Woo-Seung Kim

A one-dimensional model for frost formation on a cold flat surface

An analytical model is developed for the formulation of a frost layer on a cold flat surface by considering the molecular diffusion of water, and heat generation due to the sublimation of water-vapor in the frost layer. Heat generation in the frost layer is expressed in terms of water-vapor density and the absorption coefficient. To examine the validity of the present model the results from the present model are compared with experimental data. The predictions agree with the existing experimental data.

Effect of surface treatments on the frosting/defrosting behavior of a fin-tube heat exchanger

The effects of the heat exchanger surface treatment on the frosting/defrosting behavior in a fin-tube heat exchanger are investigated experimentally. It is found that the hydrophilic surface mainly influences the frosting behavior, while the hydrophobic surface has some influence on the defrosting behavior. In view of the frosting, a surface-treated heat exchanger with either hydrophilic or hydrophobic characteristic shows little improvement in the thermal performance rather than the bare aluminum heat exchanger. The results reveal that the heat exchanger with a hydrophobic surface treatment is more effective in view of the defrosting efficiency and time. The amount of residual water on the surface-treated heat exchangers is shown to be smaller than that of the bare heat exchanger. Therefore further improvements on the performance of re-operations are expected.

Hyperbolic heat conduction due to a mode locked laser pulse train

Abstract In situations which involve repetitive pulsing of a material with a mode locked Nd:YAG laser, the pulse duration can be sufficiently small (i.e. in the picosecond range) that the classical parabolic heat conduction equation fails to adequately predict the resulting temperature distribution in the material. In such cases, the hyperbolic heat conduction equation, which accounts for the finite time to the commencement of heat flow, is appropriate. In the present work, the hyperbolic heat conduction equation is used to predict the temperature distributions in both semi-infinite and finite isotropic media due to a train of temporally rectangular pulses which approximate the Gaussian temporal profile of mode locked laser pulses. The energy carried in the pulses is assumed to be absorbed in the surface plane of the material. The spatial profile of the pulses can be either Gaussian, doughnut or a combination of the two. The parabolic and hyperbolic models are examined for selected pulse frequencies.

Optimal shape and arrangement of staggered pins in the channel of a plate heat exchanger

Abstract The optimum shape and arrangement of staggered pins in the channel of a plate heat exchanger are studied in this paper. The following four dimensionless geometric parameters of the pins are selected as the important design variables: the distance (L), the volume (V), the angle ( β ) and the pitch (G). The pressure drop and heat transfer characteristics are examined, and an optimization is carried out to minimize a global objective function consisting of the correlation between the Nusselt number and the friction factor. The optimal geometric parameters are as follows: L =0.272 , V =0.106 , β =0.44 and G =0.195 . The optimal geometric parameters obtained from this study can be applied for a Reynolds number ranging from 500 to 1500.

A unified analysis of filling and solidification in casting with natural convection

Abstract In this paper, a unified model for simultaneous filling and solidification is applied to the two-dimensional filling and solidification of a square cavity. The effects of wall temperature and gate position on solidification are examined. The mixed natural-convection flow and residual flow resulting from the completion of filling are included in this study to investigate the coupled effects of filling and natural convection on solidification. Two different filling configurations (assisting flow and opposite flow due to the gate position) are analyzed to study the effects of residual flow on solidification. The results clearly show the necessity to carry out a coupled filling and solidification analysis, including the effect of natural convection. A numerical algorithm that uses the implicit volume of fluid (VOF) method for filling and a general, implicit, source-based method for solidification are presented.

STUDY OF THERMAL BEHAVIOR AND FLUID FLOW DURING LASER SURFACE HEATING OF ALLOYS

Abstract Transient and steady state laser melting problems are numerically simulated for steel and Al-4.5% Cu. Enthalpy and apparent capacity methods are used to solve the energy equation, and the momentum equations are solved in the liquid domain and mushy zone with the SOLA-VOF algorithm. Using a laser with a top-hat profile, a wide range of studies are performed by varying the beam power density and the beam radius. The streamline plots show that the flow pattern is dependent on the strength of the rotating cell along with heat flux. It is found that the shape of the pool, the surface velocity, and the surface temperature are quite different from those without convection in the mushy zone. Convection in the mushy zone plays an important role in heat transfer and fluid flow during laser melting.

Simulation of coupled turbulent flow and heat transfer in the wedge-shaped pool of a twin-roll strip casting process

Abstract The proper choice of nozzle in a twin-roll strip casting process is important to obtain the stabilization of the molten steel and free surface and a stable temperature distribution in a wedge-shaped pool. In this study, a numerical investigation of the coupled turbulent flow and heat transfer in a twin-roll strip casting process was performed for two patterns of melt-feed through a nozzle. In addition, the patterns for the removal of superheat for different gap thicknesses were analyzed using a local Nusselt number along the roll surface. The flow turbulence was examined using the low-Reynolds-number k – e turbulence model of Launder and Sharma. The results show that the use of a submerged nozzle may have a beneficial impact on the stabilization of the free-surface zone. The increased gap thickness yields an increased local Nusselt number in the downstream section of the wedge-shaped pool where the cross-sectional flow area is reduced.

Control mechanism of an organic self-regulating microfluidic system

The control mechanism and fluid dynamic properties of a previously developed organic pH regulation system are analyzed. The system regulates an output fluid stream to a pH of 6.7 with varying input flow rates. A pH sensitive hydrogel post acts as the feedback pH sensor and flow regulator. The control mechanism of the system is studied through numerical modeling of the regulator and the model is validated through experimentation. Analysis of the fluid dynamics at a T-channel junction, in which two buffer streams merge into one, is performed by solving the Navier-Stokes equation with commercial software. Various areas of a star-shaped orifice are occluded by a flexible membrane to throttle the rate that compensating buffer is fed back into the system. The relationship between orifice open area and volume of compensating buffer through the orifice was analyzed numerically. The axial and lateral visualization of the hydrogel post was obtained via optical microscopy. The model of the regulation system successfully predicts experimental results.

Optimization of the design factors for thermal performance of a parallel-flow heat exchanger

The heat and flow analyses of a parallel-flow heat exchanger are performed. Two models with and without considering the effects of the geometric characteristic of flat tube are used. Comparing the two models, the modeling using the heat transfer correlations of flat tubes shows the better accuracy and stability of numerical solutions. The effect of flow distribution on the thermal performance is examined with varying the design factors (i.e., the locations of separators and inlet/outlet, and the aspect ratios of microchannels of the heat exchanger). The flow uniformities along the paths of the heat exchanger are proposed, and are observed to evaluate the thermal performance of the heat exchanger. The optimization using the ALM method has been accomplished by maximizing the flow uniformity. It is found that the heat transfer rate of the optimized model is increased by 6.0% compared to that of the base type and the pressure drop by 0.4%.

A study on the optimal monolith combination for improving flow uniformity and warm-up performance of an auto-catalyst

The optimal design of an auto-catalyst needs a good compromise between the pressure drop and flow uniformity in the monolith. One of the effective methods to achieve this goal is to use the concept of a radially variable cell density. But this method has not been examined with respect to its usefulness in terms of chemical behavior and light-off performance. In this study, a multi-dimensional performance prediction of catalyst coupled with a turbulent reacting flow simulation has been used to evaluate the benefits of this method from the simultaneous viewpoint of fluid dynamics and chemical response during warm-up. The results showed that the combined monolith with 93/73 and 93/62 was very prospective for improved light-off performance and reduced the pressure loss due to both the balanced space velocity and the efficient usage of the geometric surface area of the channels. It was also found that the air distribution between the different cell densities greatly affects not only pressure loss and flow uniformity but also the light-off pattern. In this study, thermal durability was also examined by using simple correlation governed by mechanical and thermal properties. It was found that based on the thermal shock parameter, the cell combined monolith shows weaker thermal resistance than the conventional monolith.