• 대한기계학회논문집B
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• 제26권8호
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• pp.1172-1182
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• 2002

In the present study a general criterion for local thermal equilibrium is presented in terms of parameters of engineering importance which include the Darcy number, the effective Prandtl number of fluid, and the Reynolds number. For this, an order of magnitude analysis is performed for the case when the effect of convection heat transfer is dominant in a porous structure. The criterion proposed in this study is more general than the previous criterion suggested by Carbonell and Whitaker, because the latter is applicable only when conduction is the dominant heat transfer mode in a porous medium while the former can be applied even when convection heat transfer prevails. In order to check the validity of the proposed criterion for local thermal equilibrium, the forced convection phenomena in a porous medium with a microchanneled structure subject to an impinging jet are studied using a similarity transformation. The proposed criterion is also validated with the existing experimental and numerical results for convection heat transfer in various porous materials that include some of the parameters used in the criterion such as a microchannel heat sink with a parallel flow, a packed bed, a cellular ceramic, and a sintered metal. It is shown that the criterion presented in this work well-predicts the validity of the assumption of local thermal equilibrium in a porous medium.

• 동력기계공학회지
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• 제13권2호
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• pp.11-17
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• 2009

The heat transfer characteristics of flow through two inclined wire-mesh baffles in a rectangular channel were investigated experimentally with varying the mesh number of wire screens and inclination angle of the baffles. Two different types of wire meshes such as dutch and plain weaves, were used in this experiment. Three kinds of baffle plates with different mesh specifications in the dutch weave and four different kinds in the plain weave were manufactured. Baffles were mounted on bottom wall with varied angles of inclination. Reynolds number was varied from 23,000 to 57,000. It is found that the placement of inclined wire-mesh baffles in the channel affects the heat transfer characteristics by combining both jet impingement and flow disturbance. The wire screen modified the flow structure leading to a change in the heat transfer characteristics. The results show that the baffle plate with the most number of mesh (type SA) has the highest heat transfer rate.

• 한국자동차공학회논문집
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• 제15권4호
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• pp.94-100
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• 2007

The purpose of this work is to investigate the effect of properties of diesel and biodiesel fuels on the nozzle cavitation and the effect of the length/diameter(L/D) ratio on internal and external flow pattern of nozzle at the various injection conditions. In order to study the effect of the L/D ratio on the nozzle cavitation characteristics of diesel and biodiesel, the characteristics of cavitation flow in the nozzle are visualized and analyzed at the injection pressure of 0.1 MPa to 0.7 MPa by using the visualized images. It was founded that the cavitation was formed in the nozzle orifice at the low injection pressure and the breakup of the issuing liquid jet was promoted at the low L/D ratio. When the L/D ratio decrease, cavitation beginning and growth were affect by cavitation number and Reynolds number.

• 대한토목학회논문집
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• 제26권6B호
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• pp.641-649
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• 2006

본 연구에서는 PIV를 이용하여 측정한 평균유속장의 실험결과에 근거하여 10의 종횡비를 갖는 3차원 순수사각형제트의 거동을 고찰하였다. 장축상의 횡분포에서 안장형 분포가 관찰되었다. 점원개념으로부터 유도된 이론적인 중심선유속의 식은 측정된 중심선유속과 잘 일치하였으며 원류핵영역, 2차원영역, 그리고 축대칭영역의 분할을 제시하였다. 이러한 이론적인 중심선유속의 감소에 의해 분할된 2차원영역의 범위가 천이영역에 비해 상대적으로 작게 관찰되었다. 작은 종횡비를 갖는 사각형제트의 거동이나 실제 해양의 깊은 수심에서 다공확산관을 통해 방류되는 하 폐수의 거동을 예측하기 위한 2차원모형의 적용은 2차원영역을 지난 천이영역과 축대칭영역에서 중대한 오차를 야기할 수 있다. 2차원영역에서 가우스 상수가 보존되는 경향을 보였으며 2차원영역의 끝 지점에서 확장률이 감소함을 알 수 있었다. 중심선유속으로 무차원화 된 중심선 난류 강도는 초기에 급격히 증가하였고 높은 레이놀즈수에서 상대적으로 높은 난류강도를 보여주었다.

• 한국전산유체공학회지
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• 제8권2호
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• pp.1-7
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• 2003

The V2F turbulence model, which has shown very good performance in several test cases at low speeds, has been applied to supersonic ramp flow with 20. corner angle at the free stream Mach number of 2.79. The flow is known to manifest strong shock wave/turbulent boundary layer interactions. As a comparative study, low-Reynolds k-ε models are also considered. While the V2F model predicts wall-pressure distribution well, it relatively predicts larger separation bubble and higher skin-friction after the reattachment than the experimental data. Although the ellpticity of f equation is the characteristics of incompressible flows, the converged solutions are acquired in the compressible flow with shock waves. The effect of the realizability constraints used in the model is also examined. In contrast to the result of impinging jet flows, the realizability bounds proposed by Durbin deterioate the overall solutions of the supersonic ramp flow.

• 대한기계학회논문집B
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• 제32권7호
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• pp.536-541
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• 2008

The Flow characteristics of a ship's propulsion mechanism of Weis-Fogh type, in which a airfoil(NACA0010) moves reciprocally in a channel, were investigated by the PIV. Velocity vectors and velocity profiles around the operating and stationary wings were observed at opening angles of ${\alpha}=15^{\circ}$ and $30^{\circ}$, velocity ratios of $V/U=0.5{\sim}1.5$ and Reynolds number of $Re=0.52{\times}104{\sim}1.0{\times}104$. As the results the fluid between wing and wall was inhaled in the opening stage and was jet in the closing stage. The wing in the translating stage accelerated the fluid in the channel. And the flow fields of this propulsion mechanism were unsteady and complex, but those were clarified by flow visualization using the PIV.

• Journal of Advanced Marine Engineering and Technology
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• 제32권7호
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• pp.1036-1043
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• 2008

The Flow fields of a ship's propulsion mechanism of Weis-Fogh type were investigated by the PIV. Velocity vectors and velocity profiles around the operating and stationary wings were observed at opening angles of ${\alpha}=15^{\circ} and $30^{\circ}$, velocity ratios of V/U=$0.5{\sim}1.5$ and Reynolds number of Re=$0.52{\times}10^4{\sim}1.0{\times}10^4$. As the results the fluid between wing and wall was inhaled in the opening stage and was jet in the closing stage. The wing in the translating stage accelerated the fluid in the channel. And the flow fields of this propulsion mechanism were unsteady and complex, but those were clarified by flow visualization using the PIV.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2009년도 제33회 추계학술대회논문집
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• pp.492-495
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• 2009

Supersonic microjets acquire considerable research interest from a fundamental fluid dynamics perspective, in part because the combination of highly compressible flow at low-to-moderate Reynolds number is not very common, and in part due to the complex nature of the flow itself. In addition, microjets have a great variety engineering applications such as micro-propulsion, MEMS (Micro-Electro Mechanical Systems) components, microjet actuators and fine particle deposition and removal. Numerical simulations have been carried out at moderate nozzle pressure ratios and for different nozzle exit diameters to investigate and to understand in-depth of aerodynamic characteristics of supersonic microjets.

• 한국가시화정보학회지
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• 제7권2호
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• pp.35-41
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• 2010

Supersonic microjets acquire considerable research interest from a fundamental fluid dynamics perspective, in part because the combination of highly compressible flow at low-to-moderate Reynolds number is not very common, and in part due to the complex nature of the flow itself. In addition, microjets have a great variety engineering applications such as micro-propulsion, MEMS(Micro-Electro Mechanical Systems) components, microjet actuators and fine particle deposition and removal. Numerical simulations have been carried out at moderate nozzle pressure ratios and for different nozzle exit diameters to investigate and to understand in-depth of aerodynamic characteristics of supersonic microjets.

• 오토저널
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• 제8권2호
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• pp.65-74
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• 1986

A computer program was developed to predict swirling steady axisymmetric turbulent flows by extending TEACH Code. It was applied to a reciprocating engine cylinder with a intake valve on the flat head. Flows were assumed to be steady and swirling. Effects of Reynolds number, the valve lift, and the swirl ratio on flow patterns and turbulence were investigated numerically. Flow patterns were reasonably predicted in comparison with experimental results. Length of the recirculation zone was shortened with increasing valve lifts and swirl ratios. Static pressure distributions show maximum value near the reattachment point of the incoming circular jet and minimum value near the maximum width of the valve attached recirculation zone.