Byung Ha Kang’s research while affiliated with Kookmin University and other places

The lowest oscillation mode of a pendant drop has long been conceived to be the longitudinal vibration, i.e., periodic elongation and contraction along the longitudinal direction. However, here we experimentally show that the rotation of the drop about the longitudinal axis is the oscillation mode of the lowest resonance frequency. This rotational mode can be invoked by periodic acoustic forcing and is analogous to the pendulum rotation, having the frequency independent of the drop density and surface tension but inversely proportional to the square root of the drop size.

One of the fascinating prospects is the possibility of new hydrodynamics technology on micro-scale system since oscillations of micro-droplets are of practical and scientific importance. In this paper, nonlinear and forced oscillations of supported viscous droplet were focused. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at fixed amplitude. The evaporation process of oscillating droplet with thermofoil has been also observed to investigate analyzing the resonance effect on the thermal characteristics of droplet. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. During imposing the acoustic wave, the pendant droplet makes a rotating motion in its longitudinal axis which is a new shape oscillation mode. The evaporation rate of a pendant droplet at resonant frequency is significantly enhanced.

In the present study, we have performed two-dimensional CFD simulations on thermal dissipation from isothermal plate fins and offset strip fins with a specific DC fan. Computations are implemented on a single flow passage between fins due to the periodicity in the lateral direction, and analysis of thermal performance is carried out for a heat sink for CPU cooling with a width of 60 mm and a length of 65 mm. As anticipated, the offset strip fin leads to 35% higher Nusselt number at each fin surface compared to the plate fin. However, the optimum OSF heat sink has 42% wider fin pitch to compensate for increased pressure drop, which results in smaller fin surface area than the plate fin heat sink. Consequently, the overall thermal performance of the offset strip fin heat sink is about 4% lower than that of the plate fin heat sink considered in this study, irrespective of the enhanced surface geometry.

We report the relationship between the flow behavior induced by ultrasonic vibration and the consequent heat transfer enhancement in natural convection and pool boiling regimes. A thin platinum wire works as both a heat source and a temperature sensor. A high speed video imaging system is employed to observe the behavior of cavitation and thermal bubbles. Experimental results show that the effects of ultrasonic vibration on flow behavior are vastly different depending on the heat transfer regime and the amount of dissolved gas. In the natural convection and subcooled boiling regimes, behavior of cavitation bubbles strongly affects the degree of heat transfer enhancement. In saturated boiling, no cavitation occurs thus the reduced thermal bubble size at departure and acoustic streaming are major factors enhancing heat transfer rate. The highest enhancement ratio is obtained in natural convection regime where the effect of ultrasonic vibration is manifested through violent motion of cavitation bubbles.

A rivulet is a narrow stream of liquid flowing down a solid surface. When its flow rate exceeds a certain limit, it tends to meander, exhibiting instability of the interface. Here we report a perturbation analysis of this meandering rivulet. We find that the combined effects of the tangential velocity difference across the interface and the dynamic wetting are responsible for the instability. The Weber number represents the ratio of the destabilizing force, inertia, to the stabilizing force, surface tension. As the Weber number increases, both the wavenumber of maximum instability and the cutoff wavenumber increase. The effects of the capillarity are such that the wavenumber of maximum instability asymptotically increases as the sensitivity of the dynamic contact angles to the contact line speed increases. However, the cutoff wavenumber remains constant despite wetting parameter changes when the Weber number is constant.

Experiments have been carried out to investigate the heat transfer characteristics of an aluminum-foam heat sink placed on a heat source in a channel. The thermal performance of aluminum-foam heat sinks is evaluated in terms of the Nusselt number and thermal resistance of the heat sinks. The pore density of the aluminum-foam heat sinks and the Reynolds number are varied in the range of parameters: 10, 20, 40 pores per inch (PPI) and 710≤Re≤2900, respectively. It is found that thermal resistance is substantially reduced by employing an aluminum-foam heat sink with low pore density due to the relatively intense airflow through the heat sink. The aluminum-foam heat sink may provide 28% or higher thermal performance than a conventional parallel-plate heat sink of the same size. Further, the aluminum-foam heat sinks can dramatically reduce the overall mass of electronics-cooling devices owing to high porosity.

Experiments have been carried out to investigate the convective heat transfer characteristics from triangular folded fin heat sinks in a suction-type fan duct. The dimension of the triangular folded fin heat sinks is 62 mm in height with a 12 mm thick base plate, 292 mm in width, and 447 mm in length. The inlet flow velocity is varied in the range of 0.6 - 5.3 m/s. Thermal performance of triangular folded fin heat sinks is evaluated in terms of thermal resistance of heat sinks according to flow velocity and fan power. The results obtained show that the present triangular folded-fin heat sink shows a higher thermal performance compared to a conventional extruded plate-fin heat sink. Especially, a perforated slit folded-fin heat sink displays a lower thermal resistance. As the number of slit fabricated on the perforated folded fins increases, thermal performance is more pronounced.

A liquid drop that partially wets a solid surface will slide down the plane when it is tilted beyond a critical inclination. Here we report the study of the sliding velocity of such a drop. Experiments for measuring the steady sliding velocity of different liquids of drops are performed. We then construct a scaling law that predicts the sliding velocity given the physical properties, wetting characteristics, and size of the drop. When the sliding velocity is low and the drop distortion due to inclination is small, the scaling law is shown to correctly model the functional dependency of the measured sliding velocity.