Flow Field in the Vicinity of a High-Pressure-Drop Ball Valve

Abstract

The testing of gas turbine combustors requires large flow rates at high pressures and elevated temperatures. To control the flow and pressure inside the combustor, some type of control valve is required in the exhaust section of the testing system. This back-pressure valve is exposed to severe operating conditions. To understand the complex flow features in the exhaust section and provide relevant information for selecting suitable low-cost valves for new large test cells, a numerical study was carried out on a back-pressure valve that has been used in a number of testing programs. The flow fields in the vicinity of the valve and the piping sections ending at the exhaust stack were resolved for three practical operating conditions. The results indicate that because of the presence of the valve with a V-shaped opening, the flow field behind experiences a series of three-dimensional expansion and shock waves. Strong interactions exist between the flow behind the blockage and the flow passing through the opening area. More importantly, it is found that most of the pressure drop occurs immediately downstream of the valve, and its values are much larger than those provided by suppliers of valves based on shock-free flow calculation. This may explain why the valve lost its ability to function during testing and its stainless-steel seat had to be removed to maintain its ability to rotate. Based on this study, it is recommended that a second pressure-drop element be installed in the exhaust section to keep the expected lifetime of the valves and reduce the noise levels. This suggestion has been implemented in the new large test cell at the Gas Turbine Laboratory.