Fluid and Mechanical System Analysis

Numerical Flow Simulation in Modernization or Upgrading Projects Existing turbines being considered for refurbishment are often older designs completed before the benefits of CFD (Computational Flow Dynamics) analysis were available. The numerical flow simulation of such machines often provides key insights into their performance and flow-field. This enables the diagnosis of specific problems such as cavitation erosion damage, but also allows the potential of alternative solutions to be assessed.

Numerical flow analysis leads to a good understanding of the specific problems and of the hydraulic design. The assessment of alternative solutions then allows a realistic estimation of costs and benefit to be made. Based on this information the optimal solution can be defined by the operating company together with the supplier and this can then be realized by the supplier. The main strengths of CFD in the early phase of refurbishment projects are that it gives clear insight into the differing potential for hydraulic improvement of the components and allows the involvement of owners in this phase right from the beginning.

A CFD-based optimization study identifies the weaknesses and strengths of an existing plant and the potential for improvement. It delivers input for the calculation of a possible increase of annual energy output, as well as for safety issues such as cavitation, mechanical load and abrasion damage. Nowadays CFD methods include three-dimensional, viscous and turbulent effects, the rotor-stator interaction, multiphase flow and even unsteady details of the turbulent structures in the flow. For life cycle engineering the information that CFD delivers is used in order to determine the optimal interval for refurbishment. To give an example, CFD indicates the cavitation risk of a runner blade. The risk of cavitation associated with certain operating conditions can be estimated and the interval of refurbishment can be optimized. A CFD study may focus on one component for which a replacement is planned, or it may be extended to all turbine components together. In cases of high uncertainty about the performance of individual components and their interaction with other components a full study has to be carried out.

This applies especially to draft tubes in existing plants, whose performance is hard to assess without a CFD or model test. In cases of high guarantees for a project a full study (i.e. a CFD analysis of all components and a model test) has to be carried out in order to identify every single possibility to improve the performance of the machine. Partial studies are sufficient when a preliminary assessment of the components reveals no significant problems and the overall risk of the project is estimated to be medium or low.