The role of variable pitch in motion blades and variable rotational speed in an industrial fan stall

Abstract

Stall-induced vibration places fundamental limitations on industrial fan performance and remains a persistent problem in the development of industrial fans and compressors. In this paper, we present an experimental programme that utilised two strategies to study rotating stall recovery patterns in an industrial fan. The experimental facility incorporated a fan with both variable pitch in motion blades and a variable speed drive. The facility enabled us first, to study the evolution of unsteady pressure signals on the fan casing whilst varying fan blade pitch and then to examine varying fan rotational speed. Consequently, we were able to recover a fan from stall either by varying blade pitch at constant speed, or by varying speed at constant blade pitch. We also studied the physical flow phenomena associated with stall recovery by cross-correlating signals from circumferentially offset high frequency response pressure transducers and then analysed the cross-spectra. This enabled us to gain insight into the transient fluid flow behaviour associated with stall recovery in the studied industrial fan class. The transient fluid flow behaviour associated with stall recovery by varying blade pitch was indicative of both a mild and progressive transition. In contrast, stall recovery by varying fan speed occurred more suddenly. In comparison with the variable pitch transition, the variable speed transition was more indicative of recovery from mild surge. From this we may conclude that stall recovery via variable speed resulted in significantly higher unsteady mechanical stress induced in fan rotating components than recovery via variable pitch. Higher bending stresses in the fan blades are more likely to result in mechanical failure, and therefore we may conclude that the studied fan is more ‘stall tolerant’ when one operates it as a variable pitch fan rather than as a variable speed fan.