Masahiro Kawaji

Effect of Channel Length on the Gas–Liquid Two-Phase Flow Phenomena in a Microchannel

An optical measurement system was used to investigate the effect of microchannel length and inlet geometery on adiabatic gas–liquid two-phase flow. Experiments were conducted with 146-mm- and 1571-mm-long, circular microchannels of 100 μm diameter. Void fraction and gas and liquid plug/slug lengths and their velocities were measured for two inlet configurations for gas–liquid mixing: (a) reducer and (b) T-junction. The superficial gas velocity was varied from 0.03 to 14 m/s, and superficial liquid velocity from 0.04 to 0.7 m/s. The test section length was found to have a significant effect on the two-phase flow characteristics measured at the same axial location (37 mm from the inlet) in both microchannels. The mean void fraction data for the short (146 mm) microchannel with the reducer inlet agreed well with the equation previously proposed by Kawahara et al. (2002). On the other hand, the mean void fraction data for the long (1571 mm) microchannel obeyed the homogeneous flow model and Armands equation for both the reducer and T-junction inlet configurations. Many long and rapidly moving gas plugs/slugs and long, slowly moving liquid plugs/slugs were observed in the short microchannel compared to the long microchannel, leading to the differences in the time-averaged void fraction data. The mean velocity of liquid plugs/slugs generally agreed well with Hughmarks equation and the homogeneous flow model predictions, regardless of the inlet configurations and microchannel lengths. Thus, both the microchannel length and inlet geometry were found to significantly affect the two-phase flow characteristics in a microchannel.

DEVELOPMENTS ON WETTING EFFECTS IN MICROFLUIDIC SLUG FLOW

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable, and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape, and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tension forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived as realistic and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size, and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channel walls and highlight the value of numeral simulation in microfluidic design. Supplementary materials are available for this article. Go to the publishers online edition of Journal of Chemical Engineering Communications to view the supplemental file.

Gas–Liquid Two-Phase Flow Evolution in a Long Microchannel

A pair of optical void sensors and a high-speed video camera were used to investigate the evolution of adiabatic gas–liquid two-phase flow in a long microchannel. Experiments were conducted with a 1676-mm-long, circular microchannel with an inner diameter of 100 μm. Two-phase flow patterns, void fraction, and velocities of gas plug/slug and liquid slugs were measured at different axial locations between the gas–liquid mixer and microchannel exit. The pressure decreased linearly in the first half of the microchannel, and more rapidly and nonlinearly in the second half of the test section. As a result, the flow accelerated significantly in the second half of the microchannel such that the void fraction and liquid slug velocity increased nonlinearly. The measured mean void fraction and mean velocity of liquid slugs also agreed well with the homogeneous flow model predictions when the liquid flow rate was constant and the mass velocity of the gas was low.

Macroscopic and Microscale Phenomena in Multiphase Energy Storage and Transport Systems

Complex macroscale and microscale heat and mass transfer phenomena are encountered in thermal energy storage and transport systems. Those systems involving ice slurries and nanoemulsions of phase change materials can be used for either cooling or heating applications or both, which can contribute to the reduced usage of electricity during peak hours. But heat and mass transfer and stability issues are encountered in the production, transport and storage of the heat storage media. In this paper, both the heat transfer enhancement effect and detrimental effects such as Ostwald ripening and supercooling are discussed along with the flow properties.

Enhancement of Pool Boiling and Critical Heat Flux in Self-Rewetting Fluids at Above Atmospheric Pressures

Pool boiling experiments have been conducted with a self-rewetting fluid consisting of an aqueous butanol solution to study the boiling heat transfer enhancement at pressures of 1 ∼ 4 bars. Although self-rewetting fluids have been used to enhance the performance of heat pipes, boiling heat transfer characteristics are yet to be fully understood especially at pressures above atmospheric. Pool boiling experiments with aqueous butanol solutions were performed using an electrically heated platinum wire to obtain pool boiling heat transfer data up to the Critical Heat Flux (CHF). Aqueous butanol solutions with butanol concentrations 2–7% showed enhanced heat transfer coefficients and CHF data at various pressure levels. In comparison to water, aqueous butanol solutions showed 20–270% higher values of CHF at pressures up to 4 bars. The bubble sizes were also observed to be significantly smaller in self-rewetting fluids compared to those in water at the same pressure. This observation was consistent even at higher pressures. However, for the highest butanol concentration tested (7%), the CHF enhancement was diminished at higher pressures.© 2011 ASME

Effect of Tube Diameter on Flow Phenomena of Gas-Liquid Two-Phase Flow in Microchannels

Gas-liquid two-phase flows in minichannels and microchannels display a unique flow pattern called ring film flow, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In the present work, the velocity characteristics of gas slugs, ring films, and their features such as the gas slug length, flow phenomena and frictional pressure drop for nitrogen-distilled water and nitrogen-30 wt% ethanol water solution have been investigated experimentally. Four kinds of circular microchannels with diameters of 100 μm, 150 μm, 250 μm and 518 μm were used. The effects of tube diameter and physical properties, especially the surface tension and liquid viscosity, on the flow patterns, gas slug length and the two-phase frictional pressure drop have been investigated by using a high speed camera at 6,000 frames per second. The flow characteristics of gas slugs, liquid slugs and the waves of ring film are presented in this paper.Copyright

Thermal Conductivity of G-348 Isostatic Graphite

Abstract Fundamental measurements have been obtained to deduce the temperature dependence of thermal conductivity for fine-grain G-348 isostatic graphite, which has been used in thermal experiments related to gas-cooled nuclear reactors. Measurements of thermal diffusivity, mass, volume, and thermal expansion were converted to thermal conductivity. Resulting correlations for the thermal conductivity and thermal expansion are presented as functions of temperature from laboratory temperature to 1000°C.

Investigation of Ring Waves in Gas–Liquid Two-Phase Flow in a Microchannel

Gas–liquid two-phase flow in minichannels and microchannels displays a unique flow pattern called ring film, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In this paper, the behaviors of ring waves, which correspond to ring films that appeared in ring film flow and disturbed ring film flow regions, have been investigated experimentally in gas–liquid two-phase flows of nitrogen-distilled water and nitrogen/30 wt% ethanol–water solution in a 150-μm-diameter silica tube to elucidate their generation mechanism and propagation behavior. In order to clarify the existence region and characteristics of ring waves, the flow patterns observed in a microchannel were investigated and flow pattern maps were made. Furthermore, the velocity of the ring wave was also investigated and compared with the gas slug velocity. In these velocity measurements, high-speed video images were taken at 6,000 frames per second and the formation of ring films and the relationship between the wave amplitude and velocity were determined. The results indicate an interfacial instability leading to the formation and growth of ring waves with both low and high wave amplitudes. The wave velocity is correlated to the wave amplitude, with the large amplitude waves moving much faster than the low amplitude waves. As a result, coalescence of large and low amplitude waves has been observed.

Microscale Contacting of Two Immiscible Liquid Droplets to Measure Interfacial Tension

In various microfluidic devices, surface tension and interfacial tension values are necessary to analyze the fluid behavior in microchannels, and evaluating the values of interfacial tension is especially important for gas–liquid and liquid–liquid flows. A pendant drop method is commonly used to measure the interfacial tension value; however, the pendant drop method requires strict accuracy in measuring the droplet size when the droplet has a nonspherical shape, as well as an accurate value of the density difference between the two liquids. In this work, a new measurement method called the “liquid bridge-inducing microscale contact method” has been developed in which the interfacial tension can be obtained from the bridging of two liquid droplets extruded from opposing ends of glass capillary tubes or formed on the ends of round metal rods. By measuring the radii of curvature of each liquid surface and interface, we calculate the Laplace pressure on the surface and interface, and derive the interfacial tension value using the Laplace equation. The results show that the values of interfacial tension obtained from the two methods are approximately the same and that the liquid bridge-inducing microscale contact method is capable of accurate interfacial tension measurements.

Investigation of Abnormal Heat Transfer and Flow in a VHTR Reactor Core

The main objective of this project was to identify and characterize the conditions under which abnormal heat transfer phenomena would occur in a Very High Temperature Reactor (VHTR) with a prismatic core. High pressure/high temperature experiments have been conducted to obtain data that could be used for validation of VHTR design and safety analysis codes. The focus of these experiments was on the generation of benchmark data for design and off-design heat transfer for forced, mixed and natural circulation in a VHTR core. In particular, a flow laminarization phenomenon was intensely investigated since it could give rise to hot spots in the VHTR core.