Axial Fan Monitoring by Pressure Transients Close to the Blades, a Preliminary Study

As the axial fan blades move within the fan case pressure transients are developed, which can be measured and qualified by a fixed point monitor. The nature of the pressure transient pattern observed close to the fan blade varies with the location on the fan performance curve. Measurement of the pressure fluctuation around the fan blade ring reveals the minute variation in pressure associated with the movement of the air within the fan. Such observations could be the basis for an on-line fan monitoring system based on the acoustic emissions from the fan blade. This paper describes a preliminary study that was conducted to evaluate the nature of the pressure transient profiles close to the blade of a laboratory axial flow fan. A comparison is made between the transient patterns measured in the fan blade/case gap and immediately behind the fan blade ring. The experimental work involves measurements of transient pressures at various points along the fan performance curve for a series of fan speeds. Of particular interest are those points measured as the fan approaches the stall point. Analysis of the data seeks to define a relationship between the fan performance and the observations. Finally, some thoughts as to how such measurement methods could be employed in fan monitoring systems are presented