High-speed Video Imaging Techniques Research Articles

Experimental investigation of two-phase flow regimes in rectangular micro-channel with two mixer types

This study primarily aimed to explore the two-phase flow characteristics in a rectangular channel with a hydraulic diameter of 1.33 mm. High-speed video imaging techniques were used to capture two-phase flow behavior corresponding to different superficial gas and liquid velocities. To examine the effect of the gas–liquid mixer configuration on two-phase flow patterns, experiments were conducted using two types of mixers. The results show that the mixer configuration can affect the flow structure only in the slug flow regime. The present study provides a more detailed classification of flow regimes than previous studies. The flow regimes observed were classified into seven categories: slug, aerated-slug, transition, multiple, wavy-annular, smooth-annular, and dispersed-annular flows. Moreover, the quantitative characterization of flow regimes and transition boundaries in micro-channel flows using the standard deviation of differential pressure fluctuations was attempted. This study comprehensively reviews and assesses previous two-phase flow regime maps for both macro-channels and mini/micro-channels. It is shown that the two-phase flow pattern in mini/micro-channels is influenced by various factors, such as the channel hydraulic diameter, the channel cross-sectional shape, the gas–liquid mixer configuration, the difference in the hydraulic diameters between the mixer outlet and test channel, and the thermophysical properties of the working fluids. The comparison between the present experimental data and predictions of the previous flow regime maps demonstrates the necessity of developing a new flow regime map to accurately predict two-phase flow regimes in micro-channels.

Insertion and Pull Behavior of Worker Honeybee Stinger

Worker honeybee pierces animal or human skin with its ultra-sharp stinger and injects venom to defend itself. The insertion behavior is a painless transdermal drug delivery process. In this study, Apis cerana cerana worker honeybee was chosen as the research object. The geometry and structure of the stinger were observed by the Scanning Electron Microscope (SEM). High-speed video imaging technique was adopted to observe the stinger insertion and pull behavior of honeybee. The skin insertion, pull-out, in-plane buckling and out-of-plane bending forces of honeybee stinger were tested by a self-developed mechanical loading equipment. Results showed that the honeybee stinger pierces directly into skin without frequent vibration. The pull-out force (average 136.04 mN) was two orders of magnitude higher than the penetration force (average 1.34mN). Compared with the penetration force, the in-plane buckling force (average 6.72 mN) was in the same order of magnitude. The result of out-of-plane bending test showed that the stinger was elastic and it could recover after bending. The excellent geometry and structure of honeybee stinger will provide an inspiration for the further improved design of microneedle-based transdermal drug delivery system.

Engineering properties of a low-grade metamorphic limestone

A particular type of limestone in Singapore is studied in this paper. Seventeen uniaxial compressive strength (UCS) tests and seventeen Brazilian tensile strength (BTS) tests are conducted. From a comprehensive petrographical study, the generally high UCS and BTS values are found to be attributed to the respective mineralogy and texture, as a result of the prevalence of low-grade metamorphism. Altered grain boundaries resulted from pressure solution are observed. Wavy suture contacts between interlocking and interpenetrating grains are common. Signs of subsequent precipitation (cementation) processes are recognized, which reduce the primary porosity and enhance the packing of the rock. On the other hand, with the aid of a high speed video capturing technology during the loading tests, exceptional low strengths obtained in certain specimens can be accounted for. An analysis of the high speed videos reveals that the early crack initiation is often favored along calcite veins, mudstone stringers, fossil fragments and hair-line cracks etc., which appear to be local zones of weakness. We also attempt to correlate the presence of heterogeneities in the rocks with the specimen failure mode under uniaxial compression. In summary, the results presented in this technical note fill in an existing gap of the engineering geology literature of Singapore with respect to limestone. The novelty of our research approach, which integrates the microscopic study and the high speed video imaging technique, is demonstrated to account for the large strength variation of the study limestone.

Experimental and theoretical study of critical heat flux in vertical upflow with inlet vapor void

This study explores the mechanism of flow boiling critical heat flux (CHF) for FC-72 in a 2.5 mm × 5 mm vertical upflow channel that is heated along its 2.5 mm sidewall downstream of an adiabatic development section. Unlike most prior CHF studies, where the working fluid enters the channel in liquid state, the present study concerns saturated inlet conditions with finite vapor void. Temperature measurements and high-speed video imaging techniques are used to investigate the influence of the inlet vapor void on interfacial behavior at heat fluxes up to CHF as well during the CHF transient. The flow entering the heated portion of the channel consists of a thin liquid layer covering the entire perimeter surrounding a large central vapor core. Just prior to CHF, a fairly continuous wavy vapor layer begins to develop between the liquid layer covering the heated wall and the heated wall itself, resulting in a complex four-layer flow consisting of the liquid layer covering the insulated walls, the central vapor core, the now separated liquid layer adjacent to the heated wall, and the newly formed wavy vapor layer along the heated wall. This behavior in captured in a new separated control-volume-based model that facilities the determination of axial variations of thicknesses and mean velocities of the four layers. Incorporating the results of this model in a modified form of the Interfacial Lift-off CHF Model is shown to provide fairly good predictions of CHF data for mass velocities between 185 and 1600 kg/m 2 s, evidenced by a mean absolute error of 24.52%.

Measurement and Prediction of Insertion Force for the Mosquito Fascicle Penetrating into Human Skin

Mosquitoes are exceptional in their ability to pierce into human skin with a natural ultimate painless microneedle, named fascicle. Here the structure of the Aedes albopictus mosquito fascicle is obtained using a Scanning Electron Microscope (SEM), and the whole process of the fascicle inserting into human skin is observed using a high-speed video imaging technique. Direct measurements of the insertion force for mosquito fascicle to penetrate into human skin are reported. Results show that the mosquito uses a very low force (average 18 μN) to penetrate into the skin. This force is at least three orders of magnitude smaller than the reported lowest insertion force for an artificial microneedle with an ultra sharp tip to insert into the human skin. In order to understand the piercing mechanism of mosquito fascicle tip into human multilayer skin tissue, a numerical simulation is conducted to analyze the insertion process using a nonlinear finite element method. A good agreement occurs between the numerical results and the experimental measurements.

Die Untersuchung funktioneller Stimmstörungen mit Hilfe der Hochgeschwindigkeitsvideotechnik

Malfunctions of the human voice with unknown causes are denoted by "functional voice disorders". The high speed video imaging technique (HVT) allows the recording of the oscillating vocal folds in real time. A new method is presented, describing different types of oscillation and their onset mathematically. A precise classification of the kind of functional voice disorders is possible. For 71 young women, from the HVT recordings two parameters were mathematically estimated, reflecting the phonation onset dynamics: The threshold value A(th), corresponding with the myoelastic tone of the vocal folds and a maximum sound pressure level of speech L(max), characterising the efficiency of voice. Both values are sensitive for functional voice disorders and enable a differentiation between these pathologies. Significant differences of the computed parameters between the pathological groups and normal voices substantiate the presented method as a stable and objective tool for medical diagnosis.

A Method for Assessing the Importance of Body Force on Flow Boiling CHF

Experiments were performed to examine the effects of body force on flow boiling CHF. FC-72 was boiled along one wall of a transparent rectangular flow channel that permitted photographic study of the vapor-liquid interface just prior to CHF. High-speed video imaging techniques were used to identify dominant CHF mechanisms corresponding to different flow orientations and liquid velocities. Six different CHF regimes were identified: Wavy Vapor Layer, Pool Boiling, Stratification, Vapor Counterflow, Vapor Stagnation, and Separated Concurrent Vapor Flow. CHF showed significant sensitivity to orientation for flow velocities below 0.2 m/s, where extremely low CHF values where measured, especially with downward-facing heated wall and downflow orientations. High flow velocities dampened the effects of orientation considerably. The CHF data were used to assess the suitability of previous CHF models and correlations. It is shown the Interfacial Lift-off Model is very effective at predicting CHF for high velocities at all orientations. The flooding limit, on the other hand, is useful at estimating CHF at low velocities and for downflow orientations. A new method consisting of three dimensionless criteria is developed to determine the minimum flow velocity required to overcome body force effects on near-saturated flow boiling CHF.

Droplet Splashing during Arc Spraying of Steel and the Effect on Deposit Microstructure

The mechanism by which droplet deposition occurs is important when filling substrate features for the electric arc spray forming of steel tooling. Particle image velocimetry and high-speed video imaging techniques have been used to observe droplet deposition, particularly with regard to the behavior of droplets originating from splashing. Droplet splashing on deposition has been seen to be significant, and splash droplets form a large proportion of the overspray. The splash droplets are smaller and, when first created, move slower than the parent droplet. When spraying into deep features, the lateral and upward movement of splash droplets acts as a mechanism for deposit formation onto surfaces in shadow from the main spray. Microstructural study has shown that oxidation of the splash droplets before redeposition leads to a deposit with a high fraction of oxide. Simultaneous growth of deposit formed directly from the spray, and from splash droplets, results in a banded microstructure containing elongated macropores. A mechanism for such microstructural evolution is proposed.

Growth Dynamics of Single-Wall Carbon Nanotubes Synthesized by CO2 Laser Vaporization

Single-wall carbon nanotubes (SWNTs) were synthesized by the irradiation of a 20-ms CO2 laser pulse (1-kW peak power) onto a graphite−Co/Ni composite target at 25−1200 °C. Characterization of carbonaceous deposits using Raman scattering, scanning electron microscopy, and transmission electron microscopy showed that SWNTs were formed by laser irradiation even at room temperature. At 1100−1200 °C, the SWNT yield significantly increased (> 60%). A high-speed video imaging technique was used to observe the expanding vaporization plume and the emerging carbonaceous materials in an Ar atmosphere. Carbonaceous materials containing SWNTs became visible after ∼3 ms from the initiation of laser irradiation of the target. At 1000−1200 °C, blackbody emission from large carbon clusters and/or particles was observed for more than 1 s after the end of the laser pulse. We suggest that the growth of the SWNTs occurs from a liquidlike carbon−metal particle via supersaturation and segregation. A continuous supply of hot car...