Flow in a Co-Axial Control Valve

An experimental and computational investigation of the complex flow inside a co-axial flow control valve with various piston configurations was performed. A transparent full-scale prototype of a control valve was installed into an instrumented flow loop. The acrylic test article allows optical access for diagnostics, such as flow visualization and Particle Image Velocimetry (PIV). Global performance was assessed in terms of the valve flow coefficient for various piston configurations in the control valve at various flow rates by measuring the pressure drop across the valve using an electronic manometer. These results were compared to those for conventional control valves, and as expected, the co-axial design exhibited considerably lower losses (up to 30 times lower). However, the differences in piston geometry designed for different valve characteristics, such as linear, fast-opening, etc. led to different flow coefficients. Investigation of the mechanisms leading to the differences in the global performance involved PIV measurements of the velocity field in several planes within the valve. Complex piston geometries caused regions of separated flow and vortical structures to form. Companion computational studies were performed for the same valve geometries as installed in the flow loop using a commercial CFD package, FLUENT. A fully 3-D Reynolds Averaged Navier-Stokes (RANS) model employing on the order of 800,000 cells was used with a Renormalized Group theory (RNG) k-ε turbulence model. The computational results were compared qualitatively to the experimental data. The CFD results were then used to investigate details of the flow that were not accessible to the experiments, including streamlines, distributions of the static pressure and turbulent kinetic energy throughout the flow field.