14 - Unsteady Flow

Abstract

In discussing transitional change in flow quantity in a pipeline, the chapter explains that if the valve at the end of the pipe is suddenly closed, the velocity of the fluid abruptly decreases causing a mechanical impulse to the pipe due to a sudden increase in pressure of the fluid. Such a phenomenon is called water hammer. This phenomenon poses a very important problem in cases where, for example, a valve is closed to reduce the water flow in a hydraulic power station when the load on the water wheel is reduced. In general, water hammer is a phenomenon which is always possible whenever a valve is closed in a system where liquid is flowing. Velocity of pressure waves in a pipeline is also explained in this chapter. The velocity of a pressure wave depends on the bulk modulus K. The bulk modulus expresses the relationship between the change of pressure on a fluid and the corresponding change in its volume. When a small volume V of fluid in a short length of rigid pipe experiences a pressure wave, the resulting reduction in volume dV1 produces a reduction in length. If the pipe is elastic, however, it experiences radial expansion causing an increase in volume dV2. This produces a further reduction in the length of volume V.