Investigation On Unsteady Rotating Flow Field By Axial Fan In A Short Air Duct

Axial fans with short air ducts have been extensively applied in industry. The flow field rotation and fluctuation induced by the impeller can result in a difficult measurement on airflow rate and wind pressure of the cross section. Therefore, it is meaningful for guiding these measurements to research the pressure and velocity distribution of the fluctuated flow filed.Experiment was conducted on a axial fan to measure the static pressure near the air-duct wall before impeller and dynamic pressure of different radii on the outlet cross section. An numerical model was formulated to research the fluctuation, influences of period and radial and axial distributions of pressure and flow angle of different radii on multiple cross sections. Also, investigations on the relationship and development in axial direction between the cross-section average pressure and near-wall pressure were made. The axial fan is of a 2.4m diameter, four blades and works at 300r/min. Its air duct is 2.485 m in length, of which length before impeller is 0.6m and after impeller is 1.885 m.research shows that pressures of different radii on multiple cross sections are varying with the same period and of greater amplitudes near impeller in air duct axis. It was found that frequency corresponding to the maxmium RMS is related to the product of rotate speed and blade amount. Radial distribution of the fluctuated pressure on cross sections before impeller were seen a minimum value at 0.83R(R represents the radius of air duct) roundly. In the whole air duct, flow angles of different radii on multiple cross sections have the same period. Amplitudes of flow angles further away from impeller are less, which is less than 5° after 0.5D(D represents air duct diameter) in axial direction.before impeller, the ratio of cross-section average pressure and near-wall pressure are about 0.9. on the cross sections beyond 0.5D in axial direction, the ratio are about 1.2.