WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition...
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WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 T h e r m o T h e r m o d y n a m i c s d y n a m i c s Çengel Çengel Boles Boles Third Third Edition Edition 4 CHAPTER CHAPTER The First Law of Thermodynamic s: Control Volumes
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WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition Chapter Summary The mass and energy balances for any system undergoing any process can be expressed as 4-21
Transcript of WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998 Thermodynamics Çengel Boles Third Edition...
WCB/McGraw-Hill © The McGraw-Hill Companies, Inc.,1998
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4CHAPTERCHAPTER
The First Law ofThermodynamics:
Control Volumes
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Chapter Summary
• A control volume differs from a closed system in that it involves mass transfer. Mass carries energy with it, and thus the mass and energy content of a system change when mass enters or leaves.
4-20
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Chapter Summary
• The mass and energy balances for any system undergoing any process can be expressed as
4-21
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• The mass and energy balances for any system undergoing any process can be expressed in the rate form as
Chapter Summary4-22
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• Mass flow through a cross section per unit time is called the mass flow rate and is denoted m. It is expressed as
where = density, kg/m3 (= 1/v)
= average fluid velocity normal to A, m/s A = cross-sectional area, m2
Chapter Summary
.
4-23
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Chapter Summary
• The fluid volume flowing through a cross section per unit time is called the volume flow rate V. It is given by
.
4-24
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Chapter Summary
• The mass and volume flow rates are related by
4-25
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Specific Properties and Enthalpy
A specific property is an intensive quantity obtained by dividing an extensive property ( or its flow rate) by the total amount of the process material. For example the specific Volume ( m3/kg) and specific kinetic energy is ( J/kg). We will use the symbol ^ to denote the a specific property: will denote specific volume specific internal energy
The enthalpy of the system H =U+PV
The specific enthalpy
V
V
V
U
VPUH
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Chapter Summary
• Thermodynamic processes involving control volumes can be considered in two groups: steady-flow processes and unsteady-flow processes. During a steady-flow process, the fluid flows through the control volume steadily, experiencing no change with time at a fixed position. The mass and energy content of the control volume remain constant during a steady-flow process.
4-26
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• Taking heat transfer to the system and work done by the system to be positive quantities, the conservation of mass and energy equations for steady-flow processes are expressed as
where the subscript i stands for inlet and e for exit. These are the most general forms of the equations for steady-flow processes.
Chapter Summary
for each exit for each inlet
4-27
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• For single-stream (one-inlet--one-exit) systems such as nozzles, diffusers, turbines, compressors, and pumps, the steady flow equations simplify to
In the above relations, subscripts 1 and 2 denote the inlet and exit states, respectively.
Chapter Summary4-28
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Chapter Summary
• During a uniform-flow process, the state of the control volume may change with time, but it may do so uniformly. Also, the fluid properties at the inlets and the exits are assumed to remain constant during the entire process. The conservation of energy equation for a uniform-flow process reduces to
4-29
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Chapter Summary
• When the kinetic and potential energy changes associated with the control volume and the fluid streams are negligible, the conservation of energy equation for a uniform-flow process simplifies to
4-30
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Steam Tables
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(fig. 4-6)
© The McGraw-Hill Companies, Inc.,1998
Velocity Profiles for Flow in a Pipe4-1
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Volume Flow Rate4-2
Volume flow rate is the volume of fluid flowing through a cross section per unit of time
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Mass Flow, Heat, and Work Affect Energy Content
4-3
The energy content of a control volume can be changed by mass flow as well as heat and work interactions
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Control Volume May Involve Boundary, Electrical, and Shaft Work
(Fig. 4-9)
4-4
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Schematic for Flow Work
(Fig. 4-10)
4-5
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During Steady Flow Process, Volume Flow Rates are not Necessarily Conserved
(Fig. 4-19)
4-6
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A Water Heater Under Steady Operation
4-7
(Fig. 4-21). .
.
.
.
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Steady-Flow Devices Operate Steadily for Long Periods
(Fig. 4-25)
4-8
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Nozzle and Diffuser Shapes Cause Large Changes in Fluid Velocities
(Fig. 4-27)
4-9
Nozzles and Diffusers are shaped so that they cause large changes in fluid velocities and thus kinetic energies
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Schematic for Example 4-24-10
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Schematic for Example 4-44-11
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Throttling Valve Devices Cause Large Pressure Drops in Fluid
(Fig.4-32)
4-12
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Ideal Gas Temperature Does Not Change During a Throttling
4-13
The temperature of an ideal gas does not change during a throttling(h =constant) process since h = h (T)
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T-Elbow Serves as Mixing Chamber for Hot and Cold Water Steams
(Fig. 4-35)
4-14
The T-ebow of an ordinary shower serves as the mixing chamber for hot- and cold-water streams.
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Heat Transfer Via Heat Exchanger Depends on System Selection
4-15
The heat transfer associated with a heat exchanger may be zero or nonzerodepending on how the system is selected
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Schematic for Example 4-94-16
.
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Rigid Tank Charging From a Supply Line is an Unsteady-Flow Process
(Fig. 4-47)
4-17
Charging of a rigid tank from a supply line is an unsteady-flow process since it involves changes within the control volume
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Temperature of Steam Rises Entering Tank, Flow Energy Converts to Internal Energy
4-18
(Fig. 4-54)
The Temperature of Steam rises from 300 to 456°C as it enters a tank as a result of flow energy being converted to internal energy
o
o
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Enthalpy of a Saturated Vapor at a Given Pressure
4-19
In a pressure cooker, the enthalpy of the existing steam is Hg@P (enthalpy of the saturated vapor at the given pressure)
