ASME GT2009-59026 pdf free download.FATIGUE ANALYSIS AND LIFETIME ESTIMATION OF CENTRIFUGAL
COMPRESSOR IMPELLER BLADES.
A multistage centrifugal compressor installed in an airseparation plant was equipped with the inlet guide vanes (IGVs) at the first stage. The impeller blades of the first stage cracked when the compressor just ran about three months. A weakly coupled fluid-structure interaclion (FSI) is numencally calculated to analyie the high cycle fatigue (HCF) failure and estimate the litetimc of the impeller blades. In order 10 obtain the time dependent aerodynamic loads on the impeller blades. three-dimensional unsteady compressible viscous flow in the first whole stage of the centrifugal compressor is simulated. And then, a data interpolation tool is developed to transfer the physical information from fluid meshes to structure meshes. and also, time dependent data is transferred into frequency dependent data using Fast Fourier Transform (FF1’). The free vibration mode and forced vibration response of the impeller blades are calculated using finite clement method iFEM and the cyclic symmetry structure of the rotating impeller is modeled by a single segment. According to the numerical result, the HCF littinw of the impeller blades is estimated using a local stress-strain method. Numerical result shows that the predominated frequency of unsteady aerodynamic loads acting on the impeller blades is equa’ to the frequency of the IGVs-impcllcr blades interaction. Both the predicted stress concentration region and lifetime of the impeller blades agree reasonably well with factual fracture case. This study shows that it is feasible to use the weakly coupled FSI simulation to predict the fracture region and lifetime of centrifugal impeller blades.
INTRODUCTION
The requirements for high reliability, high speed to turbomachines arc growing, and along with the extension of operating range for energy saving. For this reasons, in order to design a more compact structure of turbornachine, the impeller vibration induced by unsteady flow becomes a very important design aspect in many cases to prevent high cycle fatigue (HCF) failures.
Impeller blade is mainly subjected to three different loads:
ceninfugal force, aerodynamic force and thermal stress. Centrifugal force is time-dependent when the compressor operates in an acceleration motion, such as start-up, shut down and rotational speed adjustment. But at usual operating condition with constant rotational speed. centrifugal force just only generates static stress on impeller blades. Thermal stress is an important cause that generates IICF of steam turbine blades [lJ, but for the centrifugal comprcssor. the surface temperature of impeller blade is usually not more lSOt, and thus the aerodynamic heats effects could be neglected.
Unsteady aerodynamic force is the main source that drives the centrifugal compressor impeller into destructive vibration. There exist so many causes that generate and affect unsteady aerodynamic force on impeller blades, such as potential and viscous interaction, vortex separation, secondary flow and stall flow. et al. Many researchers have contributed to understand the unsteady flow mechanism of turbomachincs. The theories of unsteady aerodynamic forces in turbomachine stage are described by Rao 121. Fast response pressure transducers are usually used to measure the unsteady pressure of discrete points on the low rotational speed impeller blades 13-51. Recently. a more advanced porous pressure seflsitivC paint (PSP) technique has been successfully applied to measure the unsteady pressure on the whole surface of rotating blade [6].
With the rapid development of computational fluid dynamics (CFD) and computer hardware, numerical studies on the three-dimensional unsteady viscous compressible flow and aerodynamic forces in centrifugal compressor have been carried out using unsteady RANS equations [7,8]and large eddy simulation[9,10].