• Tribology and Lubricants
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• 제36권1호
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• pp.39-46
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• 2020

Air stages are usually applied to precision engineering in sectors such as the semiconductor industry owing to their excellent performance and extremely low friction. Since the productivity of a semiconductor depends on the acceleration and deceleration performance of the air stage, many attempts have been made to improve the speed of the stage. Even during sudden start or stop sequences, the stage should maintain an air film to avoid direct contact between pad and the rail. The purpose of this study is to quantitatively predict the dynamic behavior of the air stage when acceleration and deceleration occur. The air stage is composed of two parts; the stage and the guide-way. The stage transports objects to the guideway, which is supported by an externally pressurized gas bearing. In this study, we use COMSOL Multiphysics to calculate the pressure of the air film between the stage and the guide-way and solve the two-degree-of-freedom equations of motion of the stage. Based on the specified velocity conditions such as the acceleration time and the maximum velocity of stage, we calculate the eccentricity and tilting angle of the stage. The result shows that the stiffness and damping of the gas bearing have non-linear characteristics. Hence, we should consider the operating conditions in the design process of an air stage system because the dynamic behavior of the stage becomes unstable depending on the maximum velocity and the acceleration time.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 한국추진공학회 2017년도 제48회 춘계학술대회논문집
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• pp.251-253
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• 2017

Numerical study of air-blast type injector for low emission aircraft engines was conducted. Volume-of-fluids approach was used to track interface of fuel and air. Primary atomization of fuel stream was visualized, and thickness and mean velocity at the injector exit was calculated. Liquid fuel injected from fuel slots joined together as a thin film on preflimer surface, and interacted with swirling air. As instability on the fuel surface increased, separation of fuel as ligaments and droplets occured. The film thickness and velocity were used to as fuel injection boundary condition for reactive flow simulation. Primary reaction zone was formed in vicinity of the fuel nozzle, creating a stable flame inside the combustor.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제22권12호
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• pp.897-901
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• 2010

A silencer has been widely used for the purpose of suppressing the noises propagated from duct of air conditioning system. The absorption type silencer is usually applied to reduce the noise with high frequency bandwidth. On the order hand, the resonance type silencer is applied to reduce the noise with low-middle frequency bandwidth. The insertion loss of silencer is measured in the reverberation room. The cut-off frequency of reverberation room is 100 Hz. The insertion loss of absorption type silencer is more larger than that for resonance type silencer regardless of changing the flow velocity.

• Nuclear Engineering and Technology
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• 제47권1호
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• pp.126-134
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• 2015

A simple Couette flow velocity profile with an appropriate correlation for the free terminal rise velocity of a single bubble in a quiescent liquid can produce reliable results for the trajectories of small spherical air bubbles in a low-viscosity liquid (water) provided the liquid remains under uniform shear flow. Comparison of the model adopted in this paper with published results has been accomplished. Based on this study it has also been found that the lift coefficient in water is higher than its typical value in a high-viscosity liquid and therefore a modified correlation for the lift coefficient in a uniform shear flow of water within the regime of the $E\ddot{o}tv\ddot{o}s$ number $0.305{\leq}Eo{\leq}1.22$ is also presented.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제9권2호
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• pp.221-228
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• 1997

A strong combustion-driven sound from a surface burner made of a perforated metal fiber plate for premixed gas was investigated to clarify the physical mechanism of its generation. A simple model was developed for the acoustic power generation in terms of the heat transfer response function and the acoustic impedance of the burner. The acoustic impedance of the perforated metal fiber placed on the open exit was measured and the heat release response of the burner to the oscillating flow associated with the acoustic disturbance was expressed in terms of a response function. It was found that the power is generated by the heat release in response to the downstream particle velocity, in contrast to the upstream velocity in the case of the Rijke oscillation driven by a heater placed in the lower half of a columm with upstream flow. The measured frequencies of the oscillation were in agreement with the estimated resonance frequencies and their excitation was varied with the combustion conditions. For the same fuel rate, the excited frequency increases with the air ratio if it is low but decreases with the ratio if not so low. Such frequency characteristics were explained by assuming a heat release response function with a time constant and it was shown that the excited frequency decreases as the time constant increases.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• 제29권4호
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• pp.175-184
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• 2017

With the improvement of living standards, the ventilation for the mitigation of indoor or outdoor air-pollution problems has recently attracted a lot of attention. Consequently, the ventilation for the supply of outdoor fresh air into a room is treated as an important building-design factor. The ventilation is generally divided into the forced and natural types; here, the former can control the ventilation rate by using mechanical devices, but it has the disadvantages of the equipment costs, maintenance costs, and noise generation, while the latter is applied to most workshops due to the absence of noise and the low installation and maintenance costs. In this experimental study, the ventilation performance of a typical rotating-type natural ventilator, which is called a "wind turbine," was investigated with the outdoor-wind velocity and the indoor/outdoor-temperature difference. From the experiment results, it was confirmed that the temperature difference of $10^{\circ}C$ corresponds to the ventilation driving force with an outdoor-wind velocity of 1.0 m/s. Additionally, the intake-opening area of a building also exerts a great effect on the ventilation rates.

• Journal of the Korea Academia-Industrial cooperation Society
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• 제11권7호
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• pp.2323-2327
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• 2010

The experimental apparatus consists of dehumidifier with multi-layer type heat exchangers to remove the moisture from automatic equipments, semiconductors, and manufacturing processes under the low temperature environment, and chemical production lines which are likely to take moisture. The major components of this system are four evaporators with different fin pitch, two compressors, two condensers and an expansion valve. In this study, the performance characteristics of dehumidifier is analyzed by the variations of frontal air velocity in the first heat exchanger(evaporator). The cooling capacity of each heat exchanger is acquired by the enthalpy calculating from measuring point of temperature and relative humidity of the first heat exchanger from 1.0m/s to 4.0m/s with increasing interval 0.5m/s, and the front air velocity. As a result, it is found that cooling capacity of the first heat exchanger showed the best cooling capacity when its frontal air velocity is 2.0 m/s.

• Journal of the Korean Institute of Gas
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• 제11권3호
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• pp.33-39
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• 2007

In this study, the effect of reformer gas(RG) on the performance development of the exhaust gas recirculation(EGR) was investigated numerically in $CH_4/air$ premixed flame. Typically EGR is used to reduce the flame temperature and NOx emission, whereas RG can be used to improve the flame stability, such as homing velocity. This competitive relationship is focused in this study. As a result, it can be identified that the adjustments of EGR and RG ratio can achieve the low NOx emission and the similar flame stability to pure $CH_4/air$ premixed flame simultaneously.

• Computers and Concrete
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• 제6권6호
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• pp.437-450
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• 2009

The effect of six different curing conditions on compressive strength and ultrasonic pulse velocity (UPV) of volcanic pumice concrete (VPC) and normal concrete (NC) has been studied. The curing conditions include water, air, low temperature ($4^{\circ}C$) and different elevated temperatures of up to $110^{\circ}C$. The curing age varies from 3 days to 91 days. The development in the pulse velocity and the compressive strength is found to be higher in full water curing than the other curing conditions. The reduction of pulse velocity and compressive strength is more in high temperature curing conditions and also more in VPC compared to NC. Curing conditions affect the relationship between pulse velocity and compressive strength of both VPC and NC.

• Journal of the Korean Society of Safety
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• 제17권2호
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• pp.112-117
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• 2002

The effects of shield gas on the structure of methane-air nonpremixed counterflow flames were numerically investigated. The near extinction flame of a low global strain rate 20 $s^{-1}$ of 19% methane diluted by 81% nitrogen by volume and undiluted air was computed. The flame shape, centerline temperature and axial velocity profiles were compared for different velocity of the shield gas and with and without the shield gas. The effects of the velocity of the shield gas were negligible for $V_{S}/V_{F}{\leq}2$ in normal gravity. Under normal gravity conditions, the flame shape and its position with the shield gas were different from those of the flame without the shield gas, whereas no discernible effects of the shield gas along the centerline were observed in zero gravity.