1. The First Law of Thermodynamics
Cengel & Boles, Chapter 4
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2. The First Law of Thermodynamics
The First Law of Thermodynamics is also known as
Conservation of energy principle
Energy balance equation
The “First Law”
In basic terms, it states:
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3. Energy Balance for a Closed System
and
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4. Energy Balance for a Closed System, cont.
The energy balance statement becomes
Rate form:
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5. Closed System Energy Balance: Special Cases
Stationary system (KE, PE = 0)
Adiabatic system (Q = 0)
No work interactions (W = 0)
Cycle (initial state = final state)
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6. Energy Balance for Control Volumes
Balance of energy transfer by work, heat, and mass flow across system boundary plus stored energy within control volume:
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7. Energy Transport by Mass
Recall from Chapter 3:
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8. Energy Balance for Steady-Flow Systems
At steady-state, dEsystem/dt = 0, thus:
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9. Energy Balance for Steady-Flow Systems, cont.
For single-stream control volumes,
Dividing by mass flow rate gives the energy balance on a unit-mass basis:
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10. Steady-Flow Engineering Devices
Nozzles and diffusers
Turbines, compressors, and pumps
Throttling valves
Mixing chambers
Heat exchangers
Pipe and duct flow
See section 4-4 in text and PDF file ME152SSdevices.pdf on website
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11. Energy Balance for Unsteady-Flow Processes
Unsteady = transient
Examples:
Filling and discharging of tanks
Start-up and shut-down of turbines, compressors, hxers, etc.
Mathematically,
need to solve 1st order differential equation
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