• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1098-1103
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• 2004

In this study, the effect of channel flow in the concave surface on local heat transfer characteristics of array jets was investigated experimentally. A TLC method is employed to determine local heat transfer coefficients on the target plate and also flow visualization has been conducted to investigate the behavior of a row of impinging jets and array of impinging jets. Two different array patterns of impinging array jets devices are tested for Reynolds number(Re=10,000). In a row of impinging jets, secondary vortex is strongly maintained by main vortex at nozzle-to-plate distance of H/d=2. Therefore, the Nusselt number slowly decreased at the mid-way region between adjacent jets. In array jets, the local maximum Nusselt number region move further in the downstream direction due to the increase of channel flow velocity.

• 대한기계학회논문집B
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• 제41권5호
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• pp.329-339
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• 2017

충돌제트는 제트가 충돌하는 정체 영역에 매우 높은 열전달을 제공하기 때문에 다양한 분야에서 적용되고 있다. 그러나 제트가 벽면에 부딪친 후 벽면 제트에 의해 야기되는 충돌 챔버 내의 횡방향 유동은 여러 개의 제트로 구성된 배열제트인 경우 하류에 있는 제트 유동을 방해하거나 휘게 할 수 있으며, 이로 인해 배열 충돌제트의 냉각 성능은 감소하게 된다. 파형 구조는 하류 제트에서의 횡방향 유동영향을 줄이기 위해 인접한 충돌 제트 사이에 있는 파형 속에 사용된 냉각 공기를 유입시키는 역할을 하며, 이러한 파형 구조에서의 유동 및 열전달 특성에 대해 수치해석을 수행하였다. 3차원, 정상상태, 비압축성 유동으로 고려하고 해석하였으며 ANSYS-CFX 15.0 코드를 사용하였다. 파형 구조의 형상 변수가 배열 충돌제트의 횡방향 유동 억제에 미치는 영향을 제시하고 분석하였다.

• 설비공학논문집
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• 제12권1호
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• pp.59-66
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• 2000

In a previous paper, we have examined the effects of nozzle configuration and jet to jet spacing on the heat transfer of 1 row of circular water jets. In this paper, experiments have been conducted to obtain the effects of nozzle to target plate distances on the heat transfer of 1 row of 3 jets and 1 row of 5 jets. The nozzle configurations are Cone type, Reverse cone type and Vertical circular type. Nozzle to target plate distance H was varied from 16 mm(H/D=2) to 80mm(H/D=10). For fixed value of mass flow rate and nozzle to target plate distance, larger values of average Nusselt number were obtained for the smaller jet to jet spacing. For the array of water jets, the average heat transfer was decreased slightly with increasing nozzle to target plate distance at low jet velocity of $\textrm{V}_{o}$=3 m/s. However, except for $\textrm{V}_{o}$=8 m/s of 1 row of 5 jets, it was increased with increasing nozzle to target plate distance at high jet velocity of $\textrm{V}_{o}$$\geq$6m/s. We proposed to apply the nozzle configuration of maximum average heat transfer to each nozzle to target plate distance for 1 row of 3 jets, and, it was Reverse cone type nozzle for 1 row of 5 jets(Reynolds number$\geq$36000).

• International Journal of Aeronautical and Space Sciences
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• 제17권3호
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• pp.296-314
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• 2016

This paper presents experimental and computational investigations of synthetic jets with a circular exit for improving flow control performance. First, the flow feature and vortex structure of a multiple serial circular exit were numerically analyzed from the view point of flow control effect under a cross flow condition. In order to improve separation control performance, experimental and numerical studies were conducted according to several key parameters, such as hole diameter, hole gap, the number of hole, jet array, and phase difference. Experiments were carried out in a quiescent condition and a forced separated flow condition using piezoelectrically driven synthetic jets. Jet characteristics were compared by measuring velocity profiles and pressure distributions. The interaction of synthetic jets with a freestream was examined by analyzing vortical structure characteristics. For separation control performance, separated flow over an airfoil at high angles of attack was employed and the flow control performance of the proposed synthetic jet was verified by measuring aerodynamic coefficient. The circular exit with a suitable hole parameter provides stable and persistent jet vortices that do beneficially affect separation control. This demonstrates the flow control performance of circular exit array could be remarkably improved by applying a set of suitable hole parameters.

• 대한기계학회논문집B
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• 제25권11호
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• pp.1606-1615
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• 2001

The present study has been conducted to investigate heat/mass transfer characteristics on a target plate fur arrays of circular impingement jets with and without effusion holes. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate. The effusion holes are located at the center of four injection holes in the injection plate where the spent air is discharged through the effusion hole after impingement on the target plate. For the array jet impingement without effusion holes, the array jets are injected into the crossflow formed by upstream spent air because the impinged jets must flow to the open exit. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to crossflow effects and re-entrainments of spent air. However, uniform distributions and enhanced values of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the crossflow has little influence on heat/mass transfer characteristics on the target palate due to the large cross-sectional open area between the injection and target plates. Therefore, the distributions and levels of heat/mass transfer coefficients are almost the same for both cases.

• 설비공학논문집
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• 제12권1호
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• pp.50-58
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• 2000

Experiments were carried out to obtain the effects of nozzle configuration and jet to jet spacing on the heat transfer characteristics of single line of circular water jets impinging on a constant heat flux plane surface. The nozzle configurations are Cone type, Reverse cone type and Vertical circular type, and the nozzle arrays are single jet(nozzle dia. 8 mm), 1 row of 3 jets and 1 row of 5 jets. Jet velocities ranging from 3m/s to 8m/s were investigated for the nozzle to target plate spacing of 80 mm. For the Cone and Reverse cone type nozzle arrays, the average Nusselt number of 1 row of 5 jets was larger than that of 1 row of 3 jets at Re$_{D}$<45000, but that of 1 row of 3 jets was larger than that of 1 row of 5 jets at $Reo\le45000$. For the Vertical circular type nozzle, however, the average Nusselt number of 1 row of 3 jets was larger than that of 1 row of 5 jets at all jet velocities. In the condition of fixed mass flow rates, the maximum heat transfer augmentation was obtained for 1 row of 5 jets and was over 2 times larger than that of the single jet for all nozzle configurations. The nozzle configurations that produce the maximum average Nusselt number are as follows: For 1 row of 3 jets, the Vertical circular type at $Reo\le45000$ and the Reverse cone type at $Reo\le45000$. But, they are the Reverse cone type at Re$_{D}$<55000 and the Vertical circular type at$Reo\le55000$ for 1 row of 5 jets.

• Journal of Mechanical Science and Technology
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• 제20권4호
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• pp.554-560
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• 2006

This paper describes the demonstration of successful fabrication and initial characterization of micromachined pressure sensors and micromachined jets (microjets) fabricated for use in macro flow control and other applications. In this work, the microfabrication technology was investigated to create a micromachined fluidic control system with a goal of application in practical fluids problems, such as UAV (Unmanned Aerial Vehicle)-scale aerodynamic control. Approaches of this work include: (1) the development of suitable micromachined synthetic jets (microjets) as actuators, which obviate the need to physically extend micromachined structures into an external flow; and (2) a non-silicon alternative micromachining fabrication technology based on metallic substrates and lamination (in addition to traditional MEMS technologies) which will allow the realization of larger scale, more robust structures and larger array active areas for fluidic systems. As an initial study, an array of MEMS pressure sensors and an array of MEMS modulators for orifice-based control of microjets have been fabricated, and characterized. Both pressure sensors and modulators have been built using stainless steel as a substrate and a combination of lamination and traditional micromachining processes as fabrication technologies.

• 한국추진공학회지
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• 제21권2호
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• pp.83-93
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• 2017

파형 구조는 배열 충돌제트 하류에서의 횡방향 유동 영향을 줄이기 위해 충돌제트 사이의 파형 속에 사용된 냉각 공기를 유입시키며, 이러한 파형 구조에서의 유동 및 열전달 특성에 대해 수치해석을 수행하였다. 모든 계산은 3차원, 정상상태, 비압축성 유동으로 고려하였으며 ANSYS-CFX 15.0 코드를 사용하였다. 제트 홀에서 평균 Reynolds 수는 10,000이며, Spanwise 단면에서 충돌제트의 경사각도는 $70^{\circ}$, $80^{\circ}$ 및 $90^{\circ}$ 이고, Streamwise 단면에서 충돌제트의 경사각도는 $70^{\circ}$, $90^{\circ}$ 및 $110^{\circ}$ 이다. 본 연구에서는 배열 충돌제트의 경사각도가 파형 구조의 유동 및 열전달 특성에 미치는 영향에 대해 고찰하였다.

• 대한기계학회논문집B
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• 제24권2호
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• pp.195-203
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• 2000

The local heat/mass transfer coefficients for arrays of impinging circular air jets on a plane surface are determined by means of the naphthalene sublimation method. Fluid from the spent jets is constrained to flow out of the system in one direction. Therefore, the spent fluid makes a crossflow in the confined space. The present study investigates effects of jet-orifice-plate to impingement-surface spacing and jet Reynolds number. The spanwise- and overall-averaged heat/mass transfer coefficients are obtained by numerical integrating the local heat transfer coefficients. The local maximum heat/mass transfer coefficients move further in the downstream direction due to the increase of crossflow velocity. At the mid-way between adjacent jets, the heat/mass transfer coefficients have a small peak owing to the collision of the adjacent wall jets and are affected strongly by the crossflow. The effect of the crossflow occurs strongly at the small orifice-to-impingement surface distance.

• 대한기계학회논문집B
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• 제21권11호
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• pp.1485-1495
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• 1997

The present study investigates heat/mass transfer around film cooling jets and circular cylinders to compare the characteristics of each other. Experiments are conducted to obtain the detailed heat/mass transfer coefficients of flat plate with injections through an array of holes and for flows around an array of protruding circular cylinders using the naphthalene sublimation technique. The inclination angles of cylinders are set to the same ones of jets; a, the angle between the jet and the surface is fixed at 30 deg. through the whole experiments and .betha., the angle between the projection of the jet on the surface and the direction of main stream is adjusted to 0 deg., 45 deg. and 90 deg. to investigate the effect of variation of injection angles. The influence of blowing rates of jets and those of cylinder length to diameter ratios are also investigated. The results indicate that the increase of angle .betha. influences the spanwise uniformity of heat/mass transfer remarkably for both jets and cylinders, but that variation of cylinder length to diameter ratios has weaker effects on heat/mass transfer coefficients than that of blowing rates.