Sarng Woo Karng

Bubble Evolution and Radiation Mechanism for Laser-Induced Collapsing Bubble in Water

Bubble behavior and the radiation mechanism from a laser-induced collapsing bubble were investigated theoretically using the Keller–Miksis equation for the bubble wall motion and analytical solutions for the vapor inside the bubble. The calculated time-dependent bubble radius is in good agreement with observed ones. Also, the calculated half width and the total emission of the luminescence pulse from the vapor bubble at the collapse point agreed well with the observed value of 8 nanoseconds and 2.3 mW, respectively. The inert gas content inside the laser-induced vapor bubble was too small to produce light emission due to bremsstrahlung.

Bubble Dynamics for Single Bubble Sonoluminescence

Problems involved in the bubble dynamics models which have been suggested to explain the sonoluminesence phenomena from a single bubble under ultrasound were discussed. One of the problems is proper choice of the time scale of the driving force, which is related to numerical artifacts due to the mismatch between the natural frequency of an oscillator (bubble) and the characteristic frequency of the applied force. Such problem may occur in a nonlinear oscillator whose behavior is crucially dependent on the frequency of the applied force. The characteristics of bubble behavior for the sonoluminescing gas bubble and the artificial resonance problem encountered during the numerical evaluation of such nonlinear system were also discussed.

Shock Pulse from a Sonoluminescing Gas Bubble.

The shock pulse emanating from a sonoluminescing gas bubble was calculated by using the analytical solutions of the conservation equations for the gas inside the bubble and the Kirkwood-Bethe hypothesis for the outgoing wave. The rise time and the magnitude of the pulse signal are in good agreement with the observed values, which may provide the approximate value of the gas pressure at near the collapse of the sonoluminescing gas bubble.

Relaxation behavior of microbubbles in ultrasonic field

The nonlinear oscillation of microbubbles under the influence of ultrasound was investigated experimentally and theoretically. The radius-time curves of bubbles including afterbounces calculated using the Rayleigh–Plesset equation with polytropic relations and with the Keller–Miksis equation using analytical solutions for the Navier–Stokes equations of the gases were compared with the results observed from light scattering. This study revealed that the dynamic behavior of microbubbles in an ultrasonic field, such as the expansion ratio of the maximum to the equilibrium radius and the bouncing motion after the first collapse (minimum), depends crucially on the relaxation time of the bubble motion with respect to the characteristic time of the applied ultrasound, and that the relaxation time decreases as the equilibrium radius of the microbubbles increases. [DOI: 10.1143/JJAP.45.317]

Pressure wave propagation inside a sonoluminescing gas bubble

The propagation of a pressure wave induced by the rapid bubble wall motion near the bubble collapse was studied analytically for a sonoluminescing gas bubble. It has been found that a pressure wave rather than a shock wave is launched into the bubble center just prior to the bubble collapse and the wave is reflected near the center. It has also been found that as the gas pressure inside the bubble increases, so the reflection point moves farther from the center. No sharp focusing developed by an inwardly moving shock wave reflected at the origin has been found. The temperature rise associated with the pressure wave developed inside the bubble turns out to be insufficient for emitting light.

A Study on Performance of Thermoelectric Air-Cooling System in Parallel Flow

Experimental and theoretical studies on cooling performance of two-channel thermoelectric air-cooling system in parallel flow are conducted. The effects of operating temperature to physical properties of thermoelectric module (TEM) are experimentally examined and used in the analysis of an air-cooling system considering thermal network and energy balance. The theoretical predicted temperature variation and cooling capacity are in good agreement with measured data, thereby validating analytic model. The heat absorbed rate increases with increasing the voltage input and decreasing thermal resistance of the system. The power consumption of TEM is linearly proportional to mean temperature differences due to variations of the physical properties on operation temperature of TEM. Furthermore thermal resistance of hot side has greater effects on cooling performance than that of cold side.

Thermal management of liquid-cooled cold plates for multiple heat sources in a humanoid robot

Thermal management for two array types of a serial circulation and a two-way parallel circulation using six mini liquid-cooled cold plates were experimentally measured in this study. In order to reduce weight of the cooling devices for humanoid robot cooling, the cold plates were covered with non-metallic material (polycarbonate, PC). Six cold plates attached on 10 x 10 mm2 copper base: 0.5 x 0.5 mm2 pin-finned surfaces of 1.5 mm high with 0.5 mm array spacing, was mounted on six copper heating blocks with isothermal conditions of 50~90°C, respectively. In order to compare thermal characteristics according to two circulation types, the surface temperatures of heating blocks and the cooling water temperatures at inlets and outlets of cold plates were measured. From the results, it was found that a two-way parallel circulation was better performance than a serial circulation in terms of total thermal resistance, total heat transfer rate, and surface temperature rises from first heating block to last one for six multiple cold plates.

Thermal performance of a thermoelectric air-cooling system with heat sinks

Thermal performance of a louver-fin and a plate-fin in a thermoelectric cooling system with a duct-flow type fan arrangement is analytically evaluated. The thermoelectric cooling system consists of a thermoelectric module and two heat exchanger fins. The analytic results show that the optimized louver fin has lower thermal resistance than plate fin. The COP and heat absorbed rate of the thermoelectric cooling system with optimized louver fins are about 10% and 6% higher than optimized plate fins, respectively.

Performance Measurement of Liquid-Cooled Cold Plates for Humanoid Robot Cooling

In this study, we compare thermal performance between one metallic cold plate and three different types of non-metallic cold plates for humanoid robot cooling. The four types of cold plates have the same dimension of 20×20 mm2 base area with 7 mm high. A metallic cold plate is made of copper. Three non-metallic PC (polycarbonate) cold plates, which are designed to reduce the overall weight of robot cooling system, are composed of a polycarbonate cover with three different base shapes. All cold plates are mounted on a 20×20 mm2 copper heating block with two cartridge heaters of 30 W/cm2 . The overall heat transfer coefficients per unit mass and thermal resistances are obtained for the liquid-cooled cold plates. It is interesting to note that the PC cold plate with an aluminum base plate with 18 channels shows the best heat transfer performance per unit mass. Most polycarbonate cold plates display fairly comparable thermal performance with more reduced weight compared to a conventional copper cold plate.Copyright

Design and Operation of a Small-Scale Hydrogen Liquefier

>> In order to accelerate hydrogen society in current big renewable energy trend, it is very important that hydrogen can be transported and stored as a fuel in efficient and economical fashion. In this perspective, liquid hydrogen can be considered as one of the most prospective storage methods that can bring early arrival of the hydrogen society by its high gravimetric energy density. In this study, a small-scale hydrogen liquefier has been designed and developed to demonstrate direct hydrogen liquefaction technology. Gifford-McMahon (GM) cryocooler was employed to cool warm hydrogen gas to normal boiling point of hydrogen at 20K. Various cryogenic insulation technologies such as double walled vacuum vessels and multi-layer insulation were used to minimize heat leak from ambient. A liquid nitrogen assisted precooler, two ortho-para hydrogen catalytic converters, and highly efficient heat pipe were adapted to achieve the target liquefaction rate of 1L/hr. The liquefier has successfully demonstrated more than 1L/hr of hydrogen liquefaction. The system also has demonstrated its versatile usage as a very efficient 150L liquid hydrogen storage tank.