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Small axial fans have become widely used as cooling devices in recent years. Because of their increasing importance, studies have been conducted on ways to improve the performance and reduce the noise of such fans. In this report, a small axial fan with a diameter of 85 mm (a type popularity used in personal computer or workstation) was selected for further examination. The influence on aerodynamic performance and noise of such frame design parameters as blade tip clearance results in a decrease of discrete frequency noise and an increase of broad-spectrum noise. As for the most suitable design refinement in terms of fan efficiency, we found that the treatment of outlet corner roundness and altering spoke skew to the direction counter to that of fan rotation was effective. Keywords : Axial Flow Fan, Performance, Efficiency, Noise, Frame 1. Introduction Small axial fans have taken an important place in thermal design as the electronic products have become increasingly miniaturized. Concurrently, huge numbers of these inexpensive fans have been produced for use in products such as personal computers. And, while improvements in the mechanical performance of a single small fan would not make much difference in immediate environmental terms, given the immense number of such fans, there are increasing demands from society that designers ensure maximum levels of efficiency. Several reports have been published about improving the efficiency and reducing the noise from axial fans, but these reports address large fans with diameters of 300 mm or greater. Kamaya et al. examined the characteristics of a small fan with a diameter of 120 mm, but few other researchers have published reports on fans of this size. The objective of the research by Kamaya et al. was reduce the noise emitted by the fan adjusting the clearance between the fan blades and the frame, and the shapes as seen from in front of the fan. In addition to the shape of the blades, determining the optimum configuration and dimensions of the frame are essential considerations for improving efficiency and reducing the noise of an axial fan. However, due to their low Reynolds number (Re≈10

Small fans are especially used in information device for its cooling. Since packaging density has been increasing, requirements of not only performance improvement but also noise reduction arise strongly. An experimental research on small multi-blade fan was carried out in order to improve performance and reduce noise. In this report, effects of scroll angle, tongue clearance and inlet diameter on small multi-blade fan with impeller diameter of 100 mm were investigated. The optimal index efficiency and noise were found by making a scroll starting point radius and a scrool termination radius into a design index, instead of the scrool design by a conventional clearance and a scroll angle. In order to reduce noise, we found that we should set the radius of scroll beginning into about 0.65 times of the outlet diameter of the impeller. And for improving efficiency, we should set the radius of scroll end into about 0.9 times of the outlet diameter of the impeller.

For decreasing the noise of turbo-machineries, making clear the generation mechanism of fluid-dynamic noise is important and an air flow having quiet noise is very useful for the study of aerodynamic noise. A "Quiet Flow Wind Tunnel", whose measuring section is in the annechoic chamber, has been developed and established. The maximum velocity of air flow from the nozzle with a section size of 400 mm×400 mm is 42 m/s and its turbulence is less than 0.5 %. The noise levels around the measuring section are less than 48 dB(A) at a velocity of 35 m/s. By using this facility, further studies have been made, "vortex sound from crossing two cylinders", "flow-noise from finned-tube bank", "noise from ventilating-gallery", "noise of inner flow in straight ducts" etc.. It has been recognised, through these experiments, that this facility can measure both phenomena of fluid-dynamics and acoustics at the same time although these are still impossible by usual wind tunnels.

In the turbulent wake behind and upstream cylinder in a uniform flow, the downstream cylinder was set in the form of an intersection with the upstream cylinder. It was found that the acoustic radiation from the two circular cylinders forming an intersection was far smaller than that of a single cylinder. The variation of the acoustic radiation was measured by varying the intersectional angle of the two cylinders, the point of mutual contact and the angle between the downstream cylinder and the uniform flow. Moreover, measurements were made on (1) the mean and fluctuating surface pressure distributions on both cylinders, and (2) the mean velocity profiles and fluctuating velocity distributions in the turbulent wake behind the upstream cylinder. The flow patterns were verified by oil-film and smoke-wire flow visualization. Then, the relationship between the flow and the noise generation was studied.

Unusual noise is sometimes caused by tube-type heat exchangers employed to recover waste heat from a gas turbine. There are a few studies on Karman vortex-induced nosie from bare tubes or bare-tube banks placed in the air flow, but there is no study concerning Karman vortex-induced noise from finned tube banks. For the purpose of this investigation leading to reduce Karman vortex-induced nosie from heat exchangers, various finned tube models were made, and Karman vortex-induced noise was compared between this model and finned tubes used in a heat exchanger. As it was confirmed that there was an acceptable parallelism between the model and the finned tubes, the effect of interference plates on Karman vortex-induced noise was determined by varing the position and eccetricity of interference plates placed at the trailing end of the finned tube banks. As a result, unusual noise due to Karman vortex-induced nosie was successfully prevented.

Low-pressure centrifugal fans often produce low-frequency discrete noise, when operating in partial discharge. In this study, an attempt was made to find the cause of low-frequency discrete noise, using various parameters, and as a result, it was demonstrated that the low-frequency noise was ascribable to the rotating stall. Comparative experiments with airfoil vanes and circular thin ones showed that the frequency of low-frequency noise due to the rotating stall was given by the equation fs = 0.72 ni, where n = rotating speed and i = number of cells and that the number of cells was variable depending on the operating point.

Study on noise reduction of the axial-flow fans was carried out, varying principal assumed parameters such as the chord length of the blade, the camber of the airfoil, the position of maximum camber, and the surface pressure distribution of the airfoil. A separated flow on the surface of the tow-dimensional airfoil was observed with use of the Schlieren divice, and measurements were made of the strength of turbulence with use of a hot wire probe so as to determine their correlation with fan noise. The airfoil profile was determined by calculation using an optimum pressure distribution. As a result, the relations between the parameters and fan noise reduction have been clarified.

The performance and noise level of the high specific speed airfoil fan are affected by the structural factors of its casing. Of such factors the following two are thought to produce particularly pronounced effects : (1) clearance and shape of the tongue; and (2) clearance and lap of the suction cone and mouth ring. The work presented here is a study to determine the effects of such structural factors. For experimental purposes two types of tongues were developed: a skew type slanting in the direction parallel to the vane outlet width; and a modified type having a clearance changed in the same direction as above. A series of experiments was performed in single suction type airfoil fans in which these parameters were incorporated. As a result, their effects on the performance, noise level and noise spectrum of the fan were clarified, thus enabling us to obtain useful information for fan design optimization.

In the turbulent wake of the upstream cylinder in a uniform flow, the downstream cylinder is set in the form of a cross with the upstream cylinder and the point of mutual contact comes at the center of the test section. It is found that the acoustic radiation from the two circular cylinders forming a cross is far smaller than that from one cylinder. In the contents of the noise reduced by adding the downstream cylinder, the broadband noise as well as the dominant frequency tone is reduced. The variation of the acoustic radiation is measured as affected by the diametral ratio of the two cylinders, distance between them and velocity of the air.