2. Unit I
Energy transfer in turbo machines
1. Application of first and second laws of
thermodynamics to turbo machines,
2. Moment of momentum equation and
Euler turbine equation,
3. Principles of impulse and reaction machines,
4. Degree of reaction,
5. Energy equation for relative velocities, one
dimensional analysis only
6. What is a Turbomachine ?
TurboMachine:- Any devices that extracts energy
from or imparts energy to a continuously moving
stream of fluid can be called a Turbomachine.
Elaborating, a turbomachine is a power or head
generating machine which employs the dynamic
action of a rotating element, the rotor; the action
of the rotor changes the energy level of the
continuously flowing fluid through the machine.
Turbines, compressors and fans are all members
of this family of machines.
7. Classification of Turbo M/Cs
• These two types of machines are governed by
the same basic relationships
including Newton's second Law of Motion
and Euler's pump and turbine equation
for compressible fluids. Centrifugal pumps are
also turbomachines that transfer energy from
a rotor to a fluid, usually a liquid, while
turbines and compressors usually work with a
gas.
8. Classification
In general, the two kinds of turbomachines
encountered in practice are open and closed
turbomachines.
1.Open machines such as propellers, windmills,
and unshrouded fans act on an infinite extent
of fluid.
2. Closed machines operate on a finite quantity
of fluid as it passes through a housing or
casing.
9. Classification of Turbo M/Cs
Turbomachines are also categorized according to
the type of flow.
1. When the flow is parallel to the axis of rotation,
they are called axial flow machines.
2. When flow is perpendicular to the axis of
rotation, they are referred to as radial (or
centrifugal) flow machines.
3. There is also a third category, called mixed flow
machines, where both radial and axial flow
velocity components are present.
10. Classification of Turbo M/Cs
Turbomachines may be further classified into two
additional categories:
1. Those that absorb energy to increase the fluid
pressure, i.e. pumps, fans, and compressors,
2. Those that produce energy such as turbines by
expanding flow to lower pressures. Of particular
interest are applications which contain pumps,
fans, compressors and turbines. These
components are essential in almost all
mechanical equipment systems, such as power
and refrigeration cycles.
11. INTRODUCTION
Q. Define Turbomachine and classify them on
the basis of fluid movement through the
machine. (Dec. 2011)
Q. What is a Turbomachine ? Classify them on
the basis of work transfer. (Dec 2010)
Q. Define Turbomachines and explain the
different types of turbomachines. (June 2010)
12. Turbomachines work on basic laws of
thermodynamics and fluid mechanics.
1. Conservation of Mass.
2. Conservation of Energy.
3. Newton’s Second law of motion.
13. CONTINUITY EQUATION
For steady flow through the control volume, the
mass flow rate, mremainsconstant
2 V 2 A 2
m 1 V 1 A 1
14. First law of Thermodynamics:-
Conservation of Energy
Q E W
Energy can neither be created nor destroyed, it
can only change from one form to another.
E E K E P U
Q EK EP UW
15. Steady Flow Energy Equation
(First law of Thermodynamics)
Q W m[h ke pe]
16. ‘Steady Flow’ means that the rates of flow of
mass and energy across the control surface
are constant.
In most engineering devices, there is a constant
rate of flow of mass and energy through the
control surface, and the control volume in
course of time attains a steady state.
17. W Wx p1v1dm1 p2v2dm2
In the rate form
d
dm
d d d
2
1
1 1 2 2
p v p v
dW
dWx dm
w1p1v1 w2p2v2
dW
dWx
d d
‘a’
18. Since there is no accumulation of energy, by the
conservation of energy, the total rate of flow
of all energy streams entering the control
volume must equal the total rate of flow of all
energy streams leaving the control volume.
d d
dW
w e
dQ
w e
1 1 2 2
Where e1& e2 - energy carried into or out of
the CV with unit mass of the fluid.
19. d
Substituting for dW from equation ‘a’.
1 1 2 2 w1p1v1 w2p2v2
dWx
w e
dQ
w e
d
d
2 2 2
2 2
1 1 1 1 1 w p v
dWx
w e w p v
dQ
w e
d
d
20. The specific energy is given by
k p
Zg u
2
2
e e e u
V
Substituting the expression for e in equation
d
dW
w2 2
Z2 u2 w2p2v2 x
V 2
V 2
w1 1
Z1 u1 w1p1v1
dQ
d 2
2
d
dW
dQ
d
Where
V 2
w2h2 2
Z2 g x
V 2
w1 h1 1
Z1g
2
2
h u pv
21. Dividing the Equation by
This Equation is Known as
STEADY FLOW ENERGY EQUATION
d
Since w w
dm
1 2
dm
d
dW
dQ V 2
h2 2
Z2 g x
V 2
h1 1
Z1g
2 dm 2 dm
23. Second Law of Thermodynamics
Clausius’ Statement of the Second Law gives:
It is impossible to construct a device which,
operating in a cycle, will produce no effect
other than the transfer of heat from a cooler
to a hotter body.
Heat cannot flow of itself from a body at a
lower temperature to a body at a higher
temperature.
24. Second Law of Thermodynamics
Entropy
• Second law of thermodynamics states that for a
fluid undergoing a reversible adiabatic process,
the entropy change is zero.
• Entropy increases from inlet to the exit, if the
fluid undergoes an adiabatic or any other
process. Due to the increase in entropy, the
power developed by a turbine is less than the
ideal isentropic power developed. Similarly the
work input to a pump is greater than the
isentropic or ideal workinput.
ds
q
T
25. Newton’s Second Law of Motion-
Momentum Equation
The moment of momentum equation is based
on Newton’s second law applied to a rotating
fluid mass system.
Previous Knowledge
Moment of momentum about an axis is known
as Angular Momentum.
The moment of a force about a point is torque.
26. The moment of momentum principle states that
In a rotating system the torque exerted by the resultant
force on the body with respect to an axis is equal to the
time rate of change of angular momentum.
[Torque exerted on the fluid by the rotating element ]=
[Angular momentum of fluid leaving out of CV]-[Angular momentum of fluid entering the CV]
T Q[(Vur)out (Vu r)in ]
Where Q= discharge, Vu = tangential component of absolute
velocity, r= moment arm of Vu
27. • The Euler’s pump and turbine equations are most
fundamental equations in the field of turbo-machinery
. These equations govern the power, efficiencies and
other factors that contribute in the design of Turbo-
machines thus making them very important. With the
help of these equations the head developed by a pump
and the head utilised by a turbine can be easily
determined. As the name suggests these equations
were formulated by Leonhard Euler in the eighteenth
century.These equations can be derived from the
moment of momentum equation when applied for a
pump or a turbine.
28. Euler Turbine Equation
• The Euler turbine equation relates the power
added to or removed from the flow, to
characteristics of a rotating blade row. The
equation is based on the concepts of
conservation of angular momentum and
conservation of energy.
29.
30. Force exerted by a Liquid Jet Striking on a Curved Vane
when the Vane is moving in the Jet Direction
31. = Rate of Change of Momentum in the
Force exerted by a Liquid Jet Striking on a Curved Vane
when the Vane is moving in the Jet Direction
The mass flow rate of liquid striking the vane is
m a(V u)
F x
direction of the force= Mass flow rate х
change in velocity in the jet direction
a(V u)[(V u) ((V u)cos]
32. Consider a Jet of liquid striking a movinf curved
vane tangentially at one of its tips. The liquid
jet strikes the vane with an absolute velocity
of V1
. Let the velocity of the vane is u . Since
the vane is moving with the a relative velocity
V r 1
which is obtained by subtracting
vectorically the velocity u from V 1 .
33.
34.
35.
36.
37. Assumptions
1. Fluid enters and leaves the vane in a direction
tangential to the vane tip at inlet and outlet.
38. Euler’s Turbine Equation
Angular Velocity of wheel (rotaional Speed)
(rad/sec) 2N / 60
Tangential Momentum of the fluid at entry
Vw1m N
39. Euler’s Turbine Equation
Moment of momentum or angular momentum
at entry N-m
Similarly Angular Momentum at the outlet
N-m
Vw1m r1
Vw2mr2
40. Euler’s Turbine equation
T= Torque on the wheel=change of angular
momentum
Vw1r1 Vw2r2 m
N-m
Workdone=rate of energy transferred
= torque х angular velocity
Vw1r1 Vw2r2 m
N m / s
41. Euler’s Turbine Equation
But
If
If
the machine is called Turbine.
the machine is called pump,
fan, compressor or blower
r1 U1;r2 U2
Workdone
Vw1u1 Vw2u2 m
W
Vw1u1 Vw2u2
Vw2u2 Vw1u1
42. Impulse and Reaction machines
Impulse turbines change the direction of flow
of a high velocity fluid or gas jet.
Reaction turbines develop torque by reacting
to the gas or fluid's pressure or mass. The
pressure of the gas or fluid changes as it
passes through the turbine rotor blades
43. Impulse and Reaction machines
A reaction turbine is a type of turbine that
develops torque by reacting to the pressure or
weight of a fluid; the operation of reaction
turbines is described by Newton's third law of
motion (action and reaction are equal and
opposite). The pressure of the fluid changes as
it passes through the rotor blades.
44. Degree of Reaction
• In turbomachinery, Degree of reaction or reaction
ratio (R) is defined as the ratio of static pressure drop
in the rotor to the static pressure drop in the stage or
as the ratio of static enthalpy drop in the rotor to the
static enthalpy drop in the stage.
• Degree of reaction (R) is an important factor in
designing the blades of a turbine
, compressors, pumps and other turbo-machinery. It
also tells about the efficiency of machine and is used
for proper selection of a machine for a required
purpose.
45. Degree of Reaction
• Various definitions exist in terms of enthalpies,
pressures or flow geometry of the device. In case
of turbines, both impulse and reaction machines,
Degree of reaction (R) is defined as the ratio of energy
transfer by the change in static head to the total energy
transfer in the rotor i.e.
Isentropic enthalpy change in rotor/Isentropic
enthalpy change in stage.
For a gas turbine or compressor it is defined as the ratio
of isentropic heat drop in the moving blades (i.e. the
rotor) to the sum of the isentropic heat drops in the
fixed blades(i.e. the stator) and the moving blades i.e.
46. Degree of Reaction
• The degree of reaction, R is defined as the
ratio of isentropic heat drop in the moving
blades to the sum of the isentropic heat drops
in the fixed and the moving blades i.e. in a
stage.
47. Degree of Reaction
Isentropic heat drop in rotor/Isentropic heat
drop in stage. In pumps, degree of reaction
deals in static and dynamic head. Degree of
reaction is defined as the fraction of energy
transfer by change in static head to the total
energy transfer in the rotor i.e.
Static pressure rise in rotor/Total pressure rise
in stage.
48. Energy Equation for Relative Velocities
Euler’s Equation
If H is the head on the machine, then energy
transfer can be written as
E (VW1U1 VW 2U2 )m
E mgH
Therefore Euler’s Equation will become
H
Vw1U1 Vw2U2
g
49. Components of Energy Transfer It is
worth mentioning in this context
that either of the Eqs. is
applicable regardless of changes
in density or components of
velocity in other directions.
Moreover, the shape of the path
taken by the fluid in moving from
inlet to outlet is of no
consequence. The expression
involves only the inlet and outlet
conditions. A rotor, the moving
part of a fluid machine, usually
consists of a number of vanes or
blades mounted on a circular
disc. Figure shows the velocity
triangles at the inlet and outlet
of a rotor. The inlet and outlet
portions of a rotor vane are only
shown as a representative of the
whole rotor
50. From the inlet velocity triangle,
or,
or,
get H (Work head, i.e. energy per unit weight of fluid, transferred between
the fluid and the rotor as) as
2 2
1 1
1 1 1
1
2 2 2
1 1 w1
r1
V V U 2U V Cos V U 2U V
2
V 2
U 2
V2
U1Vw1 1 1 r1
2
2
2
2 2
2
2 2 2
2
2
2 w2
Similarly from the outlet velocity triangle. r2 2
V V U 2U V Cos V U 2UV
2
V 2
U 2
V 2
U2Vw2 2 2 r2
1 2
1 2 1 2 r1 r2
H [(V V 2
) (U2
U 2
) (V2
V 2
)
Invoking the expressions ofU1 Vw 1 and in Euler’s Eq. , we
U 2 V w 2
• Vector diagrams of velocities at inlet and outlet correspond to two velocity
triangles, where Vr is the velocity of fluid relative to the rotor 1and2 are
the angles made by the directions of the absolute velocities at the inlet
and outlet respectively with the tangential direction, while1and2are the
angles made by the relative velocities with the tangential direction. The
angles 1 and 2 should match with vane or blade angles at inlet and
outlet respectively for a smooth, shockless entry and exit of the fluid to
avoid undersirable losses. Now we shall apply a simple geometrical
relation as follows:
51. The Eq is an important form of the Euler's equation relating to fluid
machines since it gives the three distinct components of energy
transfer as shown by the pair of terms in the round brackets. These
components throw light on the nature of the energy transfer. The
first term of Eq. is readily seen to be the change in absolute kinetic
energy or dynamic head of the fluid while flowing through the rotor.
The second term of Eq. represents a change in fluid energy
due to the movement of the rotating fluid from one radius of
rotation to another.
1 2
2
2
2
2
1
2
2
2
1 r2
r1
2g
H [(V V ) (U U ) (V V )
52. • Energy Transfer in Axial Flow Machines
For an axial flow machine, the main direction of flow is parallel to
the axis of the rotor, and hence the inlet and outlet points of the
flow do not vary in their radial locations from the axis of rotation.
Therefore, U1 U2 and the equation of energy transfer Eq. can be
written, under this situation, as
53. • Radially Outward and Inward Flow Machines
For radially outward flow machines,U2 U1 , and
hence the fluid gains in static head, while, for
a radially inward flow machine, U2 U1 and the
fluid losses its static head. Therefore, in radial
flow pumps or compressors the flow is always
directed radially outward, and in a radial flow
turbine it is directed radially inward.
54. Impulse and Reaction Machines
Impulse and Reaction Machines The relative proportion of energy
transfer obtained by the change in static head and by the change in
dynamic head is one of the important factors for classifying fluid
machines. The machine for which the change in static head in the
rotor is zero is known as impulse machine . In these machines, the
energy transfer in the rotor takes place only by the change in
dynamic head of the fluid. The parameter characterizing the
proportions of changes in the dynamic and static head in the rotor
of a fluid machine is known as degree of reaction and is defined as
the ratio of energy transfer by the change in static head to the total
energy transfer in the rotor.
Therefore, the degree of reaction,
55. Impulse and Reaction Machines
For an impulse machine R = 0 , because there is no change in static
pressure in the rotor. It is difficult to obtain a radial flow impulse
machine, since the change in centrifugal head is obvious there.
Nevertheless, an impulse machine of radial flow type can be
conceived by having a change in static head in one direction
contributed by the centrifugal effect and an equal change in the
other direction contributed by the change in relative velocity.
However, this has not been established in practice. Thus for an axial
flow impulse machine U1 U2 ;Vr1 Vr2 . For an impulse machine, the
rotor can be made open, that is, the velocity V1 can represent an
open jet of fluid flowing through the rotor, which needs no casing. A
very simple example of an impulse machine is a paddle wheel
rotated by the impingement of water from a stationary nozzle as
shown in Fig
56.
57. • A machine with any degree of reaction must have an enclosed rotor so
that the fluid cannot expand freely in all direction. A simple example of a
reaction machine can be shown by the familiar lawn sprinkler, in which
water comes out (Fig. b) at a high velocity from the rotor in a tangential
direction. The essential feature of the rotor is that water enters at high
pressure and this pressure energy is transformed into kinetic energy by a
nozzle which is a part of the rotor itself.
• In the earlier example of impulse machine (Fig. a), the nozzle is stationary
and its function is only to transform pressure energy to kinetic energy and
finally this kinetic energy is transferred to the rotor by pure impulse
action. The change in momentum of the fluid in the nozzle gives rise to a
reaction force but as the nozzle is held stationary, no energy is transferred
by it. In the case of lawn sprinkler (Fig. b), the nozzle, being a part of the
rotor, is free to move and, in fact, rotates due to the reaction force caused
by the change in momentum of the fluid and hence the word reaction
machine follows.
58. Efficiencies
The concept of efficiency of any machine comes from
the consideration of energy transfer and is defined, in
general, as the ratio of useful energy delivered to the
energy supplied. Two efficiencies are usually
considered for fluid machines-- the hydraulic efficiency
concerning the energy transfer between the fluid and
the rotor, and the overall efficiency concerning the
energy transfer between the fluid and the shaft. The
difference between the two represents the energy
absorbed by bearings, glands, couplings, etc. or, in
general, by pure mechanical effects which occur
between the rotor itself and the point of actual power
input or output.
61. Principle of Similarity and Dimensional Analysis
The principle of similarity is a consequence of nature for any
physical phenomenon. By making use of this principle, it becomes
possible to predict the performance of one machine from the
results of tests on a geometrically similar machine, and also to
predict the performance of the same machine under conditions
different from the test conditions. For fluid machine, geometrical
similarity must apply to all significant parts of the system viz., the
rotor, the entrance and discharge passages and so on. Machines
which are geometrically similar form a homologous series.
Therefore, the member of such a series, having a common shape
are simply enlargements or reductions of each other. If two
machines are kinematically similar, the velocity vector diagrams at
inlet and outlet of the rotor of one machine must be similar to
those of the other. Geometrical similarity of the inlet and outlet
velocity diagrams is, therefore, a necessary condition for dynamic
similarity.
62. The ratio of rotor and shaft energy is represented by mechanical efficiency
63. Let us now apply dimensional analysis to determine the dimensionless parameters, i.e., the π terms as the
criteria of similarity for flows through fluid machines. For a machine of a given shape, and handling
compressible fluid, the relevant variables are given in Table
Variable physical parameters Dimensional formula
D = any physical dimension of the machine as a measure of the machine's size, usually the rotor diameter L
Q = volume flow rate through the machine L3 T -1
N = rotational speed (rev/min.) T -1
H = difference in head (energy per unit weight) across the machine. This may be either gained or given by the
fluid depending upon whether the machine is a pump or a turbine respectively.
L
ρ=density of fluid ML-3
µ= viscosity of fluid ML-1 T -1
E = coefficient of elasticity of fluid ML-1 T-2
g = acceleration due to gravity LT -2
P = power transferred between fluid and rotor (the difference between P and H is taken care of by the hydraulic
efficiency
ML2 T-3
64. In almost all fluid machines flow with a free surface does not
occur, and the effect of gravitational force is negligible.
Therefore, it is more logical to consider the energy per unit
mass gH as the variable rather than H alone so that
acceleration due to gravity does not appear as a separate
variable. Therefore, the number of separate variables
becomes eight: D, Q, N, gH, ρ, µ, E and P . Since the
number of fundamental dimensions required to express
these variable are three, the number of independent π
terms (dimensionless terms), becomes five. Using
Buckingham's π theorem with D, N and ρ as the repeating
variables, the expression for the terms are obtained as,
65. We shall now discuss the physical significance and
usual terminologies of the different π terms. All
lengths of the machine are proportional to D ,
and all areas to D2. Therefore, the average flow
velocity at any section in the machine is
of the rotor is proportional to the product ND .
The first π term can be expressed as
proportional to D2 . Again, the peripheral velocity
Q
