1. Thermodynamics I Inter - Bayamon Lecture 5 Thermodynamics I MECN 4201 Professor: Dr. Omar E. Meza Castillo omeza@bayamon.inter.edu http://facultad.bayamon.inter.edu/omeza Department of Mechanical Engineering Inter American University of Puerto Rico Bayamon Campus

2. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon  To Develop the conservation of mass principle.  To apply the conservation of mass principle to various systems including steady- and unsteady-flow control volumes.  To apply the first law of thermodynamics to control volumes.  To identify the energy carried by a fluid stream crossing a control volume  To solve energy balance problems for common steady-flow devices such as nozzles, compressors, turbines, throttling valves, mixer, heaters and heat exchangers. Thermal Systems Design Universidad del Turabo 2 Course Objective

3. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Control Volumes Energy Analysis 3

4. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Topics Conservation of mass principle applied to steady- and unsteady-flow control volumes. Conservation of energy principle applied to control volumes. Energy carried by a fluid stream crossing a control surface. Energy balances for common steady-flow devices such as nozzles, compressors, turbines, throttles, mixers, heaters, and heat exchangers. Energy balances for unsteady-flow processes. 4

5. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Conservation of Mass  Conservation of mass: Mass, like energy, is a conserved property, and it cannot be created or destroyed during a process.  Closed systems: The mass of the system remain constant during a process.  Control volumes: Mass can cross the boundaries, and so we must keep track of the amount of mass entering and leaving the control volume 5

6. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Mass and Volume Flow Rates 6 The average velocity V avg is defined as the average speed through a cross section. Definition of average velocity Mass flow rate Volume flow rate

7. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Conservation of Mass Principle 7 Conservation of mass principle for an ordinary bathtub. The conservation of mass principle for a control volume: The net mass transfer for a control volume during a time interval t is equal to the net change in the total mass within the control volume during the interval. General conservation of mass General conservation of mass in rate form or

8. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Mass Balance for Steady-Flow Processes 8 During a steady-flow process, the total amount of mass contained within a control volume does not change with time (m CV = constant). Then the conservation of mass principle requires that the total amount of mass entering a control volume equal the total amount of mass leaving it. For steady-flow processes, we are interested in the amount of mass flowing per unit time, that is, the mass flow rate. Multiple inlets and exits Single stream Devices such as nozzles, diffusers, turbines, compressors, and pumps involve a single stream (only one inlet and one outlet).

9. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Flow Work 9 Flow work, or flow energy: The work (or energy) required to push the mass into or out of the control volume. This work is necessary for maintaining a continuous flow through a control volume.

10. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Total Energy of a Stream 10 The flow energy is automatically taken care of by enthalpy. This is the main reason for defining the property enthalpy.

11. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Energy Transport by Mass 11 When the kinetic and potential energies of a fluid stream are negligible When the properties of the mass at each inlet or exit change with time as well as over the cross section

12. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Steady-Flow Systems 12 Under steady-flow conditions, the mass and energy contents of a control volume remain constant. Under steady-flow conditions, the fluid properties at an inlet or exit remain constant.

13. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Balances for a Steady-Flow Process 13 Mass balance Energy balance Single stream

14. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Energy balance relations with sign conventions 14 KE and PE negligible Some energy unit equivalents Single entrance, single exit, unit time basis Unit mass basis General, unit time basis

15. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Steady-Flow Engineering Devices 15 Many engineering devices operate essentially under the same conditions for long periods of time. Therefore, these devices can be conveniently analyzed as steady-flow devices. 1.Nozzles and Diffusers 2.Turbines and Compressors 3.Throttles 4.Mixing Chambers 5.Heat Exchangers 6.Pipes and Ducts

16. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Nozzles and Diffusers 16 Nozzles and diffusers are shaped so that they cause large changes in fluid velocities. A nozzle is a device that increases the velocity of a fluid at the expense of pressure. A diffuser is a device that increases the pressure of a fluid by slowing it down.

17. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Turbines and Compressors 17 Turbine: drives the electric generator in a steam, gas, or hydroelectric power plant. As the fluid passes through the turbine, work is done against the blades, which are attached to the shaft. As a result, the shaft rotates, and the turbine produces work. Compressors, pumps and fans, are devices used to increase the pressure of a fluid. Work is supplied to these devices from an external source through a rotating shaft. A fan increases the pressure of a gas slightly. A compressor is capable of compressing the gas to very high pressures. Pumps work very much like compressors except that they handle liquids instead of gases. Energy balance for the compressor in this figure:

18. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Throttles 18 Throttling valve: flow-restricting device that cause a significant pressure drop in the fluid. The pressure drop in the fluid is accompanied by a large drop in temperature, and for that reason throttling devices are commonly used in refrigeration and air-conditioning applications. During a throttling process, the enthalpy of a fluid remains constant. Energy balance Isenthalpic

19. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Mixing Chamber 19 In engineering applications, the section where the mixing process takes place is commonly referred to as a mixing chamber. Energy balance for the adiabatic mixing chamber in the figure is: 10C 60C 43C 140 kPa

20. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Heat Exchangers 20 Heat exchangers are devices in which two moving fluid streams exchange energy without mixing. Mass and energy balances for the adiabatic heat exchanger :

21. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Pipe and Duct Flow 21 Pipe or duct flow may involve more than one form of work at the same time. Energy balance for the pipe flow shown on the right is

22. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Unsteady-Flow Processes 22 Many processes of interest, however, involve changes within the control volume with time. Such processes are called unsteady-flow, or transient-flow, processes. Uniform-flow process: The fluid flow at any inlet or exit is uniform and steady, and thus the fluid properties do not change with time or position over the cross section of an inlet or exit. Charging of a rigid tank from a supply line is an unsteady-flow process since it involves changes within the control volume. The shape and size of a control volume may change during an unsteady- flow process.

23. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Mass balance Energy balance A uniform-flow system may involve electrical, shaft, and boundary work all at once. 23

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25. 5. Energy Analysis of Control Volume Thermodynamics I Inter - Bayamon Due Date: Omar E. Meza Castillo Ph.D. Homework5  Web Page 25