Study the flow at the top gap of the axial fan impeller

The existence of the plexiglass barrel causes a problem that the intensity of the incident light is weakened, the reflected light returns to the receiving lens, and the incident optical path is deflected, so that the signal-to-noise ratio is greatly increased, and even serious problems such as difficulty in focusing and receiving no particle signal are caused. For this reason, the measuring window should not only have good light transmission, but also the shape should be as flat as possible. At present, there are three main methods for dealing with this problem at home and abroad: in order to reduce the influence of different media on the refraction, a square glass water tank is placed outside the plexiglass cylinder, filled with pure water to compensate for the influence of refraction, and the measurement window can be made. It changes from a circular surface to a flat surface. The relationship between the theoretical intersection of the laser beam and the actual measurement point is derived by theoretical derivation, and then the actual measurement point corresponding to the theoretical measurement point is calculated by the program. Here, the axial, circumferential, and radial position correction relation expressions are derived separately.

Groove at the entrance where the laser is incident, and the optical glass is attached to the slot. Plastic film was used at the beginning, but the noise in the measured data is relatively large. When the optical glass is used, the efficiency of the measurement data and the signal-to-noise ratio are greatly reduced. This method was used in this experiment. Due to the reflection of the wall, there is a section on the near wall that is a measurement blind zone. The near wall area (ie, the boundary layer area) has very important flow information, and the purpose of the experiment is to study the flow at the top gap of the axial fan impeller. Therefore, reducing the dead zone is an important issue in PDA measurement. The wall can be coated with a black light absorbing material to reduce the scattered light (the black leaves are also blackened); the filter can be used to filter out some of the scattered light; black paper on the upper and lower sides of the measurement window, or the wall surface The incident light is slightly inclined, and the reflected light is folded out of the receiving lens or the like. In addition, the wall should be processed to be smoother to reduce scattered light.