Thermodynamics Chapter 8

1
8
CHAPTER
Gas PowerCycles
2
Idealizations Help Manage Analysis of Complex
Processes
8-1
The analysis of many complex processes can be
reduced to a manageable level by utilizing some
idealizations

• (fig. 8-2)

3
P-v and T-s diagrams of a Carnot Cycle
8-2
4
Nomenclature for Reciprocating Engines
8-3

• (Fig. 8-10)

5
Reciprocating Engine Displacement and Clearance
Volumes
8-4

• (Fig. 8-11)

6
The Net Work Output of a Cycle
8-5
The net work output of a cycle is equivalent to
the product of the mean effect pressure and the
displacement volume

• (Fig. 8-12)

7
Actual and Ideal Cycles in Spark-Ignition Engines
and Their P-v Diagram
8-6

• (Fig. 8-13)

8
Schematic of a Two-Stroke Reciprocating Engine
8-7
9
T-s Diagram for the Ideal Otto Cycle
8-8

• (Fig. 8-15)

10
The Thermal Efficiency of the Otto Cycle
8-9
The thermal efficiency of the Otto Cycle
increases with the specific heat ratio k of the
working fluid

• (Fig. 8-18)

11
T-s and P-v Diagrams for the Ideal Diesel Cycle
8-10

• (Fig. 8-21)

12
Thermal Efficiency of the Ideal Diesel Cycle
8-11
The thermal efficiency of the ideal Diesel cycle
as a function of compression and cutoff rates
(k1.4)

• (Fig. 8-22)

13
P-v Diagram of an Ideal Dual Cycle
8-12

• (Fig. 8-23)

14
T-s and P-v Diagrams of Carnot, Stirling, and
Ericsson Cycles
8-13

• (Fig. 8-26)

15
An Open-Cycle Gas-Turbine Engine
8-14

• (Fig. 8-29)

16
A Closed-Cycle Gas-Turbine Engine
8-15

• (Fig. 8-30)

17
T-s and P-v Diagrams for the Ideal Brayton Cycle
8-16

• (Fig. 8-31)

18
Thermal Efficiency of the Ideal Brayton Cycle as
a Function of the Pressure Ratio
8-17

• (Fig. 8-32)

19
The Net Work of the Brayton Cycle
8-18
For fixed values of Tmin and Tmax, the net work
of the Brayton cycle first increases with the
pressure ratio, then reaches a maximum at
rp(Tmax/Tmin)k/2(k-1), and finally decreases
20
The Back-Work Ratio is the Fraction of Turbine
Work Used to Drive the Compressor
8-19

• (Fig. 8-34)

21
Deviation of Actual Gas-Turbine Cycle From
Brayton cycle
8-20
The deviation of an actual gas-turbine cycle from
the ideal Brayton cycle as a result of
irreversibilities

• (Fig. 8-36)

22
A Gas-Turbine Engine With Regenerator
8-21

• (Fig. 8-38)

23
T-s Diagram of a Brayton Cycle with Regeneration
8-22

• (Fig. 8-39)

24
Thermal Efficiency of the ideal Brayton cycle
with and without regeneration
8-23

• (Fig. 8-40)

25
A Gas-Turbine Engine
8-24
A gas-turbine engine with two-stage compression
with intercooling, two-stage expansion with
reheating, and regeneration

• (Fig. 8-43)

26
T-s Diagram of Ideal Gas-Turbine Cycle with
Intercooling, Reheating, and Regeneration
8-25

• (Fig. 8-44)

27
Turbojet Engine Basic Components and T-s Diagram
for Ideal Turbojet Cycle
8-26
28
Schematic of A Turbofan Engine
8-27

• (Fig. 8-52)

29
Illustration of A Turbofan Engine
8-28
30
Schematic of a Turboprop Engine
8-29

• (Fig. 8-54)

31
Schematic of a Ramjet Engine
8-30

• (Fig. 8-55)

32
Chapter Summary
8-31

• A cycle during which a net amount of work is
  produced is called a power cycle, and a power
  cycle during which the working fluid remains a
  gas throughout is called a gas power cycle.

33
Chapter Summary
8-32

• The most efficient cycle operating between a heat
  source at temperature TH and a sink at
  temperature TL is the Carnot cycle, and its
  thermal efficiency is given by

34
Chapter Summary
8-33

• The actual gas cycles are rather complex. The
  approximations used to simplify the analysis are
  known as the air-standard assumptions. Under
  these assumptions, all the processes are assumed
  to be internally reversible the working fluid is
  assumed to be air, which behaves as an ideal gas
  and the combustion and exhaust processes are
  replaced by heat-addition and heat-rejection
  processes, respectively.

35
Chapter Summary
8-34

• The air-standard assumptions are called
  cold-air-standard assumptions if, in addition,
  air is assumed to have constant specific heats at
  room temperature.

36
Chapter Summary
8-35

• In reciprocating engines, the compression ratio r
  and the mean effective pressure MEP are defined as

37
Chapter Summary
8-36

• The Otto cycle is the ideal cycle for the
  spark-ignition reciprocating engines, and it
  consists of four internally reversible processes
  isentropic compression, constant volume heat
  addition, isentropic expansion, and con-stant
  volume heat rejection.

38
Chapter Summary
8-37

• Under cold-air-standard assumptions, the thermal
  efficiency of the ideal Otto cycle
  iswhere r is the compression ratio and k
  is the specific heat ratio Cp /Cv.

39
Chapter Summary
8-38

• The Diesel cycle is the ideal cycle for the
  compression-ignition reciprocating engines. It is
  very similar to the Otto cycle, except that the
  constant volume heat-addition process is replaced
  by a constant pressure heat-addition process.

40
Chapter Summary
8-39

• The Diesel cycle thermal efficiency under
  cold-air-standard assumptions iswhere rc
  is the cutoff ratio, defined as the ratio of the
  cylinder volumes after and before the combustion
  process.

41
Chapter Summary
8-40

• Stirling and Ericsson cycles are two totally
  reversible cycles that involve an isothermal
  heat-addition process at TH and an isothermal
  heat-rejection process at TL. They differ from
  the Carnot cycle in that the two isentropic
  processes are replaced by two constant volume
  regeneration processes in the Stirling cycle and
  by two constant pressure regeneration processes
  in the Ericsson cycle. Both cycles utilize
  regeneration, a process during which heat is
  transferred to a thermal energy storage device
  (called a regenerator) during one part of the
  cycle that is then transferred back to the
  working fluid during another part of the cycle.

42
Chapter Summary
8-41

• The ideal cycle for modern gas-turbine engines is
  the Brayton cycle, which is made up of four
  internally reversible processes isentropic
  compression, constant pressure heat addition,
  isentropic expansion, and constant pressure heat
  rejection.

43
Chapter Summary
8-42

• Under cold-air-standard assumptions, the Brayton
  cycle thermal efficiency iswhere rp
  Pmax/Pmin is the pressure ratio and k is the
  specific heat ratio. The thermal efficiency of
  the simple Brayton cycle increases with the
  pressure ratio.

44
Chapter Summary
8-43

• The deviation of the actual compressor and the
  turbine from the idealized isentropic ones can be
  accurately accounted for by utilizing their
  adiabatic efficiencies, defined asand
  where states 1 and 3 are the inlet states,
  2a and 4a are the actual exit states, and 2s and
  4s are the isentropic exit states.

45
Chapter Summary
8-44

• In gas-turbine engines, the temperature of the
  exhaust gas leaving the turbine is often
  considerably higher than the temperature of the
  air leaving the compressor. Therefore, the
  high-pressure air leaving the compressor can be
  heated by transferring heat to it from the hot
  exhaust gases in a counter-flow heat exchanger,
  which is also known as a regenerator.

46
Chapter Summary
8-45

• The extent to which a regenerator approaches an
  ideal regenerator is called the effectiveness e
  and is defined as

47
Chapter Summary
8-46

• Under cold-air-standard assumptions, the thermal
  efficiency of an ideal Brayton cycle with
  regeneration becomes where T1 and T3 are
  the minimum and maximum temperatures,
  respectively, in the cycle.

48
Chapter Summary
8-47

• The thermal efficiency of the Brayton cycle can
  also be increased by utilizing multistage
  compression with intercooling, regeneration, and
  multistage expansion with reheating. The work
  input to the compressor is minimized when equal
  pressure ratios are maintained across each stage.
  This procedure also maximizes the turbine work
  output.

49
Chapter Summary
8-48

• Gas-turbine engines are widely used to power
  aircraft because they are light and compact and
  have a high power-to-weight ratio. The ideal
  jet-propulsion cycle differs from the simple
  ideal Brayton cycle in that the gases are
  partially expanded in the turbine. The gases that
  exit the turbine at a relatively high pressure
  are subsequently accelerated in a nozzle to
  provide the thrust needed to propel the aircraft.

50
Chapter Summary
8-49

• The net thrust developed by the turbojet engine
  iswhere m is the mass flow rate of gases,
  Vexit is the exit velocity of the exhaust gases,
  and Vinlet is the inlet velocity of the air, both
  relative to the aircraft

51
Chapter Summary
8-50

• The power developed from the thrust of the engine
  is called the propulsive power Wp and it is given
  by

.
52
Chapter Summary
8-51

• Propulsive efficiency is a measure of how
  efficiently the energy released during the
  combustion process is converted to propulsive
  energy, and it is defined as

53
Chapter Summary
8-52

• For an ideal cycle that involves heat transfer
  only with a source at TH and a sink at TL, the
  irreversibility or exergy destruction is
  determined to be