• 유체기계공업학회:학술대회논문집
• /
• 2005.12a
• /
• pp.223-230
• /
• 2005

The unsteady nature of vortex structures has been investigated by a large eddy simulation (LES) in an axial flow fan with a shroud covering only the rear region of its rotor tip. The simulation shows that the tip vortex plays a major role in the structure and unsteady behavior of the vortical flow in the fan. The movements of the vortex structures induce high-pressure fluctuations on the rotor blade and in the blade passage. Frequency characteristics of the fluctuating pressure on the rotor blade are analyzed using wavelet transform. The dominant frequency of the real-time pressure selected at the high pressure fluctuation region corresponds well to that of the fluctuating rotor torque and the experimental result of fan noise. It is mainly generated due to the unsteady behavior of the vortical flow, such as the tip vortex and the leading edge separation vortex.

• Journal of Power System Engineering
• /
• v.4 no.3
• /
• pp.25-33
• /
• 2000

This study represents three-dimensional turbulent flow characteristics around an axial fan measured at the operating point ${\varphi}=0.32$, which is equivalent to the maximum flowrate region, by using three-dimensional fiber-optic type LDA system. This LDA system is composed of a 5 W Argon-ion laser, two optics in back-scatter mode, three BSA's, a PC, and a three-dimensional automatic traversing system. A kind of paraffin fog is used for laser particles in this study. Mean velocity profiles around an axial fan along the downstream radial distance show that the streamwise and the tangential components exist as a predominant velocity and have the maximum value at the radial distance ratio 0.8, while the radial component has a small scale distribution and its flow direction is inward except a part of blade tip. The turbulent intensity profiles show that the radial component exists the most greatly. And also the turbulent kinetic energy shows about 60% as a maximum value at the radial distance ratio 0.9. Moreover, the Reynolds shear stresses do not exist at upstream flow, but the streamwise and the radial components of them show about 20% as a maximum value at the radial distance ratio 0.9 at downstream flow.

• Journal of Power System Engineering
• /
• v.12 no.5
• /
• pp.40-47
• /
• 2008

The turbulent flow characteristics around a small-size axial fan(SSAF) for a refrigerator are strongly dependent upon the operating points. Four operating points such as $\phi$ =0.1, 0.18, 0.25 and 0.32 were adopted in this study to investigate three-dimensional turbulent flow characteristics around the SSAF by using a fiber-optic type Laser Doppler Anemometer(LDA) system. Downstream mean velocity profiles of the SSAF along the radial distance show that axial and tangential velocity components exist predominantly, except $\phi$ = 0.1, and have a maximum value at $r/R{\fallingdotseq}0.8$, but radial velocity component having a relatively small value only turns flow direction to the outside or the central part of the SSAF. The turbulent intensity shows that the radial component exists most greatly after $r/R{\fallingdotseq}0.5$. Downstream turbulent kinetic energy at $\phi$ = 0.25 and 0.32 together has the largest peak value at $r/R{\fallingdotseq}0.9$.

• Nuclear Engineering and Technology
• /
• v.56 no.9
• /
• pp.3812-3825
• /
• 2024

The axial-flow-induced vibration of fuel rods in the nuclear power plant is closely related to nuclear safety. In this article, a numerical study is performed on vibration of two elastic cylinders arranged side-by-side in axial flow. Large eddy simulation is employed to predict the turbulent flow. The numerical method has been verified using the experimental root-mean-square vibration amplitude of a single cylinder. A wide range of inflow velocities u^*, incident turbulence intensity T_u and space ratio P/D have been examined, where D and P are the diameter and centre-to-centre distance of the cylinders, respectively. The results show that the vibration amplitudes increase with an increasing u^*, comparable to the case of a single cylinder in axial flow. However, the two cylinders could bend outwards during a relatively high u^* and low T_u. Although T_u significantly affects the amplitudes of the cylinders, it does not change the vibration frequency and the critical velocity at which buckling instability occurs. As the gap between the two cylinders is sufficiently small, the vibration amplitude enhances significantly due to the pronounced hydrodynamic interaction between the two elastic cylinders and surrounding fluid. The direction of buckling is no longer random but fixed.

• The KSFM Journal of Fluid Machinery
• /
• v.15 no.1
• /
• pp.58-63
• /
• 2012

Three dimensional unsteady numerical calculations were performed to investigate unsteadiness of the tip leakage flow in an axial compressor. The first stage of the four-stage low-speed research axial compressor was examined. Since this compressor has a relatively large tip clearance, the unsteadiness of the tip leakage flow is induced. Through the results from the unsteady calculations, the process of the induced unsteady tip leakage flow was investigated. It was shown that the leakage flow that occurred at a rotor blade tip clearance affected the pressure distribution on the pressure side near the tip of the adjacent blade, thus caused the fluctuation of the pressure difference between the pressure side and suction side. Consequently, the unsteady tip leakage flow was induced at the adjacent rotor blade. The unsteady feature of the tip leakage flow was changed as the operating point was moved. The interface between the tip leakage flow and the main flow only affected the trailing edge region at the design point whereas the interface influenced up to the leading edge at the low flow rate point. As the flow rate decreased, additionally, it was seen that the vortex size of the tip leakage flow increased and the relatively large length scale disturbance occurred. On the other hand, using frequency analysis, it was shown that the unsteadiness was not associated with the rotor speed and was about 40% of the blade passing frequency. This feature was explained in the rotor relative frame of reference, and the frequency decreased as the flow rate decreased.

• 유체기계공업학회:학술대회논문집
• /
• 2002.12a
• /
• pp.131-136
• /
• 2002

A computational analysis using Reynolds stress model in FLUENT is conducted to give a clear understanding of the effect of blade loading on the structure of tip leakage flow in a forward-swept axial-flow fan at design condition ($\phi$=0.25) and off-design condition ($\phi$=0.21 and 0.30). The roll-up of tip leakage flow starts near the minimum static wall pressure position, and the tip leakage vortex developes along the centerline of the pressure trough within the blade passages. Near tip region, a reverse flow induced by tip leakage vortex has a blockage effect on the through-flow. As a result, high momentum region is observed below the tip leakage vortex. As the blade loading increases, the reverse flow region is more inclined toward circumferential direction and the onset position of the rolling-up of tip leakage flow moves upstream. Because the casing boundary layer becomes thicker, and the mixing between the through-flow and the leakage jet with the different flow direction is enforced, the streamwise vorticity decays more fast with blade loading increasing. The computational results show that a distinct tip leakage vortex is observed downstream of the blade trailing edge at $\phi$=0.30, but it is not observed at $\phi$=0.21 and 0.25.

• International Journal of Fluid Machinery and Systems
• /
• v.4 no.2
• /
• pp.262-270
• /
• 2011

Experimental and numerical investigations were conducted for an internal flow in an axial flow stator of a diagonal flow fan. A corner separation near the hub surface and the suction surface of a stator blade was focused on, and further, three-dimensional vortices in separated flow were investigated by the numerical analysis. At low flow rate of 80% of the design flow rate, a corner separation of the stator between the suction surface and the hub surface can be found in both experimental and calculated results. Separation vortices are observed in the limiting streamline patterns both on the blade suction and on the hub surfaces at 80% of the design flow rate in the calculated results. It also can be observed in the streamline pattern that both vortices from the blade suction surface and from the hub surface keep vortex structures up to far locations from these wall surfaces. An attempt to explain the vortices within a three-dimensional separation is introduced by using vortex filaments.

• Transactions of the Korean Society of Automotive Engineers
• /
• v.10 no.4
• /
• pp.69-76
• /
• 2002

The aim of this study is to provide fundamental informations that make it possible to use a cool stream and a hot stream simultaneously. We changed the pressure of compressed air that flows into a tube, the inner diameter of orifice that a cold stream exits, and the mass flow rate ratio. And in each case, we measured the temperature of a cold stream and a hot stream in each exit of a tube. Also we measured the axial and the radial temperature distribution in internal spare of a tube. From the study, fellowing conclusive remarks 7an be made. First, As the number of nozzles increase, separation point move into the hot exit. Second, When we use guide vane type nozzle, the axial temperature distribution constant over the 0.75 of air mass flow rate radio. Third, When we use Spiral type nozzle, axial and radial temperature distribution in the inner space is higher than another nozzle. Fourth, Axial and radial temperature distribution in the inner space vortex-tube is determined by separation point. And separation point is moved by changing of air mass flow rate ratio. At last, A heating apparatus is possible far vortex-tube to use.

• 유체기계공업학회:학술대회논문집
• /
• 2006.08a
• /
• pp.213-216
• /
• 2006

This paper was a study about noise reduction through flow stabilization in duct using experimental method and numerical analysis at the same time. To determine the fan's type three kinds of fans(axial fan, centrifugal fan, and axial fan with centrifugal type blades) was examined to investigate the suitability for in-line duct. As a result, under the equal number of rotation 2000 RPM, performance of an axial fan with centrifugal type blades was the most superior by 55dBA at 4.3CMM among other fans. After this, analyzed the results of the numerical analysis to find out the optimum design of pitch angle such as $0^{\circ}$, $10^{\circ}$, $15^{\circ}$ and $20^{\circ}$. The intensity of turbulence was low when pitch angle was $15^{\circ}$ and air volume became peak by 5.08 CMM. It was observed that axis component of velocity increased gradually when pitch angle increased from $0^{\circ}$ to $20^{\circ}$, and embodied noise reduction and improvement of air flow rate through flow stabilization.

• Coupled systems mechanics
• /
• v.1 no.3
• /
• pp.285-295
• /
• 2012

In this paper, two types of porous burners with radial and axial flow have been modeled numerically and compared. For this purpose, governing equations were solved one-dimensionally for methane-air premix gas. The mechanism used in simulating combustion phenomenon was 15 stage reduced mechanism based on GRI3.0. In order to compare the two burners, the inlet flow rate and fuel-air ratio have been assumed equal for the two burners. The results of the study indicated that reduction in speed and increase in cross-section area in the direction of flow have a considerable influence on the behavior of radial burner in comparison to axial burner. Regarding temperature distribution inside the burner, it was observed that the two above mentioned factors can be influential in temperature of flame propagation region. Also, regarding distribution of CO and NO emission, the results indicate that the porous radial burner has lower emissions in comparison to the axial once. The output radiative heat transfer efficiency of the two burners was also compared and in this case also even the radial porous burner was found to be preferable.