First Law of Thermodynamics (Thermodynamics)

First Law of Thermodynamics [Thermodynamics]

1. “ First Law of Thermodynamics” 1 Advanced Thermodynamics and Heat Engines

2. According to the principle of conservation of energy, “Energy can neither be created nor be destroyed, however it can only be transformed from one form to another form.” Advanced Thermodynamics and Heat Engines 2 Hence the First law of thermodynamics adheres to the principle of conservation of energy, that is, Total Energy entering the system Total Energy leaving the system Change in Total Energy of the system - =

3. Advanced Thermodynamics and Heat Engines 3 Energy Transfer Heat Work Mass Transport Conduction Convection Radiation Flow work Forms of work other than flow work Internal energy Potential energy Kinetic energy Flow energy

4. Total Energy of a flowing fluid Advanced Thermodynamics and Heat Engines 4 Energy required to do flow work Energy due to the virtue of motion energy associated with the random, disordered motion of molecules Energy due to the relative position

5. Advanced Thermodynamics and Heat Engines 5 The equation can also be written as, By defining flow energy and internal energy in terms of enthalpy, Enthalpy takes care of the flow work and doesn’t needs to be defined separately. This is the main reason Enthalpy is defined.

6. Conservation of energy for a Control Volume Advanced Thermodynamics and Heat Engines 6 Conservation of mass for Control Volume where,

7. Advanced Thermodynamics and Heat Engines 7 Also in terms of rate, This conservation of energy equation for control volume serves as a generalized equation encompassing all possibilities of energy transfer, Heat, Work, and Mass transport, and can be used to derive conservation for other special cases.

8. Advanced Thermodynamics and Heat Engines 8 For Control Volume , Steady state analysis for Control Volume , Also,

9. Advanced Thermodynamics and Heat Engines 9 Commonly most steady state devices, turbine, compressor, nozzle etc., have single inlet and outlet. Let’s denote the inlet and outlet section by numbers 1 and 2 respectively. Then,

10. Unsteady state analysis for Control Volume Advanced Thermodynamics and Heat Engines 10 In unsteady state analysis of control volume both total mass and total energy is a function of time. Unlike steady-flow processes, unsteady-flow processes start and end over some finite time period instead of continuing indefinitely. Therefore in this section, we deal with changes that occur over some time interval Δt instead of with the rate of changes (changes per unit time). Conservation of mass

11. Advanced Thermodynamics and Heat Engines 11 Conservation of Energy

12. Advanced Thermodynamics and Heat Engines 12 Control Volume Applications Steady Shaft work machines Turbine Compressor, Pump, and Fan Flow devices Heat Exchanger Nozzle and Diffuser Throttle valve Unsteady Work Flow

13. Steady state Control Volume applications: Shaft Work Machines Advanced Thermodynamics and Heat Engines 13 Turbine: A turbine is a shaft work machine that takes energy from a working fluid and produces power while expelling the working fluid possessing a reduced energy. Turbines are a fundamental machine used in producing electricity, either in steam power plants or gas power plants. The working fluid flows into a row of blades, causing the rotor to rotate. There are two factors causing the rotation: The first factor is the force of the fluid impacting the blades and pushing the blades. The second factor is a pressure difference from one side of the blade to the other caused by the flow moving at different speeds over the two sides of the blades. This pressure difference forces the blades to move in the direction of the side with lower pressure. 1 2 W Turbine

14. Advanced Thermodynamics and Heat Engines 14 Compressor, Pump, and Fan: A compressor is a shaft work machine whose purpose is to use power to increase the pressure of a gas or vapor flowing through it. A pump is a shaft work machine whose purpose is to increase the pressure of a liquid flowing through it through the use of input power. These devices typically use a rotating impeller to increase the fluid pressure. Fans increases kinetic energy (by increasing velocity) of the fluid. in out W in out W compressor in out W

15. Advanced Thermodynamics and Heat Engines 15 For shaft work machines; change in potential energy of the fluid is negligible, except in the case of a hydroelectric turbine the change in kinetic energy is usually small in comparison to the change in enthalpy of the fluid 0 (adiabatic)

16. Steady state Control Volume applications: Flow devices Advanced Thermodynamics and Heat Engines 16 Heat Exchanger: Heat exchangers are used to transfer heat between two flowing fluids. In an indirect heat exchanger, the cold and hot streams do NOT mix; therefore, the mass conservation equation must be applied to each flow stream individually. The energy conservation equation may be applied to the entire heat exchanger or each flow stream individually. Fluid A CV boundary Fluid B

17. Advanced Thermodynamics and Heat Engines 17 Conservation of mass and energy equations for an heat exchanger,

18. Nozzle and Diffuser: Nozzles and diffusers are devices used to accelerate or decelerate flow by gradually changing their cross-sectional areas. In a nozzle, the flow velocity increases and the pressure decreases as the cross-sectional area of the nozzle decreases. On the contrary, in a diffuser, the flow velocity decreases and the pressure increases as the cross-sectional area of the diffuser increases. Advanced Thermodynamics and Heat Engines 18 V 1 V 2 >> V 1 V 1 V 2 << V 1

19. Advanced Thermodynamics and Heat Engines 19 For Nozzles and Diffusers,

20. Throttle Valve: Throttling valves are any kind of flow-restricting devices that cause a significant pressure drop in the fluid. Unlike turbines, they produce a pressure drop without involving any work. The pressure drop in the fluid is often accompanied by a large drop in temperature, and for that reason throttling devices are commonly used in refrigeration and air-conditioning applications. Advanced Thermodynamics and Heat Engines 20

21. Advanced Thermodynamics and Heat Engines 21 W Gas Unsteady state work Application: The mass and energy conservation for the given system is, Change in P.E, and K.E is negligible compare to the change in internal energy,

22. Advanced Thermodynamics and Heat Engines 22 Unsteady state flow Application: The mass and energy conservation for the given system is, Change in P.E, and K.E is negligible compare to the change in internal energy,

23. Conservation of energy for a Control Mass Advanced Thermodynamics and Heat Engines 23

24. Advanced Thermodynamics and Heat Engines 24 Conservation of energy for a Control Mass undergoing a cycle A process is said to be a cyclic process, if the initial state of the system is restored by a number of different processes in series. ( initial state = Final state) P V 1 2 3 For an entire cycle, Whenever a control mass is taken through a cycle then the heat transferred to the control mass is equal to the net work done by the control mass. Whenever a control mass is taken through a cycle then the heat rejected by the control mass is equal to the net work done on the control mass. OR

25. Advanced Thermodynamics and Heat Engines 25 Significance of the First Law of Thermodynamics The existence of an internal energy function. Definition of heat as energy in transit. The principle of conservation of energy.

26. Advanced Thermodynamics and Heat Engines 26