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

A Reasonable Approach to Pump Design While Avoiding Resonance

For the majority of pump application, the growing use of variable speed operation has increased the likelihood of resonance conditions that can cause excessive vibration levels, which can negatively impact pump performance and reliability. Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations (external excitation source) matches the system’s natural frequency of vibration more than it does at other frequencies. To avoid vibration issues, potential complications must be properly addressed and mitigated during the design phase.

Some of the factors that may cause excitation of a natural frequency include rotational balance, impeller exit pressure pulsations, and gear couplings misalignment. The effect of the resonance can be determined by evaluating the pumping machinery construction. All aspects of the installation such as the discharge head, mounting structure, piping and drive system will affect lateral, torsional and structural frequencies of the pumping system. It is advised that the analysis be conducted during the initial design phase to reduce the probability of reliability problems and the time and expense associated.

Natural frequencies of a pump and motor can be calculated by performing a modal analysis using the Finite Element Method Analysis (FEA). The finite element modelling and analysis techniques provide an understanding of the mechanical system behaviour, including the natural frequency values during design phase.

Understanding the predicted natural frequency values allows an evaluation of the expected separation between the pump natural frequency and excitation frequencies, such as pump operation speed. The separation is established by the pump manufacturer to avoid mechanical resonance.

The boundary conditions assumed during FEA are essential to the accuracy of predicted results. In some cases, the final as-built conditions (such as foundation stiffness) significantly affect the analysis accuracy if they differ from those conditions assumed during the analysis. In such case a pump test is recommended. Tests like that indicate that increasing the natural frequency of the system is the best solution. This increase in natural frequency could be accomplished by modifying two of the pump system’s physical characteristics, reducing mass or increasing stiffness of the system.

It is therefore important to know the type of acceptable solution that will provide the best pump operation. And this is where AxSTREAM adds significant value at the design process. Using SoftInWay fully integrated engineering platform the customers are able to optimize the pumping machine, and next to perform all the necessary structural analysis using AxSTRESS, our express structural, modal and harmonic analysis FEM solver with a customizable, automatic turbomachinery-specific mesh generation.

Source:

http://empoweringpumps.com/lateral-torsional-structural-analyses-pumps-can-avoid-vibration-problems-pump-issues-related-resonance-conditions/

http://www.pumpsandsystems.com/topics/pumps/vertical-turbine-pumps/structural-resonance-problems-vertical-pumps

The Importance of Turbulence Modelling

What is the importance of turbulence modelling in capturing accurate 3D secondary flow and mixing losses in turbomachinery? An investigation on the effect of return channel (RCH) dimensions of a centrifugal compressor stage on the aerodynamic performance was studied to answer this question by A. Hildebrandt and F. Schilling as an effort to push turbomachinery one step further.

W. Fister was among the first to investigate the return channel flow using 3D-CFD. At that time the capability of commercial software was not extended and any computational effort was limited by the CPU-capacity. Therefore, only simplified calculations that included constant density without a turbulence model (based on the Prandtl mixing length hypothesis) embedded in in-house code, were performed.

Although separated flow without a predominant flow direction could not have been calculated, the method indicated separated flow regions with relatively accurate precision, and it predicted the magnitude of loss coefficients to a higher degree than experimental data. The study was further
simplified using incompressible flow, and an axial U-turn inlet flow.AxCFD

The biggest drawback of using inverse methods for return channel design refers to the question of appropriate flow distribution across the RCH surface. Furthermore, flow separation cannot be predicted with the help of singularity methods. In order to circumvent the problem of predicting flow separation, nowadays compressible viscous 3D-CFD applied with different highly complex turbulence modeling is the state of the art even at the conceptual stage of the design.

Hildebrandt and F. Schilling analyzed three different centrifugal stages regarding the return channel system performance. All three stages featured the same impeller type, two of them being applied with a 3D-RCH at different flow coefficient and one impeller being applied with a 2D-RCH system. The 3D-RCH stage featured both CFD calculated and measured superior aerodynamics over the 2D-RCH stage regarding the overall performance as well as regarding the outlet flow angle. The comparison between the measured and the CFD-predicted performance showed agreement both when it comes to overall performance (efficiency, pressure rise coefficient) and also regarding detailed flow field (outlet flow field). The 3D secondary flow and mixing losses of the entire domain downstream the vaneless diffuser were either underestimated or overestimated by the CFD-calculations, depending on the turbulence modeling and the impeller fillet radii-modeling which affects the RCH-inlet flow conditions. The effect of fillet radii-modeling on the RCH-exit flow angle spanwise distribution was found to be significant in order to better match the experimental results.

It is worth noting that the rather simple Spalart–Allmaras turbulence model provided better agreement with the measured RCH-exit flow angle distribution than the more sophisticated k-epsilon model, which on the other hand, outputted a closer fit with the measured surface vane pressure distribution. Regarding the RCH total pressure loss distribution, none of the models showed a perfect agreement with the measurement data.

Moreover, the incident losses of the 3D-RCH system seemed to play a minor role within the overall RCH-loss which is significantly dominated by the 3D-secondary losses.

Interested in learning more? Check out AxSTREAM and AxCFD!

[1] A. Hildebrandt and F. Schilling, 2017 “Numerical and Experimental Investigation of Return Channel Vane Aerodynamics With Two-Dimensional and Three-Dimensional Vanes”, Journal of Turbomachinery Vol. 139 / 011010-1

[2] Fister, W., Zahn, G., and Tasche, J., 1982, “Theoretical and Experimental Investigations About Vaneless Return Channels of Multi-Stage Radial Flow Turbomachines,” ASME Paper No. 82-GT-209.