• Journal of the Korean Vacuum Society
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• v.17 no.3
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• pp.189-194
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• 2008

Measurements with a Langmuir probe, which are the most often used procedures of plasma diagnostics, can disturb plasma flows and change its characteristics quite a little because the probe should be inserted into thermal flowing plasmas. In this study, we calculated the characteristics of thermal plasmas with and without the probe into an atmospheric argon free-burning arc numerically, and investigated aerodynamic and thermal disturbances with temperature and axial velocity distributions. For the modelling of thermal plasmas, we have made two governing equations, which are on the thermal-flow and electromagnetic fields, coupled together with a commercial CFD package and user-coded subroutines. It was found that thermal disturbances happened to both sides of the probe, before and behind, seriously. Due to the aerodynamic disturbance, we could find that there were the stagnation point in front of the probe and the wake behind it. Therefore, aerodynamic and thermal disturbances caused by the probe insertion should be considered to increase the reliability of the probe diagnostics.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.1
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• pp.73-80
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• 2006

Present study is concerned with an experimental study on the cooling characteristics of heat-generating components arranged in channels which are made by printed circuit boards. To assess the thermal performance of the heat-generating components arranged by $5\times11$ in flow channel, three variables are used: the velocity of the fluid at the entrance, the height of channel, and row number of the component. The cooling characteristics of the heat-generating components such as the surface temperature rise, the adiabatic temperature rise, the adiabatic heat transfer coefficient, and the effect of thermal wake are compared with the result of the experiment and the numerical analysis. Based on the experiment analysis, some conclusions can be drawn: First of all, the experiment and numerical analysis are identical comparatively; the heat transfer coefficient increases as H/B decreases. Howeve., when H/B is over 7.2, the effect of H/B is rather trivial. The effect is the biggest at the first component from the entrance, and it decreases until the fully developed flow, where it becomes very consistent. The thermal wake function calculated for each row decreases as H/B increases.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.18 no.6
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• pp.509-517
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• 2006

Present study is concerned with an experimental study on the cooling characteristics of heat-generating components arranged in channels which are made by printed circuit boards. To estimate the thermal performance of the heat-generating components arranged by $5\times11$ in channel flow, three variables are used: the inlet velocity, the height of channel, and row number of the component. The cooling characteristics of the heat-generating components such as the surface temperature rise, the adiabatic temperature rise, the adiabatic heat transfer coefficient, and the effect of thermal wake are compared with the result of the experiment and the numerical analysis. The experimental result is in a good agreement with the numerical analysis. The heat transfer coefficient increases as the Reynolds number increases, while the thermal wake function calculated for each row decreases as the Reynolds number increases. In addition, it is found that Nu-Re correlation equation is Identical to the previous studies, and the empirical correlation equation between the thermal wake function and Re is presented.

• Journal of computational fluids engineering
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• v.18 no.1
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• pp.36-42
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• 2013

The performance of the wind turbine is determined by wind speed and unsteady flow characteristics. Unsteady wake flow causes not only the decline in performance but also structural problems of the wind turbine. In this paper, conceptual designs for the wind turbine tower are conducted to minimize unsteady wake flow. Numerical simulations are performed to inspect the shape effect of the tower. Through the installation of additional structures at the rear of the tower, the creation of Karman vortex is delayed properly and vortex interactions are reduced extremely, which enhance the stability of the wind turbine. From the comparative analysis of lift and drag coefficients for each structure, it is concluded that two streamwise tips with a splitter plate have the most improved aerodynamic characteristics in stabilizing wake flow.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.4
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• pp.245-253
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• 2016

The friction and deformation of a tire causes heat generation, which causes a temperature rise of the tire. This temperature rise can be a source of tire damage. The object of this study is to investigate the cooling effect of the application of a fin to the tire side to suppress the temperature rise. Eight different fin shapes were considered, and the sidewall surface temperature reduction owing to the cooling fin shape was numerically analyzed. In addition, the flow characteristics and heat transfer characteristics of the vortex of the pin rear were compared.

• Journal of the Korean Society of Propulsion Engineers
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• v.24 no.5
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• pp.34-42
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• 2020

In modern air combat, infrared signals play an important role in the detection of opponents and must be reduced to improve survivability and stealth. In particular, IR signals generated in the wake of aircraft engines have high intensity and short wavelengths, so most heat-tracking missiles detect these signals. Accordingly, the measurement and characteristic analysis of Gas radiation signals from the engine's wake were carried out in this study. Micro turbojet engine has been configured to simulate a real aircraft turbofan engine, and the characteristics of IR signal reduction by adjusting the bypass ratio were identified. Through this, the IR signal characteristics for each wavelength are analyzed and verification of signal reduction technologies is performed.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.10
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• pp.659-665
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• 2017

When an electrically conducting fluid flows through a staggered tube bank, the heat transfer and fluid flow features are changed by the externally introduced magnetic field. This study provides a numerical investigation of this phenomenon. Heat and fluid flows are investigated for unsteady laminar flows at Reynolds numbers of 50 and 100 with the Hartmann number gradually increasing from zero to 100. As the Hartmann number increases, and owing to the effects of the introduced magnetic field, the velocity boundary layer near the tube wall is thinned, the flow separation is delayed downstream, and the shrinkage of a recirculation zone formed near the rear side is observed. Based on these thermo-fluid deformations, the resulting changes in the local and average Nusselt number are investigated.

• Transactions of the Korean Society of Mechanical Engineers
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• v.15 no.1
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• pp.218-228
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• 1991

Since heat exchangers are composed of bank of tubes, the knowledge on the flow and heat transfer characteristics of the tube bank are required for the optimum design and selection of heat exchangers. In this paper, the turbulent flow fields and heat transfers normal to a staggered tube bank are solved numerically employing K-.epsilon. 2 equation turbulence model and non-orthogonal coordinate transformation for the treatment of curved surface of tubes. Predicted mean Nusselt numbers of tube bank agree reasonably well with Grimision's correlation

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.11
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• pp.1645-1654
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• 2003

When vortex shedding is locked-on to a single frequency oscillatory flow, the variations of vortex dynamics are investigated using a time-resolved PIV system. Wake regions of recirculation and vortex formation, dynamic behavior of the shed vortices and the Reynolds stress fields are measured in the wake-transition regime at the Reynolds number 360. In the lock-on state, reduction of the wake region occurs and flow energy distributed downstream moves upstream being concentrated near the cylinder base. To observe the dynamic behavior of the shed vortices, the trajectory of the vortex center extended to the inside of the wake bubble is considered, which describes well the formation and evolution processes. The Reynolds stresses and their contributions to overall force balance on the wake bubble manifest the increase of the drag force by the lock-on.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2001.05a
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• pp.135-139
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• 2001

This experimental study investigates the wake flow behind a pair of cylinders displaced normal to a free stream. In this experiment, the principal aim is to investigate the transition mechanism of the large vortex generating process in the wake having unique vortex shedding pattern. associated with the gap difference between the cylinders. The detailed visualization is carried out using the PIV measurement. The transition mechanism of the large generating vortex is clarified by showing the streak lines. the vorticity and the statistical fluctuating velocity distributions.