• Journal of Mechanical Science and Technology
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• 제16권11호
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• pp.1448-1456
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• 2002

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.

• 대한기계학회논문집B
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• 제21권7호
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• pp.882-890
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• 1997

A buoyant jet flow impinging on a circular cylinder is investigated including heat conduction through the cylinder. Temperature and flow fields are obtained by an iterative method, and the effects of the non-uniform wall temperature on the flow and heat transfer are analyzed. Effects of three-dimensionality and the traversing of the jet are also included. Nusselt number over the cylinder surface for the conjugate case is relatively small as compared with the constant wall temperature case due to the small temperature gradient. As the conductivity of the cylinder becomes lower, Nusselt number decreases due to the reduced temperature gradient. Increasing jet traversing speed causes the surface temperature of the cylinder to decrease, which increases local Nusselt number over the surface.

• 한국생산제조학회지
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• 제6권2호
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• pp.34-41
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• 1997

The objective of this study is to investigate experimentally the frequency characteristics of the hole tones generated by a circular jet of low speed impinging on a plate with a round hole. The experimental results about the sound spectrum and the time wave of the hole tone are presented and discussed in relation with the hole type, the jet velocity and the distance of the nozzle-to-plate with a round hole. From the sound spectrum and time wave measurements, it is found that the hole tone is generated not only by an interaction of convected vortices with a round hole but also by a series of vortex shedding from jets passing through a hole. The hole tones generated by a feedback mechanism consist of many frequency stages and also have a hysteresis phenomenon like an edge tone. But the hole tones generated by a series of vortex shedding have nothing with the stage characteristics. The frequencies of hole tones are influenced by the jet velocity, the distance of the nozzle-to-plate with a round hole and the hole type.

• 대한기계학회논문집B
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• 제30권11호
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• pp.1093-1100
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• 2006

Combustion stability rating of jet injector is conducted numerically using air-injection technique in a model chamber, where air is supplied to oxidizer and fuel manifolds of the model five-element injector head. A sample F(fuel)-O(oxidizer)-O-F impinging-jet injector is adopted. In this technique, we can simulate mixing process of streams flowing through oxidizer and fuel orifices under cold-flow condition without chemical reaction. The model chamber was designed based on the methodologies proposed in the previous work regarding geometrical dimensions and operating conditions. From numerical data, unstable regions can be identified and they are compared with those from air-injection acoustic and hot-fire tests. The present stability boundaries are in a good agreement with experimental results. The proposed numerical method can be applied cost-effectively to stability rating of jet injectors when mixing of fuel and oxidizer jets is the dominant process in instability triggering.

• 대한기계학회논문집B
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• 제23권10호
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• pp.1285-1295
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• 1999

An experimental study on heat transfer and flow characteristics of a circular impinging jet on a flat plate has been carried out. Of particular interests are the effects of nozzle wall thickness and nozzle exit pressure. Experimental apparatus has been designed to view heating plate coated by TLC from the opposite side of the nozzle in order to measure heat transfer rates for cases of very small nozzle to plate spacings. A visualization study of jet flows has also been performed. As the nozzle wall thickness increases at small nozzle to plate spacings, the effect of mixing is inhibited due to the confinement caused by the finite nozzle wall, consequently, heat transfer rates have been decreased. At small nozzle to plate spacings, heat transfer rates and nozzle exit pressures are increased together, therefore, enhancement of heat transfer at small nozzle to plate spacings should be considered in conjunction with the need of more fan power to generate the same Reynolds numbers.

• 대한기계학회논문집B
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• 제29권5호
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• pp.537-544
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• 2005

The present study numerically investigates two-dimensional fluid flow in the confined jet flow in the presence of applied magnetic field. Numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields in the range of $0{\le}N{\le}0.05$, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow fields become the steady state if the Stuart number is greater than a critical value. Thus the pressure coefficients at the stagnation point also vary as a function of Stuart number.

• 대한기계학회논문집B
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• 제29권6호
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• pp.653-661
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• 2005

The present study numerically investigates two-dimensional fluid flow and heat transfer ir the confined jet flow in the presence of applied magnetic field. For the purpose of controlling vortex shedding and heat transfer, numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields and for different Prandtl numbers of 0.02 (liquid metal), 0.7 (air) and 7 (water) in the range of $0{\le}N{\le}0.05$, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow and thermal quantities in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow and thermal fields become the steady state if the Stuart number is greater than the critical value. Thus the Nusselt number at the stagnation point representing the heat transfer characteristics also vary as a function of Stuart number.

• 대한기계학회논문집B
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• 제33권2호
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• pp.93-100
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• 2009

Characteristics of micro-sonic/supersonic axi-symmetric jet impinging on a flat plate with a pre-drilled hole were both experimentally and numerically studied, to observe the role of assist-gas jet to eject melted materials from the cut zone in the laser machining. For various Mach numbers of the nozzle and the total pressures of the assist gas, detailed impinging jet flow structures over the plate and the variations of mass flux through the pre-drilled hole were observed. It was found that the present experimental and numerical results show a good agreement, which proves the accountability of the present work. From the present study, it was also observed that the mass flow rate through the hole was closely related with the total pressure loss caused by the Mach disc on the work piece, and that supersonic nozzle could perform more efficient roles as blowing the assist-gas jet in the laser machining, as compared to sonic nozzles.

• 대한기계학회논문집
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• 제16권8호
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• pp.1566-1574
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• 1992

본 연구에서는 원형수직 충돌분류에 의한 가열평판의 전열특성을 해석하기 위 하여 최근 표면온도 측정에 많이 사용되고 있는 liquid crystal을 이용하였다. 즉 liquid crystal의 감온특성과 영상처리 기법을 이용하여 레이놀즈수 및 노즐과 충돌판 사이의 거리(L/D) 변화에 따른 전열면의 온도장 변화를 측정하였다.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집D
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• pp.565-571
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• 2001

The objective of this study was to investigate the characteristics of air flow and heat transfer caused by trapezoid rods array in impinging air jet system. In this study, trapezoid rods have been set up on front of flat plate to act as a turbulence promoter. Local Nusselt numbers were determined as a function of three parameters: (a) the space from rods to heating surface(C=1, 2, 4mm), (b) the pitch between each rods(P=30, 40, 50mm), (c) the distance from nozzle exit to flat plate(H/B=2, 6, 10). And this research compared the above with the experiment without trapezoid rods. As a result, heat transfer performance was best under the condition of C=1mm and as the pitch is 30mm. In this case, maximum rate of heat transfer augmentation is about 1.9 times greater compared to that without trapezoid rods.