Flow Characteristics of Backward Curved Centrifugal Fan with Rectangular Casing

E3 - Flow Characteristic of Centrifugal Fans

The flow characteristics of the backward curved centrifugal fan with a rectangular casing are investigated with experiments and simulations. The performances of the fans are examined with the experiments. The pressure, the efficiency and the noise characteristics were measured. The simulations are made by the commercial code. The numbers of elements are about 3,000,000 of tetrahedrons. The turbulent model is SST. The steady and unsteady flows were simulated.

At first, the influence of the length of the rectangular casing is examined. The section geometry of one casing is squared and another one is rectangular that is extended with the straight duct. The position of the impeller is centered of the square casing at both ones. It is cleared that the performance of the fan is a little decreased with the casing compared with the impeller only. The total pressure and the efficiency of the fan become low with casing. But they are not influenced with length of the casing. The pressure on the casing varies with the section geometry that the pressure decreased at the narrow passage and increased at the wide passage in the casing. Then the pressure distribution has third harmonic mode for circumferential direction. The pressure out of the impeller varies mainly with the distortion of the net flow that the pressure decreases only on the outlet of the fan, but increases on the opposite of the outlet. Then the distribution has first harmonic mode for circumferential direction. When the net flow out of the impeller increases, the pressure of the fan decreases. That is, the net flow distortion is closely relating to the pressure out of the impeller. The discrete frequency noise of the fan is mainly generated by the interaction of the distorted net flow and the blades.

Second the influence of the position of the impeller in the casing is examined. The position of the impeller was varied to three types for the square casing. The location of the narrow passage in the casing is varied with them. The first one is moved the impeller toward to the inner part of a casing. The second one is moved it perpendicular to the first one. The third one is moved to both directions. The performances of the fan are varied only at the second one. The pressure and the efficiency of second one are increased compared to the original (centered) position. At all cases, the efficiency in the impellers is almost same. But it is varied in the casing at second one. This is caused by the improvement of the flow in the casing.