Gaining Turbomachinery Insight Using a Fluid Structure Interaction Approach

Existing research studies for the corresponding flow-induced vibration analysis of centrifugal pumps are mainly carried out without considering the interaction between fluid and structure. The ignorance of fluid structure interaction (FSI) means that the energy transfer between fluid and structure is neglected. To some extent, the accuracy and reliability of unsteady flow and rotor deflection analysis should be affected by this interaction mechanism.

In recent years, more and more applications of FSI are found in the reliability research of turbomachinery. Most of them are about turbines, and a few of them address pumps. Kato [1] predicted the noise from a multi-stage centrifugal pump using one-way coupling method. This practical approach treats the fluid physics and the solid physics consecutively.

cent-pump
Figure 1: Multistage centrifugal pump [1].
In the CFD computations of the internal flows, Kato could successfully predict the pressure fluctuations despite turbulent boundary layer in the impeller passages was not resolved. The computed pressure fluctuations on the internal surface agreed well with the measured ones not only at the blade passing frequencies, (BPF) but also on the base level. By visualizing the distributions of the pressure fluctuations at the BPFs, it was found that the fluctuation was especially high at the second harmonics of the BPF. This was consistent with the vibration velocity measured on the outer surface. On the other hand, he overpredicted the total head by about 10%. This is because turbulent boundary layer in the impeller passage was not resolved, and therefore, the blockage effect was not taken into account appropriately at this stage of the research.

Vibration of the structure portion was then calculated by a dynamical structural analysis with the calculated pressure fluctuations on the internal surface as input data. It was clearly shown that the dominant vibrations of the pump originate from the rotor-stator interaction. The trivial vibrations were damped off over time. The vibration levels of the BPF on the outer surface of the pump structure agreed reasonably well with the  measured ones. The computations revealed the feasibility of the fluid-structure coupled simulation for flow-induced noise generated in turbomachinery.

Another example of fluid-structure interaction was presented by Pei et. Al [2] when an axial-flow pump device with a two-way passage was studied. A coupled solution of the flow field and structural response of the impeller was established using a two-way coupling method to study the distribution of stress and deformation in the impeller and quantitatively analyze that on the blade along the wireframe paths had different flow rates. This studied showed that the maximum equivalent stress and maximum total deformation in the impeller are greatly influenced by flow rate, and its values drops with an increasing flow rate and a decreasing head. In addition, the total deformation in the impeller is greater near the blade rim, where the maximum value can be found. The equivalent stress is greater near the blade hub, where the maximum value can be obtained.

The above studies are the best proof that by using the right methods, tools and expertise you can get an insight for any kind of turbomachinery. Try AxSTREAM using the CFD and FEA integrated modules to design your machine and understand the fundamentals of its operation in depth.

References:

[1] Prediction of the Noise From a Multi-Stage Centrifugal Pump, Chisachi Kato, Shinobu Yoshimura, Yoshinobu Yamade, Yu Yan Jiang, Hong Wang, Ryuta Imai, Hiroyuki Katsura, Tetsuya Yoshida and Yashushi Takano , ASME 2005 Fluids Engineering Division Summer Meeting, Volume 1: Symposia, Parts A and B, Houston, Texas, USA, June 19–23, 2005

[2] Fluid–structure coupling analysis of deformation and stress in impeller of an axial-flow pump with two-way passage, Ji Pei, Fan Meng, Yanjun Li, Shouqi Yuan, Jia Chen, National Research Center of Pumps, Jiangsu University, Zhenjiang, China

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