CFD Calculation and Experiments of Unsteady Flow on Control Valve

Abstract

A steam control valve causes vibrations of piping when the opening is in the middle condition. For the rationalization of maintenance and management in a plant, valves should be improved, but it is difficult to understand the flow characteristics in detail experimentally because the flow around the valve has a complex 3D structure and becomes supersonic (M>1). Therefore, it is necessary to clarify the cause of vibration and to develop improvements both experimentally and through CFD (Computational Fluid Dynamics) calculation. First, small-scale air experiments are performed. In this experiment, a spike-type pressure fluctuation that rotates circumferentially is observed in a middle valve opening. This fluctuation sometimes changes its rotation direction suddenly, or sometimes stays at the same position. Because of this randomness, FFT analysis cannot reproduce this peak frequency. However, another peak caused by the 1st-mode resonance in the pipe-diameter direction can be seen. After some experiments, a high-precision 3D CFD code for compressible flow, “MATIS”, was developed. This code can calculate an unsteady 3D compressible flow steadily and accurately, with TVD scheme, LU-SGS implicit scheme and inner iterative calculation. This code also includes LES turbulence model for compressible fluid in order to reproduce turbulence flow accurately. To validate this code, first, some benchmark tests such as karman vortex calculation and the calculation of a detached shock position on a sphere are performed. After benchmark tests, CFD calculation of the valve is performed under several valve opening conditions. The mesh number is about 600 thousand and y+ is about 3. CFD results agree well with those of the experiment qualitatively and quantitatively. Therefore, the validation of MATIS is confirmed. Detailed flow characteristics around the valve in a middle valve opening are investigated. Under this condition, the jet after the throat travels the valve (named “attached flow”) and strikes the jet coming from the opposite side. This phenomenon generates a high-pressure region and a pair of vortices; this region rotates circumferentially and causes cyclic side load on the valve body. CFD calculation clarifies that a circumferentially propagating spike-type pressure fluctuation is caused by attached flow along the valve, and this is though to be the cause of vibration.