• Proceedings of the KSME Conference
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• 2003.04a
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• pp.2095-2100
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• 2003

This Experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ration of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin-friction coefficients have been measured for fully developed flow of bentonite-water solution(5%) when the inner cylinder rotates at the speed $0{\sim}400rpm$. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number $R_o$ With respect to the skin friction coefficients. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. In all flow regime, the skin friction coefficient is increased by the inner cylinder rotation. The critical (bulk flow) Reynolds number $Re_c$ decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.

• Journal of Ocean Engineering and Technology
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• v.27 no.3
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• pp.73-78
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• 2013

CFD (computational fluid dynamics) analyses that considered the dynamic interaction effects between the flow and a turbine were performed to evaluate the power output characteristics of two representative vertical-axis tidal-current turbines: an H-type Darrieus turbine and Gorlov helical turbine (GHT). For this purpose, a commercial CFD code, Star-CCM+, was utilized, and the power output characteristic were investigated in relation to the scale ratio using the relation between the Reynolds number and the lift-to-drag ratio. It was found that the power coefficients were significantly reduced when the scaled model turbine was used, especially when the Reynolds number was lower than $10^5$. The power output characteristics of GHT in relation to the twisting angle were also investigated using a three-dimensional CFD analysis, and it was found that the power coefficient was maximized for the case of a Darrieus turbine, i.e., a twisting angle of $0^{\circ}$, and the torque pulsation ratio was minimized when the blade covered $360^{\circ}$ for the case of a turbine with a twisting angle of $120^{\circ}$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.8
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• pp.693-702
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• 2015

This paper presents the performance characteristics of a Darrieus-type vertical-axis wind turbine (VAWT) with National Advisory Committee for Aeronautics (NACA) airfoil blades. To estimate the optimum shape of the Darrieus-type wind turbine in accordance with various design parameters, we examine the aerodynamic characteristics and separated flow occurring in the vicinity of the blade, the interaction between the flow and blade, and the torque and power characteristics that are derived from it. We consider several parameters (chord length, rotor diameter, pitch angle, and helical angle) to determine the optimum shape design and characteristics of the interaction with the ambient flow. From our results, rotors with high solidity have a high power coefficient in the low tip-speed ratio (TSR) range. On the contrary, in the low TSR range, rotors with low solidity have a high power coefficient. When the pitch angle at which the airfoil is directed inward equals $-2^{\circ}$ and the helical angle equals $0^{\circ}$, the Darrieus-type VAWT generates maximum power.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.1
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• pp.16-25
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• 2004

The flow and heat/mass transfer in the falling-film of a heat exchanger can be influenced by the motion of the surrounding refrigerant vapor. In this study, the effect of the vapor flow direction on the absorption heat transfer has been investigated for a falling-film helical coil which is frequently used as the absorber of ammonia/water absorption refrigerators. The experiments were carried out for different solution concentration. The heat and mass transfer performance was measured for both parallel and counter-current flow. The effect of vapor flow on the heat and mass transfer is found to be increased with decreasing solution concentration. In the experiments with low solution concentration, whose vapor specific volume is great, the counter-current flow of vapor resulted in uneven distribution of falling-film and reduced the heat transfer performance of the absorber. The direction of the vapor flow hardly affected the thermal performance as the solution concentration became stronger since the specific volume of the ammonia/water vapor was much smaller than that of the water vapor.

• Proceedings of the KSME Conference
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• 2003.11a
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• pp.765-768
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• 2003

The objective of the present study was to visualize the pulsatile flow field in a branching model by using the high-resolution PIV system. A bifurcated flow system was built for the experiments in the pulsatile flow. Harvard pulsatile pump was used to generate the pulsatile velocity waveforms. Conifer powder as the tracing particles was added to water to visualize the flow fields. Two consecutive particle images at several cross sections of the flow filed were captured by the CCD cameras ($1K{\ast}1K$ and $640{\ast}480$). The results after the image processing clearly showed the recirculation zones and the formation of the paired secondary flows from the distal to the apex in the bifurcated model. The results also indicated that the flow velocities in the inner wall moved faster than those in the outer wall due to the inertial force effects and the helical motions generated in the branch flows as the flow proceeded toward the outer wall. While the PIV images from the $1K{\ast}1K$ camera were closer to the simulation results thantheimagesfromthe640${\ast}$480camera,bothresultsofthePIVexperimentsusingthetwocamerasgenerallyagreed quitewellwiththeresultsfromthenumericalsimulation.

• Journal of the Korean Society of Visualization
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• v.8 no.3
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• pp.35-40
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• 2010

We have investigated different radial temperature gradient effect on the stability of Taylor-Couette flow. The radius ratio and aspect ratio of the model was 0.825 and 48, respectively. Two heating exchangers were used for generating different temperature gradient along the radial direction. The change of flow regime in the Taylor-Couette flow was studied by increasing the Reynolds number. The results showed that: as Gr is increased in helical vortex flow regime, the vortices with the same direction of convection flow increased in size, and the vortex moving velocity also increased. It is also shown that the presence of temperature gradient obviously increased the flow instability when the Richardson number is larger than 0.0045.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.6
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• pp.763-772
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• 1998

The effects of an inserted coil on flow patterns and heat transport performance for a horizontal rotating heat pipe have been studied experimentally. Especially, the present study is to see an internally inserted helical coil inside a RHP would lead to the same kind of results as internal fins. Visualization test conducted for an acryl tube, charged water with at a volumetric rate of 20%. When the flow kept pool regime at a low RPM(less than 1,000 RPM), the movement of coil forced the water to flow in axial direction. But this pumping effect of coil disappeared, when the pool regime changed to annular one which could be created by increasing RPM. The pumping effects for RHP with an inserted coil resulted enhancement both in condensation heat transfer coefficient and heat transport limitation, as obtained in case of using internal fins. But all these effects became negligible in the range of higher RPM(above 1,000∼1,200) with the transition of flow regime to annular flow.

• International Journal of Air-Conditioning and Refrigeration
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• v.8 no.1
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• pp.50-61
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• 2000

The effects of an inserted coil on flow . patterns and heat transfer performance for a horizontal rotating heat pipe have been studied experimentally. Especially, the present study is to see an internally inserted helical coil inside a RHP would lead to the same kind of results as internal fins. Visualization test conducted for an acryl tube, charged water with at a volumetric rate of 20%. When the flow kept pool regime at a low rpm(less than 1,000rpm), the movement of coil forced the water to flow in axial direction. But this pumping effect of coil disappeared, when the pool regime changed to annular one which could be created by increasing rpm. The pumping effects for RHP with an inserted coil resulted in the enhancement in both condensation heat transfer coefficient and heat transport limitation, as obtained in case of using internal fins. But all these effects became negligible in the range of higher rpm(above 1,000-1,200) with the transition of flow regime to annular flow.

• Journal of Ocean Engineering and Technology
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• v.33 no.4
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• pp.350-357
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• 2019

The development of marine technology is expected to increase the demand for marine plants because of increasing oil prices. Therefore, there is also expected to be an increase in the demand for cylindrical structures such as URF (umbilical, riser, flowline) structures and spars, which are used operating in various seas. However, a cylindrical structure experiences vortex induced motion (VIM) in a current. In particular, for risers and umbilicals, it is important to identify the characteristics of the VIM because interference between structures can occur. In addition, various studies have been conducted to reduce VIM because it is the cause of fatigue damage to structures. The helical strake, which was developed for VIM reduction, has an excellent VIM reduction performance, but is difficult to install on structures and has a negative effect on heave motion. Therefore, the purpose of this study was to supplement the shortcomings of the helical strake and develop a high-performance reduction device. In the reduction device developed in this study, a string is placed around the structure inside the flow, causing vibration. The vibration of this string causes a small turbulence in the flow field, reducing the VIM effect on the structure. Finally, in this study, the 2-DOF motion characteristics of models without a suppression device, models with a helical strake, and models with a string were investigated, and their reduction performances were compared through model tests.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.61 no.3
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• pp.413-421
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• 2012

This paper proposes the design and implementation of fluid flow generation system by using polypropylene(PP) water capture, which harvests electric energy from the kinetic energy of tidal current or water flow and drives the desired load, and applies it to the discharge drain of Hadong thermal power plant. This experimental system is composed of water captures, driving wheel, gear trains, 10[kW] synchronous generator, and three phase rectifying circuit which drives lamp load for test. The proposed water capturing system which is composed of water captures, rope and driving wheel, rotates as caterpillar according to water flow. This system is very easy to manufacture and more economical than another type of tidal current turbines such as conventional propeller and helical type. Also, we estimated the available fluid flow energy that can be extracted from the cooling water in discharge drain based on drain's cross-sectional area. Therefore, this paper confirms the validity of proposed fluid flow generation system with water captures and the possibility of its application for renewable energy generation in discharge drain of thermal power plant, from the obtained performance characteristic of this energy conversion system.