A Global Approach for Fan Flutter Identication
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Due to progress in CFD (Computational Fluid Dynamics), it is now possible to compute and analyzesteady ow and phenomena for turbomachine design. Unsteady instability predictions are important tocertify that a turbomachine will not encounter high vibration levels in operation. Flutter is one of the mostcommon fan instabilities. Thus, the fan design is bound to respect a given Flutter Margin. It guaranteesa certain operation envelope for the engine and its fan, refered as operability. The operation envelope ofan engine is dened in the fan map mass ow rate - pressure ratio as the space in which the engine can berun in during operation. The fan map is made of isovelocities. An isovelocity is a line described by varyingmass ow rate and keeping constant the rotational speed. This domain is bounded by phenomena such asrotating stall, surge, utter, etc which are hazardous for engine mechanical integrity. Fig. 1 highlights howan operating envelope is bounded. When operation envelope is too small, the fan blade geometry needsto be modied to improve its utter behavior and therefore increase the size of the envelope. Eciency,pressure ratio and operability can be strongly impacted by the changes made. Therefore design parametersinuencing utter must be precisely spotted. This can be done if mechanisms which trigger utter are wellunderstood. Thus the blade can be reshaped to lower the contribution of a given phenomenon. However,when a phenomenon is identied, one should quantify its contribution to the globally stable or unstablebehavior. In fact, to reduce the geometrical changes and therefore consequences on operability, one shouldact on the most critical phenomenon.The study has been performed on a single fan blade with dierent congurations of back pressure androtational speed. Consequently, two kinds of utter are investigated : stall utter and transonic utter. Therst one occurs at low rotational speed. It corresponds to zone 1 in Fig. 1. It is commonly driven by owseparation. As described in,1 separation on the suction side of a blade can be responsible for utter. Article1shows that unsteady pressure and blade motion are out of phase in the separated zone. Furthermore, studiesin2 reveal that traveling Mach waves on the blade surface can destabilize it. It depends on the phase betweenthe waves and the blade motion. Acoustic interference is also studied in.1 Transonic utter occurs at higherrotational speed when the blade is shocked. It corresponds to zone 2 in Fig. 1. There are two main sourcesthat destabilize the blade: interaction between the shock waves and the boundary layer and the shock waveoscillation as presented in.3 That study divides the prole into four parts. Each part corresponds to a givenmechanism: supersonic part (stabilizing), shocked part (both stabilizing and destabilizing if the shock wavesoscillates), downstream the shock (destabilizing due to separation) on the suction side and the pressure side(stabilizing).The objective of this paper is not only to identify mechanisms responsible for utter at low and high rotationalspeed but also to follow their evolution along an isovelocity. A global approach from the mechanismsidentication to the quantication of the phenomenon is then described.The critical mechanisms responsiblefor fan blade utter for a given conguration can be pointed out.
Place, publisher, year, edition, pages
2013. , 11 p.
Trita-AVE, ISSN 1651-7660 ; 2013:01
IdentifiersURN: urn:nbn:se:kth:diva-121354OAI: oai:DiVA.org:kth-121354DiVA: diva2:618564