• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1998.10a
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• pp.28-28
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• 1998

단일 충돌제트의 경우 충돌면에서의 열전달 효과는 제트 출구와 충돌면 사이의 간격, 노즐 형상에 따른 제트출구의 초기조건, Re$_{D}$수 변화와 충돌면의 형상이나 표면거칠기 등에 따라 변화하게 된다. 그러나 수직 충돌제트의 경우 정체영역에서의 국소적인 열전달 향상은 용이하나 반경 방향으로 열전달이 점차 감소하게 된다. 따라서 충돌면의 전반적인 열전달 향상을 위해서는 열전달이 정체점보다 상대적으로 낮은 영역에서 열전달 향상이 필요하다. 또한, 실제 적용시 충돌면의 형상이나 협소한 공간에서 노즐위치의 제약으로 인하여 충돌제트가 수직으로 분사되는 경우보다 경사진 제트가 더욱 많이 응용된다. 더욱이, 수직충돌제트가 사용되는 많은 상황에서도 주위의 다른 공기 흐름의 영향으로 인하여 실제 제트가 경사각을 갖고 분사되어진다 이러한 경사충돌제트의 중요성에도 불구하고 경사충돌제트에 대한 연구는 수직충돌제트에 대한 연구보다 상대적으로 적게 이루어져 왔다.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1998.10a
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• pp.27-27
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• 1998

제트에 있어서 유동특성은 제트초기의 생성되는 불안정성이 하류에서의 와류성장에 영향을 끼치게 되기 때문에 중요하게 되며, 와류의 조절을 통해 충돌면에의 열전달 효과의 변화를 가져올 수 있게 된다 따라서 본 연구에서는 FFT를 이용하여 제트의 와류생성과 병합의 주파수 특성을 연구하고, 이에 적절한 주파수로 와류를 여기 하여 자유제트의 유동특성 변화와 이에 따른 충돌 면에서의 충돌제트의 유동 및 열전달 특성을 고찰하였다. 제트의 음향을 통한 여기를 함으로써 생성되는 와류형성 및 병합 특성 변화는 연기열선법(smoke-wire method)과 속도 및 난류강도 특성 결과를 통해 확인 할 수 있었는데, 이는 자연적으로 생성되는 와류의 주파수(고유 주파수)와 관련하여 고유주파수의 조화성분 또는 부조화성분의 주파수를 가함으로써 와류의 병합을 촉진시키거나 억제하는 효과를 나타내기 때문이다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.41 no.5
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• pp.329-339
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• 2017

Impingement jets have been applied in a wide variety of fields as they provide significantly high heat transfer on the impingement-jet stagnation zone. However, the crossflow in an impingement chamber developed by spent wall jets can disrupt and deflect the downstream jets in the array, leading to a decrease in the cooling performance of an array of impingement jets. A numerical analysis is made of the fluid flow and heat transfer characteristics in a corrugated structure that traps the spent air in the corrugations between impingement jets and reduces crossflow effects on downstream jets. All computations are performed by considering a three-dimensional, steady, and incompressible flow by using the ANSYS-CFX 15.0 code. The effects of the configuration parameters of the corrugated structure on crossflow reduction of the array of impingement jets are presented and discussed.

• Journal of the Korean Society of Propulsion Engineers
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• v.3 no.1
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• pp.34-40
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• 1999

An experimental study has been conducted to determine the effects of inclined impinging jet on the local heat transfer coefficients. A single jet with nozzle diameter of 24.6 mm was tested for Reynolds numbers from 10,000 to 70,000 and nozzle-to-plate spacings of 2~6 jet diameters. The angle of inclination of the impingement surface relative to the horizontal surface was varied from $0^{\cire}$ (normal impingement) to $60^{\cire}$. The results indicate that the point of maximum heat transfer is moved up from the geometrical stagnation point of inclined surface by Coanda effect. The local heat transfer coefficients on the minor jet region decrease more rapidly than on the major jet region, thus creating an imbalance in the cooling capabilities on the two sides.

• Journal of Energy Engineering
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• v.20 no.2
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• pp.96-102
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• 2011

This research has been proceeded over the transition region(H/B=10) of two-dimensional impinging air jet system, in which square rods has been set up in front of heating surface in order to increase heat transfer. The objective of this research was to investigate the characteristics of heat transfer and air flow, in cases the clearance from rods to heating surface(C=1, 2, 4, 6 mm) and the width of rods(W=4, 6, 8 mm) changed. And this research compared the above with the experimentation without rods. As result, heat transfer performance was best under the condition of C=1 mm, and as the width is 8 mm, it is largely influenced by eddies and acceleration in case width of rods changed.

• Journal of Energy Engineering
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• v.18 no.2
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• pp.108-113
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• 2009

An experimental study of jet impingement on aluminum foam mounted on the surface with constant heat flux is conducted with the presentation of the heat transfer rate measured when jet impinges normally to a flat plate. Effects of pore density, foam thickness and Reynolds number on the heat transfer are analyzed. Experimental results show that the significant enhancement in Nu is obtained when the aluminum foam is mounted on the heated plate and that the increase in the heat transfer due to the porous material insertion is dominated by both the increase in the heat transfer area and the decrease in the momentum flux resulted from the pressure drop.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.33 no.8
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• pp.84-91
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• 2005

An experimental investigation has been carried out to examine heat-transfer characteristics of an axisymmetric, under-expanded, sonic jet impinging on a flat plate and the local measurement of surface pressures and heat transfer coefficients on a plate have been achieved together with a visualization test of shock structure in a jet. Heat transfer coefficients on a plate have been found to be changed significantly depending on the under-expansion ratio as much as the nozzle-to-plate distance. These phenomena could be explained by the wall pressure measurement and the shock visualization.

• Journal of the Korean Society of Propulsion Engineers
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• v.1 no.2
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• pp.32-40
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• 1997

The velocity and turbulent intensity of the jet core are affected by the vortices around jet. By the control of vortex acoustically, we can expect the changes of the flow and heat transfer characteristics of free and impinging jets. On this paper, we studied the effects of vortex forcing. If vortex pairings are promoted by acoustic excitation, the turbulent intensity is increased and the high heat transfer coefficients are obtained at the small nozzle to plate distance. On the other hand, it has low turbulent intensity at the center of jet. However due to increase of potential core length, it is more effective at the large nozzle to plate distance. Therefore the excited frequency, especially its subharmonic frequency, has an important role to control the jet flows.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.540-545
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• 2001

An experimental study was conducted to compare the heat transfer characteristics of an impinging slot jet and three kinds of impinging circular jets. Thermochromic liquid crystal with an image processing system was employed to measure the temperature of impinging wall where constant heat flux condition was applied. The distribution of convective heat transfer coefficients were then evaluated for eight nozzle-to-surface distance settings for each jet cases. The cooling effect was linearly proportional to the number of nozzles for circular jet cases at the same nozzle exit speed. However, the heat transfer under constant volume flow rate was the most at single circular jet. It was concluded that the overall convective heat transfer was better at the circular jets than the slot jet.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.1447-1451
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• 2004

The laminar impinging jet thermal fields were investigated with or without magnetic fields. The transient phenomenon from steady to unsteady flow was founded at specific Reynolds number ranges. In unsteady flow region, the magnetic fields make flow stable. So the characteristics of heat transfer at impingement wall are changed