同济大学：《工程热力学》课程电子教案（讲稿）Chapter 9 Gas Power Systems

2013-3-6 Chapter 9 Gas Power Systems Learning Outcomes Perform air-standard analyses of internal combustion engines based on the Otto, Diesel,and dual cycles,including: sketching p-and T-s diagrams and evaluating property data at principal states. applying energy,entropy,and exergy balances. determining net power output,thermal efficiency,and mean effective pressure 1

2013-3-6 1 Chapter 9 Gas Power Systems Learning Outcomes ►Perform air-standard analyses of internal combustion engines based on the Otto, Diesel, and dual cycles, including: ►sketching p-v and T-s diagrams and evaluating property data at principal states. ►applying energy, entropy, and exergy balances. ►determining net power output, thermal efficiency, and mean effective pressure

2013-3-6 Learning Outcomes Perform air-standard analyses of gas turbine power plants based on the Brayton cycle and its modifications,including: sketching T-s diagrams and evaluating property data at principal states. applying mass,energy,entropy,and exergy balances. determining net power output,thermal efficiency,back work ratio,and the effects of compressor pressure ratio. Learning Outcomes For subsonic and supersonic flows through nozzles and diffusers: demonstrate understanding of the effects of area change,the effects of back pressure on mass flow rate,and the occurrence of choking and normal shocks. analyze the flow of ideal gases with constant specific heats. 2

2013-3-6 2 Learning Outcomes ►Perform air-standard analyses of gas turbine power plants based on the Brayton cycle and its modifications, including: ►sketching T-s diagrams and evaluating property data at principal states. ►applying mass, energy, entropy, and exergy balances. ►determining net power output, thermal efficiency, back work ratio, and the effects of compressor pressure ratio. Learning Outcomes ►For subsonic and supersonic flows through nozzles and diffusers: ►demonstrate understanding of the effects of area change, the effects of back pressure on mass flow rate, and the occurrence of choking and normal shocks. ►analyze the flow of ideal gases with constant specific heats

2013-3-6 Introducing Power Generation To meet our national power needs there are challenges related to Declining economically Current U.S.Electricity recoverable supplies of eration by Sourc nonrenewable energy resources. Coal al gas 48.5 Effects of global climate change and other environmental and human health and safety issues. Rapidly increasing demand for power owing to increasing Others population. Today we are heavily dependent on coal,natural gas,and nuclear,all of which are nonrenewable. Introducing Power Generation While coal,natural gas,and nuclear will continue to play important roles in years ahead,contributions from wind power solar power,and other renewable sources are expected to be increasingly significant up to mid-century at least. Table 8.2 LacaleEectric Powerrioufrom Renewable and wable Source Renewable Source Ther oal-fueled electric Yes 3

2013-3-6 3 Introducing Power Generation ►To meet our national power needs there are challenges related to ►Today we are heavily dependent on coal, natural gas, and nuclear, all of which are nonrenewable. ►Declining economically recoverable supplies of nonrenewable energy resources. ►Effects of global climate change and other environmental and human health and safety issues. ►Rapidly increasing demand for power owing to increasing population. Table 8.2 Introducing Power Generation ►While coal, natural gas, and nuclear will continue to play important roles in years ahead, contributions from wind power, solar power, and other renewable sources are expected to be increasingly significant up to mid-century at least

2013-3-6 Introducing Power Generation This table also shows that thermodynamic cycles are a fundamental aspect of several power plant types that employ renewable or nonrenewable sources. Vapor power cycles are the focus of Chapter 8.In Chapter 9 gas tu rbine power systems and interal combustion engines are studied as thermodynamic cycles.The basic building block of gas turbine cycles is the Brayton cycle. Gas power system learning resources are now provided,including Power cycle review Area interpretations for work and heat transfer Ideal gas model review Power Cycle Review The first law of thermodynamics requires the net work developed by a syste power cycle to equal the net energy added by heat transfer to the system: 巾oce=On-Oot The thermal efficiency of a power cycle is Cin

2013-3-6 4 Introducing Power Generation ►This table also shows that thermodynamic cycles are a fundamental aspect of several power plant types that employ renewable or nonrenewable sources. ►Vapor power cycles are the focus of Chapter 8. In Chapter 9 gas turbine power systems and internal combustion engines are studied as thermodynamic cycles. The basic building block of gas turbine cycles is the Brayton cycle. ►Gas power system learning resources are now provided, including ►Power cycle review ►Area interpretations for work and heat transfer ►Ideal gas model review Power Cycle Review in cycle Q W & & η = ►The first law of thermodynamics requires the net work developed by a system undergoing a power cycle to equal the net energy added by heat transfer to the system: ►The thermal efficiency of a power cycle is Wcycle = Qin – Qout · · · · · ·

2013-3-6 Power Cycle Review The second law of thermodynamics requires the thermal efficiency to be less than 100%. Thermal effici increases and/or the average temperature at which energy is rejected by heat transter decreases. Improved thermodynamic performance of power cvcles.as measured by increased thermal efficiency,for example,also accompanies the reduction of irreversibilities and losses. The extent of improve however,by constraints imposed by thermodynamics and economics. Area Interpretations for Work and Heat Transfer Ideal cycles formed from internally reversible processes are used in Chapter 9 to further understanding of reciprocating internal combustion engines and gas turbine power systems Closed systems involving expansion and compression work are used to model reciprocating engines.For these applications, the following area interpretations apply for internally reversible processes: 5

2013-3-6 5 Power Cycle Review ►The second law of thermodynamics requires the thermal efficiency to be less than 100%. ►Thermal efficiency tends to increase as the average temperature at which energy is added by heat transfer increases and/or the average temperature at which energy is rejected by heat transfer decreases. ►Improved thermodynamic performance of power cycles, as measured by increased thermal efficiency, for example, also accompanies the reduction of irreversibilities and losses. ►The extent of improved power cycle performance is limited, however, by constraints imposed by thermodynamics and economics. Area Interpretations for Work and Heat Transfer ►Ideal cycles formed from internally reversible processes are used in Chapter 9 to further understanding of reciprocating internal combustion engines and gas turbine power systems. ►Closed systems involving expansion and compression work are used to model reciprocating engines. For these applications, the following area interpretations apply for internally reversible processes:

2013-3-6 Area Interpretations for Work and Heat Transfer 周 8 Observe that these expressions give work and heat transfer per unit of mass contained within the closed system. Area Interpretations for Work and Heat Transfer One-inlet,one exit control volumes at steady state are used to model gas turbine power plants.For these applications,the following area interpretations apply for internally reversible processes: D 周.吨 T Observe that these expressions give work and heat pe unit of wing through the contro volume. 6

2013-3-6 6 Area Interpretations for Work and Heat Transfer ∫ ⎟ = ⎠ ⎞ ⎜ ⎝ ⎛ pdv m W rev int ►Observe that these expressions give work and heat transfer per unit of mass contained within the closed system. ∫ ⎟ = ⎠ ⎞ ⎜ ⎝ ⎛ Tds m Q rev int p v T s ►One-inlet, one exit control volumes at steady state are used to model gas turbine power plants. For these applications, the following area interpretations apply for internally reversible processes: Area Interpretations for Work and Heat Transfer T s p v ►Observe that these expressions give work and heat transfer per unit of mass flowing through the control volume. ∫ = ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ Tds m Q rev & int & ∫ = − ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ dp m W v rev & int &

2013-3-6 Ideal Gas Model Review ntary thermodynamic analyses of reciproca ombustion engines and gas turbi model principles,as reviewed in Table 9.1. hanges 8别 ur)-ur)-c.indr 40) nr-Mr -c(nar See TablesAooaddata. Ideal Gas Model Review sn-sp n+Rn款6：切 em号-2 s-s s》-sW n2+Rh6 0)-00-R1n2620 7

2013-3-6 7 ►Elementary thermodynamic analyses of reciprocating internal combustion engines and gas turbines use ideal model principles, as reviewed in Table 9.1. Ideal Gas Model Review Ideal Gas Model Review

2013-3-6 Ideal Gas Model Review 643 (air only) 64 640 (air only) 6.45月 Considering Reciprocating Internal Combustion Engines What are reciprocating internal combustion engines? They are reciprocating engines commonly used in automobiles.trucks,and buses. How do reciprocating internal combustion engines differ from the vapor power plants considered in Processes occur within reciprocating piston- 8

2013-3-6 8 Ideal Gas Model Review Considering Reciprocating Internal Combustion Engines ►What are reciprocating internal combustion engines? ►They are reciprocating engines commonly used in automobiles, trucks, and buses. ►How do reciprocating internal combustion engines differ from the vapor power plants considered in Chapter 8 and the gas turbines considered in later sections of Chapter 9? ►Processes occur within reciprocating piston￾cylinder arrangements rather than by mass flowing through a series of interconnected components

2013-3-6 Considering Reciprocating Internal Combustion Engines -Two Types Spark-ignition A mixture of fuel and air is ignited by a spark plua This type is advantageous for applications up to about 300hp(225kW. .lightweight and relatively low cost. predominantly used by automobiles in the U.S. Considering Reciprocating Internal Combustion Engines -Two Types Compression-ignition Air is compressed to a high pressure and temperature. Combustion occurs spontaneously when fuel is injected. This type is preferred for high-power applications and when fuel economy is required. used in heavy trucks and buses,locomotives and ships,and auxiliary power units 9

2013-3-6 9 Considering Reciprocating Internal Combustion Engines – Two Types ►Spark-ignition ►A mixture of fuel and air is ignited by a spark plug. ►This type is • advantageous for applications up to about 300 hp (225 kW). • lightweight and relatively low cost. • predominantly used by automobiles in the U.S. Considering Reciprocating Internal Combustion Engines – Two Types ►Compression-ignition ►Air is compressed to a high pressure and temperature. ►Combustion occurs spontaneously when fuel is injected. ►This type is • preferred for high-power applications and when fuel economy is required. • used in heavy trucks and buses, locomotives and ships, and auxiliary power units

2013-3-6 Introducing Engine Terminology Displacement volume:volume center Top dead center Bore- Stroke Bottom dead center Compression ratio,r:volume at bottom dead center divided by volume at top dead center Introducing Engine Terminology Combustion initiated for every two revolutions of the crankshaft Intake stroke With the intake valve open, piston stroke draws a fresh charge into the cylinder For spark-ignition includes fuel and air. Intake For compression-ignition charge is air 10

2013-3-6 10 Stroke ►Compression ratio, r: volume at bottom dead center divided by volume at top dead center ► Displacement volume: volume swept by piston when it moves from top dead center to bottom dead center Top dead center Bottom dead center Introducing Engine Terminology Four-stroke cycle Four strokes of the piston for every two revolutions of the crankshaft ►Intake stroke With the intake valve open, piston stroke draws a fresh charge into the cylinder. ► For spark-ignition engines, the charge includes fuel and air. ► For compression-ignition engines, the charge is air alone. Introducing Engine Terminology