Gaku Minorikawa

Experimental Research for Performance and Noise of Small Axial Fan

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.

Investigation of Characteristic of Flow Induced Vibration Caused by Turbulence Relating to Acoustically Induced Vibration

In the flare piping system, it is known that piping vibrations occur caused by Acoustically Induced Vibration (AIV) and Flow Induced Vibration (FIV) corresponding to high flow rate, high pressure drop and relatively thin pipe wall thickness. For FIV, turbulence generated at combining tee with high fluid velocity results in low frequency piping vibration. For AIV, large noise produced through a component with large pressure drop results in high frequency piping vibration. Carucci and Mueller shows the several cases with piping failure due to AIV and most of these cases the piping failure occurred at the combining tee. In these piping failure cases, the velocity at the combining tee would be quite high close or equal to sound speed and this means piping vibrations could occur due to FIV in addition to AIV.This paper shows the investigation results of FIV at combining tee with 90 degrees using experimental data. The results are compared to the previous study results for 45 degrees combining tee and the difference between 90 and 45 degrees tees are discussed in the view points of pressure fluctuation and piping vibration. This paper also shows that the vibration index proposed by authors is quite effective to evaluate the vibration level caused by FIV for both of 90 and 45 degrees tees. This proposed vibration index is applied to failure and no failure cases presented in Carucci and Mueller paper with some assumptions and it is suggested that the vibration indexes for failure case is relatively higher than those of no failure cases. And this suggests that not only AIV but also FIV could affect the piping failure reported in Carucci and Mueller paper.Copyright

Risk Evaluation Method of Acoustically Induced Vibration (AIV) Based on Random Vibration Theory

The widely recognized design criteria for Acoustically Induced Vibration (AIV) is basically based on the past failure examples reported by Carucci and Muller. However, the validity of these AIV criteria has not been fully investigated theoretically. The authors had developed the evaluation method for the Flow Induced Vibration (FIV) caused by turbulence in the pipe downstream of the combining tee. In this study, this FIV evaluation method developed by the authors was applied to evaluate AIV risk since both AIV and FIV cause random vibrations of the piping shell mode and the difference between AIV and FIV is just the source of the excitation force. The results of this AIV evaluation method indicates that the AIV risk increases as the pipe diameter decreases and this tendency agrees with the experiment result and conventional acoustic calculation. This AIV evaluation method can introduce the allowable power level limit which has no contradiction with the past failure examples reported by Carucci and Mueller. From the considerations mentioned above, the proposed method would be useful as one of the risk screening of AIV.Copyright

Design and Prototyping Micro Centrifugal Compressor for Ultra Micro Gas Turbine

In order to investigate the design method for a micro centrifugal compressor, which is the most important component of an ultra micro gas turbine, an impeller having the outer diameter of 20mm was designed, manufactured and tested. The designed rotational speed is 500,000 rpm and the impeller has a fully 3-dimensional shape. The impeller was rotated at 250,000 rpm in the present study. The experimental results of the tested compressor with the vaned and the vaneless diffusers were compared. It was found that the vaned diffuser attained the higher flow rate than the vaneless diffuser at the maximum pressure ratio. In addition the maximum pressure ratio was higher for the diffuser having a larger diffuser divergence angle at the high flow rate. These results were compared with those obtained by the prediction method used at the design stage.

Study on sound quality evaluation of fan noise

This study is to establish new evaluation scales of sound quality which is more suitable for the human sense of hearing than the conventional evaluation scales such as A‐weighted sound‐pressure level. The authors focus on fan noise and have investigated the meaning space of its sound quality by hearing tests and factor analyses. The hearing tests were performed using semantic differential (SD) method and their test results were analyzed to extract dominant factors of the fan noise sound quality. The factor analyses, influence of noise level and 1/3 octave band level on sound quality of fan noise were also investigated. It was found that the sound quality of fan noise had two major factors; ‘‘offensive’’ and ‘‘booming‐powerful’’ factors. The evaluation of hearing test using a paired comparison method was performed in order to investigate the correlation of these two factors with physical quantities that represent noise level and quality such as A‐weighted sound pressure level and loudness level. Analysis o...

Noise from the flow around semi-cylinder with a spherical head and its flow pattern

Developments for silent vehicles of railway are expected in Japan as the high speed railway system has a less effects on the environment with saving energies on public transportations. However, boundary layer noise on a body surface of train and separated flow noise from a wake flow of train have not been studied completely to predict the aerodynamic noise of vehicles of railway. The purpose of this study were to make clear the detail characteristics of aerodynamic noise emitted from the longitudinal air flow around a semicylinder with a quarter spherical head as a simplified model of high speed train vehicles. On this experimental study we used the small size low noise acoustic wind tunnel and measured noise and flow patterns around some testing models. The main conclusions of this study are that the spherical tail part did not affect the noise decrease in case of the long semicylinder but that the tail part affected the noise decrease in case of the shorter one, and that these phenomena were induced by the difference of shear flow patterns in the cylinder’s wake.

Developments of low-noise contractive nozzle used in acoustic wind-tunnel

The acoustic wind tunnel which has the quietter air flow is important and very useful for developing quiet and agreeable vehicles, turbo-machineries and air-conditioning systems. Some aerodynamic noises around the measuring section contribute to the performance of quietness in an acoustic wind tunnel, in which the noise from fans as the air-source is almost fully reduced. The object of this study is to decrease back ground aerodynamic noise in a measuring section of acoustic wind-tunnel in which the air-flow velocity is 20m/s to 80m/s. Some aerodynamic noise-sources around the measuring section in acoustic wind-tunnel had been experimentally studied and contributions of each sources to back ground aerodynamic noise were made clear. On measuring section of general acoustic wind-tunnel, dominant aerodynamic noise sources for background noise are vortex-releasing sound from nozzle edge, jet flow noise, and boundary layer sound on diffuser. Therefor performance of the contractive nozzle are important for developing quietter acoustic wind-tunnels. And some methods for decreasing aerodynamic noise from nozzle blowing subsonic flow have been investigated. The back ground flow-noise in the measuring section are decreased as about 6dB(A) by installing many small slender semi-cones on inside surface of the nozzle-end.