TURBOMACHINES-Introduction 2 - Malnad College of Engineering

TURBOMACHINES
VIJAYAVITHAL BONGALE
Associate Professor and Head
Department of Mechanical Engineering
Malnad College of Engineering,
Hassan – 573 201.
Mobile No:9448821954
E-mail : [email protected]
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APPLICATIONS OF FIRST AND SECOND LAW OF
THERMODYNAMICS TO TURBO MACHINES
Properties of the flowing fluid:
Fluid at rest: pressure, temperature and volume all directly
measurable.
Moving fluid: measuring devices used to measure the
properties can give different readings depending on the
speed and the measuring conditions.
1.Turbomachines involves high speed flow
2.In a high speed flow two kinds of states are defined
a. Static state
b. Total or stagnation state
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STATIC STATE:
Pressure, temperature and volume are determined at any
given fluid particle and these define the static state of the
fluid and the properties are called static properties.
The static properties are measured with instruments which
are at rest relative to the fluid.
Example:
To measure the temperature of the fluid moving with a
given speed, the measuring thermometer should
theoretically move with the same speed as the fluid particle
itself while the measurement is being made.
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STAGNATION OR TOTAL STATE:
It is defined as the terminal state of a fictitious,
isentropic and work-free thermodynamic process, during
which the macroscopic kinetic and potential energies of
the fluid particle are reduced to zero in steady flow.
The properties measured at stagnation state are called
total or stagnation properties
The initial state for the fictitious process is static state.
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Stagnation versus Static Properties:
Static Properties: Still air
•represent the properties you would
measure if you were moving with the flow
(at the local flow velocity)
•always defined in the flow’s reference
frame
Stagnation Properties: Moving air
•always defined by conditions at a point
•represent the (static) properties you’d
measure if you first brought the fluid at
that point to a stop (isentropically) with
respect to a chosen observer
•depends on observer’s reference frame
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STATIC PROPERTIES:
Pressure - P or p , Temperature – T , Specific enthalpy – h, Density – ρ etc.
STAGNATION PROPERTIES:
Pressure – P0 , Temperature – T0 , Enthalpy – h0, Density – ρ0 etc.
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Application of the FIRST law of thermodynamics:
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From equation (3) we conclude that, the energy transfer
as work per unit mass flow is therefore numerically equal
to the change in stagnation enthalpy of the fluid between
the turbomachine inlet and exit.
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Application of the SECOND law of thermodynamics:
Points 1 and
the inlet and
stator blades
Points 3 and
the inlet and
rotor blades
2 represent
exit of the
4 represent
exit of the
• In any turbomachine the energy transfer between the fluid and
the blades can occur only by dynamic action
• Work is done when the fluid flows over rotor blades and not
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over the stator blades
For flow between 1 and 2 ideally, there should be no stagnation
enthalpy changes, thus
h01 = h02
For flow between 3 and 4 however, the stagnation enthalpy
change may be positive or negative depending upon whether the
machine is power absorbing or power generating.
If the machine absorbs power, h04 > h03 and
if the machine generates power h04 < h03
If the system is perfectly reversible and isentropic, with no energy
transfer as work, no changes can occur in
1. stagnation enthalpy 2. stagnation pressure and 3. stagnation
temperature between the inlet and outlet of the machine
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In any turbomachine there will be necessarily be
a. energy transfer as work and
b. Frictional and other losses.
The effect of these losses is to
1.Reduce the stagnation pressure and increase the entropy
2.Reduce the net work output in a power generating machine and
increase the net work input in a power absorbing machine
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EFFICIENCIES:
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Mechanical losses in turbomachines are very very small, and If
mechanical efficiency of all the turbomachines are assumed to be
unity, then
ɳPG= ɳ a and ɳPA= ɳ a
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To determine adiabatic efficiency it is necessary to specify the
ideal work input or output from the fluid states at the inlet and
outlet respectively
The actual work input or output w is the quantity given by,
For power absorbing machines, w = h02 – h01 ,
whereas for power generating machine it is w = h01 – h02
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The ideal work input or output can be calculated by any one of the
following four equations:
1.On Total to Total basis
w t-t = h02’ – h01 -- for PA
= h01 – h02’ -- for PG
2. On Total to Static basis
w t-s = h2’ – h01 -- for PA
= h01 – h2’ -- for PG
3. On Static to Total basis
w s-t = h02’ – h1 -- for PA
= h1 – h02’ -- for PG
4. On Static to Static basis
w s-s = h2’ – h1 -- for PA
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Based on the calculations of mechanical work presented the
following efficiencies may be defined:
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