Impellers with two-dimensional blades are mainly employed in a centrifugal fan. The blade profile can be defined according to the distribution of flow parameters such as specified loading (velocity) and circulation. Here, velocity distribution is closely related with the increase of the boundary layers on the blade, flow separation, secondary flows and pressure pulsation, etc. So, it directly determines and controls the internal flow through the impeller. Thus, what is the most important in the blade design is to determine proper velocity distribution.
According to the literature, it is impossible to avoid secondary flow because of turning and curvature inside blade passage. However, there are several ways of decreasing the influence of secondary flow. Those are increasing the curvature radius of inlet vane and meridian surface, prevention of overlapping of the position of maximum vane curvature and the position of maximum meridian curvature, and controlling the enlargement of boundary layers inside flow passage.
A number of researches on the relative velocity distribution law of a centrifugal impeller have been conducted, but practical applications in the impeller design have not been reported. Instead, most impeller design methods considered average velocity distribution only. Velocity distribution on the lower surface, however, has an important influence on the flow pattern of impeller passage.
Kim Won Il, a section head at the Faculty of Aerospace Engineering, has proposed a calculation method for improving the impeller performance with flow control parameters such as reduction ratio of inlet area and maximum reduction ratio of mean relative flow velocity, and inlet gradient of rear relative flow velocity, and verified its effectiveness through CFD simulations.
The simulation result shows the following.
First, it is better to reduce the decrease in average flow relative velocity of gas flow, since it decreases the attack loss in the blade inlet. On the other hand, it is better to slightly increase the average relative velocity, since it suppresses the flow separation in the blade outlet. In addition, appropriate inlet gradient on the lower surface and position of maximum loading guarantees small change in the relative velocity of a real gas throughout the whole flow passage from the blade inlet to the outlet, and minimization of the thickness of the boundary layers.
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