CAVITATION
flow rate at choked flow( Q max) in order to calculate the F L
. The mixture multiphase model was used to capture the liquid and vapour phase in cavitating regime. A series of simulations were conducted by reducing the outlet pressure while maintaining a constant inlet pressure. In the non-cavitating region, the C v remained constant- the flow rate was proportional to the square root of pressure drop. However, as the pressure drop is increased by reducing the outlet pressure, the flow crossed the incipient cavitation threshold, transitioned into the cavitating region, and reached the choked condition where the flow rate doesn’ t increase any further. The Q max obtained at choked flow was then used to calculate the F L
. The CFD results closely matched the experimental data, confirming the accuracy of the simulations.
Challenges in CFD simulation
CFD simulations present several challenges, including:
• High mesh refinement: Required for accurate flow resolution. High mesh count increases computational cost, especially for complex control valve geometries.
• Multiphase simulations: These are computationally heavy, requiring multiple iterations to reach choked flow and repeated for each valve opening to generate a full F L plot.
• Slow Convergence: The pressure inlet – pressure outlet boundary conditions can be unstable, resulting in slow convergence.
• High computational time and cost: Due to the complexity of multiphase flow simulations, the overall computation time and cost are significantly high.
Proposed alternate method: pressure field correction
The proposed method leverages singlephase flow simulation results to determine F L
. In an incompressible flow
Step 1 single-phase results
Step 2 multi-phase results
simulation, the computed pressure field allows for adding or subtracting the same pressure value across all cells without affecting other flow variables. Hence, the pressure field obtained from single phase flow simulation( step-1) is adjusted to mimic a choked flow by subtracting a constant value, without any additional simulations. During this process, the pressure contours are visualized, with a color scale that distinguishes areas where the pressure is greater than the vapor pressure( blue) from areas where it falls below the vapor pressure( red). The pressure field is iteratively corrected until the contour plot shows a complete blockage of the flow path, with the red region. This condition is considered to approximate choked flow. From this corrected pressure field, the liquid
pressure recovery factor F L can be calculated using the upstream pressure, pressure drop, and vapor pressure values. When applied to the sharp-edge orifice validation case, the proposed method produced excellent agreement with the lab data as well as the results from conventional CFD simulations.
Case study: 1.5” full port ball valve
In this case study, a 1.5-inch full port ball valve was evaluated at 80 ° opening. The liquid pressure recovery factor( F L
) was calculated using both the conventional multiphase flow approach and the proposed pressure field correction method. The results from both approaches were within 10 % difference.
Pressure field correction method
54 Valve World August 2025 www. valve-world. net