Internal Jet Formation During Bubble Generation in Microchannels

Bubble generation is a very dynamic process including surface forces with fluid flow and structure interaction on short time and length scales. This study describes interaction effects during bubble generation in combination with bubble flow through a nozzle for redispersion purpose. At certain flow velocities and phase ratios, liquid jets within gas bubbles have been observed in microchannels, which origin from the rear tip of the bubble cap and penetrate through the whole bubble. The penetration of the filament or thread leads to bubble surface corrugation and causes bubble breakup, when the opposite cap of the bubble is hit. In the case of micronozzles behind the contact element, jet formation within the bubble is also caused by another bubble leaving the micronozzle and probably leading to a pressure disturbance acting on the just generated bubble. First data indicate major influence parameters in jet formation; however, systematic investigations are following.

Role of Copper Oxide Layer on Pool Boiling Performance With Femtosecond Laser Processed Surfaces

Copper pool boiling surfaces are tested for pool boiling enhancement due to femtosecond laser surface processing (FLSP). FLSP creates self-organized micro/nanostructures on metallic surfaces and creates highly wetting and wicking surfaces with permanent surface features. In this study two series of samples were created. The first series consists of three flat FLSP copper surfaces with varying microstructures and the second series is an open microchannel configuration with laser processing over the horizontal surfaces of the microchannels. These microchannels range in height from 125 microns to 380 microns. Each of these surfaces were tested for pool boiling performance. It was found that all the processed surfaces except one resulted in a decrease in critical heat flux and heat transfer coefficient compared to an unprocessed surface. It was found that the laser fluence parameter had a significant role in whether there was an increase in CHF or HTC. A cross sectioning technique was employed to study the different layers of the microstructure and to understand how FLSP could have a negative effect on the CHF and HTC. It was found that a thick oxide layer forms during the FLSP process of copper in an open-air atmosphere. The thickness and uniformity of the oxide layer is highly dependent on the laser fluence. A low fluence sample results in an inconsistent oxide layer of nonuniform thickness and subsequently an increase in CHF and HTC. A high laser fluence sample results in a uniformly thick oxide layer which increases the thermal resistance of the sample and allows for a premature CHF and decrease in HTC.

Modeling of Electro-Kinetic Motion of Janus Droplet

Electro-kinetic manipulation Janus particles and droplets has attracted attention in recent years due to their potential application in microfluidics. Due to the presence of two different zone on the surface of particles with different charge distribution, the motion of the Janus particles are quite different than the that of regular particles. Therefore; the fundamental understanding of this motion is the key element for the further development of the microfluidic systems with Janus particles. In present study, electro-kinetic motion of Janus droplets inside a micro-channel is modeled using boundary element formulation. 2D formulation is verified against the reported experimental data in the literature. Results show that the 2D boundary element formulation is successful for the prediction of the electrophoretic velocity of the Janus droplets. The current formulation has a potential to model non-spherical particles and to study particle-particle and particle-wall interactions.

Influence of Surface Roughness and Wettability on Oil-Water Flow Regimes in Circular Glass and Stainless Steel Mini-Channels

The present study investigates the effects of tube roughness and wettability on oil-water flow regimes in mini channels. The tube material examined included borosilicate glass (i.e., e = 0.1 μm) and stainless steel (i.e., e = 5 μm). Flow patterns and pressure drop were measured and presented for different combinations of oil and water superficial velocities, 0.28–3.36 m/s and 0.07–5 m/s, respectively. Stratified, annular, intermittent, and dispersed flow regimes were observed in all tubes and between tubes, many similarities in flow regime emerged. Tube wettability affected flow regime and flow transition from stratified to annular and intermittent flows. Surface roughness had an observable effect overall flow regime and particularly on pressure drop measurements as stainless steel recorded higher pressure drops.

Wavy-Channel for Microscale Heat Transfer in Macro Geometries

Microscale heat and fluid flow in macro geometries have been made practical in terms of cost and fabrication, by superimposing two macro geometries which are fabricated using readily-available CNC machining methods. Wavy-profile has been proposed to enhance heat transfer performance in the microchannel owing to the simplicity of geometry and feasibility to be fabricated using simple turning process. Experimental studies were conducted on single-phase, forced convective heat transfer using water as the working fluid for the Reynolds number range of 1300 to 4600, for a constant heat flux of 53.0 W/cm2. Three sinusoidal waves with different wavelength and same amplitude are studied to examine the effect of the total number of waves on the heat transfer and hydrodynamic performance within constant microchannel length. The maximum performance index, which evaluates heat transfer performance per unit pumping power, is 1.39, achieved by wavy profile with the shortest wavelength at Reynolds number of 2800. The performance index for all the enhanced microchannels peaks at the Reynolds number range of 2500 to 2800. Beyond that, the performance index is not a strong function of the wavelength. At lower Reynolds numbers, profile with the shortest wavelength achieves substantially higher performance indices, as the increment in pressure drop is accompanied by a comparable increment in heat transfer. Future work includes the introduction of correlations for the implementation of such geometries in industrial heat exchangers.

An Ultra-High-Throughput Flow-Focusing Microfluidic Device for Creation of Liquid Droplets in Air

In this paper a new microfluidic technique is proposed for ultra-high-throughput generation of micron-sized water droplets using a high-speed air. We use a 3D flow-focusing microchannel fabricated in PDMS by multilayer lithography process. The interaction of liquid and gas created three main flow conditions which are: Flooded, Dripping, and Jetting. We characterize the Jetting regime where a capillary jet surrounded by the air breaks up into uniform array of droplets. Frequency of generation and droplet size are reported for the jetting regime under different liquid and gas flows. It was possible to obtain 25μm diameter droplets and much higher frequencies (f≈120 kHz) compared to the state-of-the-art microfluidic systems. We believe the advantages of this platform enables many novel applications such as high-throughput screening of airborne targets and large-scale production of oil-free particles. The 3D structure of this device also eliminates the limitation of the conventional droplet-based microfluidic systems, namely clogging issues due to particle aggregation.

Droplet Coalescence and Departure on a Vibrating Film During Humid Air Condensation

This paper investigates an approach to collect evaporated water from a moist air stream, a scenario found in many power plant cooling towers which utilize evaporation to cool, thereby resulting in evaporative water losses. For example, a 500 MW power plant may lose about 27m3/h (7133 gal/h) of water to evaporation during operation. When a cooled surface is placed in a warm humid environment, water condenses on the surface. The condensed liquid forms a thermal resistance, thereby reducing the condensation rate and water collected. The concept presented in this paper is to vibrate the cooled surface, thereby rapidly removing more droplets than gravity alone. With forced movement and through droplet coalescence, new droplets can nucleate in the space created by departing water droplets. Droplet nucleation, coalescence, and departure were investigated on vibrating and stationary Teflon films (contact angle 105°) in an environmental chamber at 30°C and 50% RH. Film vibrations of approximately 100 Hz were investigated. Droplet departure diameters were approximately 2–3 mm diameter on the vibrating surface and 6 mm on the stationary surface.

Investigation of Bubble Breakup in Laminar, Transient, and Turbulent Regime Behind Micronozzles

Gas-liquid flow in microchannels has drawn much attention in the last years in research fields of analytics and applications such as oxidations or hydrogenations. High interfacial area leads to increased mass transfer and intensified reactions. Since surface forces are increasingly important on small scale, bubble coalescence is detrimental and leads to Taylor bubble flow in microchannels. To overcome this limitation, we have investigated the gas-liquid flow through nozzles and particularly the bubble breakup behind the nozzle. Two different regimes of bubble breakup were identified, laminar and turbulent with different mechanisms. Although turbulent breakup is not common in microchannels, its mechanisms were studied for the first time and can give new insight for two-phase flow mechanisms.

Synchrotron X-Ray Investigation of Membrane Water Distribution in Polymer Electrolyte Membrane Fuel Cell Applications

Water management is a critical component of extracting optimum performance and efficiency from polymer electrolyte membrane (PEM) fuel cells. During fuel cell operation, a balance needs to be maintained between excess water blocking the reactant pathways through the gas diffusion layer, and the requirement for membrane hydration. The ionic conductivity through the membrane depends strongly on the hydration of the membrane. The reactant gases in a PEM fuel cell are supplied through a humidification system to maintain appropriate levels of hydration in the membrane. The removal of the anode humidifier would significantly reduce the balance of plant costs and reduce the volume required for the fuel cell in an automotive setting. However, removing the anode humidification system could have adverse effects on membrane hydration and on fuel cell performance. In this study, the anode humidification was varied and the cell performance and the membrane resistance were monitored. Synchrotron X-ray radiography was conducted simultaneously to visualize the water distribution in the membrane, the gas diffusion layer, and the associated interfaces. It was observed that the anode humidification had a strong impact on the performance of the fuel cell, with the dry condition leading to voltage instability at a current density below 1.0 A/cm2. The membrane water content was observed to decrease with increases in operating current density.

Modeling of an AC-Electro-Osmosis Based Microfluidic Mixer

The purpose of this study is presenting an active micro-mixer, which is based on AC electro-osmotic flow driven on 3D micro wires. In order to solve governing equations of AC electroosmosis, a commercial software COMSOL Multiphysics® is implemented. Different wire configurations with various imposed electric fields and flow rates are tested for evaluating mixing efficiencies. The analyses show that mixing performance is significantly improved by number of the wires as well as wire orientation. It is also revealed that the degree of mixing can also be controlled by the tuning of the applied voltage for a given flow rate.