Don't Let Vortex Ropes Tie Up Your Hydraulic Turbine

Wout Poncelet, Product Engineering Manager at Cadence, and Remi Lestriez, Hydro CFD Consultant at Numlberica talk about characteristics of flow inside a hydraulic turbine and demonstrate reliable CFD simulations to model and analyze vortex ropes under off-design conditions. The video presentation titled Simulation of Hydraulic Turbines with Omnis CFD Platform is available at CadenceTECHTALK. 

Introduction 

In the hydropower sector, hydraulic turbines are largely used to convert the energy of the falling water into rotational mechanical energy, which is then consequently transformed into electric energy. A Francis turbine or a reaction turbine belongs to the class of hydraulic turbines that appear in the power ranges of up to 800 MW. These turbines are characterized by radial inflow, inlet vanes, runner, and axial outflow through a draft tube. They can quickly respond to any load changes over a wide range of time scales.  

Figure 1. Inlet vanes guiding flow into a Hydraulic turbine (to the left), the geometry of a GAMM Turbine (in the middle), and parts of the turbine i.e., draft tube, vanes, and runner (to the right). 

At a load of about 50% to 70% of its best efficiency point, a Francis turbine experiences the development of cavitation vortex ropes which can lead to pressure surges. This part load surging is also known as Rheinganz frequency. Here the produced frequency is about 1/3rd of the rotational speed of the turbine. This unsteady behavior in operation is not only observed at part load but also at nominal full load and overload. In this Cadence TECHTalk, the presenters talk about the different challenges due to vortex ropes on the hydraulic turbine design and how to accurately analyze vortex ropes under off-design conditions.

Challenges Due to Vortex Ropes  

A vortex rope is an unsteady phenomenon that occurs at a high Reynolds number in the draft tube cone of a Francis turbine. Two types of vortex flow are observed in the draft tube of such turbines: 

• pulsating vortex rope at full load
• helical vortex rope at part load.

The flow inside the draft tube of a hydraulic turbine operating at off-design conditions is often characterized by a complex and swirling flow which can lead to cavitation phenomena. The frequency of a vortex rope is usually in the range of 30% to 50% of the runner's rotational speed.  
 

Figure 2. Vortex rope appears in the draft tube of a hydraulic turbine. 

Vortex ropes, as shown in Figure 2, can result in pressure pulsations in the draft tube, causing structural vibrations, power swings, and pulsative pressure recovery which are a major concern in the hydropower sector because they can lead to reduced system performance or limit the operating range of the turbine. Hence, it is critical to model the vortex ropes accurately and to analyze the corresponding pressure fluctuations.

Solution 

Analyzing turbine performance under design and off-design conditions has become so much easier using CFD simulations. As a first step, the turbine design should be simulated under steady-state conditions to study the efficiency points. Then, simulate it under off-design conditions to visualize the vortices in the flow. The vortex flow in a draft tube is characterized by a region of low pressure and can be visualized by velocity streamlines (Figure 3) or an iso-surface of the static pressure. Depending on the accuracy of the user inputs and the choice of turbulence model, the vortex rope can be modeled to a frequency that is close to the literature value. For an improved result of vortex rope modeling, especially to capture the strong swirling flows, the Reynolds Stress Equation Model (RSM) is recommended.

 
Figure 3. Velocity streamlines displaying vortex rope inside a hydraulic turbine.  

Another way to visualize the vortices in the flow is by using the Q criterion (it defines vortices with vorticity magnitude greater than the rate of strain). For the same geometry, multiple design conditions can be applied, e.g., multiple openings of the vanes using the same mesh generation and simulation setup, to save an ample amount of time. 

User Benefits of the Omnis Platform  

Cadence Omnis is an end-to-end platform to mesh, simulate, and analyze turbomachinery applications - all in one environment. Omnis offers streamlined workflows for turbomachinery applications including industry best practices. The structured and unstructured meshes for a single geometry can be generated and combined in the same Omnis project. As seen in Figure 4, for a hydraulic turbine, an unstructured mesh of the draft tube is performed on Omnis Hexpress while the structured mesh of the impeller and the guide vanes are carried out on Omnis Auto Grid. 
                

Figure 4. Structured mesh of runner blades, guide vanes and stay vanes using Omnis Auto Grid (to the left) and unstructured mesh of the draft tube using Omnis Hexpress (to the right).  

While dealing with unsteady or off-design simulations, the time step for each revolution needs to be set up. To speed up the simulation, the time steps throughout the simulation can be changed using Omnis OpenLabs. Omnis Open-Pressure-Based Solver (PBS) in Openlabs, is an easy-to-use multiphysics solver that provides fast and accurate results; and is fully integrated into the Omnis platform. Omnis also provides a custom interface for post-processing, enabling different views for the same project. As shown in Figure 5, users can observe the static pressure between blade-to-blade cuts, streamlines of velocity, and the image of the 3-D flow-field inside the draft tube using a single interface.

                                     
 
Figure 5. Different views of the simulation on a single interface (to the left) and the performances in terms of power and efficiency were calculated and visualized using a hill chart (to the right). 

Conclusion 

Cadence Omnis provides an end-to-end workflow for turbomachinery applications allowing engineers to generate high-quality meshes (structured and unstructured), run fast and robust simulations, and analyze steady and unsteady CFD results. The swift workflow in Omnis is demonstrated by steady and unsteady simulation of the Francis turbine allowing users to design a hydraulic turbine with reduced design instability due to vortex ropes.  

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