• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.1
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• pp.116-122
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• 1988

Interfacial shear stresses have been determined for countercurrent stratified flow of air and water in a nearly-horizontal rectangular channel, based upon measurements of pressure drop, gas velocity profiles and mean film thickness. A dimensionless correlation for the interfacial friction factor has been developed as a function of the gas and liquid Reynolds numbers. Equivalent surface roughnesses for the interfacial friction factor have been calculated using the Nikuradse correlation and have been compared with the intensity of the wave height fluctuation on the interface. The results show that the interfacial shear stress is mainly affected by turbulent mixing near the interface due to the wave motion rather than by the roughened surface.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.5
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• pp.1300-1310
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• 1990

Naphthalene sublimation technique is employed to investigate the mass transfer processes from a square cylinder at various Reynolds numbers and various angles of attack. Distribution of the local mass transfer coefficients on each face of the cylinder changes dramatically with the angle of attack. Such variation of local mass transfer rates closely connected with the complex flow phenomena such as stagnation, acceleration, separation, reattachment and vortex shedding. The average Sherwood number has a minimum value at 12.deg.-13.deg., and a maximum value at a=20.deg.-25.deg. A comparison of present mass transfer measurement with other heat transfer measurements, using the heat/mass transfer analogy, shows good agreement in average transfer rates, same trend but notable differences in local values. Therefore, naphthalene sublimation technique can be adopted to explore heat transfer processes in the complex flow situations, which is considered to be hardly possible with the conventional heat transfer measurements.

• Journal of the Society of Naval Architects of Korea
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• v.42 no.6 s.144
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• pp.608-618
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• 2005

A new technique giving significant drag reduction in turbulent shear flows has been proposed by using the buoyancy effect to generate periodic spanwise motion. Such spanwise motion can be obtained by arranging heating and cooling strips periodically aligned in the spanwise direction of a vertical channel, where the streamwise mean flow is perpendicular to the gravity vector The strip size has been changed in order to obtain the optimum size corresponding to the maximum drag reduction. The bulk Reynolds number, $ Re_{m} = U_{m} \delta / \nu \$ is fixed at 2270 while Grashof numbers is changed between $10^{6}$ to $10^{7}$. As Grashof number increases, considerable drag reduction can be obtained, At the highest Grashof number, an optimum strip size of about 250 wail units gives drag reduction of about 35$\%$. The greater the Grashof number, the smaller the strip size attains the maximum drag reduction.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.2
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• pp.141-146
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• 2011

This study numerically carried out the propeller open water test(POW) by solving Navier-Stokes equations governing the three-dimensional unsteady incompressible viscous flow with the turbulence closure model of the ${\kappa}-{\omega}$ SST model. Numerical simulations are performed at various range of advance ratios. Corresponding to Reynolds numbers of $5.89{\times}105{\sim}6.47{\times}105$ based on free stream velocity and the chord length at 0.7 propeller radius. The present results give a good agreement with those of the experiment. The propeller induced vortical structures have been analyzed by visualizing the resultant vorticity. As the advance ratio increases, the magnitude and length of the resultant vorticity decrease significantly. As the main focus of present study, the numerical model to present the ($r-{\theta}$) plane-averaged resultant vorticity along the streamwise direction for various advance ratios has been suggested.

• International Journal of Air-Conditioning and Refrigeration
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• v.14 no.2
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• pp.49-56
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• 2006

Heat transfer performances of individual row of two-row fin and tube heat exchangers are experimentally investigated. Tested are four heat exchangers which are geometrically identical with the exception of fin shape, slit or louver, and that the fins between the first row and the second row are connected or separated. The tube diameter and fin spacing of the heat exchangers examined are 7mm and 1.4mm, respectively. All thermal fluid measurements are made using a psychrometric calorimeter. In order to evaluate air-side heat transfer coefficients of individual rows, tube-side water flow rates of individual rows are independently controlled such that the water-side temperature drops in each row remain at $5^{\circ}C$. Frontal air velocity varies in the range from 0.7m/s to 2.5m/s. Heat transfer coefficients are presented in terms of Colburn ${\jmath}-factor$. The results show that the heat transfer coefficient of the upstream row is larger than that for the downstream row at low Reynolds numbers.

• International Journal of Air-Conditioning and Refrigeration
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• v.14 no.2
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• pp.41-48
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• 2006

Absorption performance at the vertical interface between refrigerant vapor and liquid solution of $LiBr-H_{2}O$ solution was enhanced by the waves formed due to the interfacial shear stress. The present study investigated experimentally and analytically the improvements of absorption performance in a falling film by wavy film flow. The dynamic parameter was the film Reynolds numbers ranged from 50 to 150. The energy and diffusion equations were solved simultaneously to find the temperature and concentration profiles at the interface of liquid solution and refrigerant vapor. Absorption characteristics due to heat and mass transfer were analyzed for the falling film of the LiBr aqueous solution contacted by refrigerant vapor in the absorber. Absorption performance showed a peak value at the solution flow rate of $Re_{f}>100$. Absorption performance for the wavy film flow was found to be greater by approximately 10% than that for uniform film flow. Based on numerical and experimental results, the maximum absorption rate was obtained for the wavy flow caused by spring insert. The difference between the measured and the predicted results were ranged from 5.8 to 12%.

• Nuclear Engineering and Technology
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• v.24 no.3
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• pp.311-318
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• 1992

During mid-loop operation of Nuclear Power Plant, to prevent the Decay Heat Removal System (DHRS) from failure due to air entrainment of free surface vortex in the piping system, a set of simulating experiments was performed. Through these experiments, a relation between the non-dimensionalized numbers, such as H/d, Froude number, Reynolds number, was found. It was also found that the perturbation of the system by the disturbance such as pump start, valve operation, etc., has a strong effect on the free surface vortex. Furthermore, from viewpoint of reactor safety, a modified inlet device which is reducer type is strongly recommended for the prevention of air entrainment into DHRS.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.28 no.11
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• pp.1289-1301
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• 2004

Direct numerical simulation (DNS) of strongly-heated air flows moving upward in a vertical tube has been conducted to investigate the effect of gas property variations on turbulence modification. Three heating conditions(q$_1$$^{+}$=0.0045, 0.0035 and 0.0018) are selected to reflect the experiment of Shehata and McEligot (1998) at the inlet bulk Reynolds numbers of 4300 and 6000. At these conditions, the flow inside the heated tube remains turbululent or undergoes a transition to subturbulent or laminarizing flow. Consequently, a significant impairment of heat transfer occurs due to the reduction of flow turbulence. The predictions of integral parameters and mean profiles such as velocity and temperature distributions are in excellent agreement with the experiment. The computed turbulence data indicate that a reduction of flow turbulence occurs mainly due to strong flow acceleration effects for strongly-heated internal gas flows. Thus, buoyancy influences are secondary but not negligible especially for turbulent flow at low heating condition. Digital flow visualization also shows that vortical structures rapidly decay as the heating increases.s.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1607-1612
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• 2004

Numerical simulations are performed to investigate the characteristics of flow past a sphere in uniform shear. The Reynolds numbers considered are Re=300, 425 and 480 based on the inlet center velocity and sphere diameter. The non-dimensional shear rate K of the inlet uniform shear is varied from 0 to 0.15. At Re=300, the head of the hairpin vortex loop always locates on the high-velocity side in uniform shear, and the flow maintains the planar symmetry. At Re=425 and 480, the irregularity in the location and strength of the hairpin vortex appearing in uniform inlet flow is much reduced in uniform shear, but the flows still keep the asymmetry for most inlet shear rates. However, in the cases of K=0.075 and 0.1 at Re=425, the flows become planar symmetric and their characteristics of the evolution of the hairpin vortex loops are different from those of asymmetric flows. A hysteresis phenomenon switching from the planar symmetry to the asymmetry (or vice versa) depending on the initial condition is also observed at Re=425.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1325-1334
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• 1998

The objectives of the present study are to develop a systematic method rather than a conventional trial-and-error method for an optimal shape design using a mathematical theory, and to apply it to engineering problems. In the present study, an optimal condition for a minimum pressure loss in a two-dimensional curved duct flow is derived and then an optimal shape of the curved duct is designed from the optimal condition. In the design procedure, one needs to solve the adjoint Navier-Stokes equations which are derived from the Navier-Stokes equations and the cost function. Therefore, a computer code of solving both the Navier-Stokes and adjoint Navier-Stokes equations together with an automatic grid generation is developed. In a curved duct flow, flow separation occurs due to an adverse pressure gradient, resulting in an additional pressure loss. Optimal shapes of a curved duct are obtained at three different Reynolds numbers of 100, 300 and 800, respectively. In the optimally shaped curved ducts, the separation region does not exist or is significantly reduced, and thus the pressure loss along the curved duct is significantly reduced.