Small Decentralized Hydropower Program National ... - Cd3wd.com

-4dvirtiendo que la potencia P es
(32) P = rjyQH
donde q es la eficiencia y la tasa de flujc Q la
produce:
(33)
ndj2
Q = Vj 4 =
ndj2
CVG y-
entonces la veiocidad especifica de la mhquina es
(34) N,=
o, aproximadamente,
(35)
dj
2114 Gig D
(9
N, = 1.3 D
Asi, pues, la velocidad cspecifica es, aproximada-
ment.e, 1, 3 veces la relaci6n de1 digmetro de1 chorro
al diametro de la rueda. Los valores prkticos de
dj/D para las ruedas Pelton osciian entre 0,04 y 0,3.
lo que significa que N, estara en la gama de 0,05 a
0,13 (19 a 25 en unidades m&ricas). En ias turbinas
Turgo, ei digmetro reiativo de !a rueda puede ser la
mitad que el de una rueda Pelton, 10 que produce
veiocidades especificas de aproximadamente el .dobie,
de !as de disefio conventional. Son posibles
velocidades especificas m&s elevadas con disefios de
inyectores mtitiples. El aumento es proporcionai a la
raiz cuadrada de1 ntimero de inyectores.
multiple nozzle designs. The increase is propor-
tional to the square root of the number of
nozzles.
Reaction Turbines. The main difference bet-
ween impulse wheels and reaction turbines is the
fact that a pressure drop takes place in the
rotating passages. This implies that the entire
flow passage from turbine in!et to discharge a?
the tailwater must be completely fiiied. Typicai
turbine runners are illustrated in Fig. 6.
The specific speed range for reaction wheels is
much broader than for impulse wheels. A typical
installation is sketched. (Figure 14)
The main difference is the draft tube which is
considered an integral part of the machine. Its
main functions are to allow a turbine setting
above the tailwater without a reduction in net
head and to reduce the head loss at the
submerged discharge by diffusing the flow leav-
ing the turbine.
In an impulse wheel, the head represented by
setting the turbine above the tailwater is lost,
but, since impulse wheels are generally operated
at high head, this represents a small fraction of
the total head available. Referring to the diagram,
the head available to the runner is
P4- v212 v2 - Vd2
(36) H’= H-kD 3n - 3n
DiFfuser Sudden
Loss Enlargement
Turbinas de Reaccih. La principal diferencia en- Basically two losses are associated with the
tre las ruedas de impuisos y ias turbinas de reacci6n draft tube; viscous losses within the draft tube
se debe a que en 10s pasajes de rotaci6n se produce associated with flow diffusion and external mixuna
rcducci6n de la presi6n. Esto significa que todo ing losses associated with the discharge into the
el pasaje de1 flujo desde la entrada de la turbina a la tailwater. Obviously, the first loss can be
descarga en el agua de descarga el tubo ha de estar minimized by reducing the amount of diffusion at
completamente lleno. En la figura 6 se ilustran the expense of increased outlet losses.
rotores tipicos de turbina. As an illustration of the design and operation
La gama de velocidades especificas para 10s ruedos
of turbines for constant speed, refer again to Fig.
de reacci6n es mucho m8s amplia que para 10s rueda 1. The pivoted guide vanes allow for control of
de impulsos. A continuaci6n esbozamos una instala- the magnitude and direction of Vl, ie. V1 and al.
cidn tipica. (Figura 14) The relationship between blade angle, inlet
velocity, and peripheral speed is
La principal diferencia radica en la galeria de suc-
ci6n que se considera coma una parte integral de la
mgquina. Sus principales funciones consisten en permitir
la instalacidn de la turbina por encima de1 agua
de descarga sin reducir la caida neta y reducir la (37)
p6rdida de la caida en la descarga sumergida
mediante la difusi6n de1 flujo que sale de la turbina.
137
cot /3, =
VI cos q-U1
VI sin al