System environments vary for axial-flow fans, which are often used under unsuitable operating conditions. In the recent high density design of such equipment, obstacles are being more frequently distributed upstream of the fan to determine a smaller fan size. In this study, an attempt is made to clarify the influence of an obstacle on the flow characteristics of axial-flow fans. In our experiment, blockage disks with various diameters were placed upstream of a typical cooling axial-flow fan. The performance curves of the axial-flow fan with the blockage disks were measured. In addition, the flow instabilities and their cell structures were investigated under typical conditions. The main objectives of the present study are as follows: (1) to evaluate the performance degradation when obstacles are placed upstream of the test fan, (2) to elucidate the causes of a positive slope and reverse flow, and (3) to clarify the generation mechanism of flow instabilities when the blockage disks, which have a larger diameter than the test fan, are used.
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