Gas Turbines

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GAS TURBINE POWER PLANTS
• Gas turbines tend to be lighter and more
compact than the vapour power plants.
• Gas turbines are used for stationary power
generation.
• In addition, the favourable power-outputto-weight ratio of gas turbines makes them
well suited for transportation applications
(aircraft propulsion, marine power plants,
and so on).
Simple gas turbine
(a) Open to the atmosphere
(b) Closed
AIR-STANDARD BRAYTON CYCLE
Air-standard gas turbine cycle
The back work ratio for the cycle is
• A relatively large portion of the work developed by the turbine is required to drive
the compressor. Typical back work ratios
of gas turbines range from 40 to 80%.
• In comparison, the back work ratios of
vapour power plants are normally only 1 or
2%.
Ideal Air-Standard Brayton Cycle
Air-standard ideal Brayton cycle
Effect of Pressure Ratio on Performance
Thermal efficiency as a function of compressor
pressure ratio for the cold air-standard ideal
Brayton cycle, k = 1.4
Ideal Brayton cycles with different pressure
ratios and the same turbine inlet temperatures
Example 1
Air enters the compressor of an ideal air-standard
Brayton cycle at 100 kPa, 300 K, with a volumetric
flow rate of 5 m3/s. The compressor pressure
ratio is 10. The turbine, inlet temperature is 1400
K. Determine
(a) the thermal efficiency of the cycle,
(b) the back work ratio,
(c) the net power developed, in kW.
Example 2
Determine the pressure ratio across the
compressor of an ideal Brayton cycle for the
maximum net work output per unit of mass flow if
the state at the compressor inlet and the
temperature at the turbine inlet are fixed. Use a
cold air-standard analysis and ignore kinetic and
potential energy effects.
Principal Irreversibilities and Losses
Effects of irreversibilities on the simple
closed-cycle gas turbine
Example 3
Reconsider Example 1, but include in the
analysis that the turbine and compressor
each have an efficiency of 80%. Determine
for the modified cycle
(a) the thermal efficiency of the cycle,
(b) the back work ratio,
(c) the net power developed, in kW.
REGENERATIVE GAS TURBINES
Regenerative airstandard gas turbine
cycle
Temperature distributions in counterflow heat
exchangers (a) Actual (b) Reversible
Example 4
If a regenerator with an effectiveness of 80% is
incorporated in the cycle of Example 1, determine
the thermal efficiency.
Gas Turbines with Reheat
Ideal gas turbine with reheat
Example 5
Consider a modification of the cycle of Example
[1] involving reheat and regeneration. Air enters
the compressor at 100 kPa, 300 K and is
compressed to 1000 kPa. The temperature at the
inlet to the first turbine stage is 1400 K. The
expansion takes place isentropically in two
stages, with reheat to 1400 K between the stages
at a constant pressure of 300 kPa. A regenerator
having an effectiveness of 100% is also
incorporated in the cycle. Determine the thermal
efficiency.
Compression with Intercooling
Internally reversible compression processes
between two fixed pressures
Two-stage compression with intercooling
Example 6
Air is compressed from 100 kPa, 300 K 1to 1000 kPa in
a two-stage compressor with intercooling between
stages. The intercooler pressure is 300 kPa. The air is
cooled back to 300 K in the intercooler before entering
the second compressor stage. Each compressor stage
is isentropic. For steady-state operation and negligible
changes in kinetic and potential energy from inlet to
exit, determine
(a) the temperature at the exit of the second
compressor stage and
(b) the total compressor work input per unit of mass
flow.
(c) Repeal for a single stage of compression from the
given inlet slate to the final pressure.
Example 7
If the inlet state and the exit pressure are specified
for a two-stage compressor operating at steady
state, show that the minimum total work input is
required when the pressure ratio is the same
across each stage. Use a cold air-standard
analysis assuming that each compression
process is isentropic, there is no pressure drop
through the intercooler, and the temperature at the
inlet to each compressor stage is the same.
Kinetic and potential energy effects can be
ignored.
Reheat and Intercooling
Regenerative gas turbine with intercooling and
reheat
Example 8
A regenerative gas turbine with intercooling and
reheat operates at steady state. Air enters the
compressor at 100 kPa, 300 K with a mass flow rate of
5.807 kg/s. The pressure ratio across the two-stage
compressor is 10. The pressure ratio across the twostage turbine is also 10. The intercooler and reheater
each operate at 300 kPa. At the inlets to the turbine
stages, the temperature is 1400 K. The temperature at
the inlet to the second compressor stage is 300 K.
The efficiency of each compressor and turbine stage
is 80%. The regenerator effectiveness is 80%.
Determine
(a) the thermal efficiency,
(b) the back work ratio,
(c) the net power developed, in kW.
GAS TURBINES FOR
AIRCRAFT PROPULSION
Turbojet engine
schematic and
accompanying
ideal T-s diagram
Schematic of a turbojet engine with afterburner
Other examples of
aircraft engines
(a) Turboprop
(b) Turbofan
(c) Ramjet
Example 9
Air enters a turbojet engine at 0.8 bar, 240 K, and an
inlet velocity of 1000 Km/h (278 m/s). The pressure
ratio across the compressor is 8. The turbine inlet
temperature is 1200 K and the pressure at the
nozzle exit is 0.8 bar. The work developed by the
turbine just equals the compressor work input. The
diffuser, compressor, turbine, and nozzle processes
are isentropic, and there is no pressure drop for
flow through the combustor. For operation at
steady state, determine the velocity at the nozzle
exit and the pressure at each principal state.
Neglect kinetic energy at the exit of all components
except the nozzle and neglect potential energy
throughout.
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