2 Replies Latest reply on May 6, 2016 12:20 PM by Amit Katz

    Cavitation with a ball valve

    Marco Rauseo

      Hello everyone, my name is Marco, I'm a Mechanical Engineer to be and I'm currently working on a ball valve project. This valve works with water so I'm verifying the cavitation model and here occurs my problem: I'm running a simulation and at 1000 iterations (0.125 travel) it doesn't cavitate yet, even though there's a strong deltaP between inlet and outlet and I'm sure it has to cavitate. I know it is not a whole travel, but with all the other cavitation projects I did, the phenomenon started really soon! I modified all the calculation control options as tutorial said and my mesh is really well refined through appropriate volumes...so I don't know what's wrong. Could somebody help me out please?

      Thank you in advance

        • Re: Cavitation with a ball valve
          Seckin Uslu

          could you please give us more info?

           

          Like model or setting etc..

          • Re: Cavitation with a ball valve
            Amit Katz

            Have you done any hand calculations on how much cavitation you expect, or is this more of a "gut feeling"? I would go over all of this stuff (from the online help) and see if you missed anything during setup:

            Hydrodynamic cavitation usually occurs in flowing liquids when the pressure falls sufficiently low in some region of the flow. This cavitation starts with the formation of bubbles in the liquid which are filled with vapor and dissolved gases. An area of two-phase flow, also called a hydrodynamic cavitation cavern, is created downstream of this region.

             

             

            Uncontrolled, cavitation can be very destructive to process equipment such as throttling valves, pump impellers, water turbines, marine propellers, etc. A number of negative effects can be expected: alteration of the performance of the system (reduction in lift and increase in drag of a foil, fall in turbomachinery efficiency, reduced capacity to evacuate water in spillways, etc.), production of noise and vibrations, wall erosion. However, cavitation can be used in some industrial processes, for example, to control mass flow rate and to improve dispersion in injector nozzles.

             

             

            If the characteristic time-scale of the vapor formation process is much less than the characteristic time-scale of the liquid flow, the cavitation process occurs in conditions close to thermodynamic equilibrium. Therefore, the equilibrium approach can be used to simulate the liquid-vapor phase transition.

             

             

            In Flow Simulation two equilibrium models are employed:

             

             

            Equilibrium Cavitation Model (for pre-defined water only)

             

             

            This model employs a homogeneous equilibrium approach and is available for pre-defined water only.

             

             

            This model has the capability to describe phase transition caused not only by pressure reduction in case of hydrodynamic cavitation but also by increasing the temperature in case of boiling.

             

             

            Isothermal Cavitation Model (for user-defined liquids only)

             

             

            This model is based on the approach considering isothermal two-phase flows. Fluid density is defined by the barotropic equation of state. The isothermal cavitation model is only available for user-defined liquids.

             

             

             

             

             

            Recommendations for use of cavitation option:

            The Cavitation option is not applicable if you calculate a flow in the model without flow openings (inlet and outlet).

             

             

            The fluid region where cavitation occurs should be well resolved by the computational mesh.

             

             

            If you analyze a flow of water in some points of which the local static pressure can reach the saturation pressure at the local temperature causing cavitation or if a vaporization of water can occur in the water flow due to intense heating, it is recommended to use the Equilibrium cavitation model.

             

             

            If the calculation has finished or has been stopped and the Cavitation option has been enabled or disabled, the calculation cannot be resumed or continued and must be restarted from the beginning.

             

             

            Cavitation area growths slowly during calculation and there is a risk that the calculation will stop before the cavitation area develops completely. To avoid this, always specify Global Goal of Average Density and increase the Analysis interval under Goals Criteria on the Finishing tab of the Calculation Control Options dialog up to 2.5 travels. Also make sure that the other finish conditions do not cause the calculation to stop before goals are converged. The easiest way to ensure this is to select All satisfied for the Criterion to stop under the Finish Conditions on the Finishing tab of the Calculation Control Options dialog.

             

             

            To see the cavitation areas you can select Density or the Volume Fraction of Vapour as the parameter for visualization.