were solved with two level set functions for a bubble and a free surface. As a result, the Weber number was the most important parameter determining the coalescence time in low viscosity liquids. In contrast, the coalescence time in a high viscosity liquid was much greater than that of a low viscosity liquid with the same Weber number. In addition, the thresholds between bubble coalescence and bouncing were affected by liquid viscosity. For the differences in coalescence time, foam was observed at the free surface in the case of high viscosity case only the difference of liquid viscosity in the non-polar liquid. By examining the pressure distribution of the liquid film between the bubble and the free surface and the downward liquid flow, we conclude that not only the liquid film but also the liquid flow field underneath the bubble was important for bubble coalescence or bouncing.     

Critical heat flux of nanofluids inside a single microchannel: Experiments and correlations

This study investigated experimentally the CHF phenomena of aqueous-based alumina nanofluids in single microchannels, and assessed the validity of a number of microchannel based CHF correlations using experimental nanofluids data. While usual approaches for CHF enhancement are through the modification of different tube surfaces or employing different inserts, this work showed that CHF in microchannels could be enhanced significantly by the inclusion of small concentrations of nanoparticles. The CHF value was found to increase with increase of mass flux, initial subcooling and alumina nanoparticle concentrations. The maximum subcooled CHF enhancement occurred at the lowest mass flux and highest alumina concentration within the experimental range. In addition, the Lee and Mudawar correlation was modified to predict the critical heat flux of water and nanofluids. The new model was examined by experimental data and 24% and 30% mean absolute error were observed for water and alumina nanofluid respectively.     

Heat transfer and pressure drop in a ZrB2 microchannel heat sink: A numerical approach

Advances in micro-electro-mechanical systems (MEMS) resulted in the fabrication of electronic and optic devices which generate high amounts of heat in a small space. Microchannel heat sinks are a new type of heat exchangers which are capable to absorb such ultrahigh heat fluxes and ensure the proper function of such devices. In the present work, a microchannel heat sink made of ZrB₂ ceramic is investigated numerically to evaluate its feasibility to operate at such harsh conditions. The governing equations of the liquid domain (water) and solid domain (ZrB₂) were solved by the finite element method. The obtained results showed a considerable heat transfer rate from the heated surface. For example, at an ultra-high heat flux of 3.6 MW/m², the maximum temperature didn't exceed ~360 K. The high heat transfer area per volume of the applied microchannel, as well as the remarkable thermal conductivity of ZrB₂, are the main reasons for such a high heat transfer rate.     

Coalescence Deformation of Bubble Pairs Generated from Twin Nozzles in CMC Solutions

A coupled level‐set/volume‐of‐fluid method, under the consideration of the rheological characteristics of a fluid, is employed to investigate numerical coalescence deformation of bubble pairs generated at two adjacent nozzles in carboxymethyl cellulose aqueous solutions. The satisfactory agreement between numerical results and experimental measurements proves the validity of this approach in predicting the surface evolution of bubbles. Simulated results show that the bubble coalescence process involves four stages of independent growth, rapid mergence, radial expansion, and vertical stretching. The various effects of surfactant concentration, gas flow rate, nozzle spacing, and nozzle diameter on the aspect ratio depend greatly on each coalescence period.     

Formation characteristics of Taylor bubbles in a T-junction microchannel with chemical absorption

This study focuses on the effect of chemical absorption on the formation dynamic characteristics and initial length of Taylor bubbles. The temporal evolutions of neck width and length of gaseous thread and initial length with and without chemical absorption were investigated with the Capillary number and Hatta number between 0.0010–0.0073 and 1.8–5.8 respectively. The squeezing regime with typical three stages, expansion, squeezing and pinch off is observed for both two processes. Compared with the non-absorption process, the increase of formation time in the chemical absorption process arises mainly from the expansion stage, and the decrease of initial length is from the necking stage. In addition, the temporal length evolution satisfies the power-law scale with the same exponent but a smaller pre-exponential factor. The correlations of neck width for stage transition and initial length with Hatta number demonstrate the enhancement effect of chemical absorption on bubble formation dynamics and initial length at relatively high chemical reaction rates and long formation time. This study provides insight into the bubble formation mechanism and helps to regulate the bubble initial size with chemical absorption.     

Generation of highly uniform droplets using asymmetric microchannels fabricated on a single crystal silicon plate: Effect of emulsifier and oil types

Uniform droplets of soybean oil, MCT (medium-chain fatty acid triglyceride) oil and n-tetradecane with a mean diameter of 26-29 μm have been generated using a silicon 24 × 24 mm microchip consisting of 23,489 asymmetric microchannels fabricated by photolitography and deep-reactive ion etching. Each microchannel consisted of a circular 10-μm diameter straight hole with a length of 70 μm and a 50 × 10 μm rectangular microslot with a depth of 30 μm. At the constant oil flux of 10 L m^− 2 h^− 1, the percent of active channels increased with increasing the oil viscosity and ranged from 4% for n-tetradecane to 48% for soybean oil. The size distribution span for SDS (sodium dodecyl sulphate)- and Tween 20 (polyoxyethylene (20) sorbitan monolaurate)-stabilized soybean and MCT oil droplets was 0.21-022. The ability of asymmetric microchannels to generate monodisperse soybean oil droplets at the very low SDS concentration of 0.01 wt.% has been demonstrated. At the SDS concentration below the CMC, the generated droplets tend to attach to the plate surface, whereas at the higher SDS concentration they detach from the plate as soon as they are formed. The agreement between the experimental and CFD (Computational Fluid Dynamics) simulation results was excellent for soybean oil and the poorest for n-tetradecane.     

Estimations of the viscosities of binary mixtures with different equations of state and mixing rules

In this study, the Eyring's kinematic viscosity model was applied with the consideration that the activation free energy of flow was related to the excess free energy of mixtures and estimated with the equation of state (EOS). The effects of the equation of state, mixing rule, and interaction parameter on the viscosity estimations were considered.     

Estimation of Coalescence Parameters in an Agitated Extraction Column Using a Hybrid Algorithm

A method based on a genetic algorithm (GA) combined with Levenberg‐Marquardt (LM) method applicable to the population balance model for coalescence parameter estimation in a liquid‐liquid biphasic system is presented. The toluene/water system in a rotating disk contactor was taken as an example. Estimation methods for such a problem are often based on deterministic optimization models that are rather instable and divergent around a local minimum. To overcome these limitations, the present study involves the introduction of a semi‐stochastic method that is able to provide at first the estimation of coalescence parameters from the GA based on an inverse approach, exploiting the principle of GA. The LM algorithm was applied to ensure that the results are not restricted to a local minimum.     

Investigations on the role of surfactants in mechanical emulsification using a high-pressure homogenizer with an orifice valve

The objective of the research work presented in this paper is to elucidate the role of surfactants during mechanical emulsification. To find out whether droplet disruption or stabilization, or both, are influenced by emulsifiers, the time for disruption and stabilization were calculated approximately. Recent developments in the institute have shown that elongational flow is the most efficient flow regime to produce emulsions with submicron droplets. Experiments, therefore, were carried out using a high-pressure homogenizer with an orifice valve. To illustrate the emulsification process the process was videotaped using a high-speed camera. Calculations for the experimental conditions investigated show that a large number of subsequent disruption steps can take place in the elongational flow. The surfactant is capable of adsorbing at newly formed droplets between subsequent disruption steps. Thus, the total disruption process can be facilitated by surfactants. Further, comparing adsorption time and residence time in the elongational flow shows that stabilization of newly formed droplets in the elongational flow is possible. The emulsification experiments show that between subsequent disruption steps the surface load is not balanced over the droplet’s surface. The results indicate that the adsorption process is governed by adsorption from the subsurface to the surface. The pictures obtained by videotaping support the results of calculations and emulsification experiments: droplet deformation with and without surfactant is not significantly different. If stabilization is poor, droplets recoalesce and the disruption result is reversed.     

Combustion characteristics of droplets composed of light cycle oil and diesel light oil in a hot-air chamber☆

Development of gas turbines fueled with light cycle oil (LCO) and oil mixture of LCO and diesel light oil (LO) requires an understanding of the droplet burning and vaporization characteristics of those oils. The present study is devoted to comparing the burning characteristics of isolated fuel droplets composed of an LCO and an LO. The tests were conducted in an atmospheric hot-air chamber preset at 1173 K, and the examined LCO had a lower cetane number but higher volatility and aromatics content compared to LO. It was demonstrated that the burning of the LCO droplet was sootier, while that of the LO droplet was more disruptive. At the tested temperature, coke formation was indistinct for both the oils, whereas slightly higher ignition delay time was shown for the LO droplet. The microexplosive burning more or less complicated the time-series droplet size d, an explicit burning rate constant, however, was still definable according to the d²-law to show the overall regression speed of the droplet surface area d² with burning time t. The rate constant exhibited little difference for smaller LCO and LO droplets but was greater for LO when the droplet was larger. The rate constant also gradually increased with increasing the initial droplet diameter d₀, which caused the relative size d/d₀ to be unified (normalized) into a single curve by a burning time t/d₀ⁿ (1.0<n<2.0). Analysis revealed that this unification resulted from the respective overlaps of the unsteady and quasi-steady burning phases for differently sized droplets. Further, it was clarified that the unification and analysis are generally valid to isolated liquid fuel droplet burning in hot ambiences.     

Mixing and enzyme reactions in a microchannel packed with glass beads

Mixing behavior in a microchannel was investigated by means of a spectrophotometer equipped with optical fiber probes. Microreactors with a Y-shaped channel were fabricated on poly(methyl methacrylate) substrates. Glass beads were packed in the intersection and the downstream regions to improve mixing performance. An NaOH solution and a BTB solution as a pH indicator were fed to the microchannel with a syringe pump. As the mixing progressed, the color of the mixed solution changed to blue. The degree of mixing was evaluated by the change in absorbance at 623 nm of the mixing solution. The packed glass beads strongly enhanced the mixing performance in the microchannels, especially in the case of packing in the intersection. The effect of mixing in the microchannel on an enzyme reaction was also investigated. The hydrolysis of o-nitrophenyl-\-D-galactopyranoside by \-galactosidase was used as a model reaction. The results showed that the enzyme reaction was enhanced in the microreactor with glass beads compared with a batch reactor. The microreactor packed with glass beads gave the highest reaction rate.     

Analysis by droplet size classes of the liquid flow structure in a pressure swirl hollow cone spray

In this work the structure of a spray resulting from the break-up of a conical liquid sheet was investigated through experimental techniques. The disperse and continuous phase velocities and the size of droplets were measured using a phase Doppler particle analyzer. A data post-processing, applying the generalized integral method, was used to evaluate liquid volume fluxes for different droplet size classes.     

Simulation of condensation flow in a rectangular microchannel

The condensation flow of the refrigerant FC-72 in a rectangular microchannel with a 1-mm hydraulic diameter is numerically studied using the volume of fluid (VOF) model. The heat transfer related to the condensation is taken into account by a thermal equilibrium model assuming the interface temperature is at saturation. The numerical method is validated against experiments from the literature and well predicts the flow patterns along the microchannel. The vapor phase in the microchannel forms a continuous column with a decreasing diameter from upstream to downstream. Slugs are periodically generated at the head of the column. Decreasing the wall cooling heat flux or increasing the flow mass flux increases the vapor column length. Waves along the interface cause necks in the column and locally increase the vapor velocity and decrease the pressure, facilitating breakage of the vapor column into slugs. The liquid temperature is close to saturation near the interface and lower downstream and in the thin liquid layer close to the cooling surface. The initial bubble size increases with increasing flow mass flux or decreasing cooling heat flux.     

Formation of solid lipid nanoparticles in a microchannel system with a cross-shaped junction

This work presents a novel method for generating solid lipid nanoparticles (SLNs) in a microchannel system with a cross-shaped junction formed by a main microchannel and two branches. A lipid solution by dissolving the lipid in a water-miscible organic solvent was passed through the main channel, while an aqueous surfactant solution was introduced into the branches simultaneously. These two liquids met together at the cross-shaped junction and passed along the main channel. The solvent diffused from the lipid solution stream into the aqueous phase, which resulted in the local supersaturation of lipid and thus led to the formation of SLNs. The flow behaviors of lipid and aqueous phase zones were measured by a digital inversion microscope system. The mean particle diameter and the particle size distribution of the obtained SLNs were measured by dynamic light scattering (DLS) method and the particle morphology was examined by transmission electron microscopy (TEM). The effects of liquid flow velocity and lipid concentration on the properties of SLNs were investigated experimentally. The formation mechanism of SLNs in the present microchannel system was discussed and analyzed.     

不同截面纤维的毛细管特性研究

从理论上分析了毛细效应发生的条件:建立了5种不同截面纤维形成毛细管的模型,推导出了不同截面纤维形成的毛细管的大小,分析了发生毛细效应的难易程度;统计了不同截面纤维根数与形成毛细管根数的关系;总结出了不同截面纤维形成毛细管液态水毛细运输的流量;从而评价了不同截面纤维的湿传导能力,提出了优化方法。