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The irreversibility associated with a student studying and watching a movie on television, each for two hours.
A hard-working laborer, for example, may make full use of his *physical exergy* but little use of his *intellectual exergy.* That laborer, for example, could learn a foreign language or a science by listenin... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE 8–47**",
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The energy content of the universe is constant, just as its mass content is. Yet at times of crisis we are bombarded with speeches and articles on how to "conserve" energy. As engineers, we know that energy is already conserved. What is not conserved is *exergy*, which is the useful work potential of the energy. Once t... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**SUMMARY**",
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- **8–1C** What final state will maximize the work output of a device?
- **8–2C** Is the exergy of a system different in different environments?
- **8–3C** Under what conditions does the reversible work equal irreversibility for a process?
- **8–4C** How does useful work differ from actual work? For what kinds of syste... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Exergy, Irreversibility, Reversible Work, and Second-Law Efficiency**",
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**8–15** One method of meeting the extra electric power demand at peak periods is to pump some water from a large body of water (such as a lake) to a reservoir at a higher elevation at times of low demand and to generate electricity at times of high demand by letting this water run down and rotate a turbine (i.e., conv... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–14E**",
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- **8–27C** Can a system have a higher second-law efficiency than the first-law efficiency during a process? Give examples.
- **8–28** A mass of 8 kg of helium undergoes a process from an initial state of 3 m3 /kg and 15°C to a final state of 0.5 m3 / kg and 80°C. Assuming the surroundings to be at 25°C and 100 kPa, de... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Exergy Analysis of Closed Systems**",
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Thermal equilibrium is established after a while as a result of heat transfer between the blocks and the lake water. Assuming the surroundings to be at 20°C, determine the amount of work that could have been produced if the entire process were executed in a reversible manner.
**8–45** Carbon steel balls (*ρ* = 7833 k... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Exergy Analysis of Closed Systems**",
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**8–46E** A 70-lbm copper block initially at 220°F is dropped into an insulated tank that contains 1.2 ft3 of water at 65°F. Determine (*a*) the final equilibrium temperature and (*b*) the work potential wasted during this process. Assume the surroundings to be at 65°F.
**8–47** An ordinary egg can be approximated as... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–45**",
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- **8–50** Steam is throttled from 8 MPa and 450°C to 6 MPa. Determine the wasted work potential during this throttling process. Assume the surroundings to be at 25°C. Answer: 36.6 kJ/kg
- **8–51** Refrigerant-134a enters an expansion valve at 1200 kPa as a saturated liquid and leaves at 200 kPa. Determine (*a*) the te... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Exergy Analysis of Control Volumes**",
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**8–66** Air enters the evaporator section of a window air conditioner at 100 kPa and 27°C with a volume flow rate of 6 m<sup>3</sup> /min. Refrigerant-134a at 120 kPa with a quality of 0.3 enters the evaporator at a rate of 2 kg/min and leaves as saturated vapor at the same pressure. Determine the exit temperature of ... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–65**",
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**FIGURE P8–79**
**8–80** A well-insulated shell-and-tube heat exchanger is used to heat water (*cp* = 4.18 kJ/kg·°C) in the tubes from 20 to 70°C at a rate of 4.5 kg/s. Heat is supplied by hot oil (*cp* = 2.30 kJ/ kg·°C) that enters the shell side at 170°C at a rate of 10 kg/s. Dis... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–65**",
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**8–85E** A refrigerator has a second-law efficiency of 28 percent, and heat is removed from the refrigerated space at a rate of 800 Btu/min. If the space is maintained at 25°F while the surrounding air temperature is 90°F, determine the power input to the refrigerator.
**8–86** The inner and outer surfaces of a 0.5-... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Review Problems**",
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- **8–94** A crater lake has a base area of 20,000 m2 , and the water it contains is 12 m deep. The ground surrounding the crater is nearly flat and is 105 m below the base of the lake. Determine the maximum amount of electrical work, in kWh, that can be generated by feeding this water to a hydroelectric power plant. A... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–93**",
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**8–105** To control an isentropic steam turbine, a throttle valve is placed in the steam line leading to the turbine inlet. Steam at 6 MPa and 600°C is supplied to the throttle inlet, and the turbine exhaust pressure is set at 40 kPa. What is the effect on the stream exergy at the turbine inlet when the throttle valve... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–104**",
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P8–120 has a volume of 500,000 m3 , and it initially contains air at 100 kPa and 20°C. The isentropic compressor proceeds to compress air that enters the compressor at 100 kPa and 20°C until the tank is filled at 600 kPa and 20°C. All heat exchanges are with the surrounding air at 20°C. Calculate the change in the work... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–104**",
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**8–130** Can closed-system exergy be negative? How about flow exergy? Explain using an incompressible substance as an example.
**8–131** Obtain a relation for the second-law efficiency of a heat engine that receives heat *QH* from a source at temperature $T_H$ and rejects heat $Q_L$ to a sink at $T_L$ , which i... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**FIGURE P8–129**",
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- **8–134** Keeping the limitations imposed by the second law of thermodynamics in mind, choose the *wrong* statement below:
- (a) A heat engine cannot have a thermal efficiency of 100 percent.
- (b) For all reversible processes, the second-law efficiency is 100 percent.
- (c) The second-law efficiency of a heat engine... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "Fundamentals of Engineering (FE) Exam Problems",
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**8–144** Obtain the following information about a power plant that is closest to your town: the net power output; the type and amount of fuel used; the power consumed by the pumps, fans, and other auxiliary equipment; stack gas losses; temperatures at several locations; and the rate of heat rejection at the condenser.... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 3": "**Design and Essay Problems**",
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wo important areas of application for thermodynamics are power generation and refrigeration. Both are usually accomplished by systems that operate on a thermodynamic cycle. Thermodynamic cycles can be divided into two general categories: *power cycles*, which are discussed in this chapter and Chap. 10, and *refrigerati... | {
"Header 1": "**EXAMPLE 8–16** Exergy Destroyed During Mixing of Fluid Streams",
"Header 2": "**GAS POWER CYCLES**",
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Most power-producing devices operate on cycles, and the study of power cycles is an exciting and important part of thermodynamics. The cycles encountered in actual devices are difficult to analyze because of the presence of complicating effects, such as friction, and the absence of sufficient time for establishment of ... | {
"Header 1": "9-1 • BASIC CONSIDERATIONS IN THE ANALYSIS OF POWER CYCLES",
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An automotive engine with the combustion chamber exposed.
*©Idealink Photography/Alamy RF*
temperature limits. However, it is still considerably higher than the thermal efficiency of an actual cycle because of the idealizations utilized (Fig. 9–3).
The idealizations and simplifications commonly employed in the an... | {
"Header 1": "9-1 • BASIC CONSIDERATIONS IN THE ANALYSIS OF POWER CYCLES",
"Header 3": "**FIGURE 9–3**",
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The Carnot cycle is composed of four totally reversible processes: isothermal heat addition, isentropic expansion, isothermal heat rejection, and isentropic compression. The P-V and T-S diagrams of a Carnot cycle are replotted in Fig. 9–5. The Carnot cycle can be executed in a closed system (a piston-cylinder device) o... | {
"Header 1": "9-2 • THE CARNOT CYCLE AND ITS VALUE IN ENGINEERING",
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Show that the thermal efficiency of a Carnot cycle operating between the temperature limits of $T_H$ and $T_L$ is solely a function of these two temperatures and is given by Eq. 9–2.
**SOLUTION** It is to be shown that the efficiency of a Carnot cycle depends on the source and sink temperatures alone.
**Analysi... | {
"Header 1": "**EXAMPLE 9–1** Derivation of the Efficiency of the Carnot Cycle",
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In gas power cycles, the working fluid remains a gas throughout the entire cycle. Spark-ignition engines, diesel engines, and conventional gas turbines are familiar examples of devices that operate on gas cycles. In all these engines, energy is provided by burning a fuel within the system boundaries. That is, they are ... | {
"Header 1": "**EXAMPLE 9–1** Derivation of the Efficiency of the Carnot Cycle",
"Header 3": "**9–3** ■ **AIR-STANDARD ASSUMPTIONS**",
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An air-standard cycle is executed in a closed system and is composed of the following four processes:
- 1-2 Isentropic compression from 100 kPa and 27°C to 1 MPa
- 2-3 *P* = *constant* heat addition in amount of 2800 kJ/kg
- 3-4 *v* = *constant* heat rejection to 100 kPa
- 4-1 *P* = *constant* heat rejection to initi... | {
"Header 1": "**EXAMPLE 9–1** Derivation of the Efficiency of the Carnot Cycle",
"Header 3": "**EXAMPLE 9–2 An Air-Standard Cycle**",
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The net work output of a cycle is equivalent to the product of the mean effective pressure and the displacement volume.
top dead center (TDC)—the position of the piston when it forms the smallest volume in the cylinder—and the **bottom dead center** (BDC)—the position of the piston when it forms the largest volume in... | {
"Header 1": "9-4 • AN OVERVIEW OF RECIPROCATING ENGINES",
"Header 3": "FIGURE 9-12",
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The Otto cycle is the ideal cycle for spark-ignition reciprocating engines. It is named after Nikolaus A. Otto, who built a successful four-stroke engine in 1876 in Germany using the cycle proposed by Frenchman Beau de Rochas in 1862. In most spark-ignition engines, the piston executes ... | {
"Header 1": "9-5 • OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES",
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Therefore, heat transfer to and from the working fluid can be expressed as
$$q_{\rm in} = u_3 - u_2 = c_{\rm U}(T_3 - T_2)$$
(9–6a)
and
$$q_{\text{out}} = u_4 - u_1 = c_{\text{U}}(T_4 - T_1)$$
(9-6b)
Then the thermal efficiency of the ideal Otto cycle under the cold air standard assumptions becomes
$$\eta_{\r... | {
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An ideal Otto cycle has a compression ratio of 8. At the beginning of the compression process, air is at 100 kPa and 17°C, and 800 kJ/kg of heat is transferred to air during the constant-volume heat-addition process. Accounting for the variation of specific heats of air with temperature, determine (a) the maximum tempe... | {
"Header 1": "9-5 • OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES",
"Header 3": "**EXAMPLE 9-3** The Ideal Otto Cycle",
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MEP =
$$\frac{418.17 \text{ kJ/kg}}{(0.8323 \text{ m}^3/\text{kg})(1 - \frac{1}{8})} \left(\frac{1 \text{ kPa·m}^3}{1 \text{ kJ}}\right) = 574 \text{ kPa}$$
(e) The total air mass taken by all four cylinders when they are charged is
$$m = \frac{V_d}{V_1} = \frac{0.0016 \text{ m}^3}{0.8323 \text{ m}^3/\text{kg}} =... | {
"Header 1": "9-5 • OTTO CYCLE: THE IDEAL CYCLE FOR SPARK-IGNITION ENGINES",
"Header 3": "**EXAMPLE 9-3** The Ideal Otto Cycle",
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In diesel engines, the spark plug is replaced by a fuel injector, and only air is compressed during the compression process.
FIGURE 9–22 *T-s* and *P-v* diagrams for the ideal Diesel cycle.
In gasoline engines, a mixture of air and fuel is compressed dur... | {
"Header 1": "9-6 • DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES",
"Header 3": "**FIGURE 9-21**",
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An ideal Diesel cycle with air as the working fluid has a compression ratio of 18 and a cutoff ratio of 2. At the beginning of the compression process, the working fluid is at 14.7 psia, 80°F, and 117 in<sup>3</sup> . Utilizing the cold-air-standard assumptions, determine (*a*) the temperature and pressure of air at th... | {
"Header 1": "9-6 • DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES",
"Header 3": "**EXAMPLE 9–4 The Ideal Diesel Cycle**",
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*P-V* diagram for the ideal Diesel cycle discussed in Example 9–4.
**Analysis** The *P-V* diagram of the ideal Diesel cycle described is shown in Fig. 9–25. We note that the air contained in the cylinder forms a closed system.
(a) The temperature and pressure values at the end of each process can be determined by u... | {
"Header 1": "9-6 • DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES",
"Header 3": "FIGURE 9-25",
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The ideal Otto and Diesel cycles discussed in the preceding sections are composed entirely of internally reversible processes and thus are internally reversible cycles. These cycles are not totally reversible, however, since they involve heat transfer through a finite temperature difference during the nonisothermal hea... | {
"Header 1": "9-6 • DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES",
"Header 3": "9-7 • STIRLING AND ERICSSON CYCLES",
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A regenerator is a device that borrows energy from the working fluid during one part of the cycle and pays it back (without interest) during another part.
FIGURE 9–27 *T-s* and *P-v* diagrams of Carnot, Stirling, and Ericsson cycles.
The execution of the Stirling cycle requires rather... | {
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"Header 3": "FIGURE 9-26",
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Using an ideal gas as the working fluid, show that the thermal efficiency of an Ericsson cycle is identical to the efficiency of a Carnot cycle operating between the same temperature limits.
**SOLUTION** It is to be shown that the thermal efficiencies of Carnot and Ericsson cycles are identical.
**Analysis** Heat i... | {
"Header 1": "9-6 • DIESEL CYCLE: THE IDEAL CYCLE FOR COMPRESSION-IGNITION ENGINES",
"Header 3": "**EXAMPLE 9–5** Thermal Efficiency of the Ericsson Cycle",
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The Brayton cycle was first proposed by George Brayton for use in the reciprocating oil-burning engine that he developed around 1870. Today, it is used for gas turbines only where both the compression and expansion processes take place in rotating machinery. Gas turbines usually operate on an *open cycle*, as shown in ... | {
"Header 1": "9-8 • BRAYTON CYCLE: THE IDEAL CYCLE FOR GAS-TURBINE ENGINES",
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Thermal efficiency of the ideal Brayton cycle as a function of the pressure ratio. Substituting these equations into the thermal efficiency relation and simplifying give
$$\eta_{\text{th,Brayton}} = 1 - \frac{1}{r_n^{(k-1)/k}}$$
(9–17)
where
$$r_p = \frac{P_2}{P_1}$$
(9–18)
is the **pressure ratio** and k is th... | {
"Header 1": "9-8 • BRAYTON CYCLE: THE IDEAL CYCLE FOR GAS-TURBINE ENGINES",
"Header 3": "FIGURE 9-33",
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The gas turbine has experienced phenomenal progress and growth since its first successful development in the 1930s. The early gas turbines built in the 1940s and even 1950s had simple-cycle efficiencies of about 17 percent because of the low compressor and turbine efficiencies and low turbine inlet temperatures due to ... | {
"Header 1": "9-8 • BRAYTON CYCLE: THE IDEAL CYCLE FOR GAS-TURBINE ENGINES",
"Header 3": "**Development of Gas Turbines**",
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The fraction of the turbine work used to drive the compressor is called the back work ratio.
increase the production of nitrogen oxides (NO*x*), which are responsible for the formation of ozone at ground level and smog. Using steam as the coolant allowed an increase in the turbine inlet temperatures by 200°F without ... | {
"Header 1": "9-8 • BRAYTON CYCLE: THE IDEAL CYCLE FOR GAS-TURBINE ENGINES",
"Header 3": "**FIGURE 9–35**",
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A gas-turbine power plant operating on an ideal Brayton cycle has a pressure ratio of 8. The gas temperature is 300 K at the compressor inlet and 1300 K at the turbine inlet. Using the air-standard assumptions, determine (a) the gas temperature at the exits of the compressor and the turbine, (b) the back work ratio, an... | {
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"Header 3": "**EXAMPLE 9-6** The Simple Ideal Brayton Cycle",
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The actual gas-turbine cycle differs from the ideal Brayton cycle on several accounts. For one thing, some pressure drop during the heat-addition and heat-rejection processes is inevitable. More importantly, the actual work input to the compressor is more, and the actual work output from the turbine is less because of ... | {
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"Header 3": "**Deviation of Actual Gas-Turbine Cycles** from Idealized Ones",
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Assuming a compressor efficiency of 80 percent and a turbine efficiency of 85 percent, determine (a) the back work ratio, (b) the thermal efficiency, and (c) the turbine exit temperature of the gas-turbine cycle discussed in Example 9–6.
**SOLUTION** The Brayton cycle discussed in Example 9–6 is reconsidered. For spe... | {
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"Header 3": "**EXAMPLE 9-7** An Actual Gas-Turbine Cycle",
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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 counterflow heat exchanger,... | {
"Header 1": "9-9 • THE BRAYTON CYCLE WITH REGENERATION",
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Determine the thermal efficiency of the gas turbine described in Example 9–7 if a regenerator having an effectiveness of 80 percent is installed.
**SOLUTION** The gas turbine discussed in Example 9–7 is equipped with a regenerator. For a specified effectiveness, the thermal efficiency is to be determined. *Analysis* ... | {
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"Header 3": "**EXAMPLE 9–8** Actual Gas-Turbine Cycle with Regeneration",
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The net work of a gas-turbine cycle is the difference between the turbine work output and the compressor work input, and it can be increased by either decreasing the compressor work or increasing the turbine work, or both. It was shown in Chap. 7 that the work required to compress a gas between two specified pressures ... | {
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An ideal gas-turbine cycle with two stages of compression and two stages of expansion has an overall pressure ratio of 8. Air enters each stage of the compressor at 300 K and each stage of the turbine at 1300 K. Determine the back work ratio and the thermal efficiency of this gas-turbine cycle, assuming (a) no regenera... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
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Gas-turbine engines are widely used to power aircraft because they are light and compact and have a high power-to-weight ratio. Aircraft gas turbines operate on an open cycle called a **jet-propulsion cycle**. The ideal jet-propulsion cycle differs from the simple ideal Brayton cycle in that the gases are not expanded ... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
"Header 3": "**9–11** ■ **IDEAL JET-PROPULSION CYCLES**",
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Basic components of a turbojet engine and the *T-s* diagram for the ideal turbojet cycle.
the engine and the high-velocity exhaust gases leaving the engine, and it is determined from Newton's second law. The pressures at the inlet and the exit of a turbojet engine are identical (the ambient pressure); thus, the net t... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
"Header 3": "**FIGURE 9–49**",
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*T-s* diagram for the turbojet cycle described in Example 9–10.
**SOLUTION** The operating conditions of a turbojet aircraft are specified. The temperature and pressure at the turbine exit, the velocity of gases at the nozzle exit, and the propulsive efficiency are to be determined.
**Assumptions** 1 Steady operati... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
"Header 3": "FIGURE 9-51",
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The first airplanes built were all propeller-driven, with propellers powered by engines essentially identical to automobile engines. The major breakthrough in commercial aviation occurred with the introduction of the turbojet engine in 1952. Both propeller-driven engines and jet-propulsion-driven engines have their own... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
"Header 3": "**Modifications to Turbojet Engines**",
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A modern jet engine used to power Boeing 777 aircraft. This is a Pratt & Whitney PW4084 turbofan capable of producing 84,000 pounds of thrust. It is 4.87 m (192 in) long, has a 2.84 m (112 in) diameter fan, and it weighs 6800 kg (15,000 lbm).
*Reproduced by permission of United Technologies Corporation, Pratt & Whitn... | {
"Header 1": "EXAMPLE 9-9 A Gas Turbine with Reheating and Intercooling",
"Header 3": "**FIGURE 9–54**",
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ramjet engine needs to be brought to a sufficiently high speed by an external source before it can be fired.
The ramjet performs best in aircraft flying above Mach 2 or 3 (two or three times the speed of sound). In a ramjet, the air is slowed down to about Mach 0.2, fuel is added to the air and burned at this low vel... | {
"Header 1": "FIGURE 9-56 A ramjet engine.",
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The ideal Carnot, Ericsson, and Stirling cycles are *totally reversible;* thus they do not involve any irreversibilities. The ideal Otto, Diesel, and Brayton cycles, however, are only *internally reversible,* and they may involve irreversibilities external to the system. A second-law analysis of these cycles reveals wh... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
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Consider an engine operating on the ideal Otto cycle with a compression ratio of 8 (Fig. 9–57). At the beginning of the compression process, air is at 100 kPa and 17°C. During the constant-volume heat-addition process, 800 kJ/kg of heat is transferred to air from a source at 1700 K and waste heat is rejected to the sur... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**EXAMPLE 9-11** Second-Law Analysis of an Otto Cycle",
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Two-thirds of the oil used in the United States is used for transportation. Half of this oil is consumed by passenger cars and light trucks that are used to commute to and from work (38 percent), to run a family business (35 percent), and for recreational, social, and religious activities (27 percent). The overall fuel... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "TOPIC OF SPECIAL INTEREST\\* Saving Fuel and Money by Driving Sensibly",
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Aerodynamically designed vehicles have a smaller drag coefficient and thus better fuel economy than boxlike vehicles with sharp corners.
Saving fuel is not limited to good driving habits. It also involves purchasing the right car, using it responsibly, and maintaining it properly. A car does not burn any fuel when it... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**FIGURE 9–60**",
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With today's cars, it is not necessary to prime the engine first by pumping the accelerator pedal repeatedly before starting. This only wastes fuel. Warming up the engine isn't necessary either. Keep in mind that an idling engine wastes fuel and pollutes the environment. Don't race a cold engine to warm it up. An engin... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Start the Car Properly and Avoid Extended Idling**",
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Remove any snow or ice from the vehicle, and avoid carrying unneeded items, especially heavy ones (such as snow chains, old tires, books) in the passenger compartment, trunk, or the cargo area of the vehicle (Fig. 9–62). This wastes fuel since it requires extra fuel to carry around the extra weight. An extra 100 lbm de... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Don't Carry Unnecessary Weight In or On the Vehicle**",
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Keeping the tires inflated properly is one of the easiest and most important things one can do to improve fuel economy. If a range is recommended by the manufacturer, the higher pressure should be used to maximize fuel efficiency. Tire pressure should be checked when the tire is cold since tire pressure changes with te... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Keep Tires Inflated to the Recommended Maximum Pressure**",
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Avoiding high speeds on open roads results in safer driving and better fuel economy. In highway driving, over 50 percent of the power produced by the engine is used to overcome aerodynamic drag (i.e., to push air out of the way). Aerodynamic drag and thus fuel consumption increase rapidly at speeds above 55 mph, as sho... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Drive at Moderate Speeds**",
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Air conditioning consumes considerable power and thus increases fuel consumption by 3 to 4 percent during highway driving, and by as much as 10 percent during city driving (Fig. 9–67). The best alternative to air conditioning is to supply fresh outdoor air to the car through the vents by turning on the flow-through ven... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Use the Air Conditioner Sparingly**",
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You cannot be an efficient person and accomplish much unless you take good care of yourself (eating right, maintaining physical fitness, having checkups, etc.), and the cars are no exception. Regular maintenance improves performance, increases gas mileage, reduces pollution, lowers repair costs, and extends engine life... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**AFTER DRIVING**",
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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*. The most efficient cycle operating between a heat source at temperature $T_H$ and a sink at temperature $T_L$ is the Carnot cycle,... | {
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"Header 3": "**SUMMARY**",
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- 1. V. D. Chase. "Propfans: A New Twist for the Propeller." *Mechanical Engineering*, November 1986, pp. 47–50.
- **2.** C. R. Ferguson and A. T. Kirkpatrick. *Internal Combustion Engines: Applied Thermosciences*, 2nd ed. New York: Wiley, 2000.
- **3.** R. A. Harmon. "The Keys to Cogeneration and Combined Cycles." *Me... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "REFERENCES AND SUGGESTED READINGS",
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... |
- **9–1C** What are the air-standard assumptions?
- **9–2C** What is the difference between air-standard assumptions and the cold-air-standard assumptions?
- **9–3C** Why is the Carnot cycle not suitable as an ideal cycle for all power-producing cyclic devices?
- **9–4C** How does the thermal efficiency of an ideal cyc... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Actual and Ideal Cycles, Carnot Cycle, Air-Standard Assumptions, Reciprocating Engines**",
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- **9–24C** What four processes make up the ideal Otto cycle?
- **9–25C** Are the processes that make up the Otto cycle analyzed as closed-system or steady-flow processes? Why?
- **9–26C** How do the efficiencies of the ideal Otto cycle and the Carnot cycle compare for the same temperature limits? Explain.
- **9–27C** ... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Otto Cycle**",
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- **9–45C** How does a diesel engine differ from a gasoline engine?
- **9–46C** How does the ideal Diesel cycle differ from the ideal Otto cycle?
- **9–47C** What is the cutoff ratio? How does it affect the thermal efficiency of a Diesel cycle?
- **9–48C** For a specified compression ratio, is a diesel or gasoline engi... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Diesel Cycle**",
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- **9–69C** What cycle is composed of two isothermal and two constant-volume processes?
- **9–70C** How does the ideal Ericsson cycle differ from the Carnot cycle?
- **9–71C** Consider the ideal Otto, Stirling, and Carnot cycles operating between the same temperature limits. How would you compare the thermal efficienci... | {
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"Header 3": "**Stirling and Ericsson Cycles**",
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- **9–79C** What four processes make up the simple ideal Brayton cycle?
- **9–80C** For fixed maximum and minimum temperatures, what is the effect of the pressure ratio on (*a*) the thermal efficiency and (*b*) the net work output of a simple ideal Brayton cycle?
- **9–81C** What is the back work ratio? What are typica... | {
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"Header 3": "**Ideal and Actual Gas-Turbine (Brayton) Cycles**",
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**9–96E** A simple ideal Brayton cycle uses argon as the working fluid. At the beginning of the compression, *P*1 = 15 psia and *T*1 = 80°F; the maximum cycle temperature is 1200°F; and the pressure in the combustion chamber is 150 psia. The argon enters the compressor through a 3 ft<sup>2</sup> opening with a velocity... | {
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"Header 3": "**FIGURE P9–95**",
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**9–98** A gas-turbine power plant operating on the simple Brayton cycle has a pressure ratio of 7. Air enters the compressor at 0°C and 100 kPa. The maximum cycle temperature is 1500 K. The compressor has an isentropic efficiency of 80 percent, and the turbine has an isentropic efficiency of 90 percent. Assume constan... | {
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"Header 3": "**FIGURE P9–97**",
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**9–99C** How does regeneration affect the efficiency of a Brayton cycle, and how does it accomplish it?
**9–100C** Define the effectiveness of a regenerator used in gas-turbine cycles.
**9–101C** Somebody claims that at very high pressure ratios, the use of regeneration actually decreases the thermal efficiency of... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Brayton Cycle with Regeneration**",
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**9–117C** For a specified pressure ratio, why does multistage compression with intercooling decrease the compressor work, and multistage expansion with reheating increase the turbine work?
**9–118C** In an ideal gas-turbine cycle with intercooling, reheating, and regeneration, as the number of compression and expans... | {
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- **9–129C** What is propulsive power? How is it related to thrust?
- **9–130C** What is propulsive efficiency? How is it determined?
- **9–131C** Is the effect of turbine and compressor irreversibilities of a turbojet engine to reduce (*a*) the net work, (*b*) the thrust, or (*c*) the fuel consumption rate?
- **9–132*... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Jet-Propulsion Cycles**",
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**9–142** An ideal Otto cycle has a compression ratio of 8. At the beginning of the compression process, air is at 95 kPa and 27°C, and 750 kJ/kg of heat is transferred to air during the constant-volume heat-addition process. Determine the total exergy destruction associated with the cycle, assuming a source temperatur... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Second-Law Analysis of Gas Power Cycles**",
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**9–153** A four-cylinder, four-stroke, 1.8-L modern highspeed compression-ignition engine operates on the ideal dual cycle with a compression ratio of 16. The air is at 95 kPa and 70°C at the beginning of the compression process, and the engine speed is 2200 rpm. Equal amounts of fuel are burned at constant volume and... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**FIGURE P9–152**",
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- **9–155** Repeat Prob. 9–154 using constant specific heats at room temperature.
- **9–156** A Carnot cycle is executed in a closed system and uses 0.0025 kg of air as the working fluid. The cycle efficiency is 60 percent, and the lowest temperature in the cycle is 300 K. The pressure at the beginning of the isentropi... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "Answers: (b) 2100 K, (c) 15.8 percent",
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- **9–176** Compare the thermal efficiency of a two-stage gas turbine with regeneration, reheating, and intercooling to that of a three-stage gas turbine with the same equipment when (*a*) all components operate ideally, (*b*) air enters the first compressor at 100 kPa and 20°C, (*c*) the total pressure ratio across al... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**FIGURE P9–175**",
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**9–192** For specified limits for the maximum and minimum temperatures, the ideal cycle with the lowest thermal efficiency is
(*a*) Carnot (*b*) Stirling (*c*) Ericsson (*d*) Otto (*e*) All are the same
**9–193** A Carnot cycle operates between the temperature limits of 300 and 2000 K and produces 400 kW of net po... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Fundamentals of Engineering (FE) Exam Problems**",
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**9–207** The amount of fuel introduced into a spark-ignition engine is used in part to control the power produced by the engine. Gasoline produces approximately 42,000 kJ/kg when burned with air in a spark-ignition engine. Develop a schedule for gasoline consumption and maximum cycle temperature versus power productio... | {
"Header 1": "9-12 • SECOND-LAW ANALYSIS OF GAS POWER CYCLES",
"Header 3": "**Design and Essay Problems**",
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} |
**I** n Chap. 9 we discussed gas power cycles for which the working fluid remains a gas throughout the entire cycle. In this chapter, we consider *vapor power cycles* in which the working fluid is alternately vaporized and condensed. We also consider power generation coupled with process heating, called *cogeneration.*... | {
"Header 1": "**VA P O R A N D C O M B I N E D POWER CYCLES**",
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We have mentioned repeatedly that the Carnot cycle is the most efficient cycle operating between two specified temperature limits. Thus it is natural to look at the Carnot cycle first as a prospective ideal cycle for vapor power plants. If we could, we would certainly adopt it as the ideal cycle. As will be explained, ... | {
"Header 1": "**VA P O R A N D C O M B I N E D POWER CYCLES**",
"Header 3": "**10–1** ■ **THE CARNOT VAPOR CYCLE**",
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Many of the impracticalities associated with the Carnot cycle can be eliminated by superheating the steam in the boiler and condensing it completely in the condenser, as shown schematically on a *T-s* diagram in Fig. 10–2. The cycle that results is the **Rankine cycle**, which is the ideal cycle for vapor power plants.... | {
"Header 1": "10-2 • RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLES",
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All four components associated with the Rankine cycle (the pump, boiler, turbine, and condenser) are steady-flow devices, and thus all four processes
FIGURE 10–2 The simple ideal Rankine cycle.
that make up the Rankine cycle can be analyzed as steady-flow processes. The kinetic and p... | {
"Header 1": "10-2 • RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLES",
"Header 3": "**Energy Analysis of the Ideal Rankine Cycle**",
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Consider a steam power plant operating on the simple ideal Rankine cycle. Steam enters the turbine at 3 MPa and 350°C and is condensed in the condenser at a pressure of 75 kPa. Determine the thermal efficiency of this cycle.
**SOLUTION** A steam power plant operating on the simple ideal Rankine cycle is considered. T... | {
"Header 1": "10-2 • RANKINE CYCLE: THE IDEAL CYCLE FOR VAPOR POWER CYCLES",
"Header 3": "**EXAMPLE 10-1** The Simple Ideal Rankine Cycle",
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The actual vapor power cycle differs from the ideal Rankine cycle, as illustrated in Fig. 10–4*a*, as a result of irreversibilities in various components. Fluid friction and heat loss to the surroundings are the two common sources of irreversibilities.
Fluid friction causes pressure drops in the boiler, the condenser... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
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} |
A steam power plant operates on the cycle shown in Fig. 10–5. If the isentropic efficiency of the turbine is 87 percent and the isentropic efficiency of the pump is 85 percent, determine (a) the thermal efficiency of the cycle and (b) the net power output of the plant for a mass flow rate of 15 kg/s.
**SOLUTION** A s... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
"Header 3": "**EXAMPLE 10-2** An Actual Steam Power Cycle",
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(a) Deviation of actual vapor power cycle from the ideal Rankine cycle. (b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.
Pump work input:
$$w_{\text{pump,in}} = \frac{w_{s,\text{pump,in}}}{\eta_P} = \frac{V_1(P_2 - P_1)}{\eta_P}$$
$$= \frac{(0.001009 \text{ m}^3/\text{kg})[(16,000 ... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
"Header 3": "FIGURE 10-4",
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Steam exists as a saturated mixture in the condenser at the saturation temperature corresponding to the pressure inside the condenser. Therefore, lowering the operating pressure of the condenser automatically lowers the temperature of the steam, and thus the temperature at which heat is rejected.
The effect of loweri... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
"Header 3": "**Lowering the Condenser Pressure (Lowers Tlow,avg)**",
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The average temperature at which heat is transferred to steam can be increased without increasing the boiler pressure by superheating the steam to high temperatures. The effect of superheating on the performance of vapor power cycles is illustrated on a *T-s* diagram in Fig. 10–7. The colored area on this diagram repre... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
"Header 3": "**Superheating the Steam to High Temperatures (Increases Thigh,avg)**",
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Another way of increasing the average temperature during the heat-addition process is to increase the operating pressure of the boiler, which automatically raises the temperature at which boiling takes place. This, in turn, raises the average temperature at which heat is transferred to the steam and thus raises the the... | {
"Header 1": "10-3 • DEVIATION OF ACTUAL VAPOR POWER CYCLES FROM IDEALIZED ONES",
"Header 3": "**Increasing the Boiler Pressure (Increases Thigh,avg)**",
"token_count": 370,
"source_pdf": "datasets/websources/Physics_v1/Physics/pdfcoffee.com_engineering-thermodynamics-by-cengel-boles-and-kanoglu-9th-edition-pd... |
Consider a steam power plant operating on the ideal Rankine cycle. Steam enters the turbine at 3 MPa and 350°C and is condensed in the condenser at a pressure of 10 kPa. Determine (a) the thermal efficiency of this power plant, (b) the thermal efficiency if steam is superheated to 600°C instead of 350°C, and (c) the th... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
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} |
We noted in Sec. 10–4 that increasing the boiler pressure increases the thermal efficiency of the Rankine cycle, but it also increases the moisture content of the steam to unacceptable levels. Then it is natural to ask the following question:
How can we take advantage of the increased efficiencies at higher boiler pr... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "10-5 • THE IDEAL REHEAT RANKINE CYCLE",
"token_count": 917,
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Consider a steam power plant that operates on the ideal reheat Rankine cycle. The plant maintains the inlet of the high-pressure turbine at 600 psia and 600°F, the inlet of the low-pressure turbine at 200 psia and 600°F, and the condenser at 10 psia. The net power produced by this plant is 5000 kW. Determine the rate o... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "EXAMPLE 10-4 The Ideal Reheat Rankine Cycle",
"token_count": 1874,
"source_pdf": "datasets/websources/Physics_v1/Physics/pdfcoffee.com_engineering-thermodynamics-by-cengel-boles-and-kanoglu-9th-edition-pdf-fre... |
A careful examination of the *T-s* diagram of the Rankine cycle redrawn in Fig. 10–15 reveals that heat is transferred to the working fluid during process 2-2′ at a relatively low temperature. This lowers the average heat-addition temperature and thus the cycle efficiency.
To remedy this shortcoming, we look for ways... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "**10–6** ■ **THE IDEAL REGENERATIVE RANKINE CYCLE**",
"token_count": 405,
"source_pdf": "datasets/websources/Physics_v1/Physics/pdfcoffee.com_engineering-thermodynamics-by-cengel-boles-and-kanoglu-9th-edition-... |
The first part of the heat-addition process in the boiler takes place at relatively low temperatures.
The ideal regenerative Rankine cycle with an open feedwater heater.
the pump. Ideally, the mixture leaves the heater as a saturated liquid at the heater pressure. The schematic of a s... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "**FIGURE 10–15**",
"token_count": 861,
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A steam power plant with one open and three closed feedwater heaters.
> the feedwater leaves the heater below the exit temperature of the extracted steam because a temperature difference of at least a few degrees is required for any effective heat transfer to take place.
> The condensed steam is then either pumped ... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "**FIGURE 10–18**",
"token_count": 279,
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Consider a steam power plant operating on the ideal regenerative Rankine cycle with one open feedwater heater. Steam enters the turbine at 15 MPa and 600°C and is condensed in the condenser at a pressure of 10 kPa. Some steam leaves the turbine at a pressure of 1.2 MPa and enters the open feedwater heater. Determine th... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "**EXAMPLE 10–5 The Ideal Regenerative Rankine Cycle**",
"token_count": 909,
"source_pdf": "datasets/websources/Physics_v1/Physics/pdfcoffee.com_engineering-thermodynamics-by-cengel-boles-and-kanoglu-9th-editio... |
Schematic and *T-s* diagram for Example 10–5.
State 5:
$$P_5 = 15 \text{ MPa}$$
$h_5 = 3583.1 \text{ kJ/kg}$
$T_5 = 600^{\circ}\text{C}$ $s_5 = 6.6796 \text{ kJ/kg·K}$
State 6:
$$P_6 = 1.2 \text{ MPa}$$
$s_6 = s_5$ $h_6 = 2860.2 \text{ kJ/kg}$ $(T_6 = 218.4 ^{\circ}\text{C})$
State 7: $P_7 = 10 \text{ kPa}... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "**FIGURE 10-19**",
"token_count": 894,
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Consider a steam power plant that operates on an ideal reheat–regenerative Rankine cycle with one open feedwater heater, one closed feedwater heater, and one reheater. Steam enters the turbine at 15 MPa and 600°C and is condensed in the condenser at a pressure of 10 kPa. Some steam is extracted from the turbine at 4 MP... | {
"Header 1": "**EXAMPLE 10-3** Effect of Boiler Pressure and Temperature on Efficiency",
"Header 3": "EXAMPLE 10-6 The Ideal Reheat-Regenerative Rankine Cycle",
"token_count": 1920,
"source_pdf": "datasets/websources/Physics_v1/Physics/pdfcoffee.com_engineering-thermodynamics-by-cengel-boles-and-kanoglu-9th-ed... |
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