Youngbae Han

Suturing property of tough double network hydrogels for bio-repair materials

Cartilage and meniscal lesions have limited potential for spontaneous repair. Consequently, much effort has been made to develop methods for repairing such lesions. Double-network (DN) gels are new candidate-materials for repairing such lesions. They exhibit exceptional mechanical strength and toughness in spite of their high water content. In this study, we prepared highly tough DN hydrogels and investigated the mechanical properties related to clinical implant use. The mechanical properties such as Young’s modulus and suture tear-out strength were measured for the artificial replacement. The results suggest that the suture property of DN hydrogels can be adjusted by controlling the crosslinking density and monomer concentration. Finite element method was also applied to these DN hydrogels in order to check whether the fracture strength of the material is enough to meet a medical purpose.

SH Wave Scattering Problems for Multiple Orthotropic Elliptical Inclusions

A volume integral equation method (VIEM) is applied for the effective analysis of elastic wave scattering problems in unbounded solids containing general anisotropic inclusions. It should be noted that this numerical method does not require use of Greens function for anisotropic inclusions to solve this class of problems since only Greens function for the unbounded isotropic matrix is necessary for the analysis. This new method can also be applied to general two-dimensional elastodynamic problems involving arbitrary shapes and numbers of anisotropic inclusions. A detailed analysis of SH wave scattering problems is developed for an unbounded isotropic matrix containing multiple orthotropic elliptical inclusions. Numerical results are presented for the displacement fields at the interfaces of the inclusions in a broad frequency range of practical interest. Through the analysis of plane elastodynamic problems in an unbounded isotropic matrix with multiple orthotropic elliptical inclusions, it is established that this new method is very accurate and effective for solving plane elastic problems in unbounded solids containing general anisotropic inclusions of arbitrary shapes.

Liquid Film Thickness in Micro Tube Under Flow Boiling Condition

Slug flow is one of the representative flow regimes of flow boiling in micro tubes. It is well known that the thin liquid film formed between the tube wall and the vapor bubble plays an important role in micro scale heat transfer. In the previous study [1], liquid film thickness under adiabatic condition was investigated and an empirical correlation for the initial liquid film thickness based on capillary number, Reynolds number and Weber number was proposed. In the present study, the effects of wall superheat and bubble acceleration on the liquid film thickness are investigated. Under flow boiling condition, bubble velocity is not constant but accelerated, and it is necessary to consider this acceleration effect on the liquid film thickness, since it may affect the viscous, surface tension and inertia forces in the momentum equation. In addition, viscous boundary layer develops, and it may also affect the liquid film thickness. Besides, viscosity and surface tension coefficient are sensitive to temperature change. If wall superheat is high, it is crucial to consider the property change according to the temperature variation. In order to investigate these effects, laser focus displacement meter is used to measure the liquid film thickness. Ethanol, water and FC-40 are used as working fluids. Circular tubes with three different diameters, D = 0.5, 0.7 and 1.0 mm, are used. It is observed that when the wall superheat is larger than 5°C, liquid film thickness becomes thinner than the adiabatic case due to the decrease of viscosity near the wall. The increase of liquid film thickness with capillary number is restricted by bubble acceleration. Finally, an empirical correlation is proposed for accelerated flows in terms of capillary number and Bond number based on bubble acceleration.Copyright

THE EFFECT OF LIQUID FILM EVAPORATION ON FLOW BOILING HEAT TRANSFER IN A MICRO TUBE

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter to determine heat transfer coefficient. In the present study, liquid film thickness is measured under flow boiling condition and compared with the correlation proposed under adiabatic condition. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Pyrex glass tube with inner diameter of D = 0.5 mm is used as a test tube. Working fluids are water and ethanol. Laser focus displacement meter is used to measure the liquid film thickness. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. However, measured liquid film thickness becomes thinner than the predicted values in the cases of back flow and short slugs. These are considered to be due to the change of velocity profile in the liquid slug. Under flow boiling condition, liquid film profile fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.Copyright

Mixed Volume and Boundary Integral Equation Method with Elliptical Inclusions and a Circular/Elliptical Void

A mixed volume and boundary integral equation method (mixed VIEM-BIEM) is used to calculate the plane elastostatic field in an unbounded isotropic elastic medium containing multiple isotropic/orthotropic elliptical inclusions of arbitrary orientation and a circular/elliptical void subjected to remote loading. In order to investigate the influence of a circular/elliptical void on the interfacial stress field, a detailed analysis of the stress field at the interface between the matrix and the central isotropic/orthotropic inclusion is carried out for the square packing of eight inclusions and one void, taking into account different values for the orientation angles and concentration of the inclusions. The mixed method is shown to be very accurate and effective for investigating the local stresses in composites containing isotropic/anisotropic fibers and a circular/elliptical void.

LIQUID FILM THICKNESS OF OSCILLATING FLOW IN A MICRO TUBE

Oscillating flow is encountered frequently in many two-phase flow systems such as pulsating heat pipe, refrigerator with reciprocating compressor, etc. Thickness of liquid film formed between the tube wall and the vapor bubble is one of the crucial parameters to develop two-phase flow systems using micro tubes. However, liquid film formation and variation of oscillating flows are very complicated phenomena coupled with acceleration, deceleration, evaporation, condensation, etc. In the previous research, liquid film thickness in accelerating flow under adiabatic conditions was measured and compared with the correlation developed under steady condition [5]. In the present study, liquid film thickness in decelerating flow in a micro tube is investigated under adiabatic condition. Circular tubes with diameter, D = 1.0 mm, is used. Laser focus displacement meter is used to measure the liquid film thickness. Two-phase flow is obtained by introducing air from the open end of the test tube. Ethanol is used as a working fluid. At small capillary numbers, the effect of deceleration is negligible similar to the trend under accelerated condition. As capillary number increases, liquid film thicknesses in decelerated conditions become larger than the predictions of adiabatic steady correlation. However, liquid film thickness does not exceed the critical thickness at Re > Recrit . It is considered that liquid film thickness is affected by the altered velocity profiles in the liquid slug ahead of air-liquid interface according to accelerated or decelerated condition.Copyright

LIQUID FILM THICKNESS IN MICRO CHANNEL SLUG FLOW

Slug flow is the representative flow regime of two-phase flow in micro channels. It is well known that the thin liquid film formed around the confined vapor bubble plays an important role in micro channel heat transfer. In the present study, experiments are carried out to clarify the effects of parameters that affect the formation of the thin liquid film in micro channel slug flow. Laser focus displacement meter is used to measure the thickness of the thin liquid film. Air, ethanol, water and FC-40 are used as working fluids. Circular tubes with five different diameters, D = 0.3, 0.5, 0.7, 1.0, 1.3 mm, and square channels with two different sizes, 0.3 × 0.3 and 0.5 × 0.5 mm, are used. It is confirmed that the liquid film thickness is determined only by capillary number at small capillary numbers. However, the effect of inertial force and flow acceleration cannot be neglected as capillary number increases. The effect of cross sectional shape is also investigated. Experimental correlation for the adiabatic liquid film thickness in circular tubes based on capillary number, Reynolds number and Weber number is proposed. When viscous boundary layer is thin, liquid film thickness is limited by the viscous boundary layer thickness. Thus, in order to develop precise flow boiling models in micro tubes, it is important to consider the effects of inertial force and boundary layer thickness on the liquid film thickness.Copyright