Thermo chapter 2-p2

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Thermo chapter 2-p2

  1. 1. Chapter 2 Energy, Energy Transfer, and General Energy Analysis Thermodynamics I 1
  2. 2. THE FIRST LAW OF THERMODYNAMICS Energy is conserved. This is the first law of thermodynamics (also called the conservation of energy principle) • 1st law of thermodynamics: Energy can neither be created nor destroyed during a process; it can only change forms. Energy cannot be created or destroyed; it can only change forms. 2
  3. 3. Energy Balance The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process. Per unit mass: Rate form: Differential forms: (kJ) (kJ/kg) 3
  4. 4. Energy Balance The work (boundary) done on an adiabatic system is equal to the increase in the energy of the system. The energy change of a system during a process is equal to the net work and heat transfer between the system and its surroundings. 4
  5. 5. Energy Balance The work (electrical) done on an adiabatic system is equal to the increase in the energy of the system. In the absence of any work interactions, the energy change of a system is equal to the net heat transfer. The work (shaft) done on an adiabatic system is equal to the increase in the energy of the system. 5
  6. 6. Mechanisms of Energy Transfer, Ein and Eout (Left hand side of the energy balance equation) • Heat transfer • Work transfer • Mass flow (kJ) • For closed systems, energy is transferred in and out across the system boundary by two means only: by work and by heat. • The energy content of a control volume can be changed by mass flow as well as heat and work interactions. 6
  7. 7. Energy Change of a System, Esystem (Right hand side of the energy balance equation) Energy is an extensive property that includes the kinetic, gravitational potential energy, and Internal energy. Internal, kinetic, and potential energy changes 7
  8. 8. Closed System Energy Balance E2 – E1 = (Qin – Qout)+(Win – Wout) Qin – Qout =Qnet Wout – Win =Wnet or Win – Wout =-Wnet E2 – E1 = Qnet– Wnet For a closed system in a cycle: For a cycle ∆E = 0, thus Qnet= Wnet or in its rate form: 8
  9. 9. Example Discuss Example 2-10 in class: 9
  10. 10. Example Discuss Example 2-12 in class: 10
  11. 11. ENERGY CONVERSION EFFICIENCIES Efficiency is one of the most frequently used terms in thermodynamics, and it indicates how well an energy conversion or transfer process is accomplished. Efficiency of a water heater: The ratio of the energy delivered to the house by hot water to the energy supplied to the water heater. • • Generator: A device that converts mechanical energy to electrical energy. Generator efficiency: The ratio of the electrical power output to the mechanical power input. 11
  12. 12. • • • Using energy-efficient appliances conserve energy. It helps the environment by reducing the amount of pollutants emitted to the atmosphere during the combustion of fuel. The combustion of fuel produces • carbon dioxide, causes global warming • nitrogen oxides and hydrocarbons, cause smog • carbon monoxide, toxic • sulfur dioxide, causes acid rain. The efficiency of a cooking appliance represents the fraction of the energy supplied to the appliance that is transferred to the food. 12
  13. 13. Efficiencies of Mechanical and Electrical Devices Mechanical efficiency For pumps and turbines, the effectiveness of the conversion process between the mechanical work supplied or extracted and the mechanical energy of the fluid is expressed by the pump efficiency and turbine efficiency, The mechanical efficiency of a fan is the ratio of the kinetic energy of air at the fan exit to the mechanical power input. 13
  14. 14. Motor efficiency Generator efficiency Pump-Motor overall efficiency Turbine-Generator overall efficiency The overall efficiency of a turbine–generator is the product of the efficiency of the turbine and the efficiency of the generator, and represents the fraction of the mechanical energy of the fluid converted to electric energy. 14
  15. 15. Example Discuss Example 2-16 in class: 15
  16. 16. Summary • Forms of energy  Macroscopic = kinetic + potential  Microscopic = Internal energy (sensible + latent + chemical + nuclear) • • • • Energy transfer by heat Energy transfer by work Mechanical forms of work The first law of thermodynamics  Energy balance  Energy change of a system  Mechanisms of energy transfer (heat, work, mass flow) • Energy conversion efficiencies  Efficiencies of mechanical and electrical devices (turbines, pumps) 16
  17. 17. Homework #2 Due: Tuesday, Feb. 18 Problems: 2.15 2.17 2.38 2.46E 2.47 2.50 2.74 2.128 17

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