TURBINES
&
AXIAL-FLOW MACHINES
UNIT - 3
Section (12.5)
HYDEL TURBINES
(Dynamic machine to extract energy from fluid)
1. Reaction (Static pressure changes) :
Converts both Flow & Kinetic energy
(a)
- Radial or (Francis turbine)
(b)
- Axial flow or propeller turbine
(Kaplan, Bulb)
©
- Mixed flow (Francis turbine)
2. Impulse: (Static pressure unchanged)
Converts only Kinetic energy
(a) -Tangential flow on buckets (Pelton)
1a.Francis
turbine
(Radial)
Runner
1b. Axial flow (Kaplan) Turbine
(Reaction)
1b. Axial flow (Bulb) Turbine
(propeller type) (Reaction)
• Used for very
low head high
volume flow
• Ideal for tidal
power plant –
1c. Francis Turbine (Mixed flow)
(Reaction)
2. Impulse turbine
(PELTON WHEEL)
Converts
kinetic
energy
alone
IMPULSE TURBINE
• High velocity jet
discharging at atmosphere
pressure hit buckets turning
the turbine
• Momentum change is
through change in flow
direction
• No Pressure Change
• As jet velocity is depends
on the head (v ~ H0.5). So it
is unsuitable for low head.
It requires high head
• Ideal Specific speed is 110 rpm
Velocity diagram of impulse turbine
Power developed (Impulse turbine)
•
•
•
•
•
Jet velocity V1 = Cv(2gH)0.5
Tangential velocity at entry, Vt1=V1= v1+u
at exit Vt2 = u – v2cos β2;
Euler Eq. Energy transfer/mass = (u Vt1- u. Vt2)
Substituting P = (ρQ)u[v1+u –(u – v2cos β2)]
= (ρQ)u(V1 –u)(1+ k.cos β2) where k = (v2/v1).
Ideally velocity does not change in impulse turbine
blades giving k =1, but some loss may occur
• Efficiency of runner = Power/Kinetic energy of jet
= (ρQ)u(V1 –u)(1+cos β2)/[(ρQ) V12/2]
= (2u/ V12)(V1 – u) (1+cos β2)
Radial/Mixed flow Turbine
(FRANCIS TURBINE)
• In a reaction turbine (hydraulic, steam, or gas), a
part of the head is converted into KE in stationery
guide vanes. Static pressure changes in the runner
• Used for low head high flow application
• It can be mixed flow or radial type
• It gives high efficiency but is unsuitable for high
head
• Specific speed for such turbine is 10-110
AXIAL FLOW MACHINES
for air/gas
(12.2.2)
An axial flow machine
Axial Flow Machines
• Here fluid flows parallel to the machine axis
• Propeller, air circulator, table fan are examples of
axial flow pump/fan
• It is also used extensively in turbines
• Axial flow machines gives high flow at low head.
For high head series of impellers (blades) are
mounted in series.
• Guide or static vanes are used in-between to guide
the flow for optimum efficiency
• Its impellers are designed to give constant axial
velocity at all radius but varying head.
Axial flow machines
• Fluid enters and leaves at the same radial distance giving
• u1= u = u2 ; Vn1 =Vn = Vn2
• Eq.(8) is valid here. So we can write
•
u2 (u2 − Vn 2 cot β 2 ) u 2 uVn cot β 2 (14)
=
−
H=
g
g
g
• Eq. 14 suggests that head varies from axis to periphery
with u.
• Axial flow fans are designed to have same velocity at all
radius.
Can you suggest how to do this?
Axial flow turbine
• Power developed ~ Flow rate x Head available
• Axial flow turbine allow large flow rate and therefore work
with low velocity or low head
• Guide vanes gives free vortex type whirl to water (Vt ~ R-1)
• To handle large flow rate blades are large. Blade
velocity (u ~ R) increases with radius. But fluid velocity
(Vt ~ R-1). To cater for this difference the runner blades
are twisted making larger angle at the tip
• For maximum efficiency blades are designed such that
tangential component of exit velocity, Vt1 is zero
• P = ρ Q(u2Vt2 – u1Vt1) = ρ Q u2Vt2
• Specific speed is 400- 800 rpm
Multi blade Reaction Turbine
Turbine selection
• The BHP output depends on Q, H, n & D
• Specific speed Nsp compares output with Head
Nsp = N(rpm) (BHP)0.5/[{Hft))1.25]
• Optimum efficiency of turbine depends on its
specific speed
Problem on Pelton wheel
• A pelton wheel driven by two similar jets transmits
3750 kW to the shaft when running at 375 rpm. The
head from the reservoir level to the nozzles is 200 m
and there 10% loss in head for flow through the
pipelines and nozzles. The jets are tangential to a
1.45m diameter circle. The relative velocity decreases
by 10% as the water traverses the buckets, which are
so shaped that they would, if stationery, deflect the jet
through 1650. Neglecting windage losses, find:
(a) the efficiency of the runner and
(b) the diameter of each jet
• [Ans: 93.3%, 157 mm]
Home work
• An axial flow fan has a hub diameter of 1.5 m and
tip diameter of 2 m. It rotates at 18 rad/s and,
when handling 5 m3/s of air, develops a theoretical
head equivalent to 17 mm of water. Determine the
blade outlet and inlet angles at the hub and at the
tip. Assume that the velocity of flow is
independent of radius and that the energy transfer
per unit length of blade is constant. Air density =
1.2 kg/m3 and water density = 1000 kg/m3
• [11.40, 15.10, 19.50, 48.60] (Douglas p-719)