Flow instability due to a diameter reduction of limited length in a long annular passage

Flow in an annular passage is a common feature in the fuel channels of the UK Advanced Gas Cooled Reactors, and in some cases has required investigation to avoid or reduce flow-induced vibration of reactor components. A number of studies have been made of vibration induced by annular flow, and these are briefly reviewed in the first part of this paper. Background to the topics then the paper describes the investigation of the most recently recognized problem of this type, which is a flow instability caused by a diameter reduction of limited length in a long annulus. A method of eliminating the mechanism is also described. 5 refs