Measurement and Prediction of Aerodynamic Stability of an Axial-Flow Ventilation Fan near the Stall Condition

This paper attempts to look at the unsteady flow pressure on the blade tip in order to quantify the inception of instability of an axial-flow ventilation fan. A test rig was set up to measure the unsteady static pressure near the rotor blade tip region by mounting several transducers on the outer casing. The data sampling system was configured for acquisition and post analysis. The measurement results showed that, as the throttle was closed, the flow disturbance appeared firstly at the rotor inlet. The subsequent aerodynamic instability of the fan was dependent upon the time evolution of such disturbances, which will notably be attenuated due to the viscous flow effect passing through the blade passage. The disturbance at the rotor inlet was shown to be enhanced as the throttle was progressively closed, and the unstable behaviour was triggered by the large disturbance at the rotor inlet. Using a 2D aerodynamic stability model for an axial-flow turbomachine, the off-design points and the stall margin for the axial-flow ventilation fan were numerically predicted and compared with experimental results.