Turbines convert the fluid power in water into rotational power in a shaft and are subdivided into impulse and reaction machines (Douglas, Gasiorek, & Swaffield, 2001).
Impulse turbines convert the total head into kinetic energy through one or more nozzles creating jets that strike vanes attached to the periphery of a rotating wheel. Because of the rate of change in angular momentum and the motion of the vanes, work is done on the runner (impeller) by the fluid, and, thus, energy is transferred. Since the fluid energy which is reduced on passing through the runner is entirely kinetic, it follows that the absolute velocity at outlet is smaller than the absolute velocity at inlet (jet velocity). Furthermore, the fluid pressure is atmospheric throughout, and the relative velocity is constant except for a slight reduction due to friction.
In reaction turbines, the fluid passes first through a ring of stationary guide vanes in which only part of the available total head is converted into kinetic energy. The guide vanes discharge directly into the runner along the whole of its periphery, so that the fluid entering the runner has pressure energy as well as kinetic energy. The pressure energy is converted into kinetic energy in the runner, and therefore the
relative velocity is not constant but increases through the runner. There is, therefore, a pressure difference across the runner.
The degree of reaction describes a measure of the continuum from impulse to reaction turbines. It is derived by the application of Bernoulli’s equation to the inlet and outlet of a turbine. Thus, if the conditions at inlet are denoted by the use of suffix 1, and those at outlet by the suffix 2, then
where E is the energy transferred by the fluid to the turbine per unit weight of the fluid. Thus
E = + HL-Hi (3.40)
In this equation, the first term on the right-hand side represents the drop in static pressure (or potential energy) in the fluid across the turbine, whereas the second term represents the drop in the velocity head (or kinetic energy). The two extreme solutions are obtained by making either of these two terms equal to zero. Thus, if the pressure is constant, so that p1 = p2, then E = ( U2— U|) /2g and the turbine is purely impulsive. If, on the other hand, U1 = U2, then E = (p1 — p2) /pg and the device is a reaction turbine. The continuum is described by the degree of the reaction, RT, as defined by
static pressure drop total energy transfer
The static pressure drop is given by
pi — p2 = E — U2 — U2 pg 2g
and the total energy transfer is E, so that
Water turbines are used in power stations to drive electric generators. There are two well-known large-head types (Figure 3.23): the Pelton wheel, which is an impulse turbine, and the Francis type. Table 3.1 compares the two types.
In tidal applications, however, there is a minimal head, and therefore the main application is for the reaction-type crossflow (Ossberger, Savonius, Michell, or Banki) turbine and the axial-flow (Kaplan) turbines. Since most Kaplan-type reaction turbines are horizontal axis, they are considered as such below (Section 3.11). Most of the crossflow turbines are vertical axis, and are thus considered as such below (Section 3.12). Finally, hydrofoil blades are also utilized in a reciprocating
Figure 3.23 The various turbines as a function of the head (vertical axis, m) and the flow discharge (horizontal axis, m3 s-1), after British Hydro, 2008.
Table 3.1 Comparison of water turbines
manner or, as in the Kobold and Darius turbines, with crossflow reaction blades, and they are considered as hybrids.