• Korean Journal of Air-Conditioning and Refrigeration Engineering
• /
• v.25 no.5
• /
• pp.246-254
• /
• 2013

Some characteristics of nitrogen-water slug flow were optically measured, in vertical acrylic tubes of 2, 5 and 8 mm diameter. Bubble velocity, bubble and unit cell lengths were measured, by analyzing the light intensity signals from two sets of dot laser-infrared sensor modules mounted along the transparent tubes. Optical images of the bubbles were also taken and analyzed, to measure bubble shapes and liquid film thickness. It was found that the measured bubble velocities were in good agreement with the empirical models in the literature, except for those measured under high superficial velocity condition in the 2 mm tube. Bubble length was found to be the longest in the 2 mm tube, being 4 to 5 times those of the other tubes. Liquid film was found to have developed early in the 2 mm tube, which made the blunt shape of the bubble head. Liquid film thickness in the 8 mm tube was measured at almost twice those of the other tubes.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.4 no.4
• /
• pp.18-25
• /
• 2000

A two-color particle image velocimetry (PIV) has been developed for measuring two dimensional velocity flowfields and applied to a Mach 2.0 supersonic nozzle. This technique is similar to a single-color PIV technique except that two different color laser beams are used to solve the directional ambiguity problem. A green-color laser sheet (532 nm: 2nd harmonic beam of YAG laser) and a red-color laser sheet (619 nm: output beam from YAG pumped Dye laser using Rhodamine 640) are employed to illuminate the seeded particles. A high resolution (3060${\times}$2036) digital color CCD camera is used to record the particle positions. This system eliminates the photographic-film processing time and subsequent digitization time as well as the complexities associated with conventional image shifting techniques for solving directional ambiguity problem. The two-color PIV also has the advantage that velocity distributions in high speed flowfields can be measured simply and accurately by varying the time interval between two different laser beams due to its high signal-to-noise ratio and thereby less requirement of panicle pair numbers for a velocity vector in one interrogation spot. The velocity distribution in the Mach 2.0 supersonic nozzle has been measured and the over-expanded shock cell structure can be predicted by the strain rate field. These results are compared and analyzed with the schlieren photograph for the velocity distributions and shock location.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
• /
• 2008.11a
• /
• pp.241-242
• /
• 2008

The Velocity Autocorrelation Function (VAF) of the sodium ions is calculated for a range of temperature from 250K to 1000K and converted into the linear ac-conductivity and ac-susceptibility response via Fourier transformation. A peak is found in the conductivity around $6\times10^{12}$ Hz that has some of the character of a Poley absorption. Here it is shown to be due to an harmonically coupled site vibrations of the sodium atoms, which extend only over a limited range. At frequencies below the peak the conductivity tends towards a constant i.e. dc value corresponding to a constant flow of ions through the simulation cell. At high temperatures the conductivity due to this ion transport process behaves like a metal with an insulator to metal transition occurring around a specific temperature.

• Proceedings of the KSME Conference
• /
• 2000.04a
• /
• pp.900-907
• /
• 2000

The three-dimensional turbulent flow in curved pipes susceptible to flow-accelerated corrosion has been analyzed numerically to predict the pressure and shear stress distributions on the inner surface of the pipes. The analysis employs the body-fitted non-orthogonal curvilinear coordinate system and a standard $ {\kappa}-{\varepsilon}$ turbulence model with wall function method. The finite volume method is used to discretize the governing equations. The convection term is approximated by a high-resolution and bounded discretization scheme. The cell-centered, non-staggered grid arrangement is adopted and the resulting checkerboard pressure oscillation is prevented by the application of a modified version of momentum interpolation scheme. The SIMPLE algorithm is employed for the pressure and velocity coupling. The numerical calculations have been performed for two curved pipes with different bend angles and curvature radii, and discussions have been made on the distributions of the primary and secondary flow velocities, pressure and shear stress on the inner surface of the pipe to examine applicability of the present analysis method. As the result it is seen that the method is effective to predict the susceptible systems or their local areas where the fluid velocity or local turbulence is so high that the structural integrity can be threatened by wall thinning degradation due to flow-accelerated corrosion.

• Transactions of the Korean hydrogen and new energy society
• /
• v.30 no.6
• /
• pp.601-607
• /
• 2019

Ammonia would be formed in natural gas containing small amount of nitrogen reforming process in the process natural gas, which might damage the Pt catalyst and Prox catalyst. In the article, the effect of nitrogen contents on the formation of ammonia in the reforming process has been studied. In the experiments, Ru based and Ni based catalysts were used and the concentration of ammonia in the reformate gas at various gas hourly space velocity was measured. Experimental result shows that relatively higher ammonia concentration was measured with Ru based catalyst than with Ni based catalyst. It also shows that the concentration of ammonia increased rapidly after most of the methane converted into hydrogen. Based on the experimental results to reduce ammonia concentration it might be better to finish methane conversion at the exit position of the reforming reactor to minimize the contact time of catalyst and nitrogen with high concentration of hydrogen.

• International Journal of Fluid Machinery and Systems
• /
• v.3 no.3
• /
• pp.211-220
• /
• 2010

The objective of this study is to clarify the occurrence of the high-speed mode of unsteady swirling flows in straight tubes. The unsteady flows generated in the tube were measured by means of a semiconductor-type pressure transducer and an FFT analyzer. The high-speed mode measured has rotational speed which is approximately equal to or higher than the peripheral velocity of the swirling flow. The unsteady flow is due to cell rotation in the circumferential direction of the tube. The occurrence of the high-speed mode was confirmed, and the characteristics (rotational speed, pressure amplitude, and phase) of this mode were clarified. In order to understand the measured unsteady flows, the three dimensional vortex core profiles were discussed based on the distributions of the pressure amplitude and phase.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.34 no.4
• /
• pp.349-354
• /
• 2010

Fuel cells have attracted enormous interest as new power sources because the cells can be used to solve the problem of environmental pollution as well as the natural-resource exhaustion problem. In this study, hydrogen-gas flow in microchannels of different shapes was numerically analyzed to improve the efficiency of a microfuel cell. Flow characteristics in six microchannels of different shapes but under identical boundary conditions were simulated. The analysis result shows that the flow characteristics such as velocity, uniformity, and flow rate, greatly depend upon the channel shape. This implies that the efficiency of microfuel cell can be expected to be increased by adopting the optimal configuration of channel shape for hydrogen-gas flow. The experimental results show that power density of a PEMFC with a microflow channel is higher than that of a PEMFC without a microflow channel; however, a durable catalyst is required in MEA.

• Geophysics and Geophysical Exploration
• /
• v.19 no.2
• /
• pp.76-83
• /
• 2016

Migration image can be distorted due to reflected waves in the source and receiver wavefields when discontinuities of input velocity model exist in seismic imaging. To remove reflected waves coming from layer interfaces, it is a common practice to smooth the velocity model for migration. If the velocity model is smoothed, however, the subsurface image can be distorted because the velocity changes around interfaces. In this paper, we attempt to minimize the distortion by reducing reflection energy in the source and receiver wavefields through acoustic impedance homogenization. To make acoustic impedance constant, we define fake density model and use it for migration. When the acoustic impedance is constant over all layers, the reflection coefficient at normal incidence becomes zero and the minimized reflection energy results in the improvement of migration result. To verify our algorithm, we implement the reverse-time migration using cell-based finite-difference method. Through numerical examples, we can note that the migration image is improved at the layer interfaces with high velocity contrast, and it shows the marked improvement particularly in the shallow part.

• Journal of Mechanical Science and Technology
• /
• v.20 no.5
• /
• pp.710-716
• /
• 2006

Micro-PIV system with a high speed CCD camera is used to measure the flow field near the advancing meniscus of a water slug in microchannels. Image shifting technique combined with meniscus detecting technique is proposed to measure the relative velocity of the liquid near the meniscus in a moving reference frame. The proposed method is applied to an advancing front of a slug in microchannels with rectangular cross section. In the case of hydrophilic channel, strong flow from the center to the side wall along the meniscus occurs, while in the case of the hydrophobic channel, the fluid flows in the opposite direction. Further, the velocity near the side wall is higher than the center region velocity, exhibiting the characteristics of a strong shear-driven flow. This phenomenon is explained to be due to the existence of small gaps between the slug and the channel wall at each capillary corner so that the gas flows through the gaps inducing high shear on the slug surface. Simulation of the shape of a static droplet inside a cubic cell obtained by using the Surface Evolver program is supportive of the existence of the gap at the rectangular capillary corners. The flow fields in the circular capillary, in which no such gap exists, are also measured. The results show that a similar flow pattern to that of the hydrophilic rectangular capillary (i.e., center-to-wall flow) is always exhibited regardless of the wettability of the channel wall, which is also indicative of the validity of the above-mentioned assertion.

• Journal of Korea Foundry Society
• /
• v.32 no.2
• /
• pp.65-74
• /
• 2012

In this paper, we present the results of our studies on optimal die design towards development of a vacuum die casting process to fabricate fuel cell bipolar plate with micro-channel array. Cavity and overflow shape is designed by computational filling analysis of MAGMA soft. Optimal die design consists of seven overflows at the end of cavity and three overflows at each side wall of cavity. The molten metal that passed the gate and reached the side wall flowed into the side overflow, no turbulent flow occurred, and the filling behavior and velocity distribution were uniform. In addition, partially solidified molten metal passing through the channel was perfectly eliminated by overflow without back-flow. When vacuum pressure, injection speed of low and high region was 300 mbar, 0.3 m/s and 2.5 m/s respectively with Silafont 36 die casting alloy, sound sample without casting defects was obtained. The experimental results are nearly consistent with simulation results.