fluid_mechanics

12.8 Turbines 683
ω
ω
Rotor blades
ω
Rotor
Adjustable
guide vanes
Plan view of guide vanes
Adjustable
guide vane
ω
Draft tube
(a)
F I G U R E 12.31
Kaplan turbine.
(b)
(a) Typical radial-flow Francis turbine, (b) typical axial-flow
rotor. Flow across the rotor contains a major axial component. Both the inlet guide vanes and the
turbine blades can be adjusted by changing their setting angles to produce the best match 1optimum
output2 for the specific operating conditions. For example, the operating head available may
change from season to season and/or the flowrate through the rotor may vary.
F l u i d s i n t h e N e w s
Fish friendly hydraulic turbine Based on data about what actually
kills fish as they pass through hydraulic turbines, Concepts
NREC produced a rotor design that allows a larger flow passage, a
more uniform pressure distribution, lower levels of shear stress,
and other acceptable trade offs between efficiency and fish survivability.
Tests and projections suggest that the fish friendly turbine
design will achieve 90 percent efficiency, with fish survivability
increased from 60% to 98%.
Actual head available
for a turbine,
h a , is always
greater than shaft
work head, h s ,
because of head
loss, h L , in the
turbine.
Pumps and turbines are often thought of as the “inverse” of each other. Pumps add energy
to the fluid; turbines remove energy. The propeller on an outboard motor 1a pump2 and the propeller
on a Kaplan turbine are in some ways geometrically similar, but they perform opposite tasks.
Similar comparisons can be made for centrifugal pumps and Francis turbines. In fact, some large
turbomachines at hydroelectric power plants are designed to be run as turbines during high-power
demand periods 1i.e., during the day2 and as pumps to resupply the upstream reservoir from the
downstream reservoir during low-demand times 1i.e., at night2. Thus, a pump type often has its corresponding
turbine type. However, is it possible to have the “inverse” of a Pelton wheel turbine—
an impulse pump?
As with pumps, incompressible flow turbine performance is often specified in terms of appropriate
dimensionless parameters. The flow coefficient,
the head coefficient,
and the power coefficient, C p W # C Q QvD 3 ,
C shaftrv 3 D 5 H gh av 2 D 2 ,
, are defined in the same way for
pumps and turbines. On the other hand, turbine efficiency, h, is the inverse of pump efficiency. That
is, the efficiency is the ratio of the shaft power output to the power available in the flowing fluid, or
h W# shaft
rgQh a