• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.611-614
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• 2002

Rotating stall in vaneless diffusers of centrifugal compressor occurs in the diffuser wall due to flow separation at large inlet flow angle. For this reason, the critical inlet flow angles are suggested by several researchers. Beyond this critical angle, flow separates in the diffuser, and develops into rotating stall. This paper studied this critical flow angle. Rotating stall is measured through eight fast-response pressure transducers which are equally spaced around the circumference at the inlet and exit of a vaneless diffuser. Experiments are done from 20000rpm to 60000rpm for the diffuser stall. Two-cell structure which rotates at $6{\~}l0{\%}$ of impeller speed is fully developed at $20000{\~}40000rpm$, and three-cell structure which rotates at $7{\~}9{\%}$ of impeller speed is fully developed at $50000{\~}60000rpm$. This paper shows that the critical inlet flow angle is not constant but related with tip speed of impeller. As tip speed increases, so does the critical inlet flow angle.

• 한국마린엔지니어링학회:학술대회논문집
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• 한국마린엔지니어링학회 2006년도 전기학술대회논문집
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• pp.281-282
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• 2006

In wide angle diffuser, the increasement of cross-sectional area so rapidly cause possibly separate from the diffuser wall and forming areas of backflow. The flow characteristics of wide angle diffuser with guide damper was investigated for three kind of inclined angle($\alpha=7_{\circ},\;10.5_{\circ}\;and\;14_{\circ}$)of guide damper. Judging from the results, guide damper can be used as an effective means of improvement for diffuser performance and inclined angle of $\alpha=10.5^{\circ}$ was through to be the best among three cases.

• 대한기계학회논문집B
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• 제22권7호
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• pp.937-945
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• 1998

The purpose of this paper is to investigate the relationship between pressure recovery and turbulent characteristic value of velocity and pressure, in the case where a swirling flow streams into a conical diffuser. The results of both measurements of the wall pressure fluctuation and velocity fluctuation revealed them to role the large part of the total pressure loss of the flow. The cause of the fluctuation of flow was showed to be the flow separation at the inlet of diffuser at low intensity of swirl, but the flow of diffuser center was instable at high intensity of swirl. The static pressure recovery depends strongly on the magnitude of the turbulent energy in the diffuser, and that this magnitude of the turbulent energy varies as the intensity of swirl at the diffuser inlet.

• 한국전산유체공학회:학술대회논문집
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• 한국전산유체공학회 2001년도 추계 학술대회논문집
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• pp.44-50
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• 2001

A numerical study was made to choose the better turbulence model for the flow in the discharge flow path from a diffuser to a wall. In this study standard $\kappa-\epsilon$ model(SKE), RNG $\kappa-\epsilon$ model(RNG), and Reynolds stress model(RSM) were applied. In case of the flow with relatively high Reynolds number at a diffuser inlet, the pressure loss coefficients by RNG have a tendency to be near to those by SKE at small ratio(below about 0.35) of $h/D_o$, but to those by RSM at large ratio(above about 0.35). At large ratio RNG begins to enlarge the effects of rapid strain and streamline curvature. RNG & RSM are recommended as the appropriate turbulence models for this case. But it is noticeable that the velocity gradient pattern in RNG is same as in SKE, and also that the total pressure distribution in RNG is same as in RSM only at swirling flow area, same as in SKE only at main flow area.

• 한국유체기계학회 논문집
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• 제15권2호
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• pp.45-50
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• 2012

The flow characteristics in a vaneless diffuser with a backswept radial impeller have been experimentally investigated according to the variation of discharge flow rate. Particle image velocimetry(PIV) system was applied to measure velocity fields with several operating conditions and on some diffuser horizontal planes. Pressure transducers were installed on hub wall of the diffuser in order to analyze the pressure fluctuations and their corresponding velocity fields. The results show that the location of the main flow center moves from the hub to the shroud side as the flow rate decreases, and the reverse flow is locally generated on the hub side.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2006년도 제27회 추계학술대회논문집
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• pp.344-352
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• 2006

Evacuation performance, starting transient, and plume blowback at diffuser breakdown of a straight cylindrical supersonic exhaust diffuser with no externally supplied secondary flow are investigated. Pressure records in the transitional periods are measured by a small-scale cold-gas simulator. Flow-fields evolving in the diffuser-type ejector are solved by preconditioned Favre-averaged Navier-Stokes equations with a low-Reynolds number $k-{\varepsilon}$ turbulence model edited for turbulence compressibility effects. The present RANS method is properly validated with measured static wall pressure distributions and evacuation level at steady operation as well as the pressure records during the transition regime.

• 한국유체기계학회 논문집
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• 제14권6호
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• pp.54-60
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• 2011

This paper presents the steady-state performance analysis of the first stage of a multistage centrifugal pump, composed of a shrouded-impeller, a vaned-diffuser and a return-channel, using the commercially available computational fluid dynamics (CFD) code, ANSYS CFX. The detailed flow fields in the vaned-diffuser with outlet in its side wall and the return-channel are investigated by the CFD code adopted in the present study. The effect of the vaned-diffuser with a downstream crossover bend and the corresponding return-channel on the overall hydrodynamic performance of the first stage pump has also been demonstrated over the normal operating conditions. The predicted hydrodynamics for the diffusing components herein could provide useful information to match the inlet blade angle of the next stage impeller for improving the multistage pump performances.

• 대한기계학회논문집
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• 제18권4호
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• pp.933-945
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• 1994

A shock-wave in a supersonic flow can be theoretically determined by a given pressure ratio at upstream and downstream flowfields, and then the obtained shock-wave is stable in its position. Under the practical situation in which the shock-wave interacts with the boundary layer along a solid wall, it cannot, however, be stable even for the given pressure ratio being independent of time and oscillates around a time-mean position. In the present study, oscillations of a weak normal shock-wave in a supersonic diffuser were measured by a Line Image Sensor(LIS), and they were compared with the data of the wall pressure fluctuations at the foot of the shock-wave interacting with the wall boundary layer. LIS was incorporated into a conventional schlieren optical system and its signal, instantaneous displacement of the interacting shock-wave, was analyzed by a statistical method. The results show that the displacement of an oscillating shock-wave increase with the upstream Mach number and the dominant frequency components of the oscillating shock-wave are below 200 Hz. Measurements indicated that shock-wave oscillations may not entirely be caused by the boundary layer separation. The statistical properties of oscillations appeared, however, to be significantly affected by shock-induced separation of turbulent boundary layer.

• 한국추진공학회지
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• 제23권5호
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• pp.10-18
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• 2019

Center Body Diffuser (CBD)의 Center Body (CB) 형상에 의한 유동 특성 및 시동압력을 해석하였다. CB 위치와 Cone 각도의 변경을 통해 초음속 유동 특성 및 시동압력을 비교하였다. CB 위치를 변수로 하여 Diffuser 해석한 결과, 유동의 모멘텀의 형성에 따라 강한 경사충격파의 발생위치가 달라지는 것을 확인하였다. 또한 경사충격파가 발생할 경우, 초음속 유동의 방향이 디퓨저 벽면으로 유도되는 것을 확인하였다. Cone 각도 변화에 따른 경사충격파의 각도 차이로 인하여, CBD의 시동압력이 영향을 받는 것을 확인하였다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2010년도 제34회 춘계학술대회논문집
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• pp.321-325
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• 2010

Supersonic ejectors are simple mechanical components, which generally perform mixing and/or recompression of two fluid streams. Ejectors have found many applications in engineering. In aerospace engineering, they are used for altitude testing of a propulsion system by reducing the pressure of a test chamber. It is composed of three major sections: a vacuum test chamber, a propulsive nozzle, and a supersonic exhaust diffuser. This paper aims at the improvement of ejector-diffuser performance by focusing attention on reducing exhaust back flow into the test chamber, since alteration of the backflow or recirculation pattern appears as one of the potential means of significantly improving low supersonic ejector-diffuser performance. The simplest backflow-reduction device was an orifice plate at the duct inlet, which would pass the jet and entrained fluid but impede the movement of fluid upstream along the wall. Results clearly showed that the performance of ejector-diffuser system was improved for certain a range of system pressure ratios, whereas the orifice plate was detrimental to the ejector performance for higher pressure ratios. It is also found that there is no change in the performance of diffuser with orifice at its inlet, in terms of its pressure recovery. Hence an appropriately sized orifice system should produce considerable improvement in the ejector-diffuser performance in the intended range of pressure ratios.