Suppression of vortex rope oscillation and pressure vibrations in Francis turbine draft tube using various strategies

The vortex rope usually occurs in the draft tube of the Francis turbine operated under part-load conditions,to induce strong low-frequency pressure vibrations,and therefore,is very harmful to the safety of the hydropower unit.In the present work,three kinds of strategies are extensively investigated,i.e.,the installations of the ventilation and the fin,as well as the hybrid strategy of the air admission through a fin,so as to effectively suppress the vortex rope oscillation and the pressure vibration in the draft tube of a Francis turbine,whose specific speed is 125 m-kW.For the unsteady flow simulation,the Reynolds averaged Navier-Stokes(RANS)method is applied coupled with the k-ω SST turbulence model and a homogeneous cavitation model.The flow analysis confirms that the low-frequency pressure vibrations are originated from the periodical oscillation of the vortex rope,and the cavitation usually enhances the vortex rope oscillation in the draft tube.Under the part-load condition,the dominant component of the pressure vibration in the draft tube has a frequency,for example,f_(1),lower than the runner rotating frequency f_(n).It is shown that all three strategies can be adopted to alleviate the vortex rope oscillation and the pressure vibrations in the draft tube,but their suppression mechanisms are quite different.The ventilation of an adequate amount from the turbine runner cone can change the vortex rope geometry from the spiral type to the cylindrical type,suppress the vortex rope oscillation,and consequently create the homogeneous distributions of the pressure and the pressure gradient in the draft tube.On the other hand,a fin installed at the draft tube wall can induce a small extra rope,and the interaction between the main vortex rope and the extra rope changes the flow field and alleviates the pressure vibration in the draft tube.It should be noted that a fin is much more effective to suppress the pressure vibration in the draft tube under the cavitation condition than under the non-cavitation condition.A better effect of suppressing the vortex rope oscillation can be achieved by the air admission through a fin,which is studied numerically in this paper.The result indicates that the air admission can further improve the effect of a fin for suppressing the pressure vibration in the inlet cone of the draft tube.This improvement is due to the stronger interaction between the main vortex rope and the extra air rope.However,the air admission through a fin should be carefully treated because the strong interaction may induce a larger pressure vibration in the elbow of the draft tube.Finally,it is clear that any strategy for suppressing the pressure vibration hardly changes the dominant component frequency f_(1),which is in the range of 0.22 f_(n)-0.23 f_(n) due to the main vortex rope oscillation in this study.The current results may be used in various engineering applications,where the active control of the vortex oscillation and the pressure vibrations with or without the cavitation is necessary.

Estimation of the turbulent viscous shear stress in a centrifugal rotary blood pump by the large eddy particle image velocimetry method

The non-physiologic turbulent flows in centrifugal rotary blood pumps (RBPs) may result in complications such as the hemolysis and the platelet activation. Recent researches suggest that the turbulent viscous dissipation in the smallest eddies is the main factor of the blood trauma caused by the turbulent flow. The turbulent viscous shear stress (TVSS) was taken as the realistic physical force acting on the cells. However, limited by the temporal and spatial resolutions of the instrumentation currently available, very limited studies are available for the TVSS in the RBPs. In this paper, the large eddy particle image velocimetry (PIV) method is used to estimate the turbulent dissipation rate in the sub-grid scale, to investigate the effect of the TVSS on the blood trauma. Detailed flow characteristics, such as the relative velocity vectors, the estimated TVSS levels and the Kolmogorov length scales, are analyzed in three impeller phases at three constant flow rates (3 L/min, 5 L/min and 7 L/min). Over the measures range in this study, the maximum TVSS in the investigated RBP is lower than the reported critical value of stress. This study demonstrates that the large eddy PIV method is effective to evaluate the flow-dependent force on the cells. On the other hand, it is found that the TVSS is highly dependent on the flow behavior. Under severe off-design conditions, the complex flow characteristics, such as the flow separation and the vortical structures, will increase the TVSS. Thus, in order to reduce the hemolysis in the RBPs, the flow disturbance, induced by the departure of the incidence angle, should be avoided during the design of the RBPs.

Investigation on cavitating turbulent flow for the twisted NACA66 hydrofoil using a PANS model with helicity modification

In the present paper,the unsteady cavitating turbulent flow over the twisted NACA66 hydrofoil is investigated based on an modified shear stress transfer k-ωpartially averaged Navier-Stokes(MSST PANS)model,i.e.,new MSST PANS(NMSST PANS)model,where the production term of kinetic energy in the turbulence model is modified with helicity.Compared with the experimental data,cavitation evolution and its characteristic frequency are satisfactorily predicted by the proposed NMSST PANS model.It is revealed that the interaction among the main flow,the reentrant jets,and sheet cavitation causes the formation of the primary shedding cavity near the mid-span and the secondary shedding cavity at each side of the twisted hydrofoil,and further induces the remarkable pressure gradient around shedding cavities.Along with the development of the primary and the secondary shedding cavities,the great pressure gradient associated with large cavity volume variation promotes the vortical flow generation and the spatial deformation of vortex structure during cavitation evolution,and results in the primary and the secondary U-type vortices.Further,dynamic mode decomposition(DMD)analysis is utilized to confirm the interaction among the main flow,the main reentrant jet and two side reentrant jets,and cavitation.These results indicate that the proposed NMSST PANS model is suitable to simulate the complicated cavitating turbulent flow for various engineering applications.