MEC 2249- Trabajo Practico 4

MEC 2249- Trabajo Practico 4
1
P1. The rotor shown in Fig. P1 rotates with an angular velocity of 2000 rpm. Assume that the fluid enters in
the radial direction and the relative velocity is tangent to the blades across the entire rotor. Is the device a
pump or a turbine? Explain.
Fig. P1
Fig. P2
P2 A centrifugal water pump having an impeller diameter of 0.5 m operates at 900 rpm. The water enters the
pump parallel to the pump shaft. If the exit blade angle, 1see Fig. P2 is determine the shaft power required to
turn the impeller when the flow through the pump is 25o, determine the shaft power required to turn the
impeller when the flow through the pump is 0.16 m 3/s. The uniform blade height is 50 mm.
P3 A Pelton wheel turbine is illustrated in Fig. P3. The radius to the
line of action of the tangential reaction force on each vane is 1 ft.
Each vane deflects fluid by an angle 135° of as indicated. Assume all
of the flow occurs in a horizontal plane. Each of the four jets shown
strikes a vane with a velocity of 100 ft/s and a stream diameter of 1 in.
The magnitude of velocity of the jet remains constant along the vane
surface.
(a) How much torque is required to hold the wheel stationary?
(b) How fast will the wheel rotate if shaft torque is negligible and what
practical situation is simulated by this condition?
Fig. P3
Fig. P4
Fig. P5
P4. A sketch of the arithmetic mean radius blade sections of an axial-flow water turbine stage is shown in
Fig. P4. The rotor speed is 1500 rpm. (a) Sketch and label velocity triangles for the flow entering and leaving
the rotor row. Use V for absolute velocity, W for relative velocity, and U for blade velocity. Assume flow enters
and leaves each blade row at the blade angles shown. (b) Calculate the work per unit mass delivered at the
shaft.
MEC 2249- Trabajo Practico 4
2
P5. A water turbine wheel rotates at the rate of 100 rpm in the direction shown in Fig. P5. The inner radius, of
the blade row is 1 ft, and the outer radius, is 2 ft. The absolute velocity vector at the turbine rotor entrance
makes an angle of with the tangential direction. The inlet blade angle is relative to the tangential direction. The
blade outlet angle is The flowrate is For the flow tangent to the rotor blade surface at inlet and outlet,
determine an appropriate constant blade height, b, and the corresponding power available at the rotor shaft. Is
the shaft power greater or less than the power lost by the fluid? Explain.
P6. Figure P6 shows a cutaway of a cross-flow or “Banki” turbine, which resembles a squirrel cage with
slotted curved blades. The flow enters at about 2 o’clock and passes through the center and then again
through the blades, leaving at about 8 o’clock. Report to the class on the operation and advantages of this
design, including idealized velocity vector diagrams.
Fig. P6
Fig. P8
P7. A simple cross-flow turbine, Fig. P6 was constructed and tested at the University of Rhode Island. The
blades were made of PVC pipe cut lengthwise into three 120°-arc pieces. When it was tested in water at a
head of 5.3 ft and a flow rate of 630 gal/min, the measured power output was 0.6 hp. Estimate (a) the
efficiency and (b) the power specific speed if n =200 rev/min.
P8. Francis and Kaplan turbines are often provided with draft tubes, which lead the exit flow into the tailwater
region, as in Fig. P8. Explain at least two advantages in using a draft tube.
P9. Turbines can also cavitate when the pressure at point 1 in Fig. P8 drops too low. With NPSH defined by
Eq.
the empirical criterion given by Wislicenus for cavitation is
Use this criterion to compute how high z1 - z2, the impeller eye in Fig. P8, can be placed for a Francis turbine
with a head of 300 ft, Nsp = 40, and pa =14 lbf/in2 absolute before cavitation occurs in 60°F water.
P10 Water for a Pelton wheel turbine flows from the
headwater and through the penstock as shown in
Fig.P10. The effective friction factor for the penstock,
control valves, and the like is 0.032 and the diameter of
the jet is 0.20 m. Determine the maximum power output.
Fig. P10