Gas turbine and propulsion system

1. Department of
Mechanical Engineering
Session: 2015-16
Submitted by:
Sec B
Roll no.:
51,52,53,54,55
Prof. K.Rambhad

2. Difference Between open cycle &
closed cycle Gas Turbine
Open cycle Closed cycle

3. Open cycle processes
1) 1-2′ represents : irreversible adiabatic compression.
2) 2′-3 represents : constant pressure heat supply in the combustion
chamber.
3) 3-4′ represents : irreversible adiabatic expansion.
4) 1-2 represents : ideal isentropic compression.
5) 3-4 represents : ideal isentropic expansion.

4. Operation 3-4 : The air is expanded isentropically from p2 to p1, the temperature
falling from T3 to T4. No heat flow occurs.
Operation 4-1 : Heat is rejected from the system as the volume decreases from V4 to
V1 and the temperature from T4 to T1 whilst the pressure remains constant at p1. Heat
rejected = mcp (T4 –T1 )
Closed cycle processes
Operation 1-2 : The air is
compressed isentropically from the
lower pressure p1 to the upper
pressure p2, the temperature rising
from T1 to T2. No heat flow occurs.
Operation 2-3 : Heat flow into the
system increasing the volume from V2
to V3 and
temperature from T2 to T3 whilst the
pressure remains constant at p2.
Heat received = mcp (T3 – T2).

5. Gas Turbine Cycle with Inter-cooling
The cooling of air between two stages of compression is known as intercooling. This
reduces the work of compression and increases the specific output of the plant with a
decrease in the thermal efficiency. The loss in efficiency due to intercooling can be
remedied by employing exhaust heat exchange as in the reheat cycle
Specific work output =
Cycle with intercooling
Heat supplied =
If is constant and not dependent on
temperature, we can write:
Note:

6. Here heat supply and output both increases as compared to simple cycle. Because the
increase in heat supply is proportionally more, decreases.
With multiple inter-cooling and multiple reheat, the compression and expansion
processes tend to be isothermal as shown in Figure 5.3
Multiple reheat and intercool cycle
The cycle tends towards the Ericsson cycle, the efficiency is same as that of the Carnot
cycle
The use of intercoolers is seldom contemplated in practice because they are bulky
and need large quantities of cooling water. The main advantage of the gas turbine,
that it is compact and self-contained, is then lost.

7. Gas Turbine Cycle with Reheat
A common method of increasing the mean temperature of heat reception is to reheat the
gas after it has expanded in a part of the gas turbine. By doing so the mean temperature
of heat rejection is also increased, resulting in a decrease in the thermal efficiency of
the plant. However , the specific output of the plant increases due to reheat. A reheat
cycle gas turbine plant is shown in Figure
Reheat cycle gas turbine plant
The specific work output is given by
The heat supplied to the cycle is

8. Thus, the cycle efficiency,
Therefore, a reheat cycle is used to increase the work output while a
regenerative cycle is used to enhance the efficiency
Gas turbine with regenerative
The effectiveness of the heat exchanger, or regenerator, is a measure of how
well it uses the available temperature potential to raise the temperature of the
compressor discharge air. Specifically, it is the actual rate of heat transferred
to the air divided by the maximum possible heat transfer rate that would exist
if the heat exchanger had infinite heat transfer surface area. The actual heat
transfer rate to the air is mcp(Tc T2), and the maximum possible rate is
mcp(T4 T2). Thus the regenerator effectiveness can be written as
nreg = ( Tc -T2 )/( T4- T2)

9. and the combustor inlet temperature can be written as
Tc = T2 + nreg( T4- T2)
It is seen that the combustor inlet temperature varies from T2 to T4 as the
regenerator effectiveness varies from 0 to 1. The regenerator effectiveness increases
as its heat transfer area increases. Increased heat transfer area allows the cold fluid to
absorb more heat from the hot fluid and therefore leave the exchanger with a higher
Tc. On the other hand, increased heat transfer area implies increased pressure losses
on both air and gas sides of the heat exchanger, which in turn reduces the turbine
pressure ratio and therefore the turbine work. Thus, increased regenerator
effectiveness implies a tradeoff, not only with pressure losses but with increased heat
exchanger size and complexity and, therefore, increased cost.

10. The exhaust gas temperature at the exit of the heat exchanger may be determined by
applying the steady-flow energy equation to the regenerator. Assuming that the heat
exchanger is adiabatic and that the mass flow of fuel is negligible compared with the air
flow, and noting that no shaft work is involved, we may write the steady-flow energy
equation for two inlets and two exits as
q = 0 = he + hc –h2 –h4 + w = cp,gTe + cpTc –cpT2 –cp,gT4 +
0
Thus the regenerator combustion-gas-side exit temperature is:
Te = T4 –(cp/cpg)( Tc -–T2 )
While the regenerator effectiveness does not appear explicitly in Equation (5.28),
the engine exhaust temperature is reduced in proportion to the air temperature rise
in the regenerator, which is in turn proportional to the effectiveness. The
dependence of
the exhaust temperature on nreg may be seen directly by eliminating Tc from
Equation
(5.28), using Equation (5.27) to obtain
T4 –- Te = nreg (cp/cp.g)(T4 -–T2)

11. The regenerator exhaust gas temperature reduction, T4 -–Te, is seen to be jointly
proportional to the effectiveness and to the maximum temperature potential, T4 -–
T2.
The regenerator, like other heat exchangers, is designed to have minimal pressure
losses on both air and gas sides. These may be taken into account by the fractional
pressure drop approach discussed in connection with the combustor.

12. TO LIST OUT THE ADVANTAGES OF GAS TURBINES OVER IC
ENGINES AND STEAM TURBINES
ADVANTAGES OF GAS TURBINES OVER STEAM TURBINES
- The important components are compressor and combustion chamber and not the
boiler and accessories.
- A gas turbine requires less space for installation.
- The installation and running cost is less.
- The starting of gas turbine is very easy and quick.
- Its control, with the changing load conditions, is easy.
- A gas turbine does not depend on water supply.

13. ADVANTAGES OF GAS TURBINE OVER IC ENGINES
- The installation and running cost is less.
- The efficiency is higher than IC engine.
- The balancing of gas turbine is perfect and does not
require a lot of maintenance in this matter.
- The torque produced is uniform. Thus no use of flywheel
is required.
- The lubrication and ignition systems are simple which
gives huge advantage over an IC engine.
- It can be driven at a very higher speed.
- The pressures used are very low.
- The exhaust of a gas turbine is free from smoke and less
polluting.
- They are very suitable for air crafts.

14. PROPELLER JET :
• An aircraft propeller or airscrew converts rotary motion from a piston engine, a
turboprop or an electric motor, to provide propulsive force. Its pitch may be fixed
or variable. Early aircraft propellers were carved by hand from solid or laminated
wood, while later propellers were constructed of metal. Modern designs use high-
technology composite materials.
• The propeller attaches to the crankshaft of a piston engine, either directly or
through a reduction unit.
• A light aircraft engine may not require the complexity of gearing, which is
essential on a larger engine or on a turboprop aircraft.A well-designed propeller
typically has an efficiency of around 80% when operating in the best regime. The
efficiency of the propeller is influenced by the angle of attack (α).
• This is defined as:
α = Φ - θ,
where, θ is the helix angle (the angle between the resultant relative velocity and
the blade rotation direction) and Φ is the blade pitch angle.

15. • Very small pitch and helix angles give a good performance against resistance but
provide little thrust, while larger angles have the opposite effect. The best helix angle
is when the blade is acting as a wing producing much more lift than drag.
Forces acting on a propeller :-
• Five forces act on the blades of an aircraft propeller in motion, they are:
• Thrust bending force
– Thrust loads on the blades act to bend them forward.
• Centrifugal twisting force
– Acts to twist the blades to a low, or fine pitch angle.
• Aerodynamic twisting force
– As the centre of pressure of a propeller blade is forward of its centre line the
blade is twisted towards a coarse pitch position.
• Centrifugal force
– The force felt by the blades acting to pull them away from the hub when turning.
• Torque bending force
– Air resistance acting against the blades, combined with inertial effects causes
propeller blades to bend away from the direction of rotation.

16. TURBO JET PROPULSION SYSTEM :
• The turbojet is an air breathing jet engine, usually used in aircraft. It consists of a
gas turbine with a propelling nozzle. The gas turbine has an air inlet, a compressor,
a combustion chamber, and a turbine (that drives the compressor). The compressed
air from the compressor is heated by the fuel in the combustion chamber and then
allowed to expand through the turbine.
• The turbine exhaust is then expanded in the propelling nozzle where it is
accelerated to high speed to provide thrust.
• Turbojets have been replaced in slower aircraft by turboprops which use less fuel.
At medium speeds, where the propeller is no longer efficient, turboprops have been
replaced by turbofan. The turbofan is quieter and uses less fuel than the turbojet.
Turbojets are still common in medium range cruise missiles, due to their high
exhaust speed, small frontal area, and relative simplicity.
• The jet engine is only efficient at high vehicle speeds, which limits their usefulness
apart from aircraft. Turbojet engines have been used in isolated cases to power
vehicles other than aircraft, typically for attempts on land speed records.
• Where vehicles are 'turbine powered' this is more commonly by use of a turboshaft
engine, a development of the gas turbine engine where an additional turbine is used
to drive a rotating output shaft. These are common in helicopters and hovercraft.
Turbojets have also been used experimentally to clear snow from switches in rail
yards.

17. TURBO-PROP :
• A turboprop engine is a turbine engine that drives an aircraft propeller. In contrast
to a turbojet, the engine's exhaust gases do not contain enough energy to create
significant thrust, since almost all of the engine's power is used to drive the
propeller.
• The propeller is coupled to the turbine through a reduction gear that converts the
high RPM, low torque output to low RPM, high torque. The propeller itself is
normally a constant speed (variable pitch) type similar to that used with larger
reciprocating aircraft engines.
• In its simplest form a turboprop consists of an intake, compressor, combustor,
turbine, and a propelling nozzle. Air is drawn into the intake and compressed by
the compressor. Fuel is then added to the compressed air in the combustor, where
the fuel-air mixture then combusts.
• The hot combustion gases expand through the turbine. Some of the power generated
by the turbine is used to drive the compressor. The rest is transmitted through the
reduction gearing to the propeller.
• Further expansion of the gases occurs in the propelling nozzle, where the gases
exhaust to atmospheric pressure. The propelling nozzle provides a relatively small
proportion of the thrust generated by a turboprop.

18. A jet engine is a reaction engine
discharging a fast moving
jet that generates thrust by jet
propulsion in accordance with
Newton's laws of motion. This broad
definition of jet engines includes
turbojets, turbofans, rockets, ramjets,
and pulse jets
THRUST:

19. THRUST POWER:
Thrust Power
Generation of thrust in
flight requires the
expenditure of power. For
a propeller or a jet-engine
fan, the shaft power and
the thrust are related by
the definition of propeller
efficiency.

20. Image result for propulsive
efficiency
In aircraft and rocket design,
overall propulsive efficiency is
the efficiency, in percent, with
which the energy contained in a
vehicle's propellant is converted
into useful energy, to replace
losses due to aerodynamic drag,
gravity, and acceleration.
Propulsive efficiency:

21. The thermal efficiency is a
dimensionless performance
measure of a thermal device such
as an internal combustion engine,
a boiler,or a furnace for example.
The input to the device is heat or
the heat-content of a fuel that is
consumed. The desired output is
mechanical work, or heator
possibly both
Thermal efficiency: