Visualization of Impeller Internal Flow During Full Closing of Turbo Type Centrifugal Blower

Abstract

Study on centrifugal blower used for dust collection, air conditioning and other applications in homes, factories, etc. In this research, turbo type models were targeted. Its characteristic is to send high pressure air with high efficiency. However, its disadvantage is to generate large noises. It aims to elucidate this noise generation mechanism. In recent research, noise generation mechanisms other than the fully closed state have been elucidated. However, the experiment is difficult in the fully closed state, so research has not progressed much. Therefore, in this research, we conducted a visualization experiment in the fully closed state, and clarified the noise generation mechanism. Visualization experiments were carried out by injecting powder from the outer edge of the casing. It was clarified that the noise is generated by collision with the trailing edge of the blade. We also clarified that the collision against the trailing edge is caused by the relative flow caused by the difference between the impeller speed and the circulation flow velocity. In addition, we clarified that the circulation flow velocity decreases the most in the vicinity of the cut-off. As a result, it was found that the noise source in the fully closed state is in the vicinity of the blade trailing edge portion and the cut-off portion. Based on these results, the noise generation mechanism in the fully closed state is shown below. When fully closed, air does not flow between the blades. Therefore, the space between the blades is equivalent to the cavity. As the impeller rotates, the air inside the cavity swirls. Relative flow is involved in this vortex. Thereafter, the relative flow collides with the trailing edge of the blade, causing a pressure rise. This creates a pressure difference across the blades. Noise is generated by rotating the blades with pressure difference.