heat engines heat pumps physics montwood high school r. casao

Download Heat Engines Heat Pumps Physics Montwood High School R. Casao

Post on 28-Mar-2015

220 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • Slide 1

Heat Engines Heat Pumps Physics Montwood High School R. Casao Slide 2 Heat Engine Cycle A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. The first law and second law of thermodynamics constrain the operation of a heat engine. The first law and second law of thermodynamics constrain the operation of a heat engine. The first law is the application of conservation of energy to the system, and The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow. the second sets limits on the possible efficiency of the machine and determines the direction of energy flow. Slide 3 First Law of Thermodynamics The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: the change in internal energy ( U) of a system is equal to the heat (Q) added to the system minus the work (W) done by the system. The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: the change in internal energy ( U) of a system is equal to the heat (Q) added to the system minus the work (W) done by the system. Mathematically: U = Q - W Mathematically: U = Q - W Slide 4 Internal Energy Internal energy is defined as the energy associated with the random, disordered motion of molecules. Internal energy is defined as the energy associated with the random, disordered motion of molecules. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic. But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic. But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second. Slide 5 Internal Energy In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine. Slide 6 First Law of Thermodynamics Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle. Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle. Slide 7 PV Diagrams Pressure-Volume (PV) diagrams are a primary visualization tool for the study of heat engines. Since the engines usually involve a gas as a working substance, the ideal gas law relates the PV diagram to the temperature so that the three essential state variables for the gas can be tracked through the engine cycle. Pressure-Volume (PV) diagrams are a primary visualization tool for the study of heat engines. Since the engines usually involve a gas as a working substance, the ideal gas law relates the PV diagram to the temperature so that the three essential state variables for the gas can be tracked through the engine cycle. Slide 8 PV Diagrams For a cyclic heat engine process, the PV diagram will be closed loop. The area inside the loop is a representation of the amount of work done during a cycle. Some idea of the relative efficiency of an engine cycle can be obtained by comparing its PV diagram with that of a Carnot cycle, the most efficient kind of heat engine cycle. For a cyclic heat engine process, the PV diagram will be closed loop. The area inside the loop is a representation of the amount of work done during a cycle. Some idea of the relative efficiency of an engine cycle can be obtained by comparing its PV diagram with that of a Carnot cycle, the most efficient kind of heat engine cycle. Slide 9 Heat Engines A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. Thermodynamics is the study of the relationships between heat and work. A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. Thermodynamics is the study of the relationships between heat and work. The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow. The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow. Slide 10 Energy Reservoir Model One of the general ways to illustrate a heat engine is the energy reservoir model. The engine takes energy from a hot reservoir and uses part of it to do work, but is constrained by the second law of thermodynamics to exhaust part of the energy to a cold reservoir. In the case of the automobile engine, the hot reservoir is the burning fuel and the cold reservoir is the environment to which the combustion products are exhausted. One of the general ways to illustrate a heat engine is the energy reservoir model. The engine takes energy from a hot reservoir and uses part of it to do work, but is constrained by the second law of thermodynamics to exhaust part of the energy to a cold reservoir. In the case of the automobile engine, the hot reservoir is the burning fuel and the cold reservoir is the environment to which the combustion products are exhausted. Slide 11 Second Law of Thermodynamics Second Law of Thermodynamics: It is impossible to extract an amount of heat Q H from a hot reservoir and use it all to do work W. Some amount of heat Q C must be exhausted to a cold reservoir. Second Law of Thermodynamics: It is impossible to extract an amount of heat Q H from a hot reservoir and use it all to do work W. Some amount of heat Q C must be exhausted to a cold reservoir. The maximum efficiency which can be achieved is the Carnot efficiency. The maximum efficiency which can be achieved is the Carnot efficiency. Slide 12 Second Law of Thermodynamics Slide 13 Carnot Cycle The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. Slide 14 Carnot Cycle In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy. In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy. Slide 15 Carnot Cycle The conceptual value of the Carnot cycle is that it establishes the maximum possible efficiency for an engine cycle operating between T H and T C. The conceptual value of the Carnot cycle is that it establishes the maximum possible efficiency for an engine cycle operating between T H and T C. Slide 16 Combustion Engines Combustion engines: burn fuel to produce the heat input for a thermodynamic cycle. Combustion engines: burn fuel to produce the heat input for a thermodynamic cycle. Burning fuel turns chemical energy into heat energy. Burning fuel turns chemical energy into heat energy. By-products of combustion have a very high temperature and produce a very high pressure. By-products of combustion have a very high temperature and produce a very high pressure. Results: piston pushed downward and a fraction of the heat energy is converted to mechanical work. Results: piston pushed downward and a fraction of the heat energy is converted to mechanical work. Some heat energy is carried away by the high temperature exhaust gases, and some is lost to the cylinder walls. Some heat energy is carried away by the high temperature exhaust gases, and some is lost to the cylinder walls. Slide 17 First law of thermodynamics for combustion engine: Mathematically: Q H = Q C + W Mathematically: Q H = Q C + W Q H = heat input due to fuel combustion Q H = heat input due to fuel combustion Q C = heat energy lost Q C = heat energy lost W = work W = work Net heat absorbed per cycle: Net heat absorbed per cycle: Q T = Q H + Q C Q T = Q H + Q C Slide 18 First law of thermodynamics for combustion engine: Work output for combustion engine: Work output for combustion engine: W = Q H - Q C Efficiency for combustion engine: Effi

Recommended

View more >