• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.3 s.234
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• pp.322-329
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• 2005

The flow characteristics in the blade passage and in the wake region of a low speed axial flow fan have been investigated by experimental analysis using a rotating hot-wire sensor for design and off-design operating conditions. The results show that the tip leakage vortex is moved upstream when flow rate is decreased, thus disturbing the formation of wake flow near the rotor tip. The tip leakage vortex interfaces with blade pressure surface, and results in high velocity fluctuation near the pressure surface. From axial velocity distributions downstream of the fan rotor, large axial velocity decay near the rotor tip is observed at near stall condition, which results in large blockage compared to that at the design condition. Although the wake flow downstream of the rotor blade is clearly measured at all operating conditions, the trough of the high velocity fluctuation due to Karmann vortex street in the wake flow is mainly observed at a higher flow condition than the design flow rate.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.1377-1382
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• 2004

Flow fields in a half ducted propeller fan have been investigated by three-dimensional Reynolds-averaged Navier-Stokes (RANS) simulations and a vortex core identification technique. The simulation at the design operating condition shows that the tip vortex onset point is located at 30 percent tip chord of the suction surface on the blade tip. There is no interaction between the tip vortex and the adjacent blade, so that the tip vortex smoothly convects to the rotor exit. However, the high vorticity in the tip vortex causes the wake and the tip leakage flow to be twined around the tip vortex and to interact with the pressure surface of the adjacent blade. This flow behavior corresponds well with experimental results by Laser Doppler Velocimetry. On the contrary, the simulation at the low-flowrate operating condition shows that the tip vortex onset point is located at the 60 percent tip chord of the suction surface. In contrast to the design operating condition, the tip vortex grows almost tangential direction, and impinges directly on the pressure surface of the adjacent blade.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.7
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• pp.883-892
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• 2003

The experiment using three-dimensional laser Dopperr velocimetery (LDV) measurements and the computation using the Reynolds stress model of the commercial code, FLUENT, were conducted to give a clear understanding on the structure of tip leakage flow in a forward-swept axial-flow fan operating at the maximum efficiency condition. The tip leakage vortex was generated near the position of the minimum wall static pressure, which was located at approximately 12% chord downstream from the leading edge of blade suction side, and developed along the centerline of the pressure trough within the blade passages. A reverse flow between the blade tip region and the casing, induced by tip leakage vortex, acted as a blockage on the through-flow. As a result, high momentum flux was observed below the tip leakage vortex. As the tip leakage vortex proceeded to the aft part of the blade passage, the strength of tip leakage vortex decreased due to the strong interaction with the through-flow and casing boundary layer, and the diffusion of tip leakage vortex caused by high turbulence. In comparison with LDV measurement data, the computed results predicted the complex viscous flow patterns inside the tip region, including the locus of tip leakage vortex center, in a reliable level.

• The KSFM Journal of Fluid Machinery
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• v.2 no.3 s.4
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• pp.45-51
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• 1999

Centrifugal fans are widely used and the noise generated by these machines causes one of the most serious problems. In general, the centrifugal fan noise is often dominated by tones at BPF(blade passage frequency) and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cutoff in the casing. However, only a few researches have been carried out on predicting the noise due to the difficulty in obtaining detailed information about the flow field and casing effects on noise radiation. The objective of this study is to develop a prediction method for the unsteady flow field and the acoustic pressure field of a centrifugal fan and to calculate the effects of rotating velocity, flow rate, cut-off distance and the number of blades and its effects on the noise of the fan. We assume that the impeller rotates with a constant angular velocity and the flow field around the impeller is incompressible and inviscid. So, a discrete vortex method (DVM) is used to model the centrifugal fan and to calculate the flow field. The force of each element on the blade is calculated with the unsteady Bernoulli equation. Lowson's method is used to predict the acoustic source. The cut-off distance is the most important factor effecting the noise generation. Acoustic pressure is proportional to 2.8, which shows the same scaling index as the experimental result. In this paper, the cut-off distance is found to be the dominant parameter offecting the acoustic pressure.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.24 no.1
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• pp.114-123
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• 2000

The flow structure around a rotating axial-fan was experimentally investigated using a phase averaging velocity field measurement technique. The fan blades were divided into 4 different phases, for which 500 velocity fields were acquired for each phase angle with a 2-frame PTV system. Velocity field measurements were also carried out at two planes parallel to the axis of rotation, with offsets toward the radial direction of the fan. For accurate synchronization of the PTV system with the phase of the axial fan, two synchronization circuits were employed with a photo-detector attached to the rotating shaft. The phase averaged velocity fields show periodic variations with respect to the blade phase. The periodic formation of vortices at the blade tip is also observed in vorticity contour plots. Locations of local maximum turbulence intensities in the axial and radial directions are found to be located in an alternating pattern. These experimental results can be used to validate numerical calculations and to understand the flow characteristics of an axial fan.

• The KSFM Journal of Fluid Machinery
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• v.8 no.3 s.30
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• pp.26-32
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• 2005

One of noticeable features in the cross flow fan is that a working fluid passes through impeller blade twice without distinction between the inlet and exit angles. Also, it does produce higher circumferential velocity than other types of blade at the same flow rate in accordance with the application of the forward curved shape. However, a design theory for the cross-flow fall has not yet been formed owing to an eccentric vortex, which is the remarkable characteristics, occurred in a cross-flow fan. Furthermore, the eccentric vortex, which is difficult to control the size and position, is the important cause of performance decrease. In this study, experiments we carried out to estimate the similarity of the cross-flow fan with various scales and rotational velocity changes. Pressure coefficients to flow coefficients with various scales of the cross-flow fan are plotted to the application of the general similarity law of the turbomachinery in the cross-flow fan with Archimedes spiral, which is the important factor having an effect on it.

• 유체기계공업학회:학술대회논문집
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• 1998.12a
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• pp.115-120
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• 1998

Aerodynamic noise generated by automobile cooling fan is investigated. Automobile cooling fans radiate both discrete frequency noise as well as broadband noise. In the present work, the former is considered through free-wake panel method coupled with acoustic analogy fully considering the retarded time variation on the blade surface, while the latter is taken into account by three well-established broadband noise components. Experiments were performed to supplement necessary inputs as well as to provide the final comparison with the predicted noise spectrum. The predicted noise levels at blade passing frequencies agree well with the experimental data for the first few harmonics. Although the predicted broadband noise levels at higher frequencies fall below the experimental data due to the fundamental shortcomings of the utilized formulations, the analysis offers a detailed physical understanding of the fan noise generation processes.

• 유체기계공업학회:학술대회논문집
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• 1998.12a
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• pp.105-114
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• 1998

Flow field and near-field noise of a centrifugal fan has been studied with an efrcient compressible method and STAR-CD. The flow field of the centrifugal fan is assumed two-dimensional. Most of the compressible studies has been done by inviscid solver because viscous simulation shows little difference. The near field noise is estimated in term s of sound pressure level in frequency domain transformed from the computed pressure fluctuations using FFT. The simulation has been done on various design elements such as impeller blade shapes, the number of blades and cut-off clearance. The comparison shows that the number of blades has a significant effect on near-field noise without losing aerodynamic performance.

• Journal of computational fluids engineering
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• v.8 no.4
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• pp.50-57
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• 2003

Cross-flow fans are widely used in various applications, due to their large capacity of mass flow and size compactness. The flow field of the cross-flow fan is, however, complex and has many design parameters. Thus, the general design guide has not been sufficiently established yet and the design strategies of cross-flow fans have been mostly based on experiments. In the present study, the performance and their two-dimensional flow characteristics are numerically analyzed by using the STAR-CD(commercial computational fluid dynamics code). The simulation is done by varying the several design parameters such as the impeller blade shapes and the gap between the stabilizer and impeller. The computational results are compared with the experimental data at the fan outlet region. Finally, some helpful guides for the optimum design of cross-flow fans are proposed.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2002.11b
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• pp.658-664
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• 2002

Unsteady flow characteristics and associated blade tonal noise of a cross-flow fan are predicted by a computational method. The incompressible Navier-Stokes equations are time-accurately solved for obtaining the pressure fluctuations between the rotating blades and the stabilizer, and sound pressure is predicted using Curie's equation. The computed fan performance is favorably compared with experimental data, and also indicates that the performance is not significantly altered by the random pitch effect at ${\phi}>0.4$. In the present study, the narrow-band noise characteristics of three impellers with a uniform and two random Pitch (type-A and-B) blades are compared by the SPL (Sound Pressure Level) spectra, and their frequency modulation characteristics of the BPF (Blade Passing Frequency) noise are also discussed.